What is propeller slip?

What is propeller slip?

• A. A measure of the angle of propeller pitch relative to hull.
• B. Slip is the difference between the theoretical advance of a propeller and actual vessel speed; higher slip reduces thrust and efficiency.
• C. Slip equals the ratio of engine RPM to ship speed.
• D. Slip is the percentage of propeller diameter lost due to wear.

Answer: (B)

Propeller slip is the difference between the propeller’s theoretical advance per revolution (its pitch) and the actual forward movement of the vessel per revolution. In ideal, frictionless flow, one revolution would move the ship forward by the pitch distance. In real operation, the water is slowed by hull wake, viscous losses, and swirl, so the ship actually moves a bit less per revolution. The slip can be expressed as a percentage: slip% = (pitch − V/n) / pitch × 100, where V is ship speed and n is shaft rpm. A larger slip means more of the engine power is spent swirling water rather than producing forward thrust, so thrust and overall efficiency drop. The other statements aren’t correct because slip isn’t an angle, it isn’t a simple RPM-to-speed ratio, and it isn’t a measure of wear on the propeller diameter.

Propeller slip is the difference between the propeller’s theoretical advance per revolution (its pitch) and the actual forward movement of the vessel per revolution. In ideal, frictionless flow, one revolution would move the ship forward by the pitch distance. In real operation, the water is slowed by hull wake, viscous losses, and swirl, so the ship actually moves a bit less per revolution. The slip can be expressed as a percentage: slip% = (pitch − V/n) / pitch × 100, where V is ship speed and n is shaft rpm. A larger slip means more of the engine power is spent swirling water rather than producing forward thrust, so thrust and overall efficiency drop.

The other statements aren’t correct because slip isn’t an angle, it isn’t a simple RPM-to-speed ratio, and it isn’t a measure of wear on the propeller diameter.