Page 299 - Practical Ship Design

Design qfLine.7 251

To minimise the proportion of the rudder that comes out of the water in a broach
reducing the helmsman’s ability to correct the course, rudders should be made as
deep as possible.

8.7.7 Ship motion calculations
The calculation of ship motions at the design stage is a comparatively recent
practice stemming from the conjunction of the importance which is now attached
to minimising motions of ships and particularly those of the types mentioned in
98.7.1. with research work which has developed the necessary calculation methods
and the ability of modern computers to handle these fairly considerable calcul-
ations with speed and economy.
The calculations can be based on a range of environmental data using three
types of sea spectra: Pierson-Moskowitz, ITTC two parameter and Jonswap and
modelling the waves either as unidirectional (long crested) or spread with a
user-defined wave spreading function.
The calculation method used is based on strip theory with frequency domain
computations for motions, added resistance and loads from waves and time
domain calculations for hull response to slamming. The output can consist of any
or all of the following:
- the ship motions of heave, pitch, roll, yaw and sway;
- the added resistance for a variety of ship headings;
- dynamic loads imposed on the ship;
- total motions at specified places in the ship;
- possible sustained sea speed against a variety of limiting factors;
- structural responses due to slamming.
Figure 8.16, reproduced by courtesy of Kockums Computer Systems, illustrates
the required input and the range of output.

8.7.8 Course stability and manoeuvrability
Course stability is put first partly because it is a requirement that applies to all ships
and partly because it seems a natural follow-on to the discussion of broaching in
the last section. It is of course the quality that a ship should have of not deviating
from its set course unless the rudder is put over to initiate a turning movement.
Most ships whose lines have been designed to minimise resistance and promote
good flow to the propellers will have good course stability as a natural corollary
without any special measures. Ships with a low lengthheam ratio especially if this
is coupled with a full block coefficient are likely to have poor course stability
unless particular care is taken to ensure that this is satisfactory - and course
stability may set a limit to the present trend to very low L/B ratios.