Page 289 - Introduction to Naval Architecture

274 MANOEUVRING

MODIFYING THE MANOEUVRING PERFORMANCE

As with other aspects of ship performance it is difficult, and sometimes
dangerous, to generalize on the effect of design changes on a ship's
manoeuvring qualities. This is because so many factors interact and
what is true for one form may not be true for another. Broadly however
it can be expected that:

(1) Stern trim improves directional stability and increases turning
diameter.
(2) A larger rudder can improve directional stability and give better
turning.
(3) Decrease in draught can increase turning rate and improve
directional stability. This is perhaps due to the rudder becoming
more dominant relative to the immersed hull.
(4) Higher length to beam ratios lead to a more stable ship and
greater directional stability.
(5) Quite marked changes in metacentric height, whilst affecting
the heel during a turn, have little effect on turning rate or
directional stability.
(6) For surface ships at a given rudder angle the turning circle
increases in diameter with increasing speed but rate of turn can
increase. For submarines turning diameters are little affected by
speed.
(7) A large skeg aft will increase directional stability and turning
circle diameter.
(8) Cutting away the below water profile forward can increase
directional stability.

By and large the hull design of both a surface ship and a submarine is
dictated by considerations other than manoeuvring. If model tests show
a need to change the manoeuvring performance this would normally
be achieved by modifying the areas and positions of the control
surfaces and skegs.

SUMMARY

The reasons a ship requires certain levels of manoeuvrability have been
discussed and the difficulties in defining any standard parameters for
studying the matter pointed out. Various standard manoeuvres used in
defining a vessel's directional stability and turning performance have
been described. A number of rudder types and other devices for
manoeuvring ships have been reviewed. The special case of a