Page 61 - Practical Ship Design

32 Chapter 1

1.3.7 Weight data

Good weight data is vitally important to a naval architect. Almost all naval
architectural calculations depend on weights and their distribution but accurate
weight calculations require a well advanced general arrangement plan, a lines plan,
structural plans, specification and equipment lists, much of which will not be
complete until the design is well advanced. During the early stages of a design, the
uncertainties surrounding the weight estimate are almost always more significant
than those associated with hydrostatics or hydrodynamics of the vessel. Far more
ship design problems arise through bad weight estimation than from errors in other
much more difficult calculations.
There are two ways of minimising this problem: one is the collection of good
weight data and its intelligent use, and the other is the frequent iteration of the
weight estimate as better data becomes available.

1.3.8 Cost data

Cost data and such related information as man-hourskonne for steelwork etc.
should be zealously sought as it is particularly difficult to obtain since Shipyards
- almost the sole source of this data - severely restrict its circulation for obvious
commercial reasons. There is the further complication that this data gets out of date
particularly quickly for a number of reasons such as improvements in productivity,
changes in currency exchange rates etc., so always date this data!

1.3.9 Data on rules

Before starting to design a ship type with which he is not familiar, a naval architect
has in recent years had the daunting task of identifying the rapidly growing number
of rules that will apply and of familiarising himself with the more significant of
these. Fortunately, help is now at hand because Lloyd’s Register now offers a
computerised solution in “Rulefinder”, which is accessible on PCs and covers
virtually all the rules that need to be considered.

1.3.10 Making use of data

One of the best ways of storing data and of interpolating between or extrapolating
beyond available information can often be to graph it. Vital to the success of
graphing is the choice of a suitable base parameter, which must be a measure that can
readily be obtained or calculated at the stage in the design process at which the