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.