Seamanship and vessel handling
This domain tests how a hull actually behaves: where she pivots, how the propeller walks the stern, why she is sucked toward a bank, and how much water she loses under the keel in shallow water. Answers should be mechanisms, not slogans.
Pivot point and propeller effect
- With headway, a single-screw ship pivots at roughly one-quarter of her length from the bow; going astern the pivot moves to about one-quarter from the stern; stopped, it is near amidships.
- A right-handed propeller produces transverse thrust: going ahead the stern tends to walk to starboard, going astern to port. Flank speed astern therefore needs the rudder used early, before steerage flow is lost.
Squat, interaction and bank effect
- Squat grows with the square of speed, with block coefficient, and sharply in shallow or confined water. Maximum squat occurs near the bow for fine-form ships (low block coefficient) and near the stern for full-form ships.
- In a narrow channel the bank effect cants the bow toward the nearer bank and the stern toward mid-channel; reduce speed and keep to the centre line.
- Interaction with a passing or overtaking ship produces suction, strongest when the other hull is alongside; slow down and increase lateral separation.
Manoeuvring and anchoring
- Williamson turn for man overboard: hard over, alter about 60°, then hard over the other way to steady on the reciprocal course - it keeps the casualty astern and is the right choice at night or after a short delay.
- Anchor scope of roughly five to seven times the depth in good holding ground, veered under control and snubbed.
Worked example
A ship of block coefficient 0.8 proceeds at 10 knots with a draught of 8 m and a charted depth giving 9.5 m of water. In open water, squat ≈ Cb × V² / 100 = 0.8 × 100 / 100 = 0.8 m. In shallow water, with the depth-to-draught ratio around 1.2, squat may double to about 1.6 m, leaving a real under-keel clearance near 1.5 m and a risk of touching bottom in the swell. The correct reaction is to reduce speed, not to change course only.
Traps that cost marks
- Assuming the pivot point stays amidships regardless of headway.
- Reversing transverse thrust for a right-handed propeller.
- Forgetting that squat is a function of speed squared and is worst in shallow water.
- Using the Scharnow turn when the Williamson turn is asked for, and vice versa.