Considerations governing selection of power plants for yachts include determination of power for speed and performance desired. This, in turn, requires determination of the best compromise on shaft revolutions compatible with limitations of propeller diameter and possible effects of cavitation. The problem as to whether to use diesel or gasoline engines is also investigated. Curves are given which give general criteria for estimating power over a large range of speeds. Accessories including reverse gears, reduction gears, and their mode of control are discussed. Propeller design in reference to engine rating is investigated, it being pointed out that on many current engines a propeller sized for maximum power and revolutions will be much too small for the horsepower available at a continuous rating.