Page 270 - Introduction to Naval Architecture

MANOEUVRING 255

force. If the applied force is F then the resulting sideways velocity is v ~
F/Y V There will be a short period of imbalance before the ship settles
down to its new steady state.
If the sideways force is applied aft of the neutral point and to
starboard the ship will turn to port. If it is applied forward of the
neutral point the ship turns in the direction of the force. The greater
the distance the force is from the neutral point the greater the turning
moment on the ship. Thus rudders placed aft are more effective than
rudders at the bow by a factor of about five for typical hull forms. Aft
they can benefit from the propeller race aft as well and are less
vulnerable in a collision.

TURNING A SHIP

From simple mechanics it will be appreciated that to cause a ship to
move in a circle requires a force to act on it, directed towards the centre
of the circle. That force is not provided by the rudder. The rudder
exerts a moment on the ship which produces an angle of attack
between the ship's heading and its direction of advance. This angle of
attack causes relatively large forces to act on the hull and it is the
component of these directed towards the centre of the circle that turns
the ship. The fore and aft components will slow the ship down which is
a noticeable feature of a ship's behaviour in turning.

MEASURES OF MANOEUVRABILITY

These are not easily quantified although there has been much
discussion on the matter. Large ocean going ships spend most of their
transit time in the open seas, steering a steady course. They can use tugs
to assist with manoeuvring in confined waters so the emphasis will
probably be on good directional stability. Poor inherent directional
stability can be compensated for by fitting an auto pilot but the rudder
movements would be excessive and the steering gear would need more
maintenance. For ships such as short haul ferries the designer would
aim for good rudder response to help the ships avoid collision and to
assist berthing and unberthing.
If possible the parameters used to define manoeuvrability should be
directly related to the performance the master desires. This is not easy
and use is made of a number of standard manoeuvres which can be
carried out full scale and during model experiments. Other movements
can be created in a model for measuring the stability derivatives, that
cannot be directly simulated at full scale. The measures commonly
studied are now described.