Question 1

Your vessel is a single right-handed propeller ship. Your examiner asks you to describe the effect of astern propulsion on the ship's heading and why this affects manoeuvring.

Accepted Answer

Single right-handed propeller astern effects: (1) TRANSVERSE THRUST — when the propeller rotates in reverse (left-handed rotation when going astern for a right-handed propeller), the paddle-wheel effect creates a transverse force pulling the stern to PORT. This kicks the stern to port and swings the bow to starboard; (2) HELICAL DISCHARGE — the propeller in astern throws water forward in a helical pattern which strikes the bottom of the keel and underwater hull, contributing to the transverse effect; (3) RESULT: on a single right-handed propeller vessel, going astern causes the stern to go to port (bow to starboard). This is used advantageously: to turn in a restricted space, going full astern will assist swinging the bow to starboard. Coming alongside a port-side berth at slow speed: the stern kick to port when going astern actually assists bringing the stern into the berth if approaching at a slight angle. Propeller effects are fundamental ship handling knowledge at OOW level and always examined. The right-handed propeller: when viewed from astern, it rotates clockwise when going ahead — the blade going down on the starboard side produces a downward force to starboard and an upward force to port, but the transverse thrust effect is more complex. When going ahead (clockwise): stern goes slightly to starboard (bow to port). When going astern (anticlockwise for a right-handed propeller): stern goes to port (bow to starboard) — this is the dominant and practical effect. Twin-screw vessels: typically fitted with outward-turning propellers (starboard propeller right-handed, port propeller left-handed when viewed from astern) — the transverse effects cancel each other out. The OOW must know their vessel's propeller configuration and use the transverse thrust effect beneficially in manoeuvring — experienced shiphandlers deliberately use short astern bursts to kick the stern to port when turning to starboard in restricted waters. The examiner at OOW level expects the candidate to understand WHY the effect occurs, not just to memorise the direction.

Question 2

Your vessel (15,000 GT) is overtaking a smaller vessel (2,000 GT) in a narrow channel. Both vessels are at close range during the overtake. Your examiner asks what hydrodynamic interaction effects you should expect.

Accepted Answer

Hydrodynamic interaction in overtaking: when two vessels are in close proximity, each vessel's pressure field is affected by the other. During overtaking: (1) BOW SUCTION — as the larger vessel's bow approaches the smaller vessel's stern, the smaller vessel is drawn towards the larger vessel (stern of smaller vessel attracted to bow of larger); (2) MID-SHIP REPULSION — when the ships are abreast, the increased pressure between them creates a repulsive force pushing both vessels apart; (3) STERN SUCTION — as the larger vessel passes, the stern of the larger vessel sucks the bow of the smaller vessel towards it. The combined effect: the smaller vessel can sheer uncontrollably — this is "bank suction" in a channel context. Mitigating actions: reduce speed (interaction forces reduce with the square of speed — halving speed reduces interaction forces by 75%); increase lateral separation; give early warning on VHF. Hydrodynamic interaction is a core ship handling topic at OOW level and has been the cause of several collisions in confined waters. The physics: every vessel underway creates a pressure field — high pressure at the bow, low pressure amidships (in the boundary layer), and a wake astern. When a second vessel enters this field, its own pressure distribution is disturbed. The effect is proportional to the vessels' speeds, their size ratio, and the water depth (shallow water amplifies interaction). Speed reduction is the most effective mitigation: interaction forces are proportional to V² (velocity squared). In a narrow channel, there is also bank effect (vessel attracted to the near bank due to increased flow velocity between vessel and bank). Real-world case: the Kariba incident and numerous harbourmaster reports from the Suez Canal where overtaking at excessive speed caused loss of control. The OOW must consider interaction effects when planning overtaking manoeuvres in channels, not just COLREGs obligations.

Question 3

You are approaching an anchorage in a cross-wind of 25 knots and 1.5 knot tidal current. Describe how you would approach the anchorage and prepare to anchor.

Accepted Answer

Approach planning: (1) Determine the final heading at the anchor position — vessels at anchor lie to the wind or current, whichever is stronger; at anchor with 25kts wind and 1.5kts current, wind dominates so the ship will lie head-to-wind at anchor; (2) Approach heading: approach into the wind (head-to-wind) so that the ship will naturally settle into the riding position; (3) Speed: approach at slow speed so that the ground speed can be controlled — calculate leeway and set against the cross-wind to maintain the intended track; (4) Position of anchor: the anchor goes down on the side towards which you want to swing — for a head-to-wind anchorage, approach from downwind on a reciprocal heading to the lie-to heading; (5) At the drop position: slow to near-stop, allow the ship to fall back on the current/wind, walk back the cable as the ship drops back — do not drop from underway at speed; (6) Confirm scope — 5:1 water depth in open anchorage in 25kts wind minimum. Anchoring technique at OOW level covers both the decision-making before the operation and the practical execution. Wind vs current dominance: current velocity of 1.5 knots exerts significantly less force on a ship than 25 knots of wind — surface area (freeboard, superstructure) means wind exerts far more force on most merchant vessels at this wind speed. As a rule: wind dominates above approximately 10-12 knots depending on vessel type; current dominates at low wind speeds with strong tidal streams. Scope: the International method uses the ratio of cable scope to water depth. Minimum safe scope in moderate conditions: 5:1. In heavy weather or holding ground of dubious quality: 7:1 to 8:1 may be required. Cable scope = length of cable out / total water depth (depth + height of hawsepipe from waterline, typically 6-10m). Anchor walk-back: the anchor and cable should be walked back under control (using the windlass brake), not dropped free — free-running cable creates shock loads when it reaches its scope that can damage the windlass or the ship's bow.