BASIC POWERBOATING. SAFETY & RESCUE FOR SAILING INSTRUCTORS 11 HOW A PROPELLER WORKS


A rotating propeller produces thrust that moves the boat. When an engine is in forward gear, the thrust from the rotating propeller drives the boat forward. When the gear is shifted into reverse, the propeller turns in the opposite direction, driving the boat backward. Since propeller blades are optimized for forward thrust, their per- formance in reverse is drastically reduced.


Forward Reverse


Thrust


Rotating prop in


forward gear


Thrust generated from a rotating propeller drives a boat forward.


Thrust Prop turns in


opposite direction in reverse gear.


A propeller rotating in the opposite direction drives a boat backward.


When viewed from behind, if a propeller generates forward thrust by rotating in a clockwise direction, it is defined as a right-handed propeller. If it rotated in a counterclockwise direction in forward gear, it would be left-handed.


A low-pitch propeller (B) moves forward a smaller distance than a high-pitch propeller (A). The higher the pitch number, the greater the pitch (distance traveled per revolution).


Propeller size is defined by its diameter and pitch, and these factors have an important effect on the performance of a boat. Pitch is the distance that a propeller would move forward in a solid material in one full rotation. For example, a propeller with a 17-inch pitch would advance 17 inches. Since water is a fluid, the propeller would actually travel a distance less than 17 inches. Larger diameter propellers with less pitch, that rotate at lower rpm (revolutions per minute), are used for slow-speed boats or towing vessels, while smaller propellers with higher pitch, operating at higher rpm, are used for high-speed boats. A wrong propeller size may result in engine overheating and/or a boat not reaching its designed speed.


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