Candidates most commonly fail this topic by treating prop effects, squat, and interaction as isolated facts rather than forces that compound. An examiner will rarely ask about one in isolation — expect scenario questions that layer two or three simultaneously, e.g. a twin-screw vessel manoeuvring in shallow water near a berth in a cross-current. If you cannot explain why a force acts, not just what it does, you will lose marks.
Propeller Effects — Single Screw
A right-handed (clockwise ahead) single-screw vessel is the baseline the examiner tests against.
- Transverse thrust (paddle-wheel effect): The lower blade operates in denser water and generates more transverse force than the upper blade. Ahead: stern kicks to starboard (bow to port). Astern: stern kicks to port — the dominant effect when going astern, which is why a right-handed single-screw vessel will typically not back in a straight line and will sheer to starboard.
- Helical wash / prop wash ahead: Screw discharge strikes the starboard quarter of the rudder, assisting the kick to port at slow speed.
- Transverse thrust astern dominates over any rudder effect because the rudder is ineffective in a backing manoeuvre at low speed — this is the critical point candidates miss.
Twin-Screw Vessels
Outward-turning screws are most common on yachts. Ahead: transverse thrusts cancel. Going astern on one engine ahead on the other produces a very tight pivot — this is the core manoeuvring advantage. Candidates must know which screw to use to kick the stern in the desired direction.
Wind and Current
- A vessel with high freeboard and low draught (typical of many yachts) will be dominated by wind leeway in slow manoeuvres. Always identify which end will blow off first — usually the bow.
- Current acts on underwater volume; wind acts on topside profile. At low speed in a cross-current, calculate a ferry-glide angle to maintain track.
- Berthing: if possible, come alongside heading into the current for steerage control; use the current to set the vessel onto the berth.
Interaction
- Occurs between vessels passing close aboard or in confined channels. A vessel approaching another produces a pressure wave: bow repulsion, then stern attraction between the two vessels. The overtaken vessel's stern is drawn toward the overtaking vessel. Narrow channels amplify this. Keep maximum separation, reduce speed.
Squat
- In shallow water (typically when UKC is less than the vessel's draught), a vessel accelerates water flow beneath the hull, reducing pressure and sinking the vessel bodily — squat. The bow sinks more in fine-bowed vessels; the stern sinks more in full-form vessels. Squat increases rapidly with speed (proportional to speed squared). Reduce speed well in advance; do not rely on depth soundings taken underway in confined water as sole reference for UKC.