USCG Exam PracticeSingle-screw maneuvering and prop walk
A single-screw inboard vessel is backing down in a chop with weight and low freeboard aft. The coxswain increases sternway to maintain steerage. Beyond the handling considerations, what additional hazard does this situation present that is specific to inboard-powered vessels?
- A. The discharge screw current directed forward under the hull creates a low-pressure zone that reduces freeboard amidships
- ✓ Water shipped over the transom may not drain immediately, threatening stability, and inboard exhaust through the transom can draw fumes over the crew and into cabin spacesCorrect
- C. Increased sternway causes the pivot point to move so far aft that the bow swings uncontrollably into the chop, risking broaching
- D. The suction screw current draws surface water into the engine cooling intakes, risking overheating at low speed
Why B is correct
Two specific cautions apply: boats with low freeboard and weight aft can ship water over the transom in a chop, and water that does not drain immediately threatens stability. Additionally, inboards exhaust through the transom, so backing can draw exhaust fumes over the crew and into cabin spaces, requiring limited exposure and subsequent ventilation.