Three methods have been implemented to compute the added resistance, the first two are based on the work of Gerritsma and Beukelman (1972). The added resistance is related to the relative vertical velocity of the vessel compared with the wave surface and the damping coefficient. The difference between the two versions is a small difference in the expression for the relative vertical motion. The general formulation is given in the following equation:

where  is a modified section damping; and  is the relative vertical velocity, given by Equation ( 42 ).

where  is given by Equation ( 43 ).

Note that h₃, h₃ and  are the complex heave, pitch and local relative wave amplitudes, containing both phase and amplitude information.

The two methods vary only in the expression for the derivative of . Version A uses the expression in Equation ( 43 ), whilst Version B uses the expression in
Equation ( 44 ).

In the third method, described by Salvesen (1978), the added resistance is given by Equation ( 45 ).

where

and

Note that h₃ and h₃ are the complex heave and pitch amplitudes, containing both phase and amplitude information.