The Design and Control of a Variable Pitch Propeller

Recent developments in the small pleasure boat field have radically changed the demands on the usual internal combustion engine-propeller drive combination. The higher thrust requirements at low boat speeds for heavy outboards, out-drive cruisers, and waterskiers demand lower pitch propellers than had been popular on lighter hulls and family runabouts. Referring to Figure 1, the use of a propeller that had been chosen on the basis of obtaining maximum engine horsepower at maximum boat speed (the usual engine-propeller-boat hull criteria) would result in failure to obtain a planing attitude. With the choice of a lower pitch propeller which reduces the effective load on the internal combustion engine (less blade loading per propeller revolution) and allows the engine to run at higher revolutions, sufficient power (thrust) is developed at low boat speeds. However, once the boat or waterskier has maintained a planing attitude, there is a drastic reduction in drag. Blade loading being reduced, the engine increases its revolutions beyond the rpm limit of the internal combustion engine. More often than not, the engine must be throttled, thereby severely limiting the maximum boat speed. In this case, we never do have full engine horsepower available (except for one fleeting instant during acceleration before overspeeding the engine). I propose the use of a constant speed automatically controllable pitch propeller for outboard or out-drive powered planning watercraft. Full engine horsepower would be available under almost any boat speed or boat load condition. Specific advantages would be: (1.) One propeller-engine combination to be used for boat loads varying from barges to light runabouts. (2.) Propeller would automatically compensate for day to day boat loading changes - changes in number of passengers, waterskiers, or amount of equipment. (3.) Under cruise or part load condition, the engine would run at a condition of more favorable brake specific fuel consumption than with a fixed pitch propeller - higher engine rpm with a more closed throttle. This is depicted in Figure 2