Page 119 - Practical Ship Design

86 Chapter 4

(Lloyds AH 32) as compared with mild steel of 245 N/mm2 and a plating thickness
ratio based on the square root of the ratio of the yield stresses.
The optimum use of higher tensile steel is for the plating of the strength deck,
followed by the bottom shell. It can also be used for the side shell but here the
weight saving is less.

1 tonne of aluminium will replace about 2.9 tonnes of mild steel

This is based on a volumetric substitution of aluminium for steel which approx-
imates to the usual practice for those parts of a ship for which aluminium can be
considered.
Depending on the method of construction and the materials used, fibre reinforced
plastics (FRP) can also be substituted on the basis of 1 tonne FRP to about 2.9
tonnes of mild steel.
A steel-weight from this table is intended for use in the initial stages of design
and should be replaced by a more detailed estimate as soon as the design is
properly detailed with a general arrangement plan, a body plan and a midship
section or similar structural plan.
Ways of minimising structural weight and cost are discussed in Chapter 10.
In the author’s 1976 paper it was noted that there had been a reduction in the
steel-weights corresponding to a particular value of E of between 15% and 20%
from the figures that had applied in 1962.
The reasons for the reduction between 1962 and 1976 were summarised as
follows:
(i) The changes in the ratios LIB, BID, TID which had occurred meant that for
the same E number, more recent ships had a reduced length, but a larger
beam and depth than earlier ships.
(ii) There was less internal structure, fewer decks and bulkheads in the more
recent ships.
(iii) Superstructure had been reduced with the reduction in crew numbers and
simplified with the elimination of the overhanging decks which used to be
a common feature.
(iv) The reduction (almost elimination) of “owners’ extras” which were once a
common feature.
(v) Rationalisation of Classification Society rules and reductions in permitted
scantlings, some of which had followed the introduction of better calcul-
ation methods whilst others may, less desirably, have been a consequence
of competition between the Societies.
(vi) Changes in demarcation of work, which had removed from the steel-
weight some of the items mentioned in 54.1.2 which used to be made in
shipyards but were now almost invariably bought in.