Question 1

Your vessel in loaded condition has a different handling characteristic compared to light ballast condition. Your examiner asks you to describe the key differences and how you adjust your watchkeeping.

Accepted Answer

Loaded vs ballast condition differences: (1) DRAFT AND UKC — fully loaded: maximum draft, minimum UKC in shallow ports, more squat at speed; ballast: high freeboard increases wind effect dramatically on the hull; (2) WINDAGE — in ballast the exposed hull side is large, the bow catches more wind; the vessel is much more susceptible to drift in beam winds; (3) STOPPING DISTANCE — a fully loaded vessel has greater displacement (inertia) and takes longer to stop; (4) TURNING — in ballast the propeller wash may be partially above the waterline, reducing propulsive efficiency and increasing turning circle; (5) VISIBILITY — fully loaded: low freeboard, better visibility from bridge; ballast: higher deck increases bridge height advantage but reduces close-quarters visibility of small vessels ahead; (6) GM — ballast condition may have higher GM (stiffer) causing rapid, uncomfortable rolling. Watchkeeping adjustments: account for windage in ballast when planning approaches; recalculate safe speeds for UKC in loaded condition. Understanding the effect of loading condition on manoeuvring is required knowledge for OOW certificate holders under STCW A-II/1. The windage effect in ballast is often underestimated by candidates: a large bulk carrier in ballast condition may have 10-15 metres of exposed hull above the waterline — in a 30-knot beam wind this creates enormous lateral force. Many incidents in port involve vessels in ballast being set by wind more rapidly than anticipated, especially during slow-speed manoeuvring when rudder effectiveness is reduced. Propeller emergence: in ballast, the stern of the vessel is higher out of the water — on some vessels the propeller partially emerges, causing racing (unloaded propeller spinning free), loss of thrust and efficiency, and vibration. Stopping distance: a fully loaded Capesize bulk carrier at 14 knots has a stopping distance (crash stop) of 2-3nm. This must inform the planning of approach speed to anchorages and port approaches. The vessel's manoeuvring booklet should include data for both fully loaded and ballast conditions.

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.