Question 1

Your examiner asks you to explain "advance," "transfer," and "tactical diameter" in a turning circle, and why this information is important for the OOW.

Accepted Answer

Turning circle terms: ADVANCE — the distance a vessel travels in the original direction from the moment the helm is put over until the ship has turned 90°; TRANSFER — the perpendicular distance from the original course line to the ship's position when it has turned 90°; TACTICAL DIAMETER — the diameter of the turning circle at 180° (the perpendicular distance between the initial course line and the position when the ship has turned 180°). Importance for OOW: (1) When plotting a turn to a new course, the OOW must account for advance to start the turn before the waypoint — if turn started AT the waypoint, the vessel overshoots; (2) Advance and transfer determine the minimum sea room required to execute a turn; (3) IMO standards require tactical diameter ≤ 5 × ship length; (4) In confined waters, the OOW must know whether the available sea room exceeds the turning circle diameter to avoid running aground. Turning circle terminology is fundamental seamanship that every OOW must know. The practical application: when altering course in a channel with a 300m advance at sea speed, the OOW must begin the turn 300 metres before the intended turning point. Failure to account for advance leads to the vessel being set outside the safe channel during the turn — a common cause of groundings. Rudder angle and turning: a harder rudder angle produces a tighter circle but more drag (speed loss); a smaller rudder angle produces a wider circle. For precision manoeuvring in confined waters, balance between turn radius and speed loss must be considered. Pivot point: during a turn, the ship rotates around a point approximately 1/6 to 1/4 of the ship's length from the bow (the pivot point). The bow swings relatively little; the stern swings out considerably — in a narrow channel the stern sweeping into the bank or shallow water is the primary risk. At OOW level the examiner expects candidates to be able to describe these effects and relate them to practical situations such as channel transits and harbour approaches.

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.