any device that converts energy into useful work for driving a ship. Some propelling devices, such as sails, use external sources of energy, for example, wind or water currents; others are put into operation by human energy, for example, oars and poles, or by marine engines. Indirect propelling devices include screw propellers and paddle wheels, rotary-blade propellers, and airscrews. Direct propelling devices include water-jet propellers and air-breathing jet engines.
All ship propellers have the common property that the driving force, or thrust, they produce is equal to the resistance encountered by the ship when it travels through the water along a straight line at a constant speed. In the case of tugboats, the thrust is equal to the resistance encountered by the tugboat itself and by the object being towed. The efficiency of a ship propeller for a particular combination of hull and rudder is specified by the propulsive coefficient—the ratio of the towing power of the ship to the power delivered at the propeller.
The choice of a ship propeller is dictated by the function of the ship. For example, ships that work in shallow water or obstructed channels use water-jet propellers. Ships with greater maneuverability use rotary-blade propellers.
E. G. LOGVINOVICH [25–140–1 ]