For boats fitted with a rudder with speed not exceeding 25 knots, the torque of the rudder or rudders is calculated according to following formula and corrections.
It must be known that the torque necessary to manoeuvre a boat depends on:
Torque Calculation Formula for Speed below 25 Knots C = S x [ (0.4 Lg) – Lc ] x V² x K

Once the torque is known, the appropriate cylinder is selected and one or two manual pumps will be added.
Note: If the selected pump has a higher flow rate in order to reduce the number of turns lock to lock, it will be necessary to use a steering wheel with the maximum recommended diameter.
For pleasure boats with planing or semiplaning hulls and speed exceeding 25 knots, the cylinder may be selected by using the chart below:
Length of Hull  Cylinder Type – 1 Rudder  Cylinder Type – 2 Rudders 
8 metres  VHM 40 DTP  VHM 32 DTP 
10 metres  VHM 40254  VHM 40 DTP 
12 metres  VHM 40254  VHM 40 DTP 
14 metres  VHM 50 DTP  VHM 40254 