Ship Construction

Hull Structure:

Description of standard steel sections: flat plate, offset bulb

plate, equal angle, unequal angle, channel, and tee

Structural components on ships’ plans and drawings: frames,

floors, transverse frames, deck beams, knees, brackets, shell
plating, decks, tank top, stringers, bulkheads and stiffeners, pillars,
hatch girders and beams, coamings, bulwarks.
Frames: These are vertical members that make up the framing of
the vertical part of the hull. Frame type and spacing vary
considerably depending on the ship's construction.

Longitudinally framed hull : The longitudinal framing is much

better able to resist buckling when the hull is hogging. Longitudinal
framing is mandatory for very large ships, oil tankers and bulk-ore
carriers. The rings are formed of floors, deck beams and web
frames that replace the frames. These rings are farther apart than
in transverse framing. The longitudinal reinforcement members are
deck girders, girders, the keel and a large number of deck, bottom
and side longitudinal. The longitudinals are slender but there are
very many of them.

Transversely framed hull : Transverse framing is used primarily

for ships less than 120 metres in length. The floors, frames and
beams form rings spaced closely together. Longitudinal strength is
provided by the keel, centre girder, side girders, deck girders, the
entire bottom, deck and side shell plating, and the tank top.
Transverse framing ensures good cross sectional strength to
handle overall stresses, vertical loads, rolling and dry docking.
However, on very long ships, sheer stresses can cause
deformations between the rings.
Mixed framing : Mixed framing combines longitudinal and
transverse framing. One type of framing is used in one part of the
ship and the other type is used in another part. The most common
combination is longitudinal framing for the bottoms and the deck,
and transverse framing for the sides.

Definitions of hull elements

Keel: The keel is a member, or series of members, running

longitudinally that forms the structural base of a ship. The keel
always corresponds to a ship's centreline. It is a major component
in providing longitudinal strength and efficiently distributes local
stresses when the ship is dry docked. There are two types of keels
used to build ships of a certain size, the flat keel and the duct keel.

1) Flat keel
2)Duct keel

 Girders: A girder is a longitudinal member used in the

construction of the bottom of a ship. They can be watertight
or not and can be placed above the keel (centre girder) or
spaced equal distances from it (side girders). They can be
continuous or divided by floor sections (inter-costal side
girders). The centre girder is always one continuous piece
and must be fastened to the keel with a continuous weld.
Girders must extend as far as possible from the forward to
the aft end of a ship.
 Floors: These are made up of cross members that are
mounted perpendicular to the keel and girders. There are
three main types of floor: Watertight, solid/ plate and
bracket/open.
 Deck beams: These are transverse members that connect
the top ends of the frames, forming the transverse framing for
the deck.
 A strake is part of the shell of the hull of a ship which, in
conjunction with the other strakes, keeps the sea out and the
vessel afloat. It is a strip of plating in a metal one, running
longitudinally along the vessel's side, bottom or the turn of the
bilge, usually from one end of the vessel to the other.
 Garboard strakes /’A’ strake : The strake immediately
adjacent to the keel is known as the garboard strake. There
are two, one on each side of the keel.
 Sheer Strake: The longest continuous strake at the top of the
side of the vessel on the main deck is called the sheer strake.
 Others have no special name other than bottom strakes, bilge
strakes and topside strakes.
1)Plate floor
2. Watertight floor:

3. Bracket or open floor

Double Bottom Tanks: All Ocean going ships are fitted with a
double bottom (DB) which extends from Fore peak bulkhead to
almost after peak bulkhead.

Consists of an outer shell and inner skin or tank top between 1 to

1.5 mtr above the keel. At the centre of the ship, longitudinally, the
keel, centre girder and centre line strake of the tank top plating
form a very strong “I” shaped girder which forms the back bone of
the ship.

DB provides a protection in the event of damage to the bottom

shell. The tank top being continuous increases the longitudinal
strength and acts as a platform for cargo and machinery. DB
contains considerable amount of structure and is therefore useless
for cargo, hence it is used for Fresh water, Seawater ballast.

It is subdivided longitudinally and transversely into large tanks

which allow different liquids to be carried and may be used to
correct list of a ship or change the trim. Manholes are provided
which are watertight, for access to the DB.

In majority vessels, only one watertight longitudinally division

made by a centre girder of the ship. Further, if another longitudinal
non water tight girder is provided within the tank, between centre
girder and tank boundary, then it is called “side girder or inter-
costal girder”

If DB transverse length is more, it will be divided further by one

more water tight division and tanks formed are named called Inner
DB , outer DB.

A cofferdam must be fitted between FW and SW DB tanks to avoid

contamination of one with another.

Testing of the tanks is done by pressing them up until they

overflow, since overflow pipe usually extends above the weather
deck, giving a tremendous head to the tank top which sometimes
exceed load from the cargo in the hold.

In cargo holds, if cargo discharges is by grabs or by forklifts,

heavier flush plating is provided on tank top to prevent damages to
the tank top.

Under hatchways, plating of tank top is increased by 10%.

At bilges, tank top is either continuous up to shell or knuckled

down to the shell by means of a tank margin plate set an angle of
45º to the tank top and meeting the shell at right angle. Margin
plate helps flow of the liquid for drainage.

DB entry is considered as “Enclosed space entry”

Double Bottom Tanks and Flooring:

1 . Solid floor , 2. Bracket floor - Open floor- Skeleton floor
3. Center girder, 4. Side girder, 5. Margin plate, 6. Center strake
7. Inner bottom plating
8. Plate Floor, 9. Reverse frame, 10 . Main frame ,11 . Keel,

12 . A strake, 13 . Bottom plating, 14 . Bilge strake, 15 . Bilge keel

16 . Tank side bracket , 17 . Gusset plate, 18 . Gusset angle
19 . Hold frame, 20 . Hold pillar, 21 . Double plate, 22 . Bottom
ceiling, 23 . Air hole, 24 . Manhole, 25 . Limber hole, 26 . Vertical
stiffener, 27 . Vertical bar, 28 . Strut, 29 . Center bracket
30 . Bracket to margin plate, 31 . Lightening hole
Longitudinal framing – Plate floors/solid floors
Longitudinal framing – Open floors/Bracket floors

Transverse framing – Open floors/bracket floors (below)

Transverse framing – Plate floors/Solid floors
Duct Keel: A duct keel is formed by two longitudinal girders
(instead of one centre girder for the ship) up to 1.83 mtr apart. This
distance must not exceed as these girders must be supported by
the keel blocks when docking.

The structure on the each side of the girders is the normal DB

arrangement. The keel and the tank top center strake must be
strengthened either by supporting members in the duct or by
increasing the thickness of the plates considerably.

Generally duct keel starts at the engine room and extends till the
forward hold. This arrangement allows pipes ( hydraulic, SW,
heating steam, Hold bilges, ballast line) to be carried beneath the
hold spaces and protected against cargo damages. Entry is from
engine room is arranged with a trolley on rails thus allowing pipes
to be inspected and repaired any time.

This arrangement also prevents contamination of cargo if pipes

were passed through tanks, cargo holds. Duct keels are important
for insulated ships, allowing access to the pipes without disturbing
the insulation.Duct keels are not provided aft of Engine room,
since pipes may be carried through the shaft tunnel.

Duct keel entry is considered as “Enclosed space Entry”.

 Bulkheads
There are three basic types of bulkhead, watertight, non watertight
and tank.

Different types of bulkheads are designed to carry out different

functions.

The watertight bulkhead are installed to carry out several

important functions ;

1. It divides the ship into watertight compartments and

thereby limit flooding if the hull plating is damaged; and
giving a buoyancy reserve in the event of hull being
breached. The number of compartments is governed by
regulation and type of vessel.

2. Cargo separation

3. They restrict the passage of flame/fire

4. Increased transverse strength, in effect they act like ends

of a box,

5. Prevent distortion of the hull plating;

6. Longitudinal deck girders and deck longitudinal are

supported by transverse watertight bulkheads which act as
pillars;

7. Rigidly attach the tank top to the upper deck.

Number of bulkheads (cargo ship)

Length of ship (m) Number of bulkheads
Above Not exceeding Machinery midships Machinery Aft
90 105 5 5
105 115 6 5
 115 125 6 6
125 145 7 6
145 165 8 7
165 190 9 8
To be
To be considered To be considered
190 considered
individually individually
individually

The number of bulkheads depends upon the length of the ship and
the position of the machinery. But in general, there is a watertight
bulkhead (collision bulkhead) at the bow that should be located
between 0.05L and 0.075L (L = length between perpendiculars of
a ship), This must be continuous to the uppermost continuous
deck.

The stern tube must be enclosed in a watertight compartment

formed by the stern frame and the after peak bulkhead which may
terminate at the first continuous deck above the waterline. The
engine room must be contained between two watertight bulkheads
one of which may be the after peak bulkhead.
Each main hold watertight bulkhead must extend to the uppermost
continuous deck unless the freeboard is measured from the
second deck in which case the bulkhead can extend to the second
deck.

A water tight bulkhead is formed from plates attached to the shell,

deck and tank top by means of welding. The bulkheads are
designed to withstand a full head water pressure and because of
this the thickness of the plating at the bottom of the bulkhead may
be greater than that at the top. Vertical stiffeners are positioned
760mm apart except were corrugated bulkheads are used.

Watertight bulkheads must be tested with a hose at a pressure of

200 KN/m2 . The test being carried out from the side on which the
stiffeners are fitted and the bulkhead must remain watertight.

Water tight bulkheads which are penetrated by pipes, cables etc.

must be provided with suitable glands which prevent the passage
of water.
 Deck girders: These are longitudinal members that combine
with the beams to form the longitudinal framing of the deck.
 Longitudinals: A very general term to identify any small
longitudinal member that can be used for several purposes.
This term is used more specifically in longitudinal framing.
 Web frames: Oversized members that replace a frame at
certain locations on a ship.
 Bracket: A general term that identifies any part used to
connect two members.
 Beam knee: Bracket located at the end of deck beams that
connect the beam and frame to the shell plating.
 Pillar: Vertical member inside a ship that connects the deck
to the ship's bottom, where it is installed between two
tweendecks, especially around hatches. They are quite bulky
and complicate cargo handling inside holds.
 Plating: The plating of a hull is the series of plates that form
the watertight shell of the hull. There is bottom plating, deck
plating and side shell plating.
 Bilge plating: Longitudinal plating that connects the side
shell plating to the bottom plating.
 Tank top: Watertight series of plates attached to a ship's
bottom framework.
 Double bottom: The double bottom is the watertight space
between the bottom plating and the tank top. Its height varies
according to the size and type of ship, but it is generally
between 0.75 and 1.5 metres. A double bottom is divided into
several watertight compartments by watertight floors and
girders. These compartments can be used to store FW,
ballast water. They are often used to adjust a ship's list and
trim.
Bulkhead definitions
Class A : Are divisions forming bulkheads and decks that;

-Constructed of steel or equivalent

-suitably stiffened

-Prevent passage of smoke and flame to the end of one hour

standard fire test

-Insulated using non-combustible material so that average

temperature on exposed side does not rise above 140oC and
point temperature above 180oC. The time the bulkhead
complies with this governs its class
A-60 60min, A-30 30Min, A-15 15Min, A-0 0Min

Class B : These are divisions formed by bulkheads, decks,

ceilings and lining

- Prevent passage of flame for first half hour of standard fire

test.

- Insulated so average exposed side temperature does not

rise more than 139oC above original and no single point rises
more than 225oC above original. The time the bulkhead
complies with this governs its class: B-15 15Min, B-0 0Min

- Constructed of non-combustible material and all materials

entering the construction are similarly non-combustible
except where permitted

Main vertical zones Divided by Class A bulkheads and not

exceeding 40m in length

A plane bulkhead, showing connections to deck, sides and double

bottom and the arrangement of stiffeners
Collision Bulkhead :
 A corrugated bulkhead

Transverse bulkheads have vertical corrugations and fore and

aft bulkheads have horizontal ones
The basic idea of a bulkhead in addition to the water tight integrity
is to add to the girder strength of the ship beam.
Thus for a transverse bulkhead, which extends from the port to the
starboard side or vice versa, the framing is done in a vertical
manner so that the compressive and the tensile stress may be
reduced for the beam.
Similarly for a longitudinal bulkhead which runs parallel to the
shipside the framing is done vertically, again so that the additional
strength would enhance the stress compensating effect of the ship
beam.

Construction of the corrugated bulkhead

A fitted corrugated bulkhead: