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 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. |