Thursday, 21 June 2012

Rudder torque

Rudder torque
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Formulae for assessing rudder torque's are based upon the expression Ta ACpV2 Sin q where:-
T = rudder torque
C = rudder area
Cp = centre of pressure distance from centre line of rudder stock
V = velocity of ship
q = rudder angle measured from mid-ship position
In practice different constants obtained empirically are used with this expression and take into account such factors as propeller slip and wake speed as appropriate depending upon the relation of the rudder and propeller positions. The position of the centre of pressure has a significant effect upon rudder torque and hence the size of the steering gear required; the greater the distance of the C of P from the centre line of the rudder stock, the larger the torque required; therefore designers attempt to bring the C of P as near to the centre line as possible. With the simple "barn door" type rudder on some single screw ships, no adjustment can be made, but the semi-balanced and balanced-type rudders can be designed to reduce the torque required; for instance, with the spade type rudder such as fitted to twin screw ferries, the position can be adjusted by the designer to give optimum position. This lies between 30 and 32 per cent abaft the leading edge of the mean chord of the rudder. Such a rudder would have its C of P forward of the stock position at low angles of helm, would balance around 10o to 15o and drift aft of the stock at higher rudder angles.
graph of rudder torque against sterring angle
In graph above is shown a typical torque characteristics for a spade type balanced rudder and a "barn door" or unbalanced plate rudder. The astern torque's should also be calculated since this is sometimes higher than the ahead torque, this is true for spade rudders.


The peak power that a steering gear must develop is the product of the maximum torque (T) usually at hard over with the ship travelling at full speed, and the maximum speed (S) of rudder movement i.e. Power (max) a T x S. The combination of maximum power and speed only exists for 2 or 3 seconds during each manoeuvre; so clearly the average power required to operate the steering gear is considerably below the peak. Because the steering gear must have sufficient power to overcome friction and still have ample reserve of power, the value for used in the foregoing expression is significantly higher than that used in the expression for rudder torque. When considering the diameter of the rudder stock, bending and shear stresses must be taken into account.

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