Unequal blade thrust is caused by?

• A. Water depth
• B. Engine RPM
• C. Angle of propeller shaft causing different pitch angles for blades
• D. Propeller diameter mismatch

Answer: (C)

Unequal blade thrust happens when each blade does not push the water with the same efficiency because its pitch relative to the flow isn’t the same. If the propeller shaft is not aligned correctly, the propeller plane isn’t perpendicular to the shaft, and the blades end up at slightly different pitch angles as they rotate. Since thrust produced by a blade depends on its pitch, those small differences cause one blade to bite more water (more thrust) and another to bite less, leading to an uneven thrust distribution and potentially vibration.

Other options don’t inherently cause this per-blade imbalance: water depth affects cavitation and loading but not blade-to-blade pitch; engine RPM changes total thrust but not how that thrust is split among blades; propeller diameter mismatch changes overall loading and efficiency rather than creating a consistent per-blade pitch difference when the shaft is aligned.