Page 290 - Practical Ship Design

248 Chapter 8

Fig. 8.1 1. Twin skeg body plan.

The type of form which this design appears to promote has a wide waterline aft
giving a high KM value which improves stability.
Claims have been made that the twin skeg form has reduced resistance, but
available data does not altogether substantiate this even when appendage resist-
ance is taken into account.

8.6 HIGH STABILITY LINES

8.6.1 General discussion

The motive behind the design of lines as discussed so far has been the desirability
of minimising the power required either by minimising the resistance or by maxi-
mising the propulsive efficiency.
In some ship types other factors need to be considered and can be so important
that the attainment of a worthwhile improvement in them will justify the acceptance
of a penalty in powering. On passenger ships and container ships, stability can be
such a factor.
Even on other ship types the use of lines which have a high KM for a given beam
can help to improve the economics of a design.
In Chapter 4 it was noted that the ratio BID could be reduced if lines designed to
give a particularly high KM value were used, and it is worth reiterating that if two
designs with different dimensions have same cargo carrying capacity the ship with
the smaller BID ratio will probably be the cheaper to build, requiring less steel, and
possibly also requiring less power.
If there is a breadth limitation for any reason, the use of high KM lines provides
a way to increase cargo carrying capacity by: