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Hydraulic Steering Systems
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 Rudder drawing
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.