USCG Exam PracticeSingle-screw maneuvering and prop walk
On a single-screw vessel, what is the PRIMARY source of propeller side force on the stern?
- A. Aeration of the upper blade as it passes through the water surface
- B. The discharge screw current acting on the rudder blade
- ✓ Unequal blade thrust caused by different effective pitch angles of the ascending and descending bladesCorrect
- D. The frictional wake layer that the hull drags along with it
Why C is correct
USCG Boat Crew Seamanship Manual §10-A states that the largest contribution to propeller side force comes from the angle of the propeller shaft, which gives ascending and descending blades different effective pitch angles — the descending blade has the higher effective pitch and produces more thrust, making blade thrust around the arc unequal and pushing the stern to one side. The other effects listed (frictional wake, discharge current on the rudder, aeration) are contributing factors but not the largest one.