Question

Explain the consideration which governs the size and shape of a rudder.

The shape and size of the rudder plays an important part in its efficiency.

The area may be of the order of 2% of the product of ship's length and designed draught. Most rudders are semi-balanced having up to 20% of their area forward of the turning axis but balanced rudders with 25 to 30% of their area forward of the turning axis and unbalanced rudders with all area aft of the turning axis are also fitted.

Since the vertical dimensions of a rudder are necessarily restricted, the fore and aft dimension must be increased to obtain the desired area and the distance to the centre of lateral resistance from the turning axis is increased.

The torque needed to turn the rudder is thus greater and the object of balance is to reduce this by providing area forward of the turning axis which reduces the distance to the centre of lateral resistance.

[When a rudder is turned from the centreline plane to any angle, the water flows round the rudder and creates an additional resistance on that side of the centreline. The force F which acts on the rudder parallel to the centreline has two components:

(a) the force created by the formation of streamlines round the rudder, i.e. due to the change in direction of the water.

(b) the suction on the after side of the rudder caused by eddying.

This force F follows the laws of fluid friction and may be determined from the expression.

F = kA v2 N

where k = a coefficient which depends upon the shape of the rudder, the rudder angle and the density of the water. When the ship speed is expressed in m/s, average values of k for sea water vary between about 570 and 610.

A = rudder area v = ship speed.

The area of rudder is not specified by Classification Societies, but experience has shown that the area should be related to the area of the middle-line plane (i,e. length of ship x draught), and values of one sixtieth for fast ships and one seventieth for slow ships have been found successful.

i.e. Area of rudder = L x d / 60 for fast ships

= L X d / 70 for slow ships.]