The Bar, the Falling Glass and the Squat Allowance
You are master of a 24 m workboat drawing 4.20 m, inbound over a shallow bar. The chart shows a least depth of 4.80 m over the bar at chart datum. Predicted high water is 13:40 at 2.60 m; predicted low water is 07:30 at 0.40 m. At 11:30 the tide table gives a height of 1.50 m. The barometer is 988 hPa and falling; the forecast gives a 1.2 m swell and 15 kn onshore wind. Your company's minimum under-keel clearance is 1.00 m. At 7 kn in the shallowest water, estimated squat is 0.35 m; you allow 0.40 m for swell and wave set-up. It is now 11:25, the berth is 40 minutes away, and the next high water after today is at 02:10 tomorrow. The vessel has a single engine and no bow thruster.
1. Calculate the corrected height of tide and the net under-keel clearance at 11:30. Using the company limit, state whether you can cross at 11:30 and show your working.
Read the worked answer
Pressure correction: 1013 − 988 = 25 hPa; 25 × 0.01 = 0.25 m. Low pressure reduces the tide, so corrected tide = 1.50 − 0.25 = 1.25 m.
Depth over bar = charted 4.80 + 1.25 = 6.05 m.
Gross UKC = 6.05 − 4.20 = 1.85 m.
Deduct squat 0.35 and swell/set-up 0.40: net UKC = 1.85 − 0.75 = 1.10 m.
Against the company minimum of 1.00 m, 11:30 is technically acceptable by 0.10 m. But that margin is smaller than the uncertainty in the squat estimate, the swell allowance and the charted depth. A 10 cm margin is not a prudent go/no-go criterion on a bar with an onshore wind and a single engine. I would treat 11:30 as the earliest possible, not the planned crossing. I would wait at least 30-45 minutes for the tide to rise, recalculate at the actual time, and cross at reduced speed with the echo sounder watched and the anchor ready. If the berth can wait, waiting for 12:15-12:30 gives a more credible margin.
2. Which assumptions in your calculation could be wrong, and how would you reduce the risk if you decide to wait for more water?
Read the worked answer
The weak assumptions are:
- Charted depth. The survey may be old; bars silt. The charted 4.80 m is not a guarantee. A local notice to mariners, a recent survey or a tide gauge/leading line is better evidence.
- Tide. The table height is for a standard port; the bar may have its own range and time offset. The pressure correction is an approximation, and the barometer is falling, so the correction could be larger.
- Swell and set-up. A 1.2 m swell with 15 kn onshore wind can raise or lower the effective depth at the bar and create breaking water. My 0.40 m allowance is a guess.
- Squat. 0.35 m at 7 kn depends on draft, block coefficient and the actual depth. In shallow water squat increases; reducing speed reduces it.
- Draft. I would confirm actual draft by reading the marks, not assume 4.20 m, and allow for fuel and water.
Risk reduction: wait for more tide; reduce speed to 4-5 kn; use echo sounder and leading marks; have the anchor ready and engine on standby; brief the crew; monitor VHF; consider a local pilot or waiting for the next daylight high water. If the vessel cannot be stopped safely, do not enter.
3. If the engine fails just as you enter the bar, what are the immediate actions and the radio call?
Read the worked answer
If the engine fails at the bar, the priority is to avoid being set sideways onto the shoal and to control the grounding. Immediate actions:
- Steer for the deepest water and drop the anchor if there is room, paying out enough chain to hold; a single-screw vessel without a bow thruster will not steer once stopped. If anchoring is impossible, try to take the way off and accept a controlled grounding rather than a beam-on impact.
- Sound the appropriate signals and make a Mayday or Pan-Pan. If there is immediate danger of grounding or capsizing, Mayday on Ch 16: 'Mayday, Mayday, Mayday, this is [name], position [bar], engine failure, four persons on board, preparing to ground, over.' If the situation is developing but not yet grave, Pan-Pan.
- Close watertight openings, muster, lifejackets, EPIRB and liferaft ready. Brief the crew on the possibility of abandoning.
- Notify the port/coastguard and ask for a tug or immediate assistance. A fishing vessel or pilot boat may be closer.
- After grounding: check soundings, pollution, injuries, and do not attempt to refloat under power until the hull and watertight integrity are checked. The single-engine configuration is a known risk; the decision to enter should have included a contingency for engine failure at the critical moment.