For boats fitted with a rudder with speed not exceeding 25 knots, the torque of the rudder(s) is calculated according to the following formula and correction coefficients.
It should be pointed out that the necessary torque to manoeuver a boat depends on:
- The speed of the water flowing on the surface of the rudder at a certain angle
- The rudder dimensions
- The total sweep of the rudder (and part of the boat), if the rudder stock is not perpendicular
- The compensating surface of the rudder (balanced rudder)
Torque Calculation Formula for Speed below 25 Knots
C = S x [ (0.4 Lg) – Lc ] x V² x K
C = Torque in kpm schema_safran
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 according to total angle of rudder
| 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 sailing-boats | C x 0.5 |
| 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, trimarans, monohulls) | Multiply the calculated torque result by the number of rudders fitted on the boat |
Once the torque has been calculated, the appropriate cylinder(s) will be selected and the remaining components of the steering system can be defined.