Page 148 - Practical Ship Design

Wright-Bused Drsigns 1 I5

4.6.4 Margins on centres of gravity

Although the estimation of the lightship VCG and LCG will not be dealt with until
a later section, it is convenient to deal here with the margins that it is wise to have
on these figures.
A margin on the VCG is sometimes established by adding the weight margin at
a high centre of gravity but this does not really take account of the main reason for
having a margin on the VCG which is to offset a possible underestimate of the
centres of gravity of some of the weights that make up the lightship rather than to
deal with a consequence of underestimating the lightweight itself.
Whilst the use of an excessively large margin may cause design problems and
possibly increase the cost of the ship in question (if for instance it results in the
beam being increased unnecessarily) it is vitally important that the margin is big
enough to counteract errors in the VCG calculations which generally seem to result
in an underestimate - or is it only when this is the case that they are noticed? The
margin should scale to some extent but less than directly with ship size and should
be bigger in the early stages of design when the calculations are approximate and
decrease as these become more accurate. To meet the first of these criteria, it is
suggested that the margin should be a based on the square root of the moulded
depth D starting say at O.l(D)o and decreasing to 0.06 (D)0.5 as confidence grows
in the calculations.
It is unusual to apply a margin to the calculated LCG position but, on the basis
that a small increase in an estimated trim by the stern will usually be acceptable,
whereas a change from an estimated level keel to a trim by the head will not be, it
may be wise to base preliminary trim calculations on an LCG a little further
forward (say 0.5% L) than the calculated figure.

4.6.5 Deadweight and displacement - merchant ships
If a total deadweight is stipulated the required full displacement is the sum of this
and the lightweight.
From many points of view, it is better for an owner to specify the required cargo
deadweight and put the onus on the designer to allow for all the non-cargo
deadweight items needed to perform the specified service.
The non-cargo deadweight items consist of the fuel for both main engines and
generators, fresh water for all purposes, engineers sundry tanks, stores of all sorts,
crew and effects, passengers and baggage, water in swimming pools, etc.
The items which commonly make up the total deadweight are:
- cargo deadweight
- passengers and baggage
- crew and effects
- stores of all sorts