TOPICS ON MECHANICS

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 air-heaters.

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.

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FRAMING AND PLATING

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 together.

Rivets are of different sizes and shapes according to the function they are to perform. They also have different shaped heads.

Constructed 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 boats.

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