Page 164 - Practical Ship Design

130 Chapter 4

4.8.4 Detailed calculations
Finally of course the centres of gravity can be calculated using detailed weights, a
procedure which often has to be carried out again and again as the design develops
and should certainly be updated when the design nears finality but while it is still
possible to make changes to ensure satisfactory stability and trim if the calcu-
lations show these to be required. The weight components used in this calculation
have been examined under the sections dealing with structural weight, outfit
weight, machinery weight and margin. In the calculation each of these weights
must be given appropriate centres of gravity, some of which will require calcu-
lations whilst others can be lifted from an accurately drawn profile plan, but a good
knowledge of ship outfitting is needed to supplement the data shown on the plan if
accurate results are to be obtained.

4.9 WEIGHT CONTROL

Although weight control is a remove from preliminary design which has been the
theme of this chapter, the chapter has also concentrated on the importance of
weight and for this reason it seems appropriate to conclude it with a discussion of
weight control.
Weight control is the process whereby the intent of the specification in respect
to the deadweight carrying ability of the ship embodied in the design is maintained
during the development of the detailed plans, the ordering of outfit and machinery
and the building of the ship.
The effectiveness with which weight control can be carried out depends to a
major extent on the accuracy of the final weight estimate on which the load
displacement and the ship’s lines are based.
In the monitoring process thereafter the weight committed by each plan is
calculated and, if it is more than that allowed for in the estimate, the question of
whether the increase is necessary to meet a specified requirement is raised.
Similarly the weight of each bought-in item is obtained before the order for it is
confirmed and again if this exceeds the estimate the reason for this is probed.
Changes requested by the owners are evaluated for their weight effects at the
same time as their cost and delivery implications are estimated.
Sometimes weight increases, for whatever reason they arisen, have to be
accepted but in this case there must be a search for compensatory weight savings.
For some ships it is a specified requirement that every item going on board is
weighed, but it has to be admitted that the knowledge thus gained generally comes
so late in the construction process that remedial measures are very difficult and
expensive. If, however, there is going to be a serious weight increase, then even
belated knowledge from weighing is better than the awful truth only coming to