Question 1

A single right-handed-propeller vessel is going astern to take the way off while a moderate beam wind is blowing from the vessel's port side. What must the OOW assess before assuming the stern will swing in the "textbook" transverse thrust direction?

Accepted Answer

Whether the wind force on the hull is strong enough to overcome or significantly modify the transverse thrust effect — at low speed with a strong enough beam wind, wind force can dominate over propeller-induced transverse thrust, so the actual stern swing may differ from the textbook prediction and must be judged against real-time vessel behaviour, not assumed from theory alone This question tests whether a candidate treats transverse thrust as a rule to apply blindly or as one of several forces that must be weighed together. A competent shiphandler predicts the textbook effect but verifies it against what the vessel is actually doing, adjusting for wind, current, and vessel-specific behaviour rather than assuming theory perfectly predicts reality every time.

Question 2

What causes "transverse thrust" (also called paddle wheel effect) on a single right-handed (clockwise-turning, viewed from astern) propeller vessel?

Accepted Answer

The propeller blades on one side of the shaft work in deeper, less disturbed water than the blades on the other side, and this asymmetric loading, combined with the angle of the propeller race against the hull, produces a sideways force on the stern — for a right-handed propeller, this characteristically pushes the stern to starboard when going ahead and to port when going astern Transverse thrust is a fundamental single-screw handling characteristic the OOW must understand and use deliberately. The effect is most pronounced at low speed with high engine revolutions (e.g. manoeuvring astern), and a skilled shiphandler uses it intentionally — for example, using astern transverse thrust to help swing the stern in a particular direction during a turn.

Question 3

Why is transverse thrust generally far more noticeable when a single-screw vessel goes astern compared to going ahead?

Accepted Answer

Going astern, there is no rudder effect to counteract the transverse thrust (since the rudder has little or no effect on a sternway-making vessel at low speed, especially with the engine going astern), so the sideways push of the propeller dominates the vessel's behaviour, whereas going ahead the rudder is normally effective and can counter or mask the transverse thrust This is the key practical reason single-screw vessels are notoriously harder to control going astern — the rudder, the OOW's main steering tool, loses effectiveness exactly when transverse thrust becomes most influential. This is precisely why berthing and unberthing plans for single-screw vessels are built around managing astern transverse thrust.