Understanding Boat Prop Torque

A boat is propelled by horsepower. In order to propel the boat the horsepower is converted into a twisting force. This twisting force is the boat prop torque. This twisting force is the force that rotates a propeller.

The maximum torque of most boat engines occurs below the maximum RPM specified in the manual of the engine. This usually poses a problem when selecting a prop. A propeller is chosen so that the engine runs close to the specified RPM. The torque is usually 50 percent of the RPM on a high speed engine. The ideal propeller torque should be about 70-85 percent of the top RPM of the engine. This will give greater efficiency, speed, and fuel efficiency and enhance engine longevity.

When a single propeller is used to drive a boat there is a torque reaction between the prop and the hull. This reaction is called the heeling torque. When the propeller moves in one direction it causes an equal and opposite torque in the direction of the turning of the propeller. This causes the hull to heel or tilt away from the direction in which the propeller is turning. Large boats have two propellers to reduce heeling. The starboard propeller is right handed while the port propeller is left handed. The hull and type of boat impacts the heeling torque of a boat.

Torque causes the boat to roll. When a propeller turns clockwise, the water resists the clockwise movement of the prop. When water resists the clockwise rotation, the boat rolls slightly in the opposite direction, counterclockwise or down on the left port side and up on the right starboard side. To offset this imbalance the driver’s seat on a boat is placed on the starboard or right side of the boat. Different boats roll in different ways as they react to the torque of a propeller. Discussing the needs of the individual boater’s needs with the manufacturer or dealer will help the boater obtain the propeller with the ideal prop torque for the type of use, boat and the engine.

The height at which the motor is placed on the transom impacts the torque of the propeller. If the motor is placed too low on the transom, the height of the motor on the transom is the reason why the boat moves without proper direction when the boater lets go of the steering wheel. If the motor is placed too low on the transom, the boat prop torque pulls the boat on one side. If the engine is placed too high on the transom then the torque reduces. The torque will be insufficient to help the steering of the boat.

When a boat moves in restricted space, the torque is the main determining factor of its performance. Torque affects the sideways movement of a boat. When the boat is being docked, the correct torque is required for greater accuracy in steering.

Understanding torque helps boaters handle abnormal balance problems in boats but making the required adjustments to counterbalance imbalances caused by torque.