PRELIMINARY DESIGNS IN SHIP BUILDING
The method in ship-building varies
very much with the different types of ships and the conditions under
which they are constructed. Those which are to carry about two thousand
passengers with a large crew must be designed to keep speed of about
28 1/2 knots in average weather. They must have a reserve of power
against unforeseen delay, such as fog or bad weather patches. Speed
is of the greatest importance for liners and units of the merchant
fleet and as such have to be run at a profit.
Before the final design is chosen, a great deal of preliminary
calculation and drawing-office work have to be undertaken, and many
alterations must be considered.
One of the most important points to be settled when designing a
ship is the length, as it has a close relationship to the breadth,
draught and displacement of the vessel.
As the hull is curved in every direction, the plan is done by drawing
three views and since the ship is symmetrical, only half of each
view is actually drawn out.
The ship is divided longitudinally into a number of equidistant
and imaginary points known as displacement stations. At each one
of them a cross-section of the hull is drawn. Those abaft midships
are drawn as half sections on one side of the centre line and those
forward of midship are shown on the other side. These two sets of
half sections are known as fore-body and after-body sections. In
addition to the centre line, there are other imaginary reference
lines such as the water-lines, or the horizontal parallel lines
at right angles to the vertical centre lines. The bottom one, the
base line or bottom of the ship, is the top of the keel plate. Then there is a series of vertical lines,
parallel to the centre lines and spaced equidistantly from it known
as bow-lines in the fore-body and buttocklines in the after-body.
Displacement is the volume in cubic feet of a ship up to a given
load draught. This is the distance between the bottom of the ship
and the water-line or the depth of the submerged portion of the
vessel. Displacement, which is usually referred to as so many tons
weight, is really the submerged volume of the hull in cubic feet
divided by 35 since a ton of salt water occupies 35 cubic feet of
space. Thus, displacement actually represents the amount of sea
water displaced by the ship. If it were possible to build a ship
of the shape of an oblong box, the problem of displacement would
be comparatively easy. Here the displacement would merely be the
product of the length, breadth and submerged depth or draught.
Our modern great steel ocean-going ships owe much of their strength
to the welding methods which bind their parts firmly together.
Welding saves weight too. This is an important fact in figuring
the cost of building a ship, since added weight means more cost.
For instance, in old times ship bodies were riveted. In some ships
built during the second world war, welding replaced riveting. About
10 % of the ship's weight was thus removed and both the speed and
the efficiency of the ship were increased.
Welding was first used in ship-building by Henry Kaiser , who reached
a marvellous record in building speed . Three hundred days were
required to build a ship during the first world war , but the first
Kaiser ship for the second war was delivered in 226 days , then
the tenth ship of the same class was built in 146 days. Later ninety-nine
days were taken to produce the seventeenth ship. Further ships were
produced first in ten days, then in only four. In the coming years
the world was amazed by the unbelievable production rate of one
ship every day.
Kaiser accomplished the miracle of
quick ship production by introducing the welding method. A single
ship had needed 650,000 rivets. Welding reduced this number to less
than four per cent. He employed thousands of welders including many
women. He also changed many other expensive as well as slow processes.
MECHANICAL SHIP PROPULSION
The most important prime-movers in ship propulsion are reciprocating-steam
engines, gas turbines, steam turbines and internal combustion engines
including the diesel type. Nowadays nuclear reactors constitute
more than a promise in this branch of engineering.
Steam is the most valuable source of marine power. It is usually
generated by the burning of fuel oil in watertube boilers.
The Scotch-marine boiler, a fire-tube class, is no longer built,
though many old ones are still in use. The most popular boiler is
a two-drum type with bent water tubes. These boilers are equipped
with water-cooled furnace walls, superheaters and sometimes with
The day of the reciprocating steam-engine, once in universal use,
is past. Its efficiency is so much lower than that of the steam
turbine or diesel engine, that it is no longer justifiable economically.
Marine steam turbines are usually arranged in two parts, a highpressure
and a low-pressure casing, the rotors of which are both geared to
the propeller shaft.
The application of gas turbines to marine use has lagged behind
their introduction into aircraft propulsion because the great savings
in weight attainable with this type of power plant were not so significant
in ships as in planes.
Modern gas turbines have achieved acceptable levels of reliability,
maintenance cost and operating personnel requirements. With their
low weight and space advantages and their rapid starting they have
become competitive with diesel and steam turbines in a wide variety
of marine applications.
TESTS ON MODEL SHIPS
The model is placed in a large experimental tank in which it is
towed by an overhead carriage travelling at a speed proportionate
to the designed speed of the ship. The carriage is fitted with intricate
instruments by which the resistance of the model through the water
is recorded. Calculations are made from these records to determine
the power required to propel the actual ship.
In the case of the "Queen Mary", for instance, the hull
form finally adopted was the outcome of a comprehensive programme
of model tests involving making and testing some twenty-two models
on which about eight thousand experiments were carried out.
Seaworthiness and behaviour in bad
weather were also recorded by creating artificial waves in the experimental
tank and recording by cinematography the effects on the model.
Ships are generally built by a combination of longitudinal and
transverse methods of framing. The former embraces all girder forms
running in a fore and aft direction and so longitudinal strength
is given. The latter involves all girder forms that cross the longitudinal
framework at right angles, transverse strength being given.
By a carefully thought out combination of these two systems, structures
of great strength can be obtained. This framework is covered by
a metal skin known as shell plating which, in combination with the
decks greatly strengthens the framework.
In a simple example of transverse framing as may be used in a small
single-deck, double-bottom cargo vessel, a half section may consist
of a frame bar, a reversed frame bar, a floor plate, a beam and
a pillar of stanchion. In this system the frame bar is continuous
from the gunwale to the tank margin which is really the side of
the double bottom.
Along the bottom of the floor plate
there is a small angle bar serving to connect it to the bottom shell
plating and is also a continuation of the frame. The reverse frame
bar, which is the opposite member to the frame bar, also extends
across the full width of the top of the floor plate and provides
a connection to the tank top or inner bottom plating.
In a single-deck the uppermost extremities of the frames are united
by beams, that is steel bars acting both as the struts and ties.
Within the double bottom there are various fore and aft girders
of which the main one is the central girder. This, in conjunction
with the keelplate, forms the backbone of the slip.
The keel plates are placed after
the keel blocks are in position. With this step the actual construction
of the ship has begun.
The keel may consist of a series
of joined plates that are flat for about two-thirds of the length
of the ship. Towards the fore and aft ends, the plates are bent
up so that along their centre they provide a recess, or channel
to accomodate either the stem bar or the stern frame.
The stem in modern ships is generally
straight and well raked, the
curved stem found in older vessels having very few advantages as
it is a survival of sailing ship days.
Generally plates are lapped and riveted
to one another, but sometimes two plates are joined end-to-end by
fitting butt straps. These fit over the joint between the two plates
and extend far enough over each plate to allow a double row or,
in larger vessels, several rows of rivets to be driven to right
and left of the joint. This work is of the greatest importance.
Riveting may be done by hand or by means of hydraulic or pneumatic
riveters, working against heavy hammer-like tools held upon the
heads of the rivets on the other side of the plate.
The rivets are supplied hot so that when they cool they will contract
slightly and in the process they will draw the plates more tightly
Rivets are of different sizes and shapes according to the function
they are to perform. They also have different shaped heads.
by fastening a number of logs simply into a buoyant structure, the
raft proved to have more carrying capacity, stability and dryness
than the single log. However, it remains an essentially wet, uncomfortable
means of transportation and its form is not readily adapted for
efficient propulsion by oars, sails or motor.
A canoe is usually a small, light
vessel of long and narrow proportions, it is open and pointed at
both ends. It is propelled by paddles or sails and in some parts
of the world is used for conveyance and fishing by maritime peoples.
It is also used for racing, travelling, exploring and as pleasure
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