Page 159 - Practical Ship Design

126 Chapter 4

4.8 LIGHTSHIP CENTRES OF GRAVITY

4.8.1 General discussion

There can be little doubt that most errors in stability calculations arise from
incorrect estimates of the lightship VCG. Much the same could be said about errors
in trim with this being generally attributable to errors in the lightship LCG,
although fortunately these errors are rarely as serious as errors in the lightship
VCG so often are.
Estimates of the lightship centres of gravity have to be made early in the design
process and generally have to be based on incomplete plans and specifications;
at this stage few decisions have been taken about machinery and equipment
suppliers; the only structural plan available is likely to be an outline midship
section; a body plan may be available but there is unlikely to be a lines plan at this
stage.
Against this back ground, or lack of it, an estimate must be made of the lightship
centres of gravity and in particular of the VCG and a number of ways of doing this
will be discussed. The results of the stability calculations made at this early stage
in the design are used to confirm or amend the preliminary dimensions and
arrangement.
In the next stage, the design will be progressed with decisions being taken on
many of the major items of equipment and machinery; a lines plan will be drawn
and the midship section updated and possibly supplemented by an outline structural
profile and decks. Along with these developments progressively more accurate
weights and centres of gravity will become available and the design stability and
trim calculations can be updated. Provided this process can be completed before a
shipbuilding order is placed it is fairly easy, if the stability is found to be
inadequate, to take remedial measures such as increasing the beam and/or reducing
the depth by a small amount.
Unfortunately shipbuilding orders are sometimes placed on designs that have
not been fully developed, or significant changes affecting the centre of gravity may
have been made at a late stage in the tender negotiations. In these cases the time
allowed for verifying the stability in detail will often be severely limited by
production priorities demanding that steel, machinery and outfit ordering and loft
work etc be put in hand as quickly as possible. Establishing a need to make changes
in dimensions once these processes are under way is highly unpopular to say the
least, and it is not unknown for designers to turn a blind eye to revised estimates
which give unpalatable answers hoping that weight reductions can be made as the
design develops, which needless to say is a recipe for trouble.
See also $4.6. on the subject of margins.