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?
- ✓ Unequal blade thrust caused by different effective pitch angles of the ascending and descending bladesCorrect
- B. The discharge screw current acting directly on the rudder blade
- C. The frictional wake layer drawn along the hull by forward motion
- D. Aeration of the upper blade as it breaks the water surface
Why A is correct
The largest contribution to propeller side force comes from the angle of the propeller shaft, which gives the descending blade a higher effective pitch and therefore more thrust than the ascending blade, producing unequal thrust around the arc and pushing the stern to one side. The other effects listed (wake, aeration, discharge current on rudder) are contributing factors but not the largest one.