Page 77 - Practical Ship Design

48 Chapter 2

would rise to the range 100-1000. However, even this is a long way from showing
the true economic advantage of the displacement monohull as the efficiency
comparison is limited to the propulsion aspects whereas the monohull has further
very large advantages in its much smaller building and running costs.
Clearly, within a range of VFN from zero to about 1.3 there is no competition.
Above this and extending to a VFN of about 2.0, a Swath configuration must be
considered and this is particularly the case where the ship has to work at sea and
especially if this makes minimum motions in a seaway desirable. Unfortunately,
present day Swath designs require more power for the same speed than an equivalent
monohull and this is inhibiting their development -possibly temporarily.
Developed since the graph was drawn in 1980 but now demanding consider-
ation in this range and extending to a VFN of about 2.5, is the wave-piercing
catamaran. Although one of these now holds the blue ribbon of the Atlantic, the
sea-keeping ability of these vessels in other than moderate weather may still
require more convincing demonstration before they win a wider acceptance. If this
is achieved there may be a considerable place for this type of vessel as passenger
and car ferries.
In the range of volumetric Froude number from 2.0 to 3.0 the hydrofoil shows to
advantage; from a VFN of 4.0 to 5.0 the air cushion vehicle seems to have both the
best record and the best potential. Between VFNs of 3.0 and 4.0 these two types
compete.
Semi-planing and planing vessels do not show to advantage on the criterion
used, but their cheapness of construction keep them very much in the picture for
small fast pleasure boats, and of course the wave-piercing catamaran may also be a
planing vessel, so maybe this type is developing.

2.8 THE DESIGN SPIRALS

Design spirals for merchant ships and warships are shown together in Fig. 2.2. The
similarities and differences are both worth noting. The similarities predominate,
although in some cases these are disguised by the use of different names such as the
“total deadweight” of the merchant ship and the “variable weights” of the warship.
The differences start with the first spoke of the spirals, which denotes the most
important feature of the two types of ship: cargo handling in its broadest sense for the
merchant ship, and the weapons configuration in its totality for the warship. Most of
the spokes thereafter are identical or nearly so until the penultimate one which is
tonnage for the merchant ship and vulnerability and signature for the warship.
Feeding in to the merchant ship design all the way round the cycle are Classific-
ation Society rules, IMO and national rules, whilst warships are similarly guided
by the relevant naval standards.