other factors such as the relation of the propeller pitch to its diameter and the energy losses due to friction between the accelerated water and the surrounding water that make a proper relation between boat speed and propeller shaft speed essential to an efficient installation. In general, horsepower available and shaft speed determine the propeller diameter, while shaft speed and boat speed determines propeller pitch. The pitch of a propeller divided by its diameter is a term called "pitch ratio." For example, a propeller of 20" diameter and 20" pitch has a pitch ratio of 1.0, a diameter of 20" and pitch of 15" a pitch ratio of .75, etc. For best efficiency, the pitch ratio of boat propellers should be in the range of .55 to .80 for tugs and trawlers, .65 to 1.0 for heavy and average cruisers, .80 to 1.2 for medium and fast cruisers and .90 to 1.5 for exceptionally fast cruisers and runabouts. Pitch ratios outside these ranges generally will indicate an unsuitable shaft speed. The evolution of blade contour to its present high state of efficiency has followed the development of marine steam turbines, high-speed marine gasoline engines, marine diesel engines, and marine electric drives. Constantly increasing pitch ratios have necessitated continual advances in blade contour and section design. The refinements, which came as a result of this work, have been responsible for the development of the marine propeller into the high speed, precision mechanism that it is today. Some styles have elliptical blade