A constant speed propeller cannot change its pitch. Most common in wooden aircraft propellers, the constant speed propeller offers the aircraft only one pitch and, therefore, one speed capability. In a variable pitch propeller, the pilot is able to adjust the pitch of the propeller blades to produce more power and speed. This allows the pilot to adjust propeller performance to conserve fuel and to compensate for changing head or tail winds. The aircraft designed with a constant speed propeller must make adjustments to the engine’s throttle to address changing wind conditions.
In a multiple engine aircraft, a variable pitch propeller can be feathered following an engine problem. This allows the propeller blades to be turned so that they are not catching wind and are in a type of freewheeling mode. With a constant speed propeller, the propeller would continue to turn and cause the damaged engine to turn as well. This could lead to continued destruction to the engine from the constant cycling of broken internal parts. Fuel mileage would also suffer from the wind drag of the constant speed propeller being driven by the flight of the aircraft. Typically offered only in single engine airplanes, the design of the constant speed propeller is almost exclusively resigned to the wooden propeller market.
Even single engine aircraft have variable pitch propellers available as an option to the constant speed propeller, and the variable pitch units are made of composite or aluminum. While some of the largest ocean liners have variable pitch propellers, most watercraft are also users of the constant speed propeller. A military exception is the submarine, which uses a variable pitch unit to assist in diving as well as surfacing the vessel. Sport and fishing boats use the constant speed propeller for power.
The frequent contact between the propeller and underwater objects such as rocks and tree stumps make the use of a variable pitch propeller very impractical for small personal boats. The resulting potential for damage to a complex variable pitch propeller on this type of vessel would not be cost-efficient or feasible. In an effort to design a similarly performing propeller on fishing craft, the water brake is often mounted on the outboard’s lower unit. This device is mounted directly in line with the boat’s propeller and hinders the propeller’s ability to turn the water. The speed of the boat is controlled by the angle of the water brake, with all the way open equaling full speed, while fully closed allows only slow speed from the boat.