Select Page

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

It must be known that the torque necessary  to manoeuver 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 (balanced rudder)

    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 starboard70°K = 15.89
Port to starboard80°K = 17.80
Port to starboard90°K = 19.52

Torque Diagram

Corrections in function of the type of boat:
For a boat with a steering nozzleC x 2.0
For twin engine power boats with 1 rudderC 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.