What factor explains the difference in thrust between ascending and descending propeller blades?

What factor explains the difference in thrust between ascending and descending propeller blades?

• A. Propeller diameter
• B. Angle of the propeller shaft causing different pitch angles for blades
• C. Hull speed
• D. Water temperature

Answer: (B)

The difference in thrust between the ascending and descending blade passes comes from the pitch angle the blades present to the water as they rotate. Thrust is generated by the blade acting like a wing in water, and the amount of thrust depends on the blade’s angle of attack, which is set by the blade pitch. If the propeller shaft is angled, or if the pitch experience changes around the rotation, the blade on the uphill pass meets the water at a slightly different pitch than the blade on the downhill pass. That changes the lift (and thus thrust) produced during each half of the rotation, so the ascending pass can differ from the descending pass. Diameter affects overall thrust, hull speed isn’t responsible for a cyclic difference, and water temperature has only a broad, not cyclic, effect on thrust.

The difference in thrust between the ascending and descending blade passes comes from the pitch angle the blades present to the water as they rotate. Thrust is generated by the blade acting like a wing in water, and the amount of thrust depends on the blade’s angle of attack, which is set by the blade pitch. If the propeller shaft is angled, or if the pitch experience changes around the rotation, the blade on the uphill pass meets the water at a slightly different pitch than the blade on the downhill pass. That changes the lift (and thus thrust) produced during each half of the rotation, so the ascending pass can differ from the descending pass. Diameter affects overall thrust, hull speed isn’t responsible for a cyclic difference, and water temperature has only a broad, not cyclic, effect on thrust.