THE SOCIETY OF NAVAL ARCHITECTS ANO MARINE ENGINEERSOne W.rld Trade Center, Suite 1369, New York, N.Y. 10048

Pane, to be Presented at the Sh,D %br.tion Syrnm,i”mArli”@en, V.., October 16–17, 1978

Highly Skewed Propellers—Full Scale Vibration TestResults and Economic ConsiderationsN. O. Hammer! Member and R. F. McGinn, AssociateMember, Maritime Administration, Washington, DC.

ABSTRACT

This paper presents full scaletest results and discusses the effec-tiveness of three different highlyskewed propeller designs in reducingship “ibrati on. The test resultscover three merchant ship designs,including one OBO vessel and two RO/ROvessels, having installed horsepowerrangj.ng from 24,000 to 37,000 SHP.Limited discussion is presented on therelative effectiveness of installinga highly skewed propeller “ersusadding additional structural reinforce–ment on a vessel to reduce excessive“ibration. Some economic factors thatshould be considered by a ship ownercontemplating a highly skewed propel–ler for a new ship design are pre–se”ted.

INTRODUCTION

The subject matter of this paperis ccmcerned with use of highly skewedlpropeller designs to reduce shipboardvibration. One might ask consideringthe history of propeller technologywherein propellers have been installedon ships since 1837 why attentionshould be focused on this old andproven propulsive device? The answerof course is that while more than 140years of development work have goneinto increasing the efficiency andservice life of propellers and on aoverall basis the technology is welldeveloped, there still remains thepossibility for further improvementsfrom standpoints other than propul.

sive efficiency. Also the trends ofmodern ship designs wherein greaterhorsepower is being placed in singlescrew ships raises the possibilitytl-,atperhaps other design criteriabesides ‘efficiency” may be equallyimportant.

Administration becoming interested inthe possibility of using highly skewedProPeller designs on merchant vesselsstarting in 1969.

While there have been many in–stances of severe “ibration o“ shipso“er the past decades, the difficultyi“ sol”ing any specific problem has notbee” a lack of ideas concerning the“aricus potential remedial measures,b“t rather the uncertainty of successand economic costs associated with eachalternative. This paper is concernedwith the actual full scale performanceof highly skewed propellers installedon three relatively new merchant shipdesigns , including one Cre/Bulk/Oil(OBO) vessel and two Roll-On/Roll-Off(RO/RO) vessels. It should be notedthat one propeller was specificallydesigned to solve a known vibrationproblem, another designed to solve asuspected vibration problem emd thethird merely to demonstrate the fullscale performance characteristics ofthis old b“t novel propeller concept.

Within the past five years therehave been several technical papersdiscussing the design aspects of highly

by Boswell and .0. (fi~ w~%;~rsskewed propellers.

et ?.1(2) have addressed the numerousdesign considerations and potentialbenefits. Thus far hcwever only onepaper, that by Dashn?.w and Valentine (3]and one report (4) have presented full-scale performance data test results.Throughout this period, hcwever, n:anYpotential advantages have bee. cited.Others, noting the absence of full-scaleevidence bane cited an equal number ofdisadvantages associated witk highlyskewed propellers. Although the basicconcept of highly skewed propellersdates back to 1851, no full-scalel?i~hly skewed propellers for merchant

There is a second reason forexploring the use of highly skewed pro–pellers on ships and that is to attemptto cure instances of excessi”e vibra.tion after all conventional attemptsat cures have failed. This reason wasreally the primary one for the Maritime

1See Appendix A for explanation of

propeller terminology.2 Number in brackets designate

References at end of paper.

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Table 1 - “Asserted’,Disadvantages of Highly

Advantages

● Reduced ship vibration levels

. I,mproved shipboard CICW comfort

. Reduced M&R cost of navigationequlprncnt.

● Increased equipment life

. Greater propeller service lifedue to decreased blade cavit–atlon erosion

shiDs were constructed until 1974, someone:hundred andlater.

while sanecapabilities of

twenty–three years

of the performancethe first conmleted

hi~hly skewed propeller were ~resentedto the maritime industry at the FirstShip Technology and Research (STAR)symposium held Au9ust 1975, there

.

.

.

.

e

.

Advantages a,,dSkewed Propeller.

Disadvan~s

Cost. more than conventionalprOpeller$:

More susceptible to damage

Los. 5% propeller efficiencytherefore greater fuel bill

Acfdec5propeller weight requireslarger diameter tail shaft,stern tube

Shori.er propeller service lifedue t.cIncreased cavitation6?,0,,,0”

Inadequate strengi.b

still remain. some uncertaintyi“q the advantages and disadvantages of

regard–

th;sc unique appearing propellers.Table 1 s“marizes some of the “ariousad”antagcs and disadvantages that bavcbeen cited within recent years. It 1shoped that this paper will give moreinsight into the validity of theadvantages and disadvantages outlinedi“ the table.

Figure 1 - Sea Bridge Class (MarAd Design C5-S-78a)

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Table 2 - Sea Bridge Class Principal

Length Overall . . . . . . . . . . . . . . .Length Between Perpendiculars . . . . . . .Bean. Molded . . . . . . . . . . . . . . . .Depth To Main Deck At Side w , Molded . . .Draft Full Load (Scantling) , MoldedLight Ship . . . . . . . . . . . . .Passengers, Crew Effects, & Stores.Fuel Oil . . . . . . . . . . . . . .Anti-Roll Tank. . . . . . . . . . .Fresh water . . . . . . . . . . . .Refrigerated Cargo (In containers).Liquid Cargo.. . . . . . . . . . .Genera l Cargo.... . . . . . . .Total Deadweight. . . . . . . . . .Displacement, Full Load (Scantling)Cargo Volume, Bale, Cu. Ft. . . . .Containers In Hold (40 Cent. ). . .Containers on Deck. . . . . . . .Passenger Accommodations. . . . .Crew Accommodations . . . . , . .Shaft Horsepower, A.B.S.. . . . .Sveed. Knots . . . . . . . . . . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .Draft.. . . .. . . .. . . .. . . .. . . .. . . .. . . ..

Propeller 6 Blades. . . . . . . . .Propelling Machinery, Cross Compound,

THREE MERCHANT SHIP DESIGNS

The three merchant ship designsdiscussed in this paper in order ofactual ship delivery sequence are:(a) the Sea Bridge class (RO/RO)vessels constructed by Ingalls Ship-yard, (b) the San Clemente class (OBO)vessels constructed by National Steeland Shipbuilding Company and (c) theMaine class (RO/RO) vessels con-structed by Bath Iron Works.

It should be noted that thehighly skewed propeller installed onone vessel of the Sea Bridge class wasundertaken to solve a known vibrationproblem. Installation of the firsthighly skewed propel ler on the SanClemente class vessels was undertakenmerely to demonstrate the full scaleperformance characteristics of thepropeller concept. Whereas highlyskewed propellers were installed on theMaine class vessels to solve a sus-pected vibration problem that ulti-mately never materialized. Principalship characteristics of the threemerchant ship designs are outlined inTables 2, 3, and 4. Photographs fromone ship of each design are shown inFigures 1, 2, and 3.

Figure 4 has been prepared todisplay the trend of maximum horse-power levels for single screw merchantships and inspection of this figurereveals that approximately a 20,000horsepower level was maintained as amaximum plateau for the period of

Characteristics

. . . . . . . . . . 602’–0”

. . . . . . . . . . 560,–0”

. . . . . . . . . . 90 -0”

. . . . . . . . . . 59’-0’,

. . . . . . . . . . 34’–0,,

. . . . . . 10,900 L. Tons

. . . . . . . 150 L. Tons

. . . . . . 2,415 L. Tons

. . . . . . . 245 L. Tons

. . . . . . . 431 L. Tons

. . . . . . . 500 L. Tons

. . . . . . 2,300 L. Tons. . . . . 10.789 L. Tons

. . . . . . 16:830 L. Tons

. . . . . . 27,580 L. Tons

. . . . . . . . 1,300,000

. . . . . . . . . . . 245

. . . . . . . . . . . 167

. . . . . . . . . . . 12

. . . . . . . . . . . . 39

. . . . . . . . 30,000

. . . . . . . . . . . . 24. . . . . . . . . . . . 23’-0’,Double Reduction, Geared Turbine

1951-1966.3 Starting in 1967/1968however the maximum horsepower startedto climb reaching a new maximum plateauof 50,000 horsepower in 1973. Plottedon Figure 4 are the horsepower levelsfor each of the subject three merchantship designs. It will be noted thatthe first Sea Bridge vessel with a30,000 SHP machinery plant representeda new high horsepower level at time ofdelivery of the first ship in 1969.With regard to the San Clemente classand Maine class vessels, these designsentered service approximately sevenyears after the horsepower levels hadbeen established by other vessels. Thetiming of the delivery of each shipdesign relative to the maximum horse-power “line” is important because itindicates in an approximate way thedegree of risk being undertaken todesign a precedent setting vessel.This matter will be discussed in moredetail later in the section of thepaper concerning economic and riskaspects.

At the present time 7 highly skew-ed propellers have been constructedand installed on merchant ships with 5additional propellers under construc-tion. Table 5 indicates information onthe ship design, type ship, and thecorresponding number of propellers. Itwill be noted from inspection of thistable that the distribution of highly

3 See Appendix B for information onhistorical trends.

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‘Table 3 – San Clcnwnte Class Principal characteristics

892’–6,’855’–0”875,–0,’105,-9,’629-6”

45,–10,’T..Tons43,00037,000

L. TonsL. TonsL. TonsL. Tons1, TonsL. TonsL. TOnSL. I’o”sL. TonsL, Tons

2731

24,00016.5

26 ‘–O’,Turbine

4

Fiqure 2 - San Clemente class (MarAd Design 0B8-S-90a)

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Table 4 Maine Class Princioal Characteristics

Figure 3 . Maine class (MarAd Design c7-s-95a)

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Table5 - sunraryof U.S. Wrchant ShipHighlyskewed%mpsllers

I. Propellers Constructed to Date

Ship Eesiqn Ship Type

Sea Bridge RO/ROSan Clemente OBOSan Clemente TankerMaine RO/RO

11. Propellers Under Construction

Ship Desiqn Ship Type

Enterprise Container(Matson)

N.3vY AO-177 Tanker

Number of Propellers

Number of Propellers

1

i5

Maine●--- class

Sea Bridge4~- class

San Clemente.=class

I , I I I (1950 1960 1970 1980

Year of ~try Intoservice

Figure 4 – Maximum Shaft Horsepower Single screw1’

Merchant Ships

i-

i

Table 6 - Propeller Characteristics

Sea Bridge

original Skew

Shaft Horsepower 30,000 30,000RPM, Design 110 107No. of Propellers 4 1No. of Blades 6 6Diameter, Feet 23 23Skew, Degrees N/A 60°Skew, Percent N/A 100Weight, Pounds 75,700 80,000Material NiAIBz NiAIBz

skewed propellers already includes abroad range of merchant ship typessuch as : RO/RO, OBO, tanker and acontainership.

This trend is expected to contin-ue as more experience is gained withthese interesting propellers. Howeverfor purposes of this paper it shouldbe noted that the information pre-sented herein is considered to reflectfull scale performance results ofvirtually all merchant ships nowoperating with highly skewed propel-lers and represents results from3PPr0X1matelY 60 percent of the shipdesigns listed in Table 5.

Once the decision has been made tomove forward with the design andinstallation of a new shipboard feature,such as a highly skewed propeller, thetest and evaluation should be rela-tively straight forward. One firstmeasures the baseline performance ofthe ship fitted with the conventionalpropeller and then replaces the unitwith the ne” propeller repeatingloading conditions and test measure-ments. Normally in obtaining mea.c,ure–ments of full scale vibration perform–ante, one is concerned with: (a) hullgirder motion, (b) longitudinalmachinery vibration and (c) super-structure vibration. SNAME Codes c-1and c-4 (5) and (6) respectively, out-line the basic test procedures,instrumentation requirements, analysisprocedures to insure the quality of therecorded data.

Comparison of the “baseline”results with the ,,after”results thenindicates the degree of improvementthat was achieved. Since any full.scale data measurement program seemsinherently to always leave some gapsof information due to equipmentfailures, less than ideal test condi-tions, omissions of data recording, the

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San Clemente Maine

Original Skew original Skew

24,000 24,000 37,000 37,00092 92 120 120

2 2 1 45 5 6 6

26 26 22 22N/A 72° 10° 30°N/A 1004 17 50

106,000 116.000 80,000 90,000NnBz MnBz NiAIBz NiAIBz

perfect comparison is,seldom achie”edin practice.

For each of the three ship designsdiscussed in the following pages, thebasic effort has been to first obtainthe baseline measurements. The nextstep of repeating the measurements withthe highly skewed propeller has howevervaried somewhat from one project toanother. In the instance of the SeaBridge class, one ship the DEFIANCE(.X MORMACSEA) was used for all tests.With regard to the San Clemente classand the Msine class, here. sister shipswere used for test purposes. Thesevessels were the ULTRANAR and ULTRASEA,and the MAINE and NEVADA.

Design information on the conven–tional and highly skewed propellers forthe three different ship designs areoutlined in Table 6. The discussionpresented in the following pages out–lines additional background informationconcerning each vessel, describes thepropel ler design test programs, pre-sents the full-scale vibration resultsand lastly mentions any special note–worthly incidents that have occurredwith the highly skewed propellers.

1. Sea Bridae Class

Overview/Background. Early in 1969the SS MORMACSEA was delivered to Moore–McCormack Lines, the first of fouridentical ships being constructed byIngalls Shipyard. This 30,000 shafthorsepower vessel shown in Figure 5 wasat time of delivery the highest horse-power single screw merchant vessel thathad ever been constructed in the U.S.Excessive vibration was experienced atthe outset and a number of remedial

L

4 Second propeller has slight pitchcorrection and change in material fromMnB z to NiAIHz. Weight reduced to109,000 pounds.

k——

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Figure 5 - General wrangements, Sea Bridge CIa~~

corrective measures were undertaken toimprove the operating performance.Although a variety of work was con-sidered at that time with some struc-tural modifications incorporated in thevessel successfully curing some of thelesser problems, the MORMACSEA and herthree sister ships were utlimatelyaccepted by the owners without acomplete cure to the vibration problemever being achieved. Concern also hadsurfaced about the possibility of highpropeller blade erosion due to cavita-tion (7, 8, 9, 10). During 1970/1971work was carried out at the David‘raylorNaval Ship Research and Envelop-ment Center (DTNSRDC) to explore thepossibility of installing a highlyskewed propel ler on one of the “SeaBridge,’ vessels in order to reduce thelevel of ship vibration and minimizethe effects of cavitation. All fourships of the design were subsequentlysold to ~erican Export Lines andrenamed. The highly skewed propellerwork however resumed after a pause ofone year and was further developed.While utlimately another ship, theSS ULTRASEA discussed later in thepaper, was to be the first U.S.merchant vessel actually fitted with ahighly skewed propel ler, the workstarted and still ongoing for the ,’SeaBridge 8’vessels served as the basicfoundation work for all the otherhighly skewed propeller projectsdiscussed in this paper. Because ofthe special importance of the MORMACSEA/DEFIANCE work, the history of eventsand nature of the vibration problemsand overall program will be discussedin some detail.

Initial Problems and CorrectiveSteps - (1969-1971) . Early in 1969the MORMACSEA was nearing completionand extensive analytic and model workhad been completed aimed at avoidingpossible vibration problems. Due to

the high power of the vessel this workwas much more than normally undertaken.The propeller design had been testedseveral times to check performancecharacteristics and the extent ofcavitation and also special bladeerosion tests had been completed. Asthe vessel neared the time for builderstrial, three separate vibration shakertests were made on the vessel in themonths of February and March to locateand correct items subject to localvibration. At the end of March thevessel was taken out to sea for thefirst time on a builder, s trial andobservers posted to note vibrationproblem areas. After completion of thefirst official sea trial in early April,approximate ly twenty-five structuralmodifications were made upon returningto the shipyard. A second official seatrial was held some two weeks later andfive more structural changes were made.

Shortly after taking delivery ofthe MORMACSEA the owner indicated thatthere were st i 11 several shipboardlocations that appeared to be vibrating.In June a vibration survey was madeduring a voyage from Baltimore,Maryland to Elizabeth, New Jersey andnumerous shipboard locations wereindicated as being of concern. A num-ber of structural modifications weremade in the summer and during a coast-wise passage in water depths of ten tothirteen fathoms at full power thevibration was judge by the crew as beingexcessive. During the trans-Atlanticpassage under full power new locationsof vibration were discovered andpassengers complained of doors rattlingand beds shaking. The after end of theunlicensed crew rooms on the uPPerPlatform deck and master 8s stateroomdisplayed increased vibration.

In November, the MORMACSEA was atan East Coast shipyard undergoingrepairs and in order to examine theeffect of added structural stiffeningon the Bridge Ceck vibration, eightheavy vertical stanchions were installedbetween the Main Deck and the 2nd Ceck,between frames 107 and 126. Afterthese structural modifications werecompleted another vibration survey wasconducted between Baltimore and New York.TWO series of tests were conducted: ashallow water series during which thedepth of water did not exceed 100 feetand a deep water series in which thedepth was always greater than 300 feet.

Although a marked decrease inamplitudes at 105 RPM was noted, pre-viously a critical speed for the housestructure, amplitudes at top speedremained about the same. Overall housestructure behavior was such thatamplitudes tended to increase at thehigher deck levels. Also, amplitudestended to be greater at the shipcenterline and decrease closer to thedeck edge and boundary of the housestructure.

Based on the minor effect of thestructural reinforcement it wasgenerally concluded that extensivestructural modifications would beneeded to achieve further improvement.However, this was not considered apractical solution because extensivestructural work would be required infully outfitted spaces. The onlYremaining solution considered at thattime was to design and install a 5-bladed conventional propeller in lieu

.5The names of the four Sea Bridge

class vessels are: DEFIANCE (exMORMACSEA) , GREAT REPUBLIC (eX MORNAC-SKY) , RED JACKET (eX MORMACSTAR) andYOUNG AMERICA (ex MORMACSUN) .

.

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of the 6-bladed propeller that was onthe vessel. This work was never com-pleted. Concurrent with the vibrationsurveys, structural modifications, andspecial studies being made on theMORMACSEA, following sister ships werealso undergoing vibration tests andstructural modifications. During themonth of June 1969, a vibration surveywas made on the MORMACSKY (second ship) .Several structural modifications hadbeen made on this vessel and reportsfrom personnel abroad during the seatrials contained numerous subjectiveimpressions. For example one personindicated:

,,thestern area although nOiSY

(indicating propeller cavita-tion) was remarkably free ofvibration, being less than thatencountered on the Mariner qsat 22,000 SHP. Certain areasin the deck house however pre–sent problems. The deck in wayof the Master’s office, particu-larly at his desk, shakes inmost disagreeable manner. Thesingle person stateroom No. 7on the Cabin Deck also shakesuncomfortably. “

During Mid-September 1969 a vib-ration survey was conducted on theMORMACSTAR, (third ship) and distinctresonance of the bridge structure wasobserved at about 105 RPM in thevertical direction. The main differenceobserved between the MORMACSTAR and theMORMACSEA was the reduction in amplitudeat 105 RPM compared with that measuredon the first ship. However, the sameoveral 1 house structure behavior wasobserved on both ships, namely in-creasing amplitudes at the higher decklevels and greatest vertical movementat the ship centerline, decreasing asone neared the house exterior boundaries.

Near the end of 1969 serious con–sider.ation was being given to thepossibility of installing a new highlyskewed propeller design cm one of thevessels. Work was subsequent lY under-taken in 1970 to explore the potentialof using a highly skewed propellerdesign to solve the problems experiencedon the Sea Bridge vessels. A programwas undertaken encompassing the fol-lowing work: (a) design of a highlyskewed propel ler for the MORMACSEA, (b)manuf .act”reof B model propeller, (c)open-water characterization tests, (d)self propulsion tests with the skewedpropeller and the existing model hull,(e) cavitation tests behind the existingwater screen, and (f) construct ion andtesting from a strength standpoint of atwo-bladed model propeller using thesame material as for the prototype.This work was completed in 1971 and theresults documented (11). Essentiallythe following conclusions were reached

based on the extensive model test pro-gram:

●

9

●

●

●

Design propulsion characteristicswere met.

Propulsion performance was compar–able to the conventional design forthe same SHP.

The highly skewed design vi.sa visconventional design delayed cavita-tion inception by approximately twoknots.

The highly skewed design had lesstendency towards cavitation erosion.

The hiahlv skewed ProPe her hadadequa;e ;trength %or the proposedapplication.

At that point in time all thatremained to be accomplished was theconstruction and installation of theprototype propeller and full- scaletesting. However, the four ships weresubsequently sold to American ExportLines and the highly skewed propellerwork ceased at that point until thefollowing year.

Prcmeller Redesians and New TestProaram - (1972-1976) . In 1972American Export Lines (AEL), new ownersof the vessels expressed interest tothe Maritime Administration in movingforward with the highly skewed pro-peller work. While the vertical vib-ration of the accommodation spacehouse structure was a discomfort prob-lem to the crew, extensive propellerblade erosion had appeared indicatingdecreased efficiency and very shortservice life for the propellers.

BY September 1973, plans hadreached the point where AEL was author-ized to proceed with the performance ofthe firSt task of a renewed and somewhalexpanded highly skewed propellerresearch and development project penal-ing completion of a final contract.This first task of the renewed projectwas to review the earlier 1971 pro-peller design and ascertain thatimprovements developed over the now twoyear time interval would be incorporateinto the final highly skewed propellerthat would be constructed and installed .on one ship of the class. Key elementsof the overall full-scale verificationtest program included the following:(a) Measurement of hull and super-structure vibration, (b) speed, POwerand thrust measurements, (c) caV1t3ti0n .erosion tests, (d) visual and photo-graphic tests of propeller performanceand (e) progressive speed trials. Theoverall objectives of the highly skewedpropeller test program were as listedon the following page:

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* Verify the benefits of a tighlyskewed propeller versus a convel~-ticmal propeller on a high power andhigh speed vessel.

● Reduce the level of accommodationspace vibraticm o“ the Sea Bridgeclass ships.

. Increase propeller service life from5 years to 10 years.

. Ad”a”ce the state–of–the–art of pro-peller design.

. Investigate the effects of highlyskewed propeller design on cavita-tion eros, on.

The David Taylor Naval ShipResearch and Development Center em-barked o. a review of the 1971 pro-peller design. While the original 1971PrOPellCr design had been based onlinear skew and no rake, more recentNSRDC work indicated that differentskew distributions and also forwardrake should be Considered. Theharmonic content of the Sea Bridge wakewas reexamined and estimates made ofthe alternating thrust, vertical bcar–ing forces and horizontal bearingforces. Since the predominant directionof vibratory motion on the ship “eedi. gcorrection was vertical it was hopedthat a propeller designed to producelow vertical bearing forces wouldimpro”e the overall ship performance.Recognition was gi”en to the fact thatthe total forces generated by thePrOPellcr consist of both “bearing” and‘press”rem< forces with some uncertaintyremaining as to the conditions wheneither bearing or pressure forces aredominant.

A model propeller conforming to the“ew design parameters was constructedand tested in August 1974 and when thetest results became known it was dis–covered that full Power would be reachedat 102.9 RPM in lieu of the originaltarget of 107 RPM. Since the machineryplant of the ships, i.e. gears, shaft in9,etc. could not tolerate developing thefull 30,000 SHP at 102.9 RPM, (a highertorque le”s!l)the newly designed modelpropeller was unfortunately unacceptable.

Three wossible alternative coursesof action were the” considered asfollows : (a) construct the full scalepropeller based on the current modelpropeller design b“t make a pitchcorrection to increase the RPM, (b)revert back to the earlier 1971 skewedpropeller design, and lastly, (c) con–struct a new model propeller based onthe current design (with pitch changeonly) and perform “ew model tests toverify the design.

Figure 6 – sea Bridge ClassHighly Skewed Propeller Installation

The first option was judged veryrisky and also a situation would becreated wherein every PrOPeller in theentire project was sliqhtly different(i.e. model propellers different thanthe full scale propellers) therebycomplicating the correlation of modelwork with full scale work. The secondoption would revert back to the 1971skewed propeller design, however, thisdesign posed the greatest risk of fullscale blade failure due to higher pre–dieted stresses and also would haveadded complications to propellerinstallation and removal. This option

would also have adversely affected thepropeller blade erosion work then beingcarried out the most of any of thethree options The third option, thatof building another model propeller formore tests would increase costs andpose problems for timing, however thiswas the option selected. A new modelwas constructed and tested and this,the third highly skewed model propellerdesign for tbe vessels, proved to have

6 Results of the ULTRASEA tests wereknown at this time and there was doubtabout rrmdel tests gi”in’g accurate RPMpredict ion..

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12

11

10

9

8

7

6

5

4

3

2

1

Oq

(a)

03

0

0

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~. ●A0

h 0 O& ● ● I J60 70 80 90 100 110

RPM

‘Jertical Vibrationof ElevatorCasinqat upper&ck

6 (h)

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2

1.

L

00

0“00°00 00005 60 70 80 90 100 110

WM

AthwartshipVibrationof Elevatorcasingat C>bin Lkck

satisfactory performance test results(including RPM) and was used as thebasis for the prototype.

By now the DEFIANCE (ex MORMACSEA)had been selected as the test ship toreceive the new propeller and in July1974 the baseline hull and super-structure vibration performance measure-ments were obtained on a coastwisevoyage from Norfolk, Virginia to StatenIsland, New York. Work proceeded and

(c)

02

0

0

0

008

●*

●

T_daGiL50 60 70 80 90 100 110

m

VerticalVibration of Elew torcasingat Cabin &ck

KEY

0 Blade Frequency ComponentsConventional Propeller

● Blade Frequency componentsSkewed Propeller

Figure 7 - Elevator casing Vibration

4

the full-scale propeller was constructedand installed on the DEFIANCE in NewportNews, Virginia in May 1976. Figure 6shows the highly skewed propeller in–stallation. Although the originalschedule called for the full-scale testmeasurements to be made on a voyage

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from Norfolk, Virginia to Staten Island,New York, the ship was rerouted tofirst make a stop in Baltimore, Marylanddue to cargo commitments. During theshort voyage up the Chesapeake Bay

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VerticalHU1lVibrationat Stern

6 (b)

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hthwartshipH.11 Vibr.ation at Sterr!

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verticalVibrationof ‘IhrustBearlnq

KEY

0 Blade Frequency componentsConventional Propeller

6 (d).

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jd

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Athwartshi.pVibration of ThrustPearing

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“----

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050 60 70 80 90 100 110

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LongitudinalVibrationof ThrustE!earing

6

1

(f)

5

4

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LAei5_250 60 70 80 90 100 110

RFM

Ibmgitudi.nalVibrationof Gear ~s.

● Blade Frequency ComponentsSkewed Propeller

Figure 8 – Hull, Thrust Bearing and Gear Case Vibration

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L’——

12

11

10[

(b)

Om

w50 60 70 80 90 Io@ 110

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LongitudinalVibrationof HighPressureT\rbinc

KEY

o Blade Frequency ComponentsConventional Propeller

● Blade Frequency ComponentsSkewed Propeller

O 2 x Blade Freauency Componentsco”ventioneii PrO~elle>

+ 2 x Blade Frequency ComponentSkewed Propeller

9

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2

lt

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000

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●*●

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50 60 70 80 90 100 110

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bngitudinal Vibrationof LOWPressureTurbine

Figure 9 - High Pressure and Low PressureTurbine Vibration

while operating at full power in shallowwater the ship crew was very aware ofthe reduced vibration levels. Whileno test measurements had been madeduring the short voyage, the new Pro-peller had already received crewacceptance by the time the vesselreached Baltimore. In fact commentssuch aS : “this was the first trip 1have slept soundly coming up the bay,’,were heard from numerous crewmembers.On June 4, 1976, full scale vibrationmeasurements were made on the DEFIANCEconfirming the remarkable improvementof the accommodation house structure.Finally after seven years from thebuilders trial in 1969, the overalldiscomfort problem had been eliminated.

Vibration Mee.s”rements ,?mdResults .If one examines Figure 5 it will benoted that the vessel arrangement fea–tures a continuous fore and aft RO/ROdeck for vehicle traffic on the SecondCeck level. As has been discussedvertical vibration of the accommodationhouse was the primary vibration problemencountered on the Sea Bridge classvessels needing correction. Althoughnot apparent from the arrangement plans,struct”ra.lly there is little continuousvertical support throughout the centercore of the accommodation house withthe vast majority of the interior

bulkheads being of the joiner nonsupporttype. The machinery casing, whileextending from the machinery space tothe Bridge Ceck level, is divided intoport and starboard sections located out-board of the vehicle passageway at theSecond Deck level and does not mergeinto one open casing until the UpperCeck level is reached. Thus, the rigidvertical and athwartship support nor-mally provided by machinery casingboundaries on conventional ship designswas not possible on the Sea Bridge classvessels. In fact, the elevator shaftboundary bulkheads are the only struc-tural bulkheads in perfect alignmentextending from the machinery space upto the Bridge Deck level. Because ofunique aspect of the elevator casingbulkheads, two locations were selectedas the reference points for measuring,,before,,and “after,, hcmse vibration.

Figure 7 displays the results ofvibration measurements obtained onselected points on the elevator casingbulkheads and ?nspection of Figure 7Creveals that maximum amplitudes ofaPPrOxlmately 8.5 roils were reached inthe vertical direction at the Cabin Cecklevel at speeds corresponding to 107/108RPM . Athwartship measurements at thesame location and speed, Figure 7b,indicates that vibration levels were

R-14

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approximately 1.0 roilsor less.Vertical vibration of the elevatorcasing at the Upper Deck level as shownon Figure 7a, was 7.5 rnils, somewhatless than that measured two deckshigher. Superimposed on Figures 7a and7. are the results of measurements madeon the DEFIANCE after the highly skewedpropeller had been installed on thevessel and inspection of these figuresshows the remarkable improvementachieved. Vertical vibration at theCabin Deck once 8.5 roilsha. beenreduced to 2.5 roils for a 66 percentreduction. Vertical “ibration at theUpPer Deck level initially 7.5 roilsbeing reduced to 1.5 roils for a 80 per-cent improvement.

Fiq”res 8a through 9b, display theresults of ‘mbefore’band “after” measurc–ments at the ship stern, thrust bearingfo“”dat.ion, gear case, high pressureturbine, and low pressure turbine,respectively. These locatims conformto the standard locations outlined inSNAME Code C–1. Inspection of Figures8a and 8b pertaining to hull vibrationreveals substantially reduced vibrationlevels after the highly skewed propellerhad been installed. Vertical hull vib–ration at the stern initially 4.0 roilsat 105 PRM has been reduced to 1.5 roilsor less, a 62 percent improvement.Clearly a 50 percent reduction orgreater has been achieved at othershaft speeds .

With regard to the other standardl-cations, namely, thrust bearingfoundation, gear case, HP and LPturbines, Figures 8C through 9b alsore”eal the substantial improvementachieved. In fact at every measuredlocation the highly skewed propellerhas reduced the vibratory movement andif one searches for general trends Itcan be observed that tbe greatestirnprovemc”t consistently is measured atshaft speeds corresponding to localizedresonant frequencies. This appears tobe an important characteristic ofhighly skewed propeller performance andthe reader is encowaged to examinethe data and form independent con–elusions from the measured results.

11. San Cleme”te Class

Background. While substantialwork had been completed exploring thepotential benefits of installing ahighly skewed propeller on the SeaBridge vessels by 1971, the highlyskewed propeller work came to a haltmtil a shipowner was found that wouldbe willing to install one on his ship.Prospective shipowners were thereforecontacted reaardina their willingness.to consider ;nstal~ation of a hi~hlyskewed propeller .

Aries Marine Shipping Company whohad signed contracts in June 1971 withNational Steel and shipbuilding Company(NASSCO) to construct two 80,500 DwTSan Clemente Ore/Bulk/Oil (OBO) carriers,indicated a willingness to try thehighly skewed propeller concept. InApril 1973 the Maritime Administrationawarded an R&D contract to install thefirst highly skewed propeller on one ofthese 24,000 SHP merchant vessels.Arrangements of the San clement. classvessel arc shown an Figure 10.

In contrast to the situation on theSea Bridge class vessels here was a newvessel design not yet constructed withno known or predicted “ibration problems.Rather this was the first shipownerwillinq to accept the risks i“”olvedwith the development of an old but yetnew and highly inno”ati”e propellerdesign.

The basic model test program forthe San Clemente OBO was similar to thatcarri.cd out for the first Sea Bridgehighly skewed propeller design, namely:(a) a wake survey was conducted at fullload displacement , (b) a highly skewedpropeller designed, (c) a model propeller

tlgure 11 – San Clemente classHighly skewed Propeller Installation

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Figure 10 - General .lrrangements, San Clemente class

.

manufactured, and (d) open-water exper–irnents and resistance and propulsionexperiment.s conduct ed. Some additionalmodel testing work was performed in-volving measurements of the propellerinduced pressures on the model hullsurfaces abo”e the propeller locationat the full load displacements for boththe conventional and’highly skewedpropellers.

Overall the model test work pro–gressed “ithout any special problemsbeing encountered. The ProPel ler wasdesigned for the best fit in theaperture emd such that it could beremoved without pulling the tail shaftor removing the rudder. The hiqhlyskewed wheel was also designed to ha”ethe same nwnber of blades and tipclearances as the conventional pro–peller .

Baseline vibration and performancemeasurements were made i“ July 1973,on the ULTRAMAR (first ship) fittedwith the conventional propeller.Although no special “ibration problemswere encotmtered, the “.ss.1 “as foundto de”elop the full 24,000 shaft horse–power at 88 RPM, some 4 RPM below thedesign “alue of 92 RPM. The ULTRAMARwas subsequently delivered and limitedto operating at speeds no greater than80 RPM until 6 inches could be remo”edfrom the propeller tips.

Approximately ei9ht months laterthe ULTRASEA (second ship) fitted withthe highly skewed propeller was takento sea and another set of full scaleperformance measurements were taken.Figure 11 sho”s the highly skewed pro–peller installation. On this vesselit was fotmd that the full 24,000 shafthorsepower was dewloped at 98 RPM, or6 RPM greater than the target of 92 RPM.Both conventional and highly skewedpropellers had missed the design RPMby a substantial .amo””t.

While the historical events lead–lng “p to the installation of a highlyske”ed propelle, on the DEFIANCE werecomplex and of special interest, theinteresting aspects of the San Clement.class propeller project were about tobegin. On March 5, 1974, the ULTRASEAwas off the coast of San Diego,California and almost finished withthe last remaining sea trial test itemsinvol”ing forward and astern crash testnlaneu”ers. After a successful crashahead had been completed and full aheadspeed reached, a crash astern comandwas given. The engines respcmded,rapidly coming to a halt a“d started tobuild up astern po”er. AS the secondsand minutes passed both boilers werenoted to be operating in the o“erloadcondition. A shaft rate impulse a“dsound was detected and leaning o“er the

stern rail, one could “iew an erraticbubble-filled wake. Propeller shafttorque readings fluctuated and as thetest continued it became apparent thatone progeilcr blade had failed. No oneaboard was certain of exactly what hadhappened. The sea trial was terminatedand the “essel proceeded back to theshipyard.

Upon deball.asting and reaching theshipyard the propeller shaft was slowlyrotated by the jacking gear a“d itbecame clear that o“. blade had beensubstantially bent aft after strikinga hard metallic object. The “cry tipof the damaged blade was curled aft forabout a 3“x3>’area and Figure 12 shcwsa closeup view of the tip damage.

Figure 12 – Tip of DamagedProPeller Blade

Subsequent examination later showed theentire blade set aft and distorted withthe bend located about 4 to 5 feet fromthe tip.

Figure 13 indicatesthe extent of the damage .5’+OT;’’::m::ed

PrOPeller blade was subsequently heatedand faired i“ place and returned to thedesign pitch and rake, following bystress relie”ing. Fiqures 14 and 15respectively, show the damaged propellerblade partially refaired and completelyrepaired.

The ULTRASEA was subseqwntly takenout to sea again to complete the re-mal”ing test items a“d ultimatelydeli”ered to the owners at the e“d ofMarch 1974. Thereafter the ULTRASEAwas engaged in “orldwide Ser”ice andaPPrOxim.telY six month. later while in

7Numerous s“bmari”es were Operating

o“ the surface in the “ear “ici”ity ofthe ship and some months later it waslearned that the propeller damage mayha”e been caused by striking a sub.rncrged submarine.

R-17

Figure 13 - Overview of Damaged Propeller

port it was observed that approximately4 feet8was missing from one propellerblade . A photograph of the broke.PrOPcller is .hown in Figure 16. Themissing tip was discovered by a matereading vessel draft marks and theexact date of the failure is not known.

In November 1974 the hiqhly skewedPrOPcller WaS removed from the ULTRASEAand replaced with a conventional pro–peller. The damaged hiqhly ske,:edpro–peller was then returned to the manu–facturcr and a new tip section installed

Figure 14 – Partially RcfairedPropeller Blade

After completion of repairs the pro–pcller was then installed in October1975 on the ULTRAMAR, the sister shipto the ULTRASEA, and the propelleragal” returned to service. Sometimein October 1977 some two years laterthe ULTRA!!AR struck a submerged objectwhile docking in the port of corpusChristi, Texas. The “,ss,1 departedfor New Orleans and while underway atlow RPM an unusual noise and vibrationwas not,ced. During the voyage the“essel stopped due to a machi”cry prob-lem and in the maneuver involvingreversing of the engines from 60 RPMahead to approximately 20 RP:Iasternthe ProPeller nut backed off and thePrOPeller WaS lost at sea where itstill remains.

Vibration Measurement Results.Baseline hull girder and machineryvibration measurements made on theULTRIJIAR (first ship) fitted with theco””entional propeller are shown onFigure.s17a through 18b. Limited super.structure rnea,s”rementsmade o“ the “E”deck of this vessel are shown o“ Figure19. Superimposed on Figures 17athraugh 19c are the results of repeatmeasurements made o. the ULTRASEA fitte<with the highly skeweti propeller.

8The propeller blade broke appruxi–

nmtel.y at the k“ucklc shown in Figure1!,,

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no vibration Fr0blC7.., ex+.,.nat~on ‘fFiqures 17f through 18d and Figure 18cfor the LP turbine, reveals that themain machinery components of theULTRAMAR were approaching a longitudinalresonant can~.tlon. when the hiqhlY

skewed propc:llcr was tested on theULTRASEA one ... observe that thevessel passed thr=ugh the criticalspeed at approximately 96 RPM and thatthe vibration le”els dimenxshed there–after.

In the ...5.of the San ClcmenteOBO vessels even though all tests havebeen completed there still remal:s un–Certainty regarding a determlnat LOn ofthe amount of improvement due to theh].,gllyskewed PrOPeller. The reason

for this assessment is that the fullscale highlY skewed prOPcllcr Ope~atcdat some 6 RPM hiqher than the dcslgnvalue, while the full. scale conven-tional propeller operated some 4 RPMon the low side resulting in a 10 RPMdiffcrence between the two propellers.Essentially this mea”. if one attemptsto make a comparison at co”st.ant RPM

Figure 15 – Propeller Aftercompletion of RePalrs

Figure 16 – Broken Propeller Blade

“al”es, propeller induced power levelswill be substantially higher fOr thecon”cntional propeller than the skewedpropeller thus precluding . truc com–parison. on the other hand ~f Oneattempts to make a comparison on thebasis of equal shaft horsepower, thepropeller Rp!+ (and blade frequency) forthe highly skewed propeller will begreater and inasmuch as sore shiplocations and components may be highlyfrequency dependent, this also voids~ true comparison.

While there cannot be a posltlvedetermination of the degree of improve–m,nt made by the highly skewed PrO:pcller, the authOrs’ are ‘f ‘he ‘p’n’onthat “ibration lewls were reduced byone–half overall. It should be noted

that in every location where rncasure–ments were taken, the highly skewedpropeller always resulted in reducedlevels of “ibratio” regardless ,Of RPMand direction of vibratory motion. Insane insta”ccs, where the ship wasoperating near to, Or at criticalfrequencies, reductions were one–fourthto one–sixth of the original value.Again it should be observed that thehighly skewed propeller shows gre:kestimpro”emcnt vis a “is tbe :O:”entlonalProPeller at resonant c:nd~tl:ns of“ibratlon. The reader ,. lnv,ted ‘0

test rcs”lts and formexamine the tindependent Conclu.lon. .

111. Maine Class

o“er”iew/Background. In contrast

to the rather complicated history ofe“c”ts associated with the Sea Brl,lgeand san clemente class highly skewedl>l.-opellerprojects, the design andtestinq of the highly skewed PrOPellersfor the Maine class “cssels as well aSservice experience has been very routxncin nature.

After construction contracts hadbeen awarded for the subject .hiPs andabout midway through development of thedetailed engineering phase concern wasexpressed about the rigidity of the aft

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Figure 19 - House Vibration

peak stern structure because of the veryfine after body hull lines.

Extensive vibration analyses wereundertaken and the results of this workindicated the strong possibility thatexcessive lateral vibration would beencountered in the stern area. Esti-mates were made of the performance ofboth 5-bladed and 6-bladed propellerswith the decision made to move forwardwith a 6-bladed conventional designpropeller. Dverall arrangements ofthis 37,000 shaft horsepower RO/ROvessel are shown on Figure 20.

A proposal was later made to con-sider the use of highly skewed pro-pellers on the ships to reduce theexcitation forces in the stern areaand subsequently a change under contractwas issued for the design, test andmanufacture of four DTNSRDC designedhighly skewed propellers. Because of

lead times, launch dates, etc. the planwas that in the event the conventionalpropeller proved satisfactory on thetrials of the first ship, that pro-peller would be retained aboard as theservice propeller. Three of the highlyskewed propellers would, however, beinstalled on the following three ships,with the fourth highly skewed propellerbecoming the spare.

Propeller model testing work forthis design took place in the summer of1974, approximately the same time asthe Sea Bridge model work was takingplace. While the propeller design andconstruction of a model propeller werecarried c,”tat DTNSRDC, the actual

model test program work was carried cmt .

at the Netherlands Ship Model Basin.It should be noted that construction of

the model propeller was completed ab~utthe same time as the model of tihesecondSea Bridge highly skewed propeller

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Figure 20 - General Arrangements, Maine Class

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Figuxe 21 - Maine ClassHighly Skewed Propeller

design and in fact both propeller modelswere in the model shop at the same time.Differences between the two designs weremost striking. A photograph of theMaine class highly skewed propeller isshcw” in Figure 21 and the readershould compare Figure 21 with Figure 6from the DEFIANCE.

Due to the vessel design, prO-peller weights and other factors, con–siderable attention had to be given toshaft alignment on the Maine classvessels. while severe stern tubebearing problems were encountered onthe MAINE (first ship) fitted with theconventional propeller, these bearingproblems are beyond the scope of thispaper addressing the effectiveness ofhighly ske”ed propellers and will notbe discussed.

Vibration Measurement Results.Results from the baseline “ibrationmeasurements obtained on the NAINE areshown on Figures 22a through z4e.Superimposed on these figures are theresults of measurements obtained on theNEVADA (second ship) fitted with thehighly skewed propeller.

Since the amount of skew o“ theMaine class highly skewed propellers,is much less than that for the SeaBridge and San Clemente projects, onewould expect that the amount of improve-

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Figure 22 - Hull Vibration

ment on the NEVADA vibration le”eIs

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Figure 24 - Low Pressure Turbine, Condenser and House Vibration

R-26+ -–

60 percent or greater. For example,

examination of Figures 23f through 24c,reveals that movement of the high-pres-sure turbine was reduced from 5.5 roilsto 1.5 roils for a 63 percent reduction,movement of the low-pressure turbinewas reduced from a peak value of 8.0roils to 2.5 roils for a 70 percentreduction, and movement of the condenserreduced from 6.5 roils to 2.5 roils for a62 percent reduction.

Improvement in vibration levels onthe structural measurement points tendedto range from virtually no improvementin the athwartship vibration at thestern, Figure 22b, to only a 25 percentreduction in the athwartship vibrationof the house as shown in Figure 24e.Considering that the Maine class pro-pellers were designed to reduce thelateral movement of the after peakstern structure, review of the measure-ments could lead one to conclude thatthe highly skewed propeller failed tomeet its objective. Conversely, onecould also say that the highly skewedpropeller performed exactly as expectedsince vibration levels of 2.0 roils andless are very small indeed and thatthe conventional propeller performedmuch better than any of the detailedvibration analyses predicted. Regard-less of which view point the reader maytake the evidence is clear that theMaine class highly skewed propeller froma vibration standpoint performed muchbetter than the conventional propeller.The character stic of highly skewedpropel lers to show greatest irr.rrovementat resonant conditions of vibration wasagain demonstrateed.

STRUCTURAL REINFORCEWENT VIS A VISHIGHLY SKEWED PROPELLER

The purpose of this section is toaddress some of the factors involvedin situations where it is found neces-sary to correct excessive vibration byadding Structural reinforcement.Clearly it is impossible to address alltYPeS Of problems, e.g. excessive hullgirder, local vibration of large sub-structures, machinery vibration, etc.and possible cures. To a limited degreethe authors, hope that the full- Scaleperformance data and discussion providedthus far on the three highly skewedpropeller projects can give the readersome insight into hull girder, housestructure and machinery behaviorrelative to this matter.

For illustrative purposes, oneactual example of a severe longitudinalmachinery vibration will be presentedwherein a problem was corrected bystructural reinforcement. “Bef ore’8 and‘,after” vibration measurements will bepresented with information on theextent of steel reinforcement added tocorrect the vibration. Following this

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some estimates will be presented on therange of possible improvement that mighthave been achieved if a highly skewedpropeller alternative been followed inlieu of the structural stiffeningapproach.

ExamDle - Vessel with SeVeI?e Lonui–tudinal I.fachinerv Vibration. Figure 2Sdisplays the results of baseline measure-ments on the bull gear shaft of a 28,500SHP container ship that experienced .severe longitudinal machinery vibration.Upon discovering the problem, the ship-owner was requested to limit the opera-tion of the vessel to 95 RPM and below.After consultation with the turbine andgear manufacturer, 10 roils single

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Figure 26 - Hull and Thrust Bearing Vibration

amplitude was established as the maximwnvibration level at which the main pro-pulsion machinery could tolerate with-out speed restrictions. The object ivethen became to find a cure that wouldreduce the vibration level to the 10roils target level.

In situations such as cited bythis example, there are many potentialremedial measures that can lead to atechnical solution. Unfortunately therealways remains substantial uncertaintyabout the degree of success one mightexpect with each alternative. Also, akey element affecting the decision pro-cess is the reluctance on the part ofeither the shipowner or the shipbuilderto take the lead i“ choosing the spe-cific corrective action that should betaken. This situation always existsbecause of the possibility of failureand the fact that the cost of the

failure must be paid be someone. Whileultimately a structural cure was found,many alternatives were considered by theparticipants associated with tbe subjectcontainership.

To illustrate the range of alter-natives, Table 7 has bee” prepared tooutline some of the advantages and dis-advantages associated with eight alter-native courses of action that were con-sidered to eliminate the excessive vib-ration enco””teced on the container ship.These alternatives included such conceptsas structural reinforcement, relocation .of the thrust bearing foundat ion, pro-peller modifications, and even consid-eration of a speed restriction.

The overall effort including dis-cussions. meetinas. a“alvtic studies,. . . .exchanges of correspondence, etc.consumed S1ightly more than two months.

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Table 7 - Alternative Courses of Action to CorrectExcessive Longitudinal Machinery Vibration

Alternatives

Cut and decrease thrust

bearing foundation

stiffness.

Cut propel ler tips andreduce diameter (increaseRPM )

Twist propeller blades(increase pitchldecreaseRPM ) .

Add additional thrustbearing foundationreinforcement.

Relocate thrusting foundat ion.

bear-

New propeller

Restrict service speed

Combinations of above

Advantages

Simple modificationvery little delay inship delivery. Slightcost in testing modifi-cation.

Can be accomplishedwith little delay inship delivery. Slightcost increase.

Reduction in shaft speedwill avoid steep part ofresponse curves.

Will raise critical fre-quency of shafting system.Little delay in shipdelivery.

Enables thrust bearinato be keyed into -strongest supportstructure.

Maximum blade frequencycan be moved belowcritical frequency.Depending on whetherconventional or highlyskewed propeller followed,excitation forces may bereduced.

No changes to vesselneeded. May decreasefuel oil cost.

Selected items fromalternatives.

However, once the decision point hadbeen reached to move forward with struc-tural reinforcement of the thrust bear-ing foundation, the actual ship modifi-cations were completed within onlyapproximately one and one-half weeks.The reinforcement work included theaddition of two new sloping bulkheadsin the machinery space and relocationof piping and wiring in the affectedarea. New bulkheads, approximately 26

R-29

Disadvantacies

Accepts critical operatingcondition in range of pro- ,peller RPM. Increasedstatic movement of prOpel -ler shafting and gear trainbetween ahead and asternconditions.

?+cceptscritical frequencyin range of propeller RPM.Increased rotational speedof turbines and gears mayvoid warranty. May in-crease propeller bladecavitation.

Increased shaft torque andgear loading. May voidwarranty for gears and tur-bines. All blades may nothave equal pitch aftermodification.

Difficult to add newstructure in existingengine room. Relocationof some machinery compo-nents, piping, etc.required.

May require new line shaftsections. Substantialripout of existing struc-ture. High costs involved.

New propeller requires 12-18 months lead time. Shipforced to operate atreduced power until replace-ment. Cost of new prOpel-ler plus other potentialcost items.

Ship has more power thancan be used. Speed lessthan upon which designand economics based. Moneyinvested in excess powerplant.

Selected ikems from above.

feet long and 10 feet high, effectivelyextended the existing shaft alleylongitudinal bulkheads into the machin-ery space, terminating at the thrustbearina foundat ion. The total struc-tural reinforcement required about 6$tons of 3/4 inch plate and associated

&

stiffeners.

Figures 26a through 27b displaythe ,,before,, and ‘,after”results Of

,+-

18

17

16 [

(i)

15

14

13

,1

00

12

m 1110 MiIs lk.xinum

kvei

-i

i ‘;: 0

0

$8. 00

!)’- 0*

6 w

5

1

#

00**4

●

3

I

●

Oul

2●

0001***

o I 1 1 I , I5 60 70 80 90 100 110

!w61

~Wit~inal vibrationof ThrustElearimg

KEY

o Blade Frequency ComponentsBefore structural Modifications

9

0

0

00

Mils Maxim

“e~0

0●

0

9d

●

00

00°

~om

Oa

.0 ~

t 160 70 80 90 100 110

P.m

LongitudinalVibrationof Forward~d of EU1l Gear shaft

Blade Frequency ComponentsAfter Structural Modifications

Figure 27 - Thrust Bearing and Bull Gear Vibration

measurements at selected locations onthe vessel. Figures 26a and 26b indi-cate vertical and athwartship vibratoryamplitudes of the hull girder asmeasured at the stern of the vessel,and Figures 26c through 27a indicatethe thrust bearing foundation movementin the three principal directions.Figure 27b presents the measurementstaken on the forward end of the bullgear shaft. Superimposed on thesecurves are the results of measurementsmade after extensive structural rein-forcement had been made to the thrustbearing foundat ion.

From inspection of Figure 27b itca” be seen that the initial maximumlongitudinal propeller shaft movementwas reduced from a maximum amplitude of17.5 roils to 10.5 roils or a 40 percentreduction at the maximum 28,500 horse-power of the vessel. Looking at thelongitudinal movement of the thrustbearing foundat ion, as shown on Figure27a, maximum movement was reduced fcom12.5 roils to approximately 6.5 roilsor a48 percent reduction. Both figuresindicate the extent of test scatter onecan encounter when operating close to acritical vibration frequency.

.

R-30 &----

Inspection of Fiaures 26a and 26bdisplayi~g respectively vertical andathwartship vibration of the hullgirder, ,shows that at some propellerspeeds vibration was not reduced, butrather increased significantly over thebaseline values.

One key observation to be madefrom the measurements is that it isextremely difficult to achieve a 50percent reduction in longitudinalmachinery vibration by rather massivestructural stiffening. Also, while thestiffening may reduce the movement atthe location of concern, ,the additionalstructural members may redistribute theexcitation forces throughout the vesselwith some locations actually increasingin the amount of movement.

Based on test results presentedfor the Sea Bridge vessels, the SanClemente vessels and the Maine classvessels a substantially differentresult would he expected if a skewedpropeller solution had been followed.Bull gear shaft motion would probablyhave been reduced 60 to 65 percentwitlnin tbe range of the critical speedband (95-105 RPM) and perhap 50 percentat speeds below 95 RPM. Elsewhere,vibration levels would have decreased50 percent instead of rather increasedin level.

If one were given the option todayto correct the original condition, whichpath should be followed? From a timeand cost standpoint the structural cureis the better choice. If one had in-stalled a highly skewed propeller atthe outset, would there have been aproblem in the first place? Probablynot.

EcONOMIC ASPECTS AND RISKS

While the discussion on highlyskewed propellers up to this point hasconcentrated primarily on technial as-pects, the matter of economics and riskswill now be discussed. There are severalintangible factors for which it is dif-ficult to place a dollar amount withgreat certainty. For example, if vib-ration levels are reduced through use ofa highly skewed propeller, how muchmoney will indeed be saved on an annualbasis? Since it is not possible toquantify with great precision the‘,savings,,that a shipowner might expect,the discussion presented herein willconcentrate primarily on estimating themagnitude of possible cost increasesthat may be expected for a given situa-tion. By following this path, it ishoped that sufficient baseline informa-tion will be presented to enable readersto prepare a cost/benefit analysistailored to tbe specific situation inwhich they may be considering the use ofhighly skewed propellers.

Cost Considerations

Figure 28 outlines some of the costfactors associated with the design, testand manufacture of conventional andhighly skewed nickel aluminum bronzepropellers for a range of shaft horse-power levels. It should be noted thatthese cost figures represent approximatecost levels as of January 1, 1978, forpropellers corresponding to thosealready installed on the merchant shipsdiscussed in this paper. Cost amountsin Figure 28a represent the situationwhere only 1 propeller is designed,manufactured and installed aboard avessel whereas cost amounts presentedin figure 28b represent the situation

Table 8 - Ship Construction Situation Scenarios

Alternatives

~ Situation

1 No problems expected,and none encountered.

2 No problems expected,however problemencountered.

3 Problems expected butnot encountered.

4 Problems expected andencountered.

Convent ional

Baseline Costs

Unexpected correc-tive work Cost in-creases.

some structural cor-rective action may berequired.

Corrective actionsneeded. Perhaps prob-lem not cureable byconventional methods.

H. S. ProDeller

cost increase. Lowervibration levels.

Problem may be avoided.

similar to Case No. 1.

Minimizes extent of .

problem. Further cor-

rective action required.

R-31

400

300

100

0

PrOF1ler costs – 1 of ]

Cs Cs Cs

o 10 20 30 40ShaftHorsew!,er-Tlxx,sand,

and 13ighly

where a series of four propellers aredesigned, manufactured and installedaboard a series of vessels. Because ofthe history associated with the back.ground of these designs, where primaryconcern was to minimize risk of failure.they therefore do not represent designsoptimized on the basis of both perfor.mance and cost. In other words, thecost differential for follow-on designscould be less in the future because ofthe experience gained from the propel-lers outlined in Table 5.

Figures presented as design costsare based upon government and industryestimates and the authors found thatthere is little or no difference indesign costs for skewed propellersversus conventional propellers. If,however, the skewed propeller is beingdesigned with special considerationtowards vibrational problems it ispossible some additional costs could beincurred.

Figures presented as shipyard costsare based on the cost of installationof the propeller plus the freight cost

R-32

PrOwller Costs - 1 of 4

CScscs

o 10 20 30 40shaftHorsepower-Ttwusanci5

Figure 28 - Cost Factors for ConventionalSkewed Propel lers

of delivering the propeller to the ship-yard. Some cost differential is foundfor skewed versus conventional, andthis is a result of some additional costfor installation for the ske”ed propeller.

The figures presented as propellermanufacturing costs are the costs ofmaterials and construction for the pro-peller. It should be noted that inasmuchhighly skewed propel lers tend towardsgreater weight there is a correspondinggreater cost.

Nevertheless, using the cost infor-mation developed for the existing pro-peller designs, it can be noted that ahighly skewed propeller will cost some-whers between 11 and 13 percent morethan the baseline conventional propeller.However, this clearly may not be thecomplete story, because several potentialcost items have not been considered.These cost items are: cost for larger &tail shaft, cost for larger stern tubebearing, cost for larger stern tube,realignment of shafting system. All ofthese items are affected by the magnitudeof the increase in weight of the highly

skewed propeller over that of the con-ventional propeller. Fortunately forall of the projects described in thepaper, physical changes to the tailshaft, stern tube bearing and sterntube were not required based on resultsof the engineering analysis. Adjust-ments in shafting alignment were neededfor only one of the three ship projects.

Moving forward, one must considerthe possible choices facing a shipowneror shipbuilder when contemplating pro-peller selection for a new ship design.Basically, the tendency in the pastdecade has been that ship specificationrequirements with regard to vibrationhave become more specific and detailed.This situation has arisen because ofnumerous unpleasant experiences in thepast with new ship designs. Againfocusing on Figure 4 pertaining to thehorsepower growth curve it is clearthat ship designs breaking new grouhdas plotted on this figure face the riskof encountering unacceptable vibrationlevels. Indeed, experience has shownthat even if the design does not breaknew ground, the state-of -the-art issuch that one cannot guarantee problemswi11 not be encountered.

Consider then four possible situa-tion scenarios outlined in Table 8.Case Number 1 portrays the situationwhere ,,noproblem was expected andnone was encountered”. This situationcorresponds to that described in theSan Clemente proj eCt where the ULTRANARand ULTRASEA could ultimately operatesuccessfully from a vibration stand-point without restrictions. Vibrationlevels were clearly lower on the vesselfitted with the highly skewed propeller,and both propellers were off the designRPM .

Case Number 2 represents the sit-~=tion Where ,,nOproblems were expected

but severe problems were encountered”.This situation coincides with thatdescribed concerning the Sea Bridgeclass. A cure could not be found fol-lowing the path of structural modifica-tions because a complete redesign ofan existing structure would have beennecessary. Installation of highlyskewed propellers at the outset wouldhave avoided the problem completely.Given the identical circumstancestoday, investment of $35,000 or 11.5percent more than the baseline costwith the conventional propeller wouldhave avoided the problem.

The situation described concerningthe containership longitudinal machineryvibration becomes somewhat of a toss-up.Investment of $40,000 or 11-13 percentmore than the baseline cost initiallywould have avoided the problem. However

once the problem had been encountered,a structural cure was found within amatter of weeks as opposed to 12-18months if it had been decided to fallowthe path of designing and constructinga highly skewed propeller.

If on the other hand the structuralmodifications had not solved the problemand one then was forced to proceed withthe design and manufacture of a highlyskewed propeller at that point in theproject, an additional outlay of$350,000 would have been required. Alsoa SitU?itlOn would have been createdwherein there would have resulted atleast one unusable conventional propel-ler posing a disposal problem and fur-ther increasing costs.

Case Number 3 represents the situa-tion where ,nproblemswere expected butnone encountered’,. This was the situa-tlOn relative to the Naine class ROIROvessels. Although the vessels did notexceed previously established horse-power levels, elements of the shipdesign were significantly advanced topose real concern about possibleexcessive vibration. Again, recognitionmust be given to the state-of-the-artof “ibration prediction technologywherein it is still not possible toplace total confidence in predictionresults. If the situation anticipatedhad been encountered, namely excessivevibration, the cure had already beendeveloped and was cm hand for promptcorrective action.

Case Number 4 represents to adegree an unusual situation. Here,,aproblemsare expected and indeedencountered ,’. This could have beenthe situation with the Maine class ves-sels, but it was not. Discussion onthis situation is very difficult to pre-sent, because it must be tailored tothe specifics of the problem. In otherwords, the first step of resolution mustbe to identify the possible courses ofaction that could be taken to rectifythe problem. The alternatives couldinclude rearrangement of structural mem-bers, movement of machinery components,all the conventional remedies such aschange of propeller RPM by cutting bladetips, etc. If the problems were antici-pated, those participating in theengineering decisions would most likelyhave some idea of possible cures. Infact in some instances where a problemis expected (although minor in nature)it is routine practice of some shipyardsto delay the cure until there is posi -tlVe confirmation of the existance ofthe problem. If the magnitude of theprojected problem is great, however, L

there may not be any inexpensive curesavailable or on the horizon. Thus far,the performance of highly skewed Pro-pellers has been so remarkable that ifthis choice of propeller type had been

Q1 .

Q2 .

Q3.

Q4 .

Q5 .

Q6 .

Q7.

Q8.

Table 9 . Shipomer Experience with HighlySkewed Propellers

Questions

1s there any notice-able speed differencebetween ship (s) fittedwith highly skewed prop-eller (s) and conven-tional propeller (s)?

1s there any notice-able fuel consumptiondifference betweenship (s) fitted withhighly skewed propel.lers (s) and conven-tional propeller (s)?

Has the crew ever ccnn-mented on reducednoiselvibration levelsof ship (s) fitted withhighly skewed propel-lers?

Have highly skewed pro.pellers shown a tendencyfor greater or reducedpropeller blade cavita-tion erosion relative toconventional propellers?

Has any highly skewedpropeller shown more SUS.ceptibility for damagefrom f lost ing debrisrelative to a conventionalpropeller?

Shipowner No. 1

No.

Yes. Appreciablereduction noticedat maximum powerand operation inshallow water.

No. To early tojudge.

No.

Has there been any reduc - No.tion in equipment failuressuch as radars, controllers,etc. on ships fitted “ithhighly skewed propellers?

Have ship masters com- No.rnented on any noticeabledifferences is asternbacking power on shipsfitted with highly skewedpropellers relative toconventional propellers?

How many ship months of 22 monthsservice operation haseach highly skewed pro-peller seen?

Shivowner No. 2 ShiDowner No. 3

There is no speed No.

difference betweenships fitted withhighly skewed ?.scompared to conven-tional propellers.

There is no notice. No.

able fuel consumptiondifference on betweenships fitted withhighly skewed as com-pared to conventionalpropellers.

All crews have com- No.

mented very favorablyon reduced noise/vib-ration levels o“ shipsfitted with highlyskewed propellers ascompared to the conven-tional propellers.

Believe that the highly No.skewed propeller has areduced tendency to pro-peller blade cavitationerosion as compared tothe conventional propellers.

Had only one occurrence, No.

i.e. blade damage and dis-count the feeling thata highly skewed propellerwould be more susceptibleto floating damage as com-pared to a conventionalpropeller.

There has been a drastic No.

marked reduction in navi-gational communication,bridge equipment failureson vessels fitted withhighly skewed propellersas compared to conventionalpropellers .

Ship masters have com- No.

mented that highly skewedpropellers in a light bal-last condition there is anoticeable difference inback power by approximately25 percent as compared to aconventional propeller. Inladen condition, i.e. withfull propeller immersionthere is no difference.

1. 12 months 1. 15 months

2. 24 months2. 13 months

3. 16 months 3. 7 months

52 months 35 months

.

*-R-34

‘.

(

Table 9 (Cent‘d) - Shipowner Experience with HighlySkewed Propel lers

Q9 .

Q1O.

.Quest~ons Sj Shipowner No. 2 Shi!mwner No. .3

If you were to build Yes Yes, without re- Highly skewedadditonal new ves- servation. propeller.sels would you pre-fer a highly skewedpropeller over a con-ventional propeller?

As conventional DrO- Yes. for 3hiDs Yes. without Yespellers near the”end with vibrati~n reservation.of their service life problems.would you replacethem with highly skewpropellers?

selected and the propeller installedwithout achieving a complete cure tothe projected problem, it would indeedbe a difficult problem. While thissituation is conceivable, it is notlikely. However, the investment inthe more expensive highly skewed pro-peller would already have substantiallyreduced the number of shipboard loca-tions experiencing the excessive vib-ration, thereby diminishing the magni-tude of the corrective effort.Generally however situations such asthis are never resolved from a technicalstandpoint. The vessel is placed inservice with a speed restriction withwhich it must operate for the remainderof its useful life. Surely a skewedpropeller would ‘,payoff‘fin this situa-tion.

CONCLUSIONS

Thus far considerable discussionhas been outlined in preceding pages onfull- scale vibration test results,alternative solutions to solving vib-ration problems, and lastly some eco-“ornicand risk considerations regardingthe choice of propeller type. The pur-pose of this section is to present anoverall assessment of the performanceof highly skewed propellers and outlinesome general conclusions that theauthors’ have reached.

It may be recalled in the initialportion of the paper, specificallyTable 1, a number of merits and dis-advantages were cited regarding highlyskewed propellers. Some of the dis–advantages cited were: higher costs,greater fuel bills, more susceptibilityto damage, shorter propeller life, etc.The merits for the highly skewed pro-pellers were: reduced vibration levels,improved crew comfort, reduced repairsto navigation equipment, etc.

In order to examine these allegedmerits and disadvantages more fully aquestionnaire outlining ten fundamental

questions was prepared and dist~ibutedto each of the three shipownerspresently operating ships fitted withthe skewed propellers. The questionsand shipowner responses are summarizedin Table 9.

Based upon examination of operatingexperience ranging from 22 to 55 months,it is apparent that each shipowner isplease with the overall performance ofthe highly skewed propellers and whenfaced with the decision in the futurewill most likely choose a highly skewedpropeller over the conventional type.Also, based upon the experienceaccumulated thus far there does notaPPear to be any speed or fuel penaltyassociated with the highly skewed pro-pellers.

Therefore based upon the vibrationmeasurement results, shipowner experi–ence, and lastly economic cost and riskconsiderations the authors have reachedthe following conclusions:

● Highly skewed propellers reduce Over-all ship vibration levels approxi-mately 50 percent. Greatest improve-ment appears to coincide withresonant conditions where highlyskewed propellers may reduce vib-ration levels 65 percent or more.

● Crew comfort has improved on allships fitted with highly skewed pro-pellers, however the crew is onlyaware of this on only two of thethree projects discussed in tbepaper.

● Highly skewed propellers can beinstalled on vessels for about40,000–$50, 000 increased over

9 AS Of April 1978, Farrell Lines be-.

came owner of all American Export Linesvessels including the Sea Bridge classships.

R-35

conventional designs. This diffen. Stern tube bearinas and tail shafts

●

9

●

●

●

●

●

●

●

●

●

tial Should narrow as greater effortis made to design for desired per-formance at minimum cost.

Operating results indicate there isno noticeable speed penalty on ves-sels fitted with highly skewed pro-pellers.

Operating results indicate there isno noticeable fuel cost increase onships fitted with highly skewed pro-pellers.

Operating results indicate thathighly skewed (H.S.) propellers donot ap?ear to be more susceptible todamage than conventional propel lers.However once damaged, repair costswill be greater on highly skewedpropellers.

Highly skewed propellers have notyet clearly demonstrated less cavit-ation erosion than conventionaldesigns, bowever the blade erosionis about the same as conventional.

Prediction of propeller RPM is im-proving, however model testingmethods may miss target propellerRPM up to 5 percent.

Full potential of H.S. benefits athorsepower levels greater than40,000 SHP remains to be demonstrated.

Maximum horsepower for single screwships fitted with H.S. or conven-tional DroDellers is not known.However” it”appears to be in excessof 50,000 SHP.

Service life of H.S. propellers froma fatigue standpoint has not beendemonstrated to be greater or lessthan conventional designs.

Structural cures of vibration prob-lems may increase levels of vib-ration elsewhere in a vessel whereasthere is no evidence this occursusing highly skewed propeller cures.

Propeller design technology has notyet developed analytical or modeltechniques for determining optimumskew angles and/or distributionsprimarily because of unknown con-tribution of pressure forces.

In regard to effectiveness of thethree highly skewed propellersdesigns in reducing ship vibration,the San Clemente class propellerprobably had excessive skew, theSea Bridge class propeller close b“tslightly more than optimum ske”, andthe Maine class propellers toolittle skew.

need not be large= than those forconvent ional propellers provided thedetailed engineering phase contem-plates heavier propellers at theoutset.

In regard to a final assessment oroverall conclusion the authors 1 muststate that the successful developmentof highly skewed propellers for merchantships represents the single most impor-tant advance in propeller. induced shipvibration reduction technology withinthe past decade and perhaps within thelast century. while the ultimatepotential of these novel propellerdesigns to operate with less cavitationerosion has not yet been demonstrated,this aspect may from an economic stand-point ultimately prove to be thegreatest benefit resultant from thehighly skewed propeller concept. There-fore propeller designers are encouragedto continue with their work and ship-Owners encouraged to install such pro-pellers on their vessels.

ACKNOWLEDGEMENTS

The authors wish to acknowledge theassistance and information furnished by 4numerous companies and individuals thathav~ contributed so greatly to thispaper. In particular, the authors wishto express their appreciation to NationalSteel Shipbuilding company, Bath IronWorks, and Messrs. Frank Dsshna”,Constantine Foltis, and Fred Seibold ofthe Naritirne Administration for thevarious photographs used in tbe paper.Technical information and guidance wasfurnished by wr. Peter Constas of ApexWarine, Mr. T. J. Wu of Farrell Lines,Nr. Eugene R. Miller, Jr. of Hydronautics,Inc. , Mr. P. F, Brunner of FergusonPropeller and Reconditioning, Ltd. ,Dr. W. B. Morgan of DTNSRDC, andMr. E. A. Frank of states steamshipCompany. Tbe authors would also liketo thank Messrs. C. B. Cherrix andR. Schubert for their editorial commentsand Nr. W. C. North of Cunard Line Ltd.for information on the record breakingsingle screw vessels UMBRIA and ETRURIAbriefly mentioned in Appendix B. Lastly,the authors would like to express theirthanks to MrS . Louella Smith for herextraordinary patience and assistancein preparing the several drafts andfinal manuscript.

REFERENCES

1.

2.

R. J. Boswell and G. G. Cox, “DeS19nAnd Model Evaluation Of A Highly .Propel ler For A Cargo Ship”,Marine TechnolcrqY, January 1974.

R. A. cunning, W. B. Morgan andR. J. Boswell, ‘,Highly Skewed Pro-pellers,’, =. SNAWE, 1972.

R-36 p---

—-

3.

4.

5.

6.

7.

8.

9.

D. T. Valentine and F. J. Da.hnaw,,,~lgh~~ skewed Propeller For ‘an

Clemente Class Ore/Bulk/Oil Car-rier Design Considerations, Modeland Full-Scale Evaluation”, ~Ship Technoloqv And Research(STAR) SvmDosiun, SNAME, 1975.

,,~igh~~_skewed propeller Research

Program - San Clemente Ore/Bulk/011 (OB0) Carriers”, NationalTechnical Publications Department,NASSCO Engineering, November 197b.

,,Code For shipboard vibration Mea-~urement ,,T&R Code c-1, SN~E,

January 1975.

,,LOC=I Ship~ard structures And

Machinery Vibration Measurements’(T&R Code C-4, SNAME, December 1976.

A. H. Weaver and W. G. Day, “AnalysisOf Wake Survey Data For A CargoShip Represented BY Model 5091”,NSRDC Report P-162-H-02, March1967.

N. E. Oliver and R. R. Hunt, ,KPOWer-ing Characteristics For A ContainerRoll-On/Roll-Off Cargo ship Repre-sented BY Model 5091 Fitted WithDesign Propel ler‘L,NSRDC ReportP-162-H-03, JUIY 1967.

A. A. Campo, ,,Cavitation Performance4295,,,NSRDC Report P-162-H-04,August 1967.

10. A. A. Campo, ,,Special Erosion TestsFor Propeller 4295,,, NSRDC ReportP-162-H-06, September 1967.

11. R. J. Boswell, G. G. Cox, D. T.Valentine and G. S. Belt, ‘,DesignAnd Model Test Evaluation Of AHighly-Skewed Propel ler For The78a Class Cargo Ship”, NSRDC ReportNo. 434-H-01, June 1971.

12. ,,De51gn And Evaluation Of A HighlY

Skewed Propeller a’,U.S. Departmentof Comnerce, Maritime Administra-tion, March 1978. (UnpublishedReport )

13. J. J. Nelka, “Experimental Evalua-tion Of A Series Of Skewed Propel-lers With Forward Rake: OpenWater Performance, Cavitation Per-formance, Field-Point Pressures,And Unsteady Propeller Loading,’,NSRDC Report No. 4113, July 1974.

14. J. Bourne, A Treatise On the ScrewProoeller, screw Vessels And screwEnaines, As AdaDted For PurDosesOf Peace And War, Longmans, Greeneand Company, 1867.

15. R. J. Boswell and R. D. Kader, “TheDesign Of A Skewed Propeller ForThe states Steamship Company Roll-On Roll-Off Cargo Vessels,,, NSRDCReport No. SPA-P-547-02, August1974.

16. E. L. Corthell, Maritime Commerce,Past-Present-Future, Bowne andCompany, 1899.

17. Otto Schlick, ,,OnThe vibratiOn of

Steam Vessels”, -. INA, 1884.

18. Otto Schlick, ‘-onw Apparatus ForMeasuring And Registering TheVibrations of Steamer s,,, ~.INA, 1893.

lg. C. H. cramp, O!Evolution Of The

Atlantic Greyhound,’, -. SNAME,1893.

zo. F. M. Lewis. ‘tTorsional Vibration InThe Diesel Engine,,,~. SNANE,1925.

Z1. F, M. Le”is, ,,Vibration And Engine

Balance in Diesel Ships’s,-.SNAME, 1927.

APPENDIX A

HIGHLY SKEWED PROPELLER DESIGN CONSIDER-ATIONS

Propel ler blade skew has been de-fined as the displacement at the mid-chord point of the blade section fromthe blade reference line along thepitch helix. Generally, the value ofthe angular displacement in the pro-jected view at the tip of the blade isused as a measure of the skew (13).Skew can, perhaps, be most clearly de-fined by Figure Al.

The amount of skew for any givenpropeller is calculated by measuringthe angular displacement of all theblades and dividing that displacementby the number of blades. For example,if the angular displacement of the

R-37

bladesoof a certain six-bladed propel~eris 180 then; 1) the skew angle is 30 ,and 2) the skew is 50 percent. Skews of50 percent or greater are considered tobe highly skewed by propeller designers.skews greater than normal amounts aregenerally considered highly skewed byshipowners and shipyards.

skewed propellers were illustratedby Bourne over one-hundred years ago.Bourne presented two concepts, one byBeadon proposed in 1851, Figure A2, andanother by Hirsch proposed in 1860,

Beadon proposed essentially.

Figure A3.a two-blade$ propeller with 100 Percentskew or 180 . The object of this con-cept was that “inasmuch as the cuttngedge, by not coming into such direct andrapid contact with the water, will not

j---

tare the source of the hull vibration.

.7-+-- The unsteady forces and moments can

PROJECTED VIEW

Figure Al - Definition of skew

experience so much resistance’,, appearsto be a first attempt at reducing pro.pelle.r induced forces. Bourne statedthat Beadons s propeller did not appearto be as strong as the common form ofscrew propeller. Hirsch proposed aleft handed propeller that used skewdistributed reversely to that usedtoday. In his concept, the blade tipsection would come into contact withthe water first with successively lowerblade sections coming into contactgradually (14).

The hydrodynamic force (lift) gen-erated by a screw propeller which pro-pels a ship through the water alsocauses propeller induced vibration.The lift, developed by the propellerblades is periodic and when the bladesrotate in the non uniform velocityfield behind the vessel the lift be.comes unsteady. This action causes thegeneration of unsteady forces andmoments. The above forces and moments

Figure A2 - Beadon 1s Screw Propeller

be divided into two groups, pressure.forces and bearing-forces. Pressure-forces are transmitted to the hull bythe action of the water against theshel1 and appendages. Bearing-forcesare transmitted to the hull through theshafting to the bearing and then to thehull . Efforts to avoid propeller in-duced vibration in the past, have gen-erally consisted of selecting the bladenumber and RPM to avoid critical hullfrequencies and providing adequateclearances in the propeller aperture.Studies conducted at DTNsRDC have in-dicated that substantial propeller bladeskew may significantly reduce propeller-induced vibration, both hull pre.ssure-forces and bearing-forces (15). Skewinga ProPeller blade allows each section ofthe blade to enter the wake at a dif-ferent instant, thus reducing the peakforces. That is the skewed propellerblade, which is rotating in the wake, ismore gradually introduced into the waterflow thereby affecting decreased forcescompared to a conventional propeller.

Figure A3 - Hirsch8s Screw PrOpellel

In addition model highly skewedpropellers have been found to have adecreased susceptibility of the propellerto cavit?,tio” when operating in a wake.To properly evaluate the reduction inpropeller-induced vibration obtained byaPPIYing skew angle, the total vibrationexcitation force, which is a vector sumof the pressure-force and bearing-forcecomponents, must be considered (15).

With regard to bearing forces ithas been found that skew effectivenessis different for propellers with odd oreven number of blades. For vessels ofa certain design high skew tends to bemost effective in reducing the forces

.

that are the largest for unskewed pro-pellers. Thrust a“d torque are reducedfor even-bladed skewed propellers butvertical and transverse horizontal sidefOrces are reduced for odd-bladed skewedpropellers.

R-38,& —

At this time it is beyond the

state-of-the-art to determine preciselythe magnitude and distribution of skewan91e that Will minimize the total,pressure plus bearing, vibration excit.ation force in a specified direction.A skew angle can be designed though,that wil1 reduce the magnitude of the

APPENDIX

HISTORICAL BACKGROUND

Historical records citing instancesof excessive ship vibration date backto the days of sail and wooden vessels.For purposes of this Appendix howeverthe starting point will be approximatelythe year 1850 when steam engines andscrew propellers started to come intopopular usage.

Fiq”re B1 outlines the growth ofinstalled horsepower for single screwmerchant ships for the period 1850 to1978. Also shown on this figure areapproximate dates for major technoloq.ical advances affecting ship vibrationeither from a excitation or responsestandpoint. Introduction of new engine

UmbrialEtruria

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bearing forces. Boswell and Cox (1)found that the data they studied in-dicated that skew at the blade tip of100 percent is generally desirable.But, they also found little guidance asto the proper tip skew and radial dis–tribution of skew that should be applied.

B

tYPes, greater horsepower levels, wouldbe considered excitation factors: where-as changes of hull materials, construc-tion methods, etc. would representtypical vibration response factors.

While most naval architects haveread in the literature some facts aboutthe remarkable steam vessel the GREATEASTERN propelled by toth paddles anda screw propeller, it is interesting tonote that this 680 foot vessel built in1858 had paddle engines of 1000 horse-power and screw engines of 1700 horse-power (14). The propeller constructionwas years ahead of its time with thefour bladed propeller being 24 feet indiameter coinciding roughly with thepropeller diameters for the three highly

Maximum Horsepower Line, .’Single screw

Merchant Ships

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Paddle 1850 1900YEARS 1950 2000

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Figure El - Growth of Single Screw Merchant ShipHorsepower and Vibration ExcitationResponse Factors

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skewed propeller projects discussed inthis paper. Figure B2 displays thepropeller/hull stern configuration usedon the GREAT EAsTERN and also showssome propeller construction details.

According to Corthell (16) in theyear 1848 there were 128 steamer* inexistence with only 36 of these builtof iron. For comparison purposes, atthe same period there were 10,579 sail-ing vessels with but 79 of these vesselsbuilt of iron. By the year 1883, theconstruction of new vessels had reachedthe point where 83 percent were con-structed of iron with 15 percent beingconstructed of steel. Only six yearslater hull material had changed to where92 percent of the new vessels were con-structed of steel and but 8 percent ofiron.

By the year 1884 the installedhorsepower of single screw ships reacheda record high of 14,700 when the CunardLine single screw passenger vesselUMBRIA conunenced trans-Atlanti. service.The steam vessels UMBRIA and ETRURIA(her sistership) operated successfullyalmost 25 years until 1910 and 1909,respectively when they were laid up andultimately scrapped. Although theauthorss have not located any recordsciting specific vibration or propellerblade erosion problems these’vesselsmay have had, ,the UMBRIA and ETRURIAare known to have operated with but twobroken propeller shaft failures duringtheir years of service. As an interest-ing item, reportedly there were 11engineers, one electrician and 109 fire-men needed to operate these vessels andin March 1887 the ETRURIA conmleted aneastbound crossing at an aver;ge speedof 19.45 knots.

Thus after only approximately 35years follo”ing introduction of thescrew propeller, steam propelled vesselshad developed rapidly to the limits oftechnology for single screw vessels andthe UMBRIA and her sistership theETRURIA appear to have held a recordhorsepower level that was not surpasseduntil the Mariner class ships ‘wereintroduced in 1951, some 66 years later.

Tbe UMBRIA and ETRURIA were thelast of the high-powered single-screwpassenger vessels and in the period1885-1890 twin screw vessels such asthe CITY OF PARIS and OCEANIC appearedfurther extending the horsepower growthcurve. Although such famous vessels arenot plotted on Figure El which is limitedto single- screw vessels it should benoted that the 68,000 horsepower levelwas reached by the quadruple-screwsteamers MAURITANIA and LUSITIANA in1907.

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Figure 92 - GREAT EASTERN Propeller/HullStern Configuration

While the historical records tendto discuss propeller and vibrationproblems in O“lY descriptivegenexalterms, it is clear that some of theproblems however were very real in thatmany instances had been encounteredwhere vibration had loosened rivets inthe sternpost of vessels and shipownerswere concerned about the possibilityof a calimity.

In 1884. Otto Schlick Dresented alandmark paper a,onthe Vibr;iion ofSteam Vessels’, to the Institution of

.

Naval Architects (17). He opened thepaper with the comments:

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,,AII s~eamers, without excep-tions, shake to a more orless degree when the enginesare in motion. Thisphenomenon is usually con-sidered as so natural, thatin most cases little or noattention is paid to it,and when ships with com-paratively powerful enginesshow an unusually strongvibration, it is lookedupon as quite natural, orthe phenomenon is simplyaccounted for by sayingthat the ship is of tooweak construct ion.”

Schlick continued and outlined hisideas on hull natural frequencies,elasticity, and comment on the forcesproduced by a working e~gine, citingthe following: Forward thrust, turningcouple of the engine, sideward pressureof the propeller, pressure or recipro-cating masses, and the pressure ofrotating masses. He even commented onthe aspect of what should be done inorder to avoid or to diminish violentvibration, citing a change in propeller,alteration of number of revolutions ofthe engine, redistribution Of cargo, ?.Spossible alternatives.

It should be noted that while theundesi.rabi2i@of vi~raticmwas kncwn,instruments for measurement of actualfull scale vibration had not beendeveloped. Thus comments about exces-sive vibration during and before the1884 time period were subjective innature.

I“ 1893 schlick presented anothersignificant paper (18) describing an:~~mtus fOr measuring ship vibration

Thus until Schlick’s inventionthere was no instrument available thatcould be used to measure or analyzeship vibration.

While earlier reports frequentlycited stern movements in the order of1 or 2 feet or more, actual measurementsof full scale vibration using Schlickrsdevice confirmed that previous subjec-tive estimates were vastly overstatedand that true values were but a smal1fraction of earlier repcuted amounts.

In 1893, C. H. Cramp, (19) indi-cated ‘8YOU cannot put more than 12,000lHP through one screw”. Further more,,,wheneveryo” require more than 12, 000lHP you must have 2 screws and if YOUfind it necessary to exceed 24,000 lHPthree screw will be required t’.

In light of the existence of theUMBRIA/ETRURIA at this time period onecan only believe that these vessels mayhave had substantial shipboard vibrationand/or p,bopeller problems or that Cramp’s

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proposed rule of thumb was unfounded.

By the year 1910 motorships started

to appear and a new source of vibrationexcitation appeared. The seagoing motor-ship-oil tanker, the VULCANUS having 480horsepower and speed just over 6 knotsappeared. At approximately this tlMeperiod major emphasis was concentratedon reducing excitation forces created byreciprocating steam engines and dieselengines.

By 1925, in response to numerousshafting failures that had been exper-ienced, F. M. Lewis presented his firstpaper on torsional vibration of dieselengines (20). Only two years laterLewis presented another paper (21) fur-ther discussing vibration and enginebalance in diesel ships.

Probably by the beginning of 1930all major types of propeller induced ormachinery induced vibration problems hadbeen encountered on merchant ships.While the literature since then containsnumerous articles on ship vibration, thegrowth of horsepower, ship size and majortechnical advances that took place inthe period of 1885 to 1920 is truelyremarkable.