Page 147 - Practical Ship Design

114 Chapter 4

4.6 MARGIN, DEADWEIGHT AND DISPLACEMENT
4.6.1 General
The final item required to make up the lightship is the margin. The purpose of
having a margin is to ensure the attainment of the specified deadweight even if
there has been an underestimate of the lightweight or an overestimate of the load
displacement. The size of the margin should reflect both the likelihood of this
happening and the severity of the penalties which may be exacted for non-
compliance. When the design is well detailed and clearly specified and the light-
weight has been calculated by detailed methods, the margin should in principle be
reduced. When the ship type is novel or the design and/or the specification are
lacking in precision, larger margins are appropriate.

4.6.2 Margin - merchant ships

Where the lightweight forms a high proportion of the load displacement and the
deadweight a correspondingly low proportion, the percentage loss of deadweight
which would result from an error in the lightweight estimation can be very serious,
and a prudent designer will therefore wish to provide a higher than normal margin.
However, it is in just this sort of ship that there is likely to be the greatest pressure
to limit the displacement to minimise the power required.
In the 1976 paper, the figure recommended for the margin for merchant ships
was 2% of the lightweight. Subject to the qualifications made above this still seems
as good advice as can be given.
An alternative to a single percentage weight margin would be the aggregation of
a margin based on different percentages of the various weight items depending on
the accuracy with which each of these weights is known.

4.6.3 Margin - warships

As well as a margin of the type described above, whose purpose is essentially to take
care of errors in the weight estimation, two other types of margin are applied to
warship weight estimates to British Ministry of Defence (MOD) rules. These are:
1. A board margin - this is an allowance for additional weight due to changes
in the design which may be made by Naval staff during the course of
construction; and
2. A growth margin - this is an allowance for the increase in weight due to
additions and alterations which may be made during the life of the vessel
and to the “natural” weight growth due to the accretion of paint, etc.
A possible figure for the board margin might be 2%, whilst the growth margin
might be based on 1/2% per annum of the intended life, in both cases based on the
lightweight.