11/09/2020
What Materials Are Used For Building Ships?
The economic aspect of running a merchant
vessel is of prime importance as a shipowner
requires a build which maximises the returns for
his initial investment and covers his running
costs.
This implies that the final design takes into
account the economic conditions at the time of
building, and also those that are likely to
develop within the life of the ship.
Apart from this, the safety of seafarers on
board, the type of vessel, the operational
logistics of the voyages is taken into serious
consideration while planning and executing the
shipbuilding operations.
Various classification societies which are based
out of different maritime nations have come into
existence for initial and continual inspection of
ships . These are well reputed and reliable
organizations which assess and maintain a ship’s
seaworthiness and award them a classification.
The Classification Societies Include the following;
Great Britain—Lloyd’s Register of Shipping
France—Bureau Veritas
Germany—Germanischer
Lloyd Norway—Det Norske Veritas
Italy—Registro Italiano Navale
United States of America—American Bureau of
Shipping
Russia—Russian Register of Shipping
Japan—Nippon Kaiji Kyokai.
Classification societies publish rules and
regulations concerned with the provision of
adequate equipment, the reliability of the
machinery used on board, the strength of the
ship, etc. The vessel may be built in any country
and are not restricted to classification only by
the relevant society of that country; they can
follow regulations in accordance with the rules
of any particular classification society.
Related Read: What is the International
Association of Classification Societies (IACS)?
Most of the world’s merchant ships are currently
being built by Japan and Korea, which together
made about 77 percent of the gross tonnage
delivered in the year 2000.
While classification is not compulsory for all
ships, it is a common industry practice as it is a
good endorsement for the company and vessel.
Shipowners with an unclassed ship must satisfy
governmental regulating bodies to ensure that
the vessel has necessary structural strength for
assigning a load line, and the issuing of a safety
construction certificate.
Related Read: What ISM Certificates You Require
to Start a Shipping Company?
Most commonly used materials for shipbuilding :
Steel:
This is a highly versatile ship construction
material and is used extensively on ships for the
making of its integral structure and parts.
Steel has been in use for over 150 years in the
shipbuilding industry, thanks to its excellent
mechanical properties and low cost.
One major drawback of employing steel in ship
constructions is the weight of steel.
For the construction of the hull of a ship, mild
steel containing 0.15% to 0.23 % carbon, and
reasonably high manganese content is used.
Sulphur and phosphorus contents in the mild
steel are kept to a minimum (less than 0.05%)
as higher contents of each hamper the welding
properties of the steel.
Related Read: Common Welding Methods And
Weld Defects In Shipbuilding Industry
Furthermore, cracks and such can develop easily
during the rolling process if the sulphur content
is high.
High Tensile Steels Steels which have higher
strength than that of mild steel are employed in
the more stressed regions of large tankers ,
container ships and bulk carriers . They are often
used for the deck and bottom regions of larger
tankers as well. As this leads to a reduction in
the scantlings of these structural items, it proves
advantageous both for the shipbuilder and
owner.
Some integral parts of the ship that are made of
steel include; the thickness stringer plate,
rounded gunwales, sheerstrake, Bilge strake,
deck strake in the way of the longitudinal
bulkheads, main deck plating, bottom plating,
keel, and the upper strake of longitudinal
bulkheads. Mast tables, crosstrees, etc., may be
fabricated from welded steel plates and
sections. Derrick booms, as a rule, are welded
lengths of seamless tubular steel.
Related Read: Shipbuilding Process – Plate
Stocking, Surface Treatment and Cutting
Aluminium Alloys:
There are three main advantages which
Aluminium alloys have over mild steel in the
construction of ships.
1. Aluminium is lighter than mild steel and
studies suggest that up to 60 percent of the
weight of a steel structure can be saved by using
Al alloys. For merchant vessels, this is a key
advantage for increasing the cargo carrying
capacity of ships.
2. Aluminium is highly resistant to corrosion
3. Its non-magnetic properties and low-cost
maintenance
The most often used Al-alloys used in
shipbuilding are the 5083 type for plates and
6082 for extrusions. These alloys are reliable in
marine service as well as during manufacture. It
has estimated that the selection of Al-Mg
(Aluminium-Magnesium) type alloys brings a
potential for at least 10% lower costs in respect
of the heat-treatable, and appears favourable
after a total estimation for applicability in
shipbuilding
Excellent corrosion properties of aluminium can
be used easily, but correct maintenance
procedures and careful insulation from the
adjoining steel structures are necessary when
using this material.
A major disadvantage of the use of aluminium
alloys is their high initial cost (They are
estimated to cost 8 to 10 times the price of steel
per tonnage). This high initial cost must be
absorbed by an increase in the earning capacity
of the vessel or a major increase in passenger
accommodation on the same draft.
Superstructure – Credits: Hervé Cozanet/
wikipedia.org
Aluminium alloys can replace carbon steels of
normal strength. The weight saved by using Al
alloys improves the ship stability – and allows
the design of narrower ships, which in turn
enhances fuel efficiency.
Related Read: Ship Stability: Intact Stability
Criteria and Inclining Experiment
Materials Used on Different Parts of a Ship
The Shell Plating:
This forms the watertight skin of the ship
contributes to the longitudinal strength of the
structure and resists vertical shear forces. The
bottom and side shell plating consist of several
flat and curved steel plates are butt welded
together. They are of greater length than
breadth.
Insulation:
A steel hull structure is an excellent conductor of
heat. Some form of insulation must be provided
at the boundaries where there is a requirement
to maintain desired temperatures, such as the
refrigeration compartments.
Cork, glass fibre, and different kinds of foam
plastics in sheet or granulated form may be
used for insulating purposes. Air spaces, which
are less efficient, may be provided. Glass fibre is
widely used in modern ships as it has several
advantages over the other materials. It is light in
weight, tends to be vermin-proof, does not
absorb moisture and is fire-resistant.
Related Read: Properties Of Membrane Tanks
For Transportation Of LNG Cargo On Ships
Superstructures:
The introduction of aluminium alloy
superstructures has provided increased
passenger accommodation on the same draft,
and/or a lowering of the lightweight centre of
gravity with improved stability. These are hence
used on passenger ships and cruise ships . It is
possible to accept more significant deformation
in these superstructures than would be possible
with steel. This is brought about by the lighter
weight of the aluminium structure.
Watertight Doors:
In some instances, it is necessary to provide
access between compartments on either side of
a watertight bulkhead . Hence watertight doors
are fitted for this purpose. For example, in a
cargo ship, direct means of access is required
between the engine room and the shaft tunnel.
In passenger ships, watertight doors are found
where passengers are allowed to pass between
one point of the accommodation and another.
Mild steel or cast steel watertight doors are
fitted below the water line, which prevents
flooding of the compartments when shut while
providing adequate strength in the case of
emergencies.
Weathertight Doors
The Stern Frame:
This structure supports the rudder and the
propeller. The stern itself may be cast, forged,
or fabricated from steel plate and sections.
Modern rudders are also fabricated from steel
plates, with plate sides that are stiffened by
internal webs. To prevent corrosion, the internal
surfaces are suitably coated, and the rudder
may be filled with inert plastic foam.
Related Read: What Is Advanced Outfitting in
Shipbuilding?
Rudder Pintles:
The rudder pintle is a bolt or pin which is
inserted into a gudgeon to attach the rudder to
the ship. Older ships may have a brass liner or
bronze liner shrunk on the pintles which turn in
hardwood (Lignum Vitae) bearings, fitted in the
gudgeons. In these days, the industry practice is
to use synthetic materials like Tufnol for the
bearings, and in some cases stainless steels for
the liners. In either, the water which immerses
the bearing is used to lubricate it.
Rudder Stock:
The stock may be of cast or forged steel, with its
diameter as determined by the torque and any
bending moment it is to withstand.
Related Read: How Does A Rudder Help In
Turning A Ship?
Propellers:
As they have to withstand the corrosive effects
of saltwater, ship propellers are constructed
from copper alloys such as brass. These are
designed to minimize cavitation, which happens
when a propeller working under heavy load
creates a region of low pressure. Bubbles of
water vapour form suddenly and then burst next
to the propeller blades, blasting little pits into
the surface and wearing it away.
The fitting of zinc plates in the way of bronze
propellers and other immersed fittings being
used as sacrificial anodes is common practice in
shipbuilding. These anodes are metals or alloys
attached to the hull, which have more anodic
potential than steel when immersed in sea
water.
Hence these anodes supply cathodic protection
current and get consumed in doing so. Regular
maintenance and replacement are hence
required in such systems for protection.
Modern anodes are based on alloys of zinc,
aluminium, or magnesium which have
undergone many tests to examine their
suitability; high purity zinc anodes are also used.
Sacrificial anodes are fitted with the hull and
also often in ballast tanks as well.
Should any part of the anode fall and strike the
tank structure where gaseous conditions exist,
an explosion could result and hence magnesium
anodes are not used in the cargo-ballast tanks
of oil carriers owing to spark hazards.
Aluminium anode system is employed in
tankers, and they are only fitted in locations
where the potential energy is less than 28 kg.m.
Related Read: Understanding Design Of Ship
Propeller
Paints:
Maintenance of a ship requires that its hull and
parts be painted regularly to avoid corrosion and
provide resistance to other natural elements.
Paints consist of a pigment dispersed in a liquid
which is referred to as the ‘vehicle’. It is spread
out thinly and overtime the vehicle changes to
form an adherent dry film.
Paints which inhibit corrosion of steel have the
following vehicle types:
(1) Bitumen or pitch solutions available in
naphtha or white spirit solvent.
(2) Oil-based: These consist mainly of vegetable
drying oils, such as linseed oil and tung oil. To
accelerate the drying by the natural reaction
with oxygen, driers are added.
(3) Oleo-resin: In this case, the vehicle consists
of natural or artificial resins incorporated into
drying oils. Some of these resins may react with
the oil to give a faster drying vehicle. Other
resins do not react with the oil but heat is
applied to dissolve the resin and cause the oil to
stick to the body
(4) Alkyd resin: These vehicles provide a shorter
drying time and improved film forming
properties of drying oils. The name alkyd arises
from alcohols and acids, which are the chemical
ingredients. These may not be made from oil, as
an oil fatty acid or an oil-free acid may be
used. Vehicle types (2) and (4) are not suitable
for underwater service, and only certain kinds of
(c) are ideal for such service.
(5) Chemical-resistant : Vehicles of this type show
high resistance to severe conditions of exposure.
A number of important vehicle types come
under this category which includes: Epoxy resins
Chemicals, coal tar/epoxy resin, Chlorinated
rubber and isomerized rubber, polyurethane
resins, Vinyl resins, and Zinc-rich paints.
(6) Anti-fouling paints: These are a category of
underwater hull paints (known as bottom paints)
which are specialized coatings applied to slow
the growth and/or facilitate detachment of
subaquatic organisms that attach to the hull of
the ship and affect a vessel’s performance and
durability. Copper oxides and biocides are
commonly used in anti-fouling paints. Another
type of hard bottom paint includes Teflon and
silicone coatings, which are too slippery for
growth to stick.
Related Read: Understanding Function of Anti-
fouling Paint and Factors to Apply it
Floating of Metal Structures
To construct a ship that floats when immersed
water, marine engineers and naval architects
rely on the Archimedes principle. This principle
describes an upward buoyant force that is
exerted on a body that is fully or partially
immersed in a fluid, which is equal to the
weight of the fluid displaced by the body. This
buoyant force acts at the centre of mass of the
displaced fluid and in the upward direction. For
an object to float in water, the amount of water
it displaces should be equal to the weight of the
object.
This volume of water which is displaced by a
ship depends not only on the weight of the
object but also on its shape and size. It can be
observed that an iron nail sinks in water, while
the same material (iron) arranged in different
size and form, i.e. a boat or ship, floats in
water.
Related Read: Intact Statical Transverse Stability
Of Displacement Vessels
We can see that if the construction of the
structure is such that the density of the vessel is
less than that of water, the ship will float in
water. Hence a seaworthy steel vessel will have
a lower average density than water, which
enables it to float. The shipbuilders also have to
consider the intact stability and damage stability
while designing the ship.
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