Practical Ship Hydrodynamics

Ship manoeuvring 179
ž Rudders outside of the propeller slipstream are ineffective at small or
zero ship speed (e.g. during berthing). In usual operation at forward
speed, rudders outside of the propeller slipstream are far less effective.
Insufficient rudder effectiveness at slow ship speed can be temporarily
increased by increasing the propeller rpm, e.g. when passing other ships.
During stopping, rudders in the propeller slipstream are ineffective.
ž Bow rudders not exceeding the draft of the hull are ineffective in ahead
motion, because the oblique water flow generated by the turned rudder
is redirected longitudinally by the hull. Thus, transverse forces on a bow
rudder and on the forward moving hull cancel each other. The same
generally applies to stern rudders in backward ship motion. The yaw
instability of the backward moving ship in one example could not be
compensated by rudder actions if the drift angle exceeded ˇ D 1.5°. To
improve the manoeuvrability of ships which frequently have to move
astern (e.g. car ferries), bow rudders may be advantageous. In reverse
flow, maximum lift coefficients of rudders range between 70% and 100%
of those in forward flow. This force is generally not effective for steering
the ship astern with a stern rudder, but depending on the maximum astern
speed it may cause substantial loads on the rudder stock and steering gear
due to the unsuitable balance of normal rudders for this condition.
The rudder effectiveness in manoeuvring is mainly determined by the
maximum transverse force acting on the rudder (within the range of
rudder angles achievable by the rudder gear). Rudder effectiveness can be
improved by:
ž rudder arrangement in the propeller slipstream (especially for twin-screw
ships)
ž increasing the rudder area
ž better rudder type (e.g. spade rudder instead of semi-balanced rudder)
ž rudder engine which allows larger rudder angles than the customary 35°
ž shorter rudder steering time (more powerful hydraulic pumps in rudder
engine)
Figure 5.12 defines the parameters of main influence on rudder forces and
moments generated by the dynamic pressure distribution on the rudder surface.
The force components in flow direction ˛ and perpendicular to it are termed
drag D and lift L, respectively. The moment about a vertical axis through the
leading edge (nose) of the rudder (positive clockwise) is termed QN. Ifthe
leading edge is not vertical, the position at the mean height of the rudder is
used as a reference point.
The moment about the rudder stock at a distance d behind the leading edge
(nose) is:
QR D QN C L Ð d Ð cos ˛ C D Ð d Ð sin ˛
The stagnation pressure:
q D 2 Ð V 2
and the mean chord length cm D AR/b are used to define the following nondimensional
force and moment coefficients: