Shells & Decks

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The Side Shell

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 Functions of Shell Plating

— Forms watertight skin of the ship.

— Contributes to longitudinal strength.

— Resists vertical sheer forces.

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 Shell Plating arrangement

— Composed of many strakes or plates arranged in a fore & aft direction and welded

together.

— A strake is a longitudinal course of plating made up of several plates welded together.

— Internal strengthening of the shell plating maybe both transverse and longitudinal.

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 Shell Plating

— Horizontal weld joints between

strakes are termed “seams”.
— Vertical weld joints between plates
within a strake are termed “butts”

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 Shell Expansion Plan

— A shell expansion plan is a drawing made for every ship so that each individual strake of
shell plating can be identified and its individual size and thickness immediately identified.
— The shell expansion plan helps in identification of shell plates which can be utilised during
repair work.

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 Seam butt

Garboard strake

Coffin plate

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 Typical strakes of a vessel at amidship

— Keel strake
— Garboard strake
— B strake
— C strake
— Bilge strake
— Sheer strake
— Stringer plate

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 Shell plate identification

— Identification of individual shell plates is made by lettering of strakes from the garboard
strake adjacent to the keel plate, around the girth to the sheer strake.
— Numbering of plates in each strake from aft to forward.
— Example of plate identification:
— Plate A5, B7 etc.

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Connections with bottom structure

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 Sheer Strake

— The strake of side plating nearest to the deck is known as the Sheer
strake
— It has increased thickness or high tensile steel maybe used.

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 Gunwale

— Two different ways to connect the

sheerstrake to the stringer plate:
— By fillet weld, forming a
perpendicular joint.
— By butt weld, forming a “rounded
gunwale”.

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 Bilge Keel

— A bilge keel is fitted along the bilge radius either side

of the ship on the parallel middle body to damp the
tendency of a ship to roll.
— Constructed of steel section and is fastened to the
hull on doubler plates located on the shell.
— Attached in such a manner that the bilge keel will
break off without damaging the hull in an event of
collision or fouling.
— Fitted at right angle to the bilge radiused plating but
does not extend beyond the extreme breadth line.

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 Bilge Keel

— Fitted at right angle to the bilge radiused plating but

does not extend beyond the extreme breadth line.
— It must penetrate the boundary layer of water along
the hull but should not be deep enough to have
forces acting upon it.

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 Function of Bilge Keel

— Damp rolling motions

— Provide protection to bilge area in an event of grounding
— Increase longitudinal strength at the bilge.

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 Shell plating

Provides the ship Has different Can be shown in Consists of

Longitudinal Thickness at Shell expansion

Bottom shell plating Side shell plating
strength different locations drawing

Watertight skin Due to different Meant for Refers to Refers to

Horizontal plating Vertical plating from

Loading and Identifying location,
from keel to turn of upper turn of bilge
resulting stresses size and thickness.
bilge to weather deck

Is made up of Is made up of

Keel strake Sheer strake

Garboard strake

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Decks

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— A deck is generally a horizontal plated structure supported by transverse or
horizontal beams (depending upon the adopted framing system), deck girders, webs,
bulkheads and ship sides.
— Function of a deck is dependent on the location.
— Provides a solid working platform capable of supporting any loads resting upon it.
— Where the deck is pierced by hatches, special coamings or surrounds are provided.

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 Types of decks

— Weather decks
— Strength decks
— Tween decks
— Platform decks
— Accommodation decks

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 Weather decks

— Deck of vessel which is exposed to weather, normally the highest deck.

— Protects space below the deck against weather.
— All openings on this deck must be fitted with watertight coves
— Camber on weather deck to provide for better drainage of water.

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 Strength deck

— A strength deck is a deck which is included in the longitudinal strength calculation.

— It is the uppermost bulkhead till which the watertight bulkheads are extended.

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 Tween deck

— In dry cargo ships, there may be more than one deck in cargo holds. These are called
tween decks.
— Allow better storage of cargo.
— Prevent cargoes at the bottom from being crushed.

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 Platform deck

— A platform deck is not included in the longitudinal strength calculation.

— Platform decks are normally very short
— Found mainly in engine room to house auxiliary engines, pumps, compressors, boilers
etc.

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 Main Deck arrangement

— Deck camber to enable water to run down the sides of ship.

— Openings for cargo and human access.
— Bulwarks and guard rails on exposed decks are fitted for safety of crew.
— Freeing ports fitted on bulwark to free decks of water.
— Scuppers fitted on decks to give effective drainage.

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 Deck framing

— Decks maybe framed transversely or longitudinally.

— When longitudinally framed, deck longitudinals are supported by deck transverses.
— When transversely framed, deck beams are supported by longitudinal girders

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 Stealer Plate

— Towards the ends of the ship, as the

cross-section reduces the various
strakes of plating will taper in width.
— Where these plate widths become
small, a stealer plate or strake is
used.

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— The plates or strakes on the deck nearest to the deck edges are termed “stringer
plates”. They are of greater scantling as they form the important joint between the
side shell and deck plating
— The large openings in the deck for hatchways, engine casing, pump room entrances
etc require compensation to maintain the section modulus of the material.
— The deck plating around such openings may be increased in thickness.
— However the plating between the hatches of a cargo ship is thinner than the rest of
the plating and contributes little to the longitudinal strength.

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 Bulwarks

— Fitted on weather decks as protection for

personnel and not intended as major structural
feature.
— Freeing ports are cut in bulwarks in order to
facilitate quick drainage of green seas.
— Must be at least 1 m in height on the exposed
decks however a reduced height maybe
permitted if it is found to be interfering with the
operation of the ship.

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 Discontinuities in structure

— Whenever the cross section area of a structural member changes suddenly a

structural discontinuity exists.
— When a discontinuity in a structural member occurs, the stress in the member in
the neighbourhood of the discontinuity goes up considerably.

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 Examples of discontinuities in structure

— Deck holes or openings like access hatches.

— Cargo hatch corners
— End of superstructures
— End of girders such as bottom structure girders

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 Effects of discontinuities in structure

— Discontinuities in structure will cause a change in the distribution of stresses.

— The stress concentration due to discontinuities in structure maybe so great as to

cause direct local failure of the material.
— In instances where a member is subjected to alternating loads, even if the local
stresses were below the yield point, it could lead to failure due to fatigue.

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 Discontinuities in structure

Discontinuities in structure cause an increase in stressesand should be avoided.

Measures need to be taken to reduce the adverse effects, such as:
— Fitting doubler plates
— Gradual tapers of structural members
— Well-rounded corners
— Increase in plate thickness.

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 Hatch Openings

Great care must be taken to compensate for any discontinuities in shell or deck' plating resulting from
doors, hatchways, etc. Where the loss of longitudinal material results, this compensation is of particular
importance.
Well-radiused corners must be used and sometimes the fitting of doubling plates or thicker insert
plates, at
the corners of all openings. Any sharp corner can produce a notch which after stressing, could result in a
crack.
The edges of all hatch openings are framed by hatch coamings. On the weather deck the coamings must
be at a minimum height of 600 mm according to the load line regulations. This is to reduce the risk of
water entry to the holds. Internal coamings, e.g. those within the superstructure or holds, have no
height specified and in tween deck holds particularly are often made flush with the deck for
uninterrupted cargo stowage. The weather deck coaming must be a minimum of 11 mm thick, and
where the height is in excess of 600 mm it must be stiffened by a horizontal stiffener and vertical
brackets must be fitted not more than 3 m apart. An edge stiffener must also be provided.

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Hatch Openings

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Hatch Opening

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Hatch Opening

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Hatch Opening

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Strengthening at cargo hatch openings

insert insert

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Scuppers

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 Superstructure& Deckhouse

— Superstructure is defined as those erections above the freeboard deck which

extend to the ship’s side or almost to the side.
— Deckhouse is defined as an enclosed erection on or above the weatherdeck that
does not extend from side to side of the ship
— Of great structural importance is the strength of the vessel where superstructures
and deckhouses terminate and are non-continuous

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 Design philosophy

— The design background of a ship’s superstructure is completely different than

that of a hull.
— The hull of a ship needs to be built from a hydrodynamic point of view.
— Structurally, a hull is vastly different than a superstructure, as it goes without
saying that the strengthening considerations to withstand the seas will be of
much more concern than the case of a superstructure which lies above. They
hardly have contact with the water (except in exceptional cases like clement
weather conditions or hefty wave surges where effects like ‘green water’, ‘wave
slap’ which momentarily affect the superstructure.)

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 Requirements of good superstructure

— Operational requirement fulfillment in terms of adequate volume.

— Provision of adequate deck space and height.

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 Superstructure end

Measures need to be taken to reduce the adverse effects of high stress at

superstructure ends:
— One of the foremost measures that need to be implemented is the construction of
these house ends in line with bulkheads, both longitudinal and transverse. This is
to maintain the continuity of the load path and reduce discontinuities
— Thickness of the sheer strake at the ends of superstructure is increased by about
50% and thickness of the stringer plate by about 25%.
— Bridge side plating is gradually tapered to the level of the bulwark.

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Superstructure

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