notes on ship construction

LECTURE NOTES ON SHIP CONSTRUCTION

Ship Construction – General

General knowledge of the principal structural members of a ship and the

proper name for the various parts is essential for controlling the

operation of the ship and care for persons on board at the operational

level and to Maintain seaworthiness of the ship and actions to ensure

and maintain the watertight integrity of the ship are in accordance with

accepted practice. Stability conditions comply with the IMO intact

stability criteria under all conditions of loading.

UNDERSTANDING THE VARIOUS STRUCTURAL MEMBERS: i. Bracket – support the girder, etc

ii. Bulkheads – a vertical partition between compartments

iii. Center girder – In lieu of longitudinal, provide longitudinal strength

. Longitudinal framing in DB –

iv. Floor – A vertical athwartships member in way of the double-

bottom. It will run from the center girder out to the margin plate

on either side of the vessel.

v. Frame – Internal support member for the shell plating. Vessel may

be framed longitudinally or transversely.

vi. Gusset – triangular plate for joining angle bar to a plate

vii. Intercostals Side girder – A side girder in the fore and aft line sited

either side of the keel. Integral connection with the tank top and

the ship’s bottom plating and rigidly connected by floors

viii. Keels – center line plate from stem to the stern frame.

. Flat plate keels – Generally used keel. Center girder is

attached to the keel and inner bottom plating by continuous

welding and no scallops permitted

a. Duct keels – a form of flat plate keel with tow center girder.

Often fitted between collision bulkhead and forward engine

room bulkhead to provide tunnel for pipes and additional

buoyancy.

ix. Lightening holes – holes cut into floors or intercostals to reduce

weight and to provide access to tank areas

x. Longitudinal – A fore and aft strength member connecting the

athwartships floors. It must be continuous for ship > 215m.

Additional longitudinal are to be found in pounding area

xi. Margin plate – a fore and aft plate sited at the turn of the bilge. The

upper edge is normally flanged to allow connection to the tank top

plating, while the opposite end is secured to the inside of the shell

plate by an angle-bar connection. It provides an end seal to the

double bottom tanks, having all the floors joining at right angles,

up to the collision bulkhead.

xii. Panting beams – athwartships members in the forepart introduced

to reduce the in & out tendency of the shell plating, caused by

varying water pressure on the bow.

xiii. Panting stringers – internal horizontal plates secured to the shell

plating and braced �� athwartships by the panting beams.

xiv. Scantlings – used to indicate the thickness of plates, angles and

flanges.

xv. Sheer strake – the continuous row of shell plates on a level with

the uppermost continuous deck.

VARIOUS STRESSES ENCOUNTERED BY A SHIP

Bending, Shear, Hogging, Sagging, Racking, Pounding, Panting. DEFINITIONS: HOGGING When the peak of a wave is amidships, causing the hull to bend so the ends of the keel are lower than the middle. The opposite of sagging. SAGGING: When the trough of a wave is amidships, causing the hull to deflect so the ends of the keel are higher than the middle. The opposite of hogging.

PANTING:

The pulsation in and out of the bow and stern plating as the ship alternately rises and plunges deep into the water POUNDING: Pounding, as a boat will do in short seas when the bow lifts clear of the water then comes crashing down. The term is local to English ports on the North Sea. RACKING: The effect of distortion of a vessel into a parallelogram shape due to the low transverse strength of the vessel is called as racking. Racking stress occurs in a seaway when the vessel is sailing in heavy weather and encountered heavy accelerations due to roll and heave.

Racking stress and its causes

In a seaway as a ship rolls from one side to the other the different areas

of the ship have motion which are dependent on the nature of the subject

area. The accelerations are thus not similar due to the various masses of

the different sections (although joined together). These accelerations on

the ships structure are liable to cause distortion in the transverse

section. The greatest effect is under light ship conditions.

Local Stresses

Panting

This is a stress, which occurs at the ends of a vessel due to variations in

water pressure on the shell plating as the vessel pitches in a seaway. The

effect is accentuated at the bow when making headway.

Pounding:

Heavy pitching assisted by heaving as the whole vessel is lifted in a

seaway and again as the vessel slams down on the water is known as

pounding or slamming. This may subject the forepart to severe blows

from the sea. The greatest effect is experienced in the light ship

condition.

Stresses caused by localized loading

Localized heavy loads may give rise to localized distortion of the

transverse section.

Such local loads may be the machinery (Main engine) in the engine room

or the loading of concentrated ore in the holds.

xvi. Panting beams – athwartships members in the forepart introduced

to reduce the in & out tendency of the shell plating, caused by

varying water pressure on the bow.

xvii. Panting stringers – internal horizontal plates secured to the shell

plating and braced athwartships by the panting beams.

xviii. Sheer strake – the continuous row of shell plates on a level with

the uppermost continuous deck.

SOME IMPORTANT QUESTIONS & ANSWERS:

Q. describes racking stress and its causes

Q. describes what is meant by 'panting' and states which parts of the ship is affected

Q. describes what is meant by 'pounding' or 'slamming' and states which part of the ship is affected

Q. describes what is meant by 'hogging' and by 'sagging' and distinguishes between them

Q. describes the loading conditions which give rise to hogging and sagging stresses

1. Describe the circumstances which cause panting and pounding

stresses.

i. Panting stresses is an in and out motion of the plating in the

bows of a ship and is caused by unequal water pressure as the

bow passes through successive waves.

ii. Pounding stresses is exist when ships is pitching. Ship’s bows

lift clear of the water and come down heavily. It causes damage

to the bottom and girder at the bow.

2. Sketch a transverse section through the forward part of a large

cargo vessel, showing the structural arrangements which resist the

stresses.

i. Panting stress

Tiers of panting beams are fitted forward of the collision

bulkhead below the lowest deck. These are similar to deck

beam and are connected to frames by beam knees, but are

only fitted at alternative frames. Tiers of beams are spaced 2

meters apart vertically and supported by wash plates or

pillars.

Panting stringers, similar to deck stringers, are laid on each

tier of beams.

To stiffen the joint between each beam and the inner edge of

the stringer, the plate edge may be shaped or gussets fitted.

At intermediate frame without beams, the stringer is support

by a beam knee of half its depth.

At fore ends, the stringers are joined by flat plate called

“Breasthooks”.

ii. Pounding stress is resisted by strong cellular double bottom.

For a large cargo vessel, longitudinally framed bottom is used.

The outer bottom plating covering the flat of the bottom must

be thickened.

The connections of the shell and inner bottom girder-work

are made stronger

Plate floors are fitted at alternate frames

Longitudinal are stronger than normal

Side girders are no more than 2.1 meters apart.

Members compensating stress

Racking Heavy weight

Water pressure

Local Stress

Hoging & Sagging SF BM Dry-

docking Pounding Panting

Beam knee � � �

Beams � � � �

Bulkheads � � � � � �

Decks � � � � �

Floors � � � � � �

Frames � � � �

Long' girders � � � � �

Pillars � � � �

Shell plating � � � � � � � � � �

SOME MORE EXAMINATION QUESTIONS

i. Describe the circumstances which cause panting and pounding

stresses.

a. Panting stresses is an in and out motion of the plating in the

bows of a ship and is caused by unequal water pressure as

the bow passes through successive waves.

b. Pounding stresses is exist when ships is pitching. Ship’s

bows lift clear of the water and come down heavily. It causes

damage to the bottom and girder at the bow.

ii. Sketch a transverse section through the forward part of a large

cargo vessel, showing the structural arrangements which resist

the stresses in (1).

a. Panting stress

i. Tiers of panting beams are fitted forward of the

collision bulkhead below the lowest deck. These are

similar to deck beam and are connected to frames by

beam knees, but are only fitted at alternative frames.

Tiers of beams are spaced 2 meters apart vertically

and supported by wash plates or pillars.

ii. Panting stringers, similar to deck stringers, are laid on

each tier of beams.

iii. To stiffen the joint between each beam and the inner

edge of the stringer, the plate edge may be shaped or

gussets fitted.

iv. At intermediate frame without beams, the stringer is

support by a beam knee of half its depth.

v. At fore ends, the stringers are joined by flat plate

called “Breasthooks”.

b. Pounding stress is resisted by strong cellular double bottom.

For a large cargo vessel, longitudinally framed bottom is

used.

i. The outer bottom plating covering the flat of the

bottom must be thickened.

ii. The connections of the shell and inner bottom girder-

work are made stronger

iii. Plate floors are fitted at alternate frames

iv. Longitudinal are stronger than normal

v. Side girders are no more than 2.1 meters apart.

iii. Sketch a longitudinal section of a bulk carrier, showing and

naming all the main compartments. // Explain the reasons for

this arrangement of compartments.

a. Large, clear holds without tween deck – to load and

discharge cargo quickly

b. Large hatches with steel covers for safety

c. Engine place aft

d. Topside tank – enable water ballast to be carried high up to

reduce GM

e. Sloping side tanks at the bilge – assist in handling bulk

cargo since it helps the self-trimming of cargo

iv. List the structural members of a ship which are designed to

resist the main longitudinal stresses in a ships hull. // State

briefly how structural continuity is maintained in these

members to enable them to perform their designed function.

a. Longitudinal stress: hogging, sagging

b. Longitudinal work in the double bottom:

1. Deck stringer and sheer-strake thicken

2. Deck girder and longitudinal bulkhead

3. Special steel for sheer-strake and bilge strake

4. Longitudinal frames and beams in the bottom

and under the strengthen deck

5. Stress is greatest amidships, so strengths of the

parts is made greater amidships

c. Hull is strengthen at about the half-depth of the ship to

resist the shearing stress

v. It is useless to make one part very strong if an adjacent part which

has to resist the same stress is weak. Hence, it is important to

maintain structural continuity

vi. When material has to be cut away, compensations must be made

to preserve continuity of strength. Square corners should be

avoided as far as possible since it has been found that these are

always a source of weakness.

vii. Parts which are very strong compared to the neighboring parts

should not be ended suddenly, as there would be a tendency for

them to tear away where they end. They should be gradually

tapered off �� and merge into the weaker parts.

viii. List the functions of : bilge wells, stern tubes.

a. Stern tube: to support the shaft and to make a watertight

joint where the shaft enters the hull.

i. Steel tube. The fore end with flange bolted to after

peak bulkhead and a large nut in aft end

ii. Inside tube, a brass bush which has grooves in it

iii. Strips of lignum vitae in grooves act as bearing for

shaft

iv. Studding box to prevent water getting into hull

b. Bilge well: when the cellular double bottom extends out to

the ship’s side there are no proper bilges. In this case the

holds drain into bilge well, which are sunken compartments

in the double bottom. It is to collect water from rain, cargo

sweat, ship sweat, refrigerated container water, etc

ix. Sketch a cross section of a rudder carrier.

x. Sketch a transverse midship section of a general cargo ship

constructed on a combined framing system. List the functions

of longitudinal framing.

xi. Functions of longitudinal framing: Resist hogging and sagging,

water pressure, pounding, dry-docking and shear stresses.

xii. Draw a sketch showing the contact between a steel hatch cover

and the hatch coaming indicating how watertightness is

achieved.

a. The lower rollers are mounted on an eccentric bush which

enables them to be raised or lowered. This enables the hatch

covers to be raised for rolling and stowage, or lowered so that

they can be secured and made watertight.

b. The hatches are made watertight by rubber jointing, being

pull down by cleats and cross-joint wedges.

xiii. Sketch a longitudinal section of a VLCC showing and naming all

the main compartments. Explain the reasons for this

arrangement of compartments.

xiv. List the functions of floors and double bottoms in a general

cargo ship.

Floor: resist water pressure, dry-docking stresses, heavy weights,

local stresses, racking, vibration and pounding.

Double bottom: resist pounding, etc

xv. Draw a sketch showing the layout of the bilge piping

arrangement in a general cargo ship with the engine room

amidships. // Explain the reason for separate pipelines to

each bilge.

a. Not pass through deep tank, double bottom tanks and oil

fuel bunkers

b. Bilge pipes are fitted with non-return valve to avoid hold

flooding

c. Suction are usually placed at the aft end of each hold since

ship normally trim by the stern and water can be collected at

the aft end of the bilges

d. Separate pipelines to each bilge since:

xvi. Sketch a transverse section through a cargo vessel showing and

naming the structural members which resist: Racking stresses

and Water pressure.

a. Racking – Resisted by tank side brackets and beam knees.

Also, transverse bulkhead, web frames or cantilever frames,

floor, shell plating and pillar. (Ship racked by wave action or

rolling. Stresses come on the corners.)

b. Water pressure – Resisted by bulkheads and by frames and

floors, also beam, deck, longitudinal girder, pillar and shell

plating (water pressure push-in the side and bottom of ship)

xvii. List and briefly describe the main drawings and plans available

on board ship.

xviii. Draw sketches showing the transverse stresses which can be

exerted on a ship’s hull and name the structural members

which resist these stresses.

xix. Sketch and list out the various part of a general dry container.

Describe the various types of container, their sizes, and their

usage. State the precautions that should be observed on

Container Stowage before and after operations in a cellular

container ship.

xx. List the function of : longitudinal bulkheads and longitudinal

framing

xxi. Sketch a transverse section of a double bottom tank in way of

a bracket floor, naming the main structural members.

xxii. Sketch a transverse midship section of a general cargo ship

constructed on a combined framing system.

(OR) List the three basic types of ship construction.

a. Transverse system – closely spaced transverse frames to

hold the planks together so that the seams could be caulked.

It provides considerable transverse strength to resist the

racking stress. Mostly for small ship and sailing ships

b. Longitudinal system – has longitudinal frame at the

bottom, sides and decks, supported by widely spaced

transverse web. Strong longitudinal strength resists hogging

and sagging stresses for long ships.

c. Combination system – longitudinal frames in the bottom

and strength deck, transverse frames on the ship side where

longitudinal stresses are smaller. Plate floor and transverse

beams are fitted at intervals to give transverse strength.

Q. Sketch a transverse section of a double bottom tank in way of

a plate floor.


a bracket floor.


a solid water tight floor.


a solid not-tight floor.

SHEAR FORCE AND BENDING MOMENTS:

When a section such as a beam is carrying a load there is a tendency for some parts to be

pushed upwards and for other parts to move downwards, this tendency is termed

Shearing.

The Shear force at a point or station is the vertical force at that point. The shear force at a

station may also defined as being the total load on either the left hand side or the right

hand side of the station; load being defined as the difference between the down and the

upward forces, or for a ship the weight would be the downward force and the buoyancy

would be the upward thrust or force.

The longitudinal stresses imposed by the weight and buoyancy distribution may give rise

to longitudinal shearing stresses. The maximum shearing stress occurs at the neutral axis

and a minimum at the deck and keel. Vertical shearing stresses may also occur.

Bending Moment

The beam, which we have been considering, would also have a tendency to bend and the

bending moment measures this tendency.

Its size depends upon the amount of the load as well as how the load is placed together

with the method of support.

Bending moments are calculated in the same way as ordinary moments that is

multiplying force by distance, and so they are expressed in weight – length units.

As with the calculation of shear force the bending moment at a station is obtained by

considering moments either to the left or to the right of the station.

Hogging and sagging

Hogging – When a beam is loaded or other wise is subjected to external forces such that

the beam bends with the ends curving downwards it is termed as hogging stress.

For a ship improper loading as well as in a seaway when riding the crest of a wave the

unsupported ends of the ship would have a tendency similar to the beam above.

Sagging – In this case the beam is loaded or other wise subjected to external forces

making the beam bend in such a way that the ends curve upwards, this is termed as

sagging.

Similar with a ship if improper loaded or when riding the trough of a wave – with crests

at both ends then the ship is termed to be sagging.

For Hogging the ship ends to curve downwards would mean that the weight/ load

amidships is much less than at the end holds/ tanks.

For Sagging the ship would have been loaded in such a manner that a greater percentage

of the load is around the midship area.

In a seaway the hogging and the sagging stresses are amplified when riding the crests and

falling into the troughs. Thus especially for large ships there are two conditions in the

stability software – Sea Condition and Harbour condition.

A ship loaded while set in the harbour condition may allow loading with hogging/

sagging stresses reaching a high level, when this state of loading is transferred to a Sea

condition in the software the results would be catastrophic since now the wave motions

have also been incorporated.

Thus planning a loading should always be in the Sea Condition.

Discharging in port may be planned in the Harbour Condition.

Hogging and sagging cause compressive and tensile stresses on the ship beam – notably

on the deck and the keel structure.

Water pressure and Thrust

Pressure is force per unit area and water pressure is dependent on the head of the water

column affecting the point of the measurement of the pressure.

Let us assume an area of 1sq.m. then this area of water up to a depth of 1 m below the

surface would have a volume of 1sq.m. x 1m = 1cbm and the weight of this volume

would be 1cbm x density of the water = 1MT (assuming that it is FW) or 1000kgf,

therefore the pressure exerted by this mass would be 1000kgf/sq.m.

Similarly if now the depth of measurement is increased to 3m then the volume of this

area subtending up to the 3m mark would be 1sq.m x 3 = 3cbm and the weight of the

water would be 3MT or 3000kgf and the pressure exerted would be 3000kgf/sq.m.

If now the liquid had not been FW but any other then the weight would be found by

multiplying the volume by the density of the liquid. And thus the pressure exerted would

be found.

If we now increase the area of the square of water plane would it make a difference in the

pressure?

Let us consider a area of 2000sq.m then the volume of this water at a depth of 1 m would

be 2000cbm and the weight would be 2000MT (consider FW) and the pressure exerted

would be 2000,000kgf/ 2000sq.m which would give us again 1000kgf/sqm, thus the

pressure is independent of the area of the water plane.

Thrust however is different, thrust is taken to be the total weight of the liquid over an

area. Thus for the previous example the thrust would be 2000 tonnes.

Thus the thrust is given by: the area of the water plane x pressure head x density of the

liquid.

Thrust always acts at right angles to the immersed surface and for any depth the thrust in

any of the directions is the same. The pressure head which is used in the above

calculation of thrust is the depth of the geometrical centre of the area below the surface of

the liquid.

For a ship the thrust on the ship side changes as the depth increases, however the bottom

is affected uniformly for a set depth.

Centre of pressure of an area is the point on the area where the thrust could be considered

to act. It is taken that the centre of pressure is at 2/3rds the depth below the surface for

ordinary vertical bulkheads and at half the depth in the case of collision bulkheads.

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notes on ship construction