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8/6/2019 7174460 Basics of Ship Resistance
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Chap 7 Resistance and Powering of Ship
Objectives
Prediction of Ships Power
- Ships driving system and concept of power
- Resistance of ship and its components
frictional resistance
wave-making resistance
others
- Froude expansion
- Effective horse power calculation
Propeller Theory
- Propeller components and definitions
- Propeller theory- Cavitation
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Ship Drive Train and Power
Ship Drive Train System
Engine ReductionGear
Bearing Seals
ScrewStrut
BHP SHP DHP
THP
EHP
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Brake Horse Power (BHP)
- Power output at the shaft coming out of the engine before
the reduction gears
Shaft Horse Power (SHP)
- Power output after the reduction gears
- SHP=BHP - losses in reduction gear
Horse Power in Drive Train
Ship Drive Train and Power
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Delivered Horse Power (DHP)- Power delivered to the propeller
- DHP=SHP losses in shafting, shaft bearings and seals
Thrust Horse Power (THP)
- Power created by the screw/propeller
- THP=DHP Propeller losses
Relative Magnitudes
BHP>SHP>DHP>THP>EHP
E/G R/G
BHP SHP
ShaftBearing Prop.
DHP THP EHP
Hull
Ship Drive Train and Power
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Effective Horse Power (EHP)
EHP : The power required to move the ship hull at a givenspeed in the absence of propeller action
(EHP is not related with Power Train System)
EHP can be determined from the towing tank experiments at
the various speeds of the model ship.
EHP of the model shipis converted into EHP of the full scale
ship by Froudes Law.
VTowing Tank Towing carriage
Measured EHP
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Effective Horse Power (EHP)
0
200
400
600
800
1000
Eff
ectiveHorsep
ower,EHP
(H
0 2 4 6 8 10 12 14 16
Ship Speed, Vs (Knots)
POWER CURVEYARD PATROL CRAFT
Typical EHP Curve of YP
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Effective Horse Power (EHP)
Efficiencies
Hull Efficiency
THP
EHPH =
-Hull efficiency changes due to hull-propeller interactions.- Well-designed ship :
- Poorly-designed ship :
1H
1H
Well-designed
Poorly-designed
-Flow is not smooth.- THP is reduced.
- High THP is needed
to get designed speed.
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Screw
Effective Horse Power (EHP)
Efficiencies (contd)
Propeller Efficiency
DHPTHPpropeller=
Propulsive Coefficients (PC)
SHP
EHPp =
propellerdesignedfor well6.0p
SHP DHP
THP
EHP
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Total Hull Resistance
Total Hull Resistance (RT)
The force that the ship experiences opposite to the motion of
the ship as it moves.
EHP Calculation
=
P
ST
P
s H
ft lb
s
ft
V(lb)R)EHP(H
550 shipofspeedV
resistancehulltotal
S ==TR
( )
P
ST
Hatts
Wattss
J
s
ftlb
s
ftlbVR
550/1W1
:
=
===
Power
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Total Hull Resistance (cont)
Coefficient of Total Hull Resistance- Non-dimensional value of total resistance
5.02
SV
RC
s
TT
=
hullsubmergedtheonareasurfacewetted
shipofSpeed
densityFluidresistancehullTotal
watercalminresistancehulltotaloftCoefficien
====
=
S
V
R
C
S
T
T
dimension-nonlb
2
2
4
2
ftsft
ftslb
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Total Hull Resistance (cont)
Coefficient of Total Hull Resistance (contd)
-Total Resistance of full scale ship can be determined using
ST VSC ,, and
TST CSVlbR =2
5.0)(
speedshipscaleFull
formofCurvesfromobtained
tablepropertywaterfromavailable
testmodelthebydetermined
:
:
:
:
S
T
V
S
C
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Total Hull Resistance (cont)
Relation of Total Resistance Coefficient and Speed
0
5000
10000
15000
20000
TotalR
esistance,
0 2 4 6 8 10 12 14 16
Ship Speed, Vs (knots)
TOTAL RESISTANCE CURVEYARD PATROL CRAFT
speedhighat5ot
speedlowat2from
2
=
n
V
VCR
n
S
STT
speedhighat6to
speedlowat3from
2
=
n
V
VVCVREHP
n
S
SSTST
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Components of Total Resistance
Total Resistance
AWVT RRRR ++=ResistanceViscous:R
V
ResistanceMakingWave:RW ResistanceAir:RA
Viscous Resistance
- Resistance due to the viscous stresses that the fluid exerts
on the hull.
( due to friction of the water against the surface of the ship)
- Viscosity, ships velocity, wetted surface area of ship
generally affect the viscous resistance.
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Components of Total Resistance
Wave-Making Resistance
- Resistance caused by waves generated by the motion of the ship- Wave-making resistance is affected by beam to length ratio,
displacement, shape of hull, Froude number (ship length &
speed)
Air Resistance- Resistance caused by the flow of air over the ship with no
wind present
- Air resistance is affected by projected area, shape of the ship
above the water line, wind velocity and direction
- Typically 4 ~ 8 % of the total resistance
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Components of Total Hull Resistance
Total Resistance and Relative Magnitude of Components
Viscous
Air Resistance
Wave-making
Speed (kts)
Res is t a
nce( lb)
-Low speed : Viscous R- Higher speed : Wave-making R
- Hump (Hollow) : location isfunction of ship length and speed.
Hump
Hollow
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Why is a Golf Ball Dimpled?
Lets look at a Baseball (because thats what I have
numbers for) At the velocities of 50 to 130 mph dominant in baseball the air
passes over a smooth ball in a highly resistant flow.
Turbulent flow does not occur until nearly 200 mph for a smoothball
A rough ball (say one with raised stitches like a baseball) inducesturbulent flow
A baseball batted 400 feet would only travel 300 feet if it wassmooth.
A non-dimpled golf ball would really hamper Tiger Woods longgame
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Coefficient of Viscous Resistance
Viscous Flow around a ship
Real ship : Turbulent flow exists near the bow.
Model ship :Studs or sand strips are attached at the bow
to create the turbulent flow.
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Coefficient of Viscous Resistance (cont)
Coefficients of Viscous Resistance
- Non-dimensional quantity of viscous resistance- It consists of tangential and normal components.
FF KCC +=+= normaltangentialV CCC
Tangential Component :
- Tangential stress is parallel to ships hull and causesa net force opposing the motion ; Skin Friction
- It is assumed can be obtained from the experimental
data of flat plate.
FC
flow shipbow stern
FC
tange
ntial
norm
al
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Coefficient of Viscous Resistance (cont)
Sn
n
F
FV
LVR
RC
CC
=
=
=
)2(log
075.0
2
10
ofComponentTangential
Semi-empirical
equation
watesaltfor
watfreshfor
/sft101.2791
/sft101.2260
/s)(ftViscosityKinematic
)Speed(ft/sShip
(ft)L
NumberReynolds
25-
25-
2
pp
=
=
=
=
==
S
n
V
L
R
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Coefficient of Viscous Resistance (cont)
Tangential Component (contd)
- Relation between viscous flow and Reynolds number
Laminar flow :In laminar flow, the fluid flows in layers
in an orderly fashion. The layers do not mix transversely
but slide over one another.
Turbulent flow :In turbulent flow, the flow is chaotic and
mixed transversely.
Laminar Flow Turbulent Flow
Flow overflat plate
5
105 aboutRn
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Normal Component
- Normal component causes a pressure distribution along the
underwater hull form of ship
- A high pressure is formed in the forward direction opposing
the motion and a lower pressure is formed aft.-Normal component generates the eddy behind the hull.
- It is affected by hull shape.
Fuller shape ship has larger normal component than slender
ship.
Full shipSlender ship
large eddy
Coefficient of Viscous Resistance (cont)
small eddy
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Normal Component (contd)
- It is calculated by the product ofSkin Friction with Form Factor.
23
)()(
)()()()(ft19K
K
=
==
=
ftLftB
ftTftBftL
C
CKC
F
Fv
FactorForm
Coeff.FrictionSkin
ofComponentNormal
Coefficient of Viscous Resistance (cont)
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23
)(
)(
)()()(
)(ft19K
=
ftL
ftB
ftTftBftL
FF CKC+=+= normaltangentialV
CCC
2
10 )2(log
075.0
=
n
FR
C
Summary of Viscous Resistance Coefficient
watesaltfor
watfreshfor
/sft101.2791
/sft101.2260
/s)(ftViscosityKinematic
)Speed(ft/sShip
(ft)L
NumberReynolds
25-
25-
2
pp
=
=
=
=
==
=
S
n
Sn
V
L
R
LVR K= Form Factor
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Reducing the Viscous Resistance Coeff.
- Method :
Increase L while keeping the submerged volume constant
1) Form Factor K Normal component KCF Slender hull is favorable. ( Slender hull form will create
a smaller pressure difference between bow and stern.)
2) Reynolds No. Rn CF KCF
Summary of Viscous Resistance Coefficient
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Wave-Making Resistance
Typical Wave Pattern
Bow divergent waveBow divergent wave
Transverse wave
L
Wave Length
Stern divergent wave
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Wave-Making Resistance
Transverse wave System
It travels at approximately the same speed as the ship.
At slow speed, several crests exist along the ship length
because the wave lengths are smaller than the ship length.
As the ship speeds up, the length of the transverse wave
increases.
When the transverse wave length approaches the ship length,
the wave making resistance increases very rapidly.This is the main reason for the dramatic increase in
Total Resistance as speed increases.
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Wave-Making Resistance (cont)
Transverse wave System
Wave Length
Wave
Length
Slow
Speed
High
Speed
Vs < Hull Speed
Vs Hull Speed
Hull Speed: speed at which the transverse wave length equals
the ship length.
(Wavemaking resistance drastically increases above hull speed)
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Divergent Wave System
It consists ofBow and Stern Waves.
Interaction of the bow and stern waves create theHollow or
Humpon the resistance curve. Hump : When the bow and stern waves are in phase,
the crests are added up so that larger divergent wave systems
are generated. Hollow : When the bow and stern waves are out of phase,
the crests matches the trough so that smaller divergent wave
systems are generated.
Wave-Making Resistance (cont)
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Calculation of Wave-Making Resistance Coeff.
Wave-making resistance is affected by
- beam to length ratio
- displacement
- hull shape- Froude number
The calculation of the coefficient isfar difficult and inaccurate
from any theoretical or empirical equation.
(Because mathematical modeling of the flow around shipis very complex since there exists fluid-air boundary,
wave-body interaction)
Therefore model test in the towing tank and Froude expansion
are needed to calculate the Cw of the real ship.
Wave-Making Resistance (cont)
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Reducing Wave Making Resistance
1)Increasing ship length to reduce the transverse wave
- Hull speed will increase.
- Therefore increment of wave-making resistance of longer
ship will be small until the ship reaches to the hull speed.-EX :
FFG7 : ship length 408 ft Which ship requires more
hull speed 27 KTS horse power at 35 KTS?
CVN65 : ship length 1040 fthull speed 43 KTS
Wave-Making Resistance (cont)
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Reducing Wave Making Resistance (contd)
2)Attaching Bulbous Bow to reduce the bow divergent wave
- Bulbous bow generates the second bow waves .
- Then the waves interact with the bow wave resulting inideally no waves, practically smaller bow divergent waves.
-EX :
DDG 51 : 7 % reduction in fuel consumption at cruise speed
3% reduction at max speed.
design &retrofit cost : less than $30 million
life cycle fuel cost saving for all the ship : $250 mil.
Tankers & Containers : adopting the Bulbous bow
Wave-Making Resistance (cont)
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Bulbous Bow
Wave-Making Resistance (cont)
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Coefficient of Total Resistance
AllowancenCorrelatio
1
:C
CCK)(C
CCCC
A
AWF
AWVT
+++=++=
Coefficient of total hull resistance
Correlation Allowance
It accounts for hull resistance due to surface roughness,
paint roughness, corrosion, and fouling of the hull surface.
It is only used when a full-scale ship prediction of EHP is madefrom model test results.
For model,
For ship, empirical formulas can be used.
.0 smoothissurfacemodelSince=AC
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Other Type of Resistances
Appendage Resistance
- Frictional resistance caused by the underwater appendages
such as rudder, propeller shaft, bilge keels and struts
- 224% of the total resistance in naval ship.Steering Resistance
-Resistance caused by the rudder motion.
- Small in warships but troublesome in sail boats
Added Resistance-Resistance due to sea waves which will cause the ship
motions (pitching, rolling, heaving, yawing).
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Other Resistances
Increased Resistance in Shallow Water-Resistance caused by shallow water effect
- Flow velocities under the hull increases in shallow water.
: Increment of frictional resistance due to the velocities: Pressure drop, suction, increment of wetted surface area
Increases frictional resistance- The waves created in shallow water take more energy from
the ship than they do in deep water for the same speed.
Increases wave making resistance
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Basic Theory Behind Ship Modeling
Modeling a ship
- It is not possible to measure the resistance of the full-scale ship- The ship needs to be scaled down to test in the tank but
the scaled ship (model) must behave in exactly same way
as the real ship.
-How do we scale the prototype ship ?- Geometric and Dynamic similarity must be achieved.
?
Dimension
Speed
Force
prototype Model
prototype shipmodel ship
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Basic Theory behind Ship Modeling
Geometric Similarity
- Geometric similarity exists between model andprototype if the ratios of all characteristic dimensions
in model and prototype are equal.
- The ratio of the ship length to the model length is typically
used to define the scale factor.
Volume:
Area:
:
FactorScale
3
33
2
22
)(ft
)(ft
)(ftS
)(ftS
(ft)L
(ft)L
M
S
M
S
M
S
=
=
=
=
Length
ModelM
shiscalefullS
:
p:
B i Th b hi d Shi M d li
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Basic Theory behind Ship Modeling
Dynamic Similarity
- Dynamic Similarity exists between model and prototypeif the ratios of all forces in model and prototype are the
same.
- Total Resistance : Frictional Resistance+ Wave Making+Others
S
MSM
M
S
S
MSM
M
M
S
S
M
MM
S
SS
nMnSnMnS
nWnV
L
LVV
L
L
v
vVV
gL
V
gL
V
v
VL
v
VL
FFRR
FfCRfC
==
==
====
,
,
)(),(
,
B i Th b hi d Shi M d li
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Basic Theory behind Ship Modeling
Dynamic Similarity (contd)
- Both Geometric and Dynamic similarity cannot be achieved
at same time in the model test because making bothRn and
Fn the same for the model and ship is not physically possible.
)(1
100
10)(10
kts
ft
ftkts
LLVV
S
MSM
=
=
=
)(100
)(assume10
100)(10
kts
vvft
ftkts
L
L
v
vVV
SM
M
S
S
MSM
=
==
=
Example
Ship Length=100ft, Ship Speed=10kts, Model Length=10ft
Model speedto satisfy both geometric and dynamic similitude?
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Basic Theory behind Ship Modeling
Dynamic Similarity (contd)
- Choice ?
Make Fn the same for the model.
Have Rn different
Incomplete dynamic similarity- However partial dynamic similarity can be achieved by
towing the model at the corresponding speed
- Due to the partial dynamic similarity, the following
relations in forces are established.
WSWM CC =
VSVM CC
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Basic Theory behind Ship Modeling
Corresponding Speeds
M
M
S
SnMnS
gLV
gLVFF == ,
-Example :
Ship length = 200 ft, Model length : 10 ft
Ship speed = 20 kts, Model speed towed ?
ktsktsV
LLV
L
LVV
S
MS
S
S
MSM
47.420
120
1
/
1
===
==
(ft)L
(ft/s)V
(ft)L
(ft/s)V
M
M
S
S =
1kt.=1.688 ft/s
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Basic Theory behind Ship Modeling
Modeling Summary
AWFAWVT CCKCCCCC +++=++= )1(AMWMMFMTM CCKCC +++= )1(
)5.0*(
550
)(
)1(
2sSSTSTS
STS
ASWSSFSTS
VSCRVR
hpEHP
CCKCC
=
=
+++=
AMFMTMWM CKCCC += )1(Froude
Expansion
Measured in tank
smooth)isModel(0
)givenorCalculated.factorscaletodue(
d)(calculate,
)//V,( S
==
===
AM
MS
FSFM
MMSnMnSWMWS
C
KK
CC
gLVgLFFCC
1)
2)
3)