Speed reduction in waves occur because of two factors. These are known as voluntary and involuntary speed loss.

Voluntary speed loss is due to a decision, by the captain, to reduce speed in order to reduce the motions, slams, deck wetness etc. to within acceptable limits. This may also necessitate a change of course.

Involuntary speed loss arises from the fact that a vessel travelling through waves will have a greater resistance due to its motions. Also it is likely that the propeller will be less efficient due to the motions and oscillations of the onset stream. These effects may reduce the vessel's speed by around two to three knots but this may be sufficient to result in substantial financial losses for a merchant ship.

The added resistance is given the symbol R_aw and is often expressed as:

where z₀ is the wave amplitude.

The total added resistance in irregular waves may be calculated in the same way that the motion in irregular waves is calculated — by integrating the contribution of all the individual components, see below. Note that the factor of 2 arises from the fact that we are looking at the amplitude of each sine-wave component , and not the energy associated with that component.