Torque calculation

For boats fitted with a rudder, the torque of the rudder or rudders is calculated as per the following formula and corrections.

It must be known that the torque necessary to manoeuvre a boat depends on:

• the speed of the water flowing on the surface of the rudder at a certain angle,
• the rudder size,
• the total sweep of the rudder (and part of the boat), if the rudder shaft is not perpendicular,
• the compensating surface of the rudder (balance).

C = S x [ (0.4 Lg) – Lc ] x V² x K

C	= Torque in kgm
S	= Total surface of rudder (H x Lg) in sq. m
H	= Height of rudder in m
Lg	= Width of rudder in m
Lc	= Compensation width in m
V	= Maximum speed of the boat in knots
K	= Coefficient in function of total rudder angle
	Port to starboard 70° K = 15.89 Port to starboard 80° K = 17.80 Port to starboard 90° K = 19.52
	Corrections in function of the type of boat:
	For a boat with a steering nozzle C x 2.0 For twin engine power boats with 1 rudder C x 0.5 For boats fitted with several rudders (catamarans, monohulls) Multiply the calculated torque result by the number of rudders fitted on the boat.