Question 1

Your 300m tanker has a draught of 16.5m. You are transiting a channel where the charted depth is 19m but where dredging may have created uneven bottom. Discuss your UKC calculation and minimum clearance policy.

Accepted Answer

UKC CALCULATION — 300M TANKER IN SHALLOW CHANNEL: UKC COMPONENTS: Under-Keel Clearance = Charted Depth − Vessel Draught − Dynamic Allowances. STATIC CALCULATIONS: (1) Chart datum: the charted 19m is referenced to Chart Datum (Lowest Astronomical Tide). Current tidal height must be added; (2) Current draught: 16.5m; (3) Static clearance at chart datum: 19 - 16.5 = 2.5m; (4) During tidal rise: if tide = +1.5m: effective depth = 20.5m; clearance = 20.5 - 16.5 = 4.0m before dynamic allowances; DYNAMIC ALLOWANCES — THESE REDUCE THE EFFECTIVE UKC: (1) SQUAT: a vessel moving through water sinks below its static waterline due to: (a) Pressure distribution under hull; (b) Speed-dependent. For large vessels in shallow water (depth/draught ratio < 3): squat can be calculated using the Barras formula. At 10 knots in a depth/draught ratio of ~1.15 (19/16.5), squat for a 300m tanker may be 1.0-1.5m; (2) HEEL: when turning or in wind — the vessel lists, increasing draught at the lower side. For 1° heel at 32m breadth: heel squat ≈ 0.28m; (3) WAVE RESPONSE: in wave action, the vessel heaves and pitches — the lowest point may be deeper than the static waterline; (4) CHART ACCURACY: the stated 19m may not reflect uneven dredging. A safety margin of 0.5-1.0m should be added; MINIMUM SAFE CLEARANCE POLICY: many companies specify 10-15% of draught = 1.65-2.5m after all dynamic deductions. At 10 knots with 1.5m squat and heel — the available clearance BEFORE wave response and chart inaccuracy is: 2.5m (static) - 1.5m (squat) - 0.28m (heel) = 0.72m. THIS IS INSUFFICIENT for a 300m tanker. REDUCE SPEED to reduce squat. Static UKC is a starting point only: option B correctly identifies static UKC as 2.5m — but fails to account for any dynamic deductions. For a VLCC/large tanker in a shallow channel, squat alone can be 1.0-2.0m at normal transit speeds. The net dynamic UKC after squat, heel, and wave response may be less than 0.5m — critically insufficient. Pilot responsibility (option C): the pilot advises on local conditions and may have experience-based knowledge of the channel. However, the master retains ultimate responsibility for the vessel's safety (MSA 1995 S.22 and SOLAS V/34). The master should calculate UKC independently and compare with the pilot's recommendations. If the master's calculation shows inadequate UKC — they must raise this with the pilot. Speed and squat relationship: option D is dangerously wrong. Increasing speed INCREASES squat (squat is proportional to approximately speed squared). The correct response is to REDUCE SPEED to reduce squat. Many VLCCs transit shallow ports at very slow speed (4-6 knots) specifically to manage squat. Company UKC policy: most major oil company and bulk carrier operators have formal UKC policies specifying minimum clearances after all dynamic deductions. These are typically: (a) At sea: 10% of draught minimum (but often 1.5-2.0m absolute minimum); (b) In restricted waters: 15% of draught after deductions. The Barras formula: B2/Cb × V²/Lpp gives a squat estimate. This is the standard formula used by many harbour masters and pilots. Masters of large vessels should be familiar with its application.

Question 2

Your 180m bulk carrier is in Force 10 seas in the North Atlantic on a westbound Atlantic crossing. What are your strategic options and how do you decide?

Accepted Answer

FORCE 10 HEAVY WEATHER MANAGEMENT — STRATEGIC OPTIONS: FORCE 10 CONDITIONS: Beaufort 10 = 48-55 knots wind, seas 9-12m, very high waves with overhanging crests. For a 180m bulk carrier: (a) Green water on deck; (b) Risk of structural damage (hatch covers, mooring equipment); (c) Risk of cargo shift (particularly if bulk cargo has high moisture content); (d) Risk of parametric rolling (if heading or speed is near resonance frequency); STRATEGIC OPTIONS: (1) MAINTAIN COURSE — if the vessel can make safe headway, maintaining course at reduced speed may be appropriate. Rule: speed such that no excessive structural stress occurs. Fuel consumption penalty; (2) HEAVE-TO: bring the vessel to approximately 30-40° off the sea on the bow (head to sea, minimal headway). Most comfortable option for weathering out a storm. Significant time and fuel cost; (3) RUNNING BEFORE: stern to sea. Faster progress but: stern-on boarding seas risk; pooping risk; loss of rudder effectiveness in following seas (surf-riding, broaching risk); (4) AVOIDANCE — alter course to skirt around the storm. Use weather routing service data and ECMWF forecast to find the storm track and avoid it; MASTER's DECISION PROCESS: (1) ASSESS STORM TRACK: is it moving, and can you outrun or avoid it?; (2) VESSEL CAPABILITY: what is this vessel's seakeeping in these conditions?; (3) CARGO RISK: is the cargo susceptible to moisture or shifting?; (4) CREW CONDITION: fatigue level; (5) DESTINATION URGENCY: is there a berth window, laycan, tidal window that has commercial significance?; (6) NOTIFY DPA: commercial implications of significant deviation. Full speed in Force 10 (option B): maintaining full service speed in Force 10 conditions risks: (a) Excessive slamming (bow slamming in following steep seas); (b) Loss of propeller in following seas (RPM surge when propeller emerges); (c) Structural overstress on hatch covers, deck fittings; (d) Cargo damage. The correct principle is to reduce speed to what the vessel can safely sustain — which in Force 10 may be as low as 3-5 knots for safety. Running before the storm (option C): stern-on to following seas can cause: (a) Surf-riding — the vessel accelerates uncontrollably down a wave face, rudder loses effectiveness; (b) Broaching — a sudden involuntary turn athwart the sea; (c) Pooping — a following sea breaks over the stern. These are extremely dangerous. Running before the storm is appropriate only in certain vessel types and conditions — it is not universally the safest option. Weather routing authority (option D): the master bears responsibility for the safety of the vessel under SOLAS V/34 (passage planning). Weather routing services provide expert guidance — but they cannot compel the master to follow their recommendations. The master assesses the weather data and makes the decision. The weather routing service cannot be on the bridge to observe actual conditions. Parametric rolling — bulk carriers: certain vessel hull forms (bulkers, container ships) are susceptible to parametric rolling in specific sea states where the encounter frequency matches the natural roll period. The master must be aware of this and alter course or speed if the vessel exhibits parametric rolling characteristics.

Question 3

Your container vessel is in confused seas with periods of severe rolling. The chief officer believes the vessel may be experiencing parametric rolling. How do you respond?

Accepted Answer

PARAMETRIC ROLLING — RECOGNITION AND RESPONSE: WHAT IS PARAMETRIC ROLLING?: parametric rolling is a resonance phenomenon where: (a) The vessel's natural roll period matches the wave encounter period; (b) Stability changes cyclically as the vessel moves between wave crests (reduced stability) and wave troughs (increased stability); (c) This creates a reinforcing cycle that can develop VERY rapidly into extreme roll angles (40-60° in minutes); (d) Most dangerous in HEAD SEAS or following seas when the encounter period matches the roll period; WHICH VESSELS ARE SUSCEPTIBLE?: (a) Container ships with large bow flare and flat stern overhang — hull geometry causes large stability changes as waves pass; (b) RO-RO vessels; (c) Bulkers to a lesser extent; SIGNS OF PARAMETRIC ROLLING: (a) Double-frequency roll — the vessel rolls twice for each wave; (b) Roll amplitude increasing rapidly; (c) Rolls that are well synchronised with wave encounter; (d) Rapid onset with no increase in sea state; MASTER's RESPONSE: (1) IMMEDIATELY ALTER SPEED OR COURSE: change the wave encounter period. Even 5-10° of course change may break the resonance; (2) REDUCE SPEED: reducing speed changes the encounter period; (3) DO NOT TURN BROADSIDE: altering to a beam sea significantly increases roll amplitude — approach the course change through the bow or stern; (4) CARGO: container lashings should be checked — parametric rolling causes extreme transverse forces that can exceed lashing system design limits; (5) LOG AND REPORT: record the event and severity. MAIB-reportable if a significant accident or near miss results. High GM and parametric rolling: option B is incorrect. Parametric rolling is primarily a RESONANCE phenomenon — it depends on the ENCOUNTER PERIOD matching the ROLL PERIOD, not on high GM. In fact: high GM = short roll period. If the encounter period is short (head seas at speed), a high GM vessel may be MORE susceptible to parametric rolling. Reducing GM slows the roll period — which might move the resonance frequency, but it also reduces the stability reserve, which is dangerous. Adjusting course and speed is the correct response, not ballasting underway. Self-limiting (option C): parametric rolling is NOT self-limiting under constant sea and course/speed conditions. It can continue to amplify until lashings fail, containers are lost, or the vessel is in severe danger. It is an ACTIVE intervention situation. Turning broadside (option D): this is the most dangerous advice. Beam-on to large ocean swells creates extreme rolling in any sea state. In a parametric rolling scenario — transitioning through a beam sea to reach a new heading should be done as quickly as possible, not maintained there. The correct approach to course-changing in heavy weather is to assess the timing relative to wave sets and make the turn briskly. Container lashing limits: the CSS Code and shipping company lashing manuals specify the design loads for container lashings (typically ±20-25° transverse acceleration loads). Parametric rolling at 40-60° amplitude produces forces FAR exceeding these design limits — container loss overboard becomes likely. The master must act before amplitudes reach these extreme values.