Question 1

Your vessel is transiting a channel where the under-keel clearance (UKC) is 1.5m. Your examiner asks what shallow water effects you should expect and how they affect manoeuvring.

Accepted Answer

Shallow water effects with 1.5m UKC: (1) SQUAT — the vessel sinks bodily and by the stern (or bow for full-form ships) due to reduced water flow under the hull at speed. Barras formula for stern squat: Squat (m) ≈ 2.4 × Cb × V² / √(depth × 10). At speed, squat may reduce the 1.5m UKC to dangerously small values — check calculation and reduce speed if necessary; (2) INCREASED DRAG — water in the channel cannot flow freely around the hull, increasing resistance; more power needed to maintain speed; (3) REDUCED STEERING EFFECTIVENESS — the rudder is less effective in shallow water due to altered flow dynamics; (4) BANK SUCTION — in a narrow channel, flow between the vessel and the bank increases, drawing the vessel towards the bank; counter with rudder but avoid over-correcting; (5) WAVE WASH — speed limit considerations to avoid bank erosion and destabilising small vessels. Squat is one of the most examined ship handling topics at OOW level because of its direct safety implication — many groundings have been caused by underestimating squat in shallow water. The Barras formula (or simplified versions) is the standard calculation method: Squat increases with the square of speed (V²) and the block coefficient (Cb — how "full" or "boxy" the hull form is). A large bulk carrier with Cb = 0.82 will squat more than a container ship with Cb = 0.65 at the same speed. At 12 knots in shallow water with Cb = 0.82: squat could be 1.0-1.5m — which would completely eliminate the 1.5m UKC and ground the vessel. The response: calculate the maximum safe speed that keeps squat within the available UKC margin (typically safety margin = 10-15% of the vessel's draft or 0.5m minimum). Many port authorities specify maximum transit speeds for this reason (e.g., suez transit maximum speed in the canal). UKC policy: companies and port authorities set minimum UKC standards — typically 10% of draft at sea, 0.5m for ports.

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.