Question 1

Your vessel is arriving at a port where two tugs will assist with berthing. As OOW under the master's direction, what do you need to know about the tugs and how should communication be managed?

Accepted Answer

Tug information needed: (1) Bollard pull of each tug (to assess whether tug power is adequate for vessel size and wind conditions); (2) Type of tug — conventional (towing astern with tow line on bow), tractor tug (pushes or pulls from various angles), ASD (azimuthing stern drive — highly manoeuvrable, can push from any direction); (3) Tug attachment method — bow tug typically on ship's forward tug point; stern tug on ship's aft tug point; (4) Communication: establish radio communication on the working VHF channel (typically Ch 09, 12, or port-specific) before arrival; confirm the master's instructions will be given directly to tug masters in English (or through the pilot); ensure bridge team knows the tug assignments. Operational: log tug connection time; watch for tug lines fouling the propeller; never go astern when a tug is pushing from forward without checking the tug is clear. Tug operations are examined at OOW level because the OOW is often on the bridge wing or forecastle during berthing and is part of the communication chain. Bollard pull: the force a tug can exert, measured in tonnes. A typical harbour tug: 20-40 tonnes BP; larger tractor tugs: 60-80 tonnes. This must be matched to the vessel's wind area and drift speed — a 20t BP tug cannot hold a 200m, 30,000 GT vessel in 30+ knots of beam wind. Tug types affect handling: conventional tugs are limited to pushing on one side or pulling in the direction of the towline; tractor tugs (Voith Schneider or azimuthing drive) can thrust in any direction and are far more flexible. The ISM Code requires the vessel's procedures to include: pre-arrival briefing (including tug arrangements), communication procedures, and clear assignment of duties. The pilot typically coordinates tugs, but the master retains authority and the OOW must understand the plan to support it.

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.