Page 16 - Ship Construction.DJ Eyres 6Ed

Ch01-H8070.fm Page 5 Wednesday, October 18, 2006 6:51 AM
Basic Design of the Ship 5
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 mini-
mum depth is controlled by the draft plus 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 scant-
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lings (i.e. plate thickness/size of stiffening members etc.). 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
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
advantage.
Many vessels are required to make passages through various canals and
straits and pass under bridges within enclosed waters and this will place a
limitation on their dimensions. For example locks in the Panama Canal and
St Lawrence Seaway limit length, breadth and draft. The maximum dimen-
sions of ships that can transit the Panama Canal are 294 metres in length
(or lesser for some ship types), 32.2 metres in breadth and 12 metres draft
in tropical fresh water. At the time of writing the Malacca Straits main ship-
ping channel is about 23 metres deep and the Suez Canal could accommo-
date ships with a beam of up to 64 metres and maximum draft of 16 metres.
ึ
A maximum air draft on container ships of around 40 metres is very close to
clear the heights of the Gerard Desmond Bridge, Long Beach, California
and Bayonne Bridge, New York. Newer bridges over the Suez Canal at
65 metres and over the Bosphorous at 62 metres provide greater clearance.
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