BASIC DESIGN OF THE SHIP

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The dimensions are primarily influenced by the cargo carrying capacity

of the vessel. In the case of the passenger vessel, dimensions are influenced

by the height and length of superstructure containing the accommodation.

Length where not specified as a maximum should be a minimum consistent

with the required speed and hull form. Increase of length produces higher

longitudinal bending stresses requiring additional strengthening and a

greater displacement for the same cargo weight. Breadth may be such as to

provide adequate transverse stability. A minimum depth is controlled by

the draft plus a statutory freeboard; but an increase in depth will result in

a reduction of the longitudinal bending stresses, providing an increase in

strength, or allowing a reduction in scantlings. Increased depth is therefore

preferred to increased length. Draft is often limited by area of operation

but if it can be increased to give a greater depth this can be an advantage.

Many vessels are required to make passages through various canals and

this will place a limitation on the dimensions. The Suez Canal has a draft

limit, locks in the Panama Canal and St. Lawrence Seaway limit length,

beam and draft. In the Manchester Ship Canal locks place limitations on

the main dimensions and there is also a limitation on the height above the

water-line because of bridges.

2. Displacement is made up of lightweight plus deadweight. The light-

weight is the weight of vessel as built, including boiler water, lubricating oil,

and cooling water system. Deadweight is the difference between the light-

weight and loaded displacement, i.e. it is the weight of cargo plus weights of

fuel, stores, water ballast, fresh water, crew and passengers, and baggage.

When carrying weight cargoes (e.g. ore) it is desirable to keep the lightweight

as small as possible consistent with adequate strength. Since only cargo

weight of the total deadweight is earning capital, other items should be kept

to a minimum as long as the vessel fulfils its commitments.

3. In determining the dimensions statical stability is kept in mind in order

to ensure that this is sufficient in all possible conditions of loading. Beam and

depth are the main influences. Statutory freeboard and sheer are important

together with the weight distribution in arranging the vessel’s layout.

4. Propulsive performance involves ensuring that the vessel attains the

required speeds. The hull form is such that it economically offers a minimum

resistance to motion so that a minimum power with economically lightest

machinery is installed without losing the specified cargo capacity.

A service speed is the average speed at sea with normal service power

and loading under average weather conditions. A trial speed is the average

speed obtained using the maximum power over a measured course in calm

weather with a clean hull and specified load condition. This speed may be a

knot or so more than the service speed.

Unless a hull form similar to that of a known performance vessel is used,

tank tests of a model hull are generally specified nowadays. These provide

the designer with a range of speeds and corresponding powers for the hull

form, and may suggest modifications to the form. Published data from

accumulated ship records and hull tests may be used to prepare the hull

form initially.

The owner may often specify the type and make of main propulsion

machinery installation with which their operating personnel are familiar.