Under-keel clearance decision on a depth-limited approach channel
MV Caspian Trader (LOA 190 m, 28,500 t deadweight) is inbound to Shahid Rajaee, with the pilot boarding at 05:00 for a 06:00 berth. Departure drafts are 12.40 m forward and 12.85 m aft. The critical section of the approach channel carries a charted depth of 14.2 m below chart datum over a rock patch; the predicted height of tide for the transit is 0.6 m. The manager's under-keel clearance (UKC) policy requires not less than 10 per cent of the maximum draft beneath the keel at all times, and that figure must already include allowances of 0.9 m for squat at the planned 12 knots and 0.3 m for heel and swell response. The channel is 300 m wide, with a soft mud bank close on the starboard side and a passing lane used by outbound tankers. Slowing to 8 knots would reduce squat to about 0.4 m but adds 35 minutes and loses the berth slot; waiting for the next high water, 0.4 m higher, costs about six hours.
1. Calculate the net UKC for the planned 12-knot transit and for an 8-knot transit, and state whether either complies with the manager's policy. What decision should the master take?
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Water available at the critical section Charted depth 14.2 m + predicted tide 0.6 m = 15.0 m. Maximum draft (aft) = 12.85 m, so gross clearance = 15.0 - 12.85 = 2.15 m. Required UKC = 10% x 12.85 = 1.285 m, and the 0.9 m squat and 0.3 m heel/swell allowances must be deducted before comparing.
At 12 knots Net UKC = 2.15 - 0.90 - 0.30 = 0.95 m, which is 0.34 m short of the 1.285 m policy figure. The transit as planned is non-compliant.
At 8 knots Squat varies approximately with the square of speed, so 0.9 x (8/12)^2 = 0.40 m. Net UKC = 2.15 - 0.40 - 0.30 = 1.45 m, which clears 1.285 m with 0.17 m in hand.
Decision Transit at a speed giving a compliant UKC. Eight knots is the defensible choice; nine knots gives 2.15 - 0.51 - 0.30 = 1.34 m, technically compliant but with only 6 cm of margin against tide prediction error, silting or a passing ship's suction. Best practice is 8 knots on the present tide, or 8 knots on the later high water (2.15 + 0.4 - 0.4 - 0.3 = 1.85 m), which is the strongest option if the berth can be rebooked. The berth slot is a commercial interest and cannot override the company's own UKC limit.
2. Explain why squat reduces as speed falls, and identify the factors in this particular channel that the simple policy figure does not capture.
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Why squat follows speed squared A moving hull pushes water aside and the return flow is squeezed between the keel and the seabed. By Bernoulli, the increased local velocity lowers pressure under the hull, the water level around the ship drops and the ship settles bodily, with a bow-up or bow-down trim depending on block coefficient and depth/draft ratio. Because the pressure change is driven by the square of the relative flow velocity, squat approximately follows V^2 for a given ship and depth, which is why the step from 12 to 8 knots cuts squat by more than half. Below a depth/draft ratio of roughly 2.5 the effect becomes non-linear and grows sharply; here 15.0/12.85 is close to 1.17, an extremely shallow ratio, so real squat may exceed the policy figure.
What the policy number misses
- Bank effect: the soft mud bank on the starboard side produces suction, sheering the bow toward the bank and the stern away; the ship may need speed for steerage while speed is exactly what must be limited.
- Passing ships: an outbound tanker in the lane generates interaction, transient sinkage and a yaw moment at close lateral separation.
- Pitch and roll dynamics: the 0.3 m allowance is a static figure; in a swell the bow can dip below the mean UKC.
- Tide uncertainty: meteorological surge, river outflow and barometric effects can put the actual level below prediction.
- Chart accuracy and silting: the sounding over the rock patch may pre-date recent sedimentation; a rock patch allows no tolerance at all.
- Water density: less buoyant water close to an outfall or after heavy rain deepens the draft.
3. What should the master record and communicate, and what should he do if the pilot or terminal insists on keeping 12 knots to save the berth?
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Before the transit Attach the UKC calculation, with the tide source and the squat allowance, to the passage plan and note the intended speed limit in the bridge order book. Inform the pilot at boarding of the chosen speed and the reason, and confirm the planned timing with port control or VTS so that the channel slot is reserved at the slower speed.
If pressed to keep 12 knots The pilot's presence is advisory; the conduct and safety of the ship remain with the master (STCW, and the ISM Code's clear statement of the master's overriding authority). The master should:
- Refuse the speed politely but unambiguously, stating the figure: 0.95 m of net UKC against a 1.285 m limit.
- Ask for the instruction to be given in writing, or record it verbatim in the log with the time and the name of the person giving it.
- Note a letter of protest to the charterer, terminal and agent, copied to the manager and DPA, stating that the delay arises from a safety limit.
- Report the deviation from the company UKC policy through the SMS and record the alternative arrangements.
Records Log the drafts, the calculated UKC, the tide height actually observed against prediction, the speed made good through the critical section, the engine movements in the bell book, and the times of any exchange with the pilot, VTS or terminal. If the passage is made at the higher tide instead, record the reason for the wait so that the delay is attributable to the safety case, not to poor planning.