When moving through waves the resistance experienced by a ship is increased and, in general, high winds mean increased air resistance. These factors cause the ship speed to be reduced for a given power output, the reduction being aggravated by the less favorable conditions in which the propeller is working. Other unpleasant features of operating in such as motions, slamming and wetness are generally eased by a reduction in speed so that an additional speed reduction may be made voluntarily.