Page 281 - Practical Ship Design

Design of Lines 239

cruise liners. Another, but rather unusual reason for retaining a cruiser stern
applied to the fishery inspection ship the design of which is described in Chapter
16, 0 16.5. In this case it was preferred to a flat transom in case the ship had to go
astern in ice.
In warships the transom stern was introduced not for cost cutting reasons but
because it improved the hydrodynamic performance giving a less turbulent wake
particularly at high speeds. As in merchant ships, the resulting increase in KM was
appreciated for stability reasons and the additional deck area because it improved
the arrangement. In fact in present warship practice the full midship beam is often
maintained right to the transom and from upper deck level to very nearly the
waterline.
A further development in the stems of high-speed ships is the transom wedge or
flap illustrated in Fig. 8.6. This reduces the high stern wave that used to build up at
the stern and thereby reduces the resistance.

8.3 DESIGNING LINES TO MINIMISE POWER

8.3.1 The LCB position

The next item to be considered is the location of the centre of buoyancy. In some
ship types this is dictated by the disposition of weight and the need to achieve a
satisfactory trim, but in most ships it should be governed by a wish to minimise
power requirements.
A very experienced tank superintendent who read this commented that he
wished this were so, but had found designers almost always saying that the LCB
position had been dictated by trim requirements.
The author believes that the LCG position is closely linked to the LCB position
(see, for example, Fig. 4.7) and that unless a ship is being designed to have a
particularly heavy local weight, it will trim satisfactorily almost automatically
provided reasonably careful thought is given to the disposition of tanks. Through-
out his career he has therefore positioned the LCB of his designs where he thought
best from a powering point of view.
While it may be wrong to speak of an optimum position of the LCB, it is
certainly correct to think in terms of an optimum range. The optimum range of
LCB position depends mainly on the Froude number and block coefficient, which
as has been shown, are themselves linked.
The range differs for ships with normal and bulbous bows, as the LCB on a form
with a bulb will be anything from 0.5-1 % further forward than that of an otherwise
very similar form with a normal bow.
The range also differs for twin-screw ships for which the optimum range is
further aft than it is for single-screw ships, reflecting the fact that the lines of a