Bar crossing with a marginal tidal window
MV Elmhaven, a 22,000 gt general cargo ship, is inbound to the estuary port of San Rafael with a mean draft of 8.40 m on an even keel. The bar has a charted depth of 6.5 m below chart datum over its shallowest 300 m. Company under-keel-clearance policy requires 10% of draft over the bar, plus allowances for squat and swell that the master must justify in the passage plan. Trials data give 0.45 m of squat at the planned 8 knots over the bar. The harbour master's notice states that today's 1.5 m swell adds up to 0.30 m of effective rise over the bar. Predicted tides are HW 3.9 m at 0210 and LW 0.6 m at 0835. Pilot boarding is two hours before the bar and pilotage is daylight-only until 0600. The agent wants the ship alongside by 0500.
1. Calculate the minimum height of tide required over the bar and state, with figures, whether the vessel can enter on the 0210 high water.
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Build the required depth from the bottom up, then compare it with the charted depth plus predicted tide.
- Draft: 8.40 m
- UKC allowance (10% of draft): 0.84 m
- Squat at 8 knots: 0.45 m
- Swell allowance: 0.30 m
- Total water depth required: 9.99 m, say 10.00 m
Required tide over the bar = 10.00 − 6.50 = 3.50 m. Predicted HW is 3.90 m, leaving an apparent margin of only 0.40 m.
So the answer is yes in arithmetic, but only just, and only if the ship is actually over the shoalest 300 m at the top of the tide. The margin is smaller than the uncertainty in the calculation:
- Squat in shallow water is larger than trial figures taken in deep water, because of the blockage effect over a shoal; a bar crossing can double the deep-water value, so 0.45 m may be optimistic.
- Squat rises roughly with the square of speed. If the ship pushes to 10 knots to make the berth window, squat grows by about a quarter to a half, wiping out most of the margin.
- Tide predictions carry meteorological error of the order of 0.3 m or more; an offshore wind can produce a set-down on a bar.
- A 1.5 m swell does not simply add 0.30 m of water; it produces vertical motion at the ship, so the allowance is a statistical figure, not a guarantee.
Conclusion: the vessel can enter only at the top of the tide, at the planned 8 knots or less, with the echo sounder watched and an abort plan ready. On the figures it should not be treated as a comfortable passage.
2. The agent presses for the ship to cross at 2330 instead, to guarantee the 0500 berth. How should the master handle this?
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At 2330 the tide is still about two and a half hours short of HW, so on a rising curve the height available is roughly 2.9 to 3.0 m against a requirement of 3.50 m. The crossing would be short by about half a metre of water - that is, the ship would be running the bar with something like 8.9 m of depth under a 8.4 m draft, roughly 0.5 m of clearance before squat and swell are counted. That is a grounding risk, not a commercial inconvenience.
The master's reasoning should be recorded, not just decided:
- State the calculation and the company UKC policy in writing to the agent, the charterer and the office, so the pressure is documented.
- Offer the alternatives that preserve the commercial outcome: hold at the outer anchorage and take the 0210 tide; reduce to minimum steerage speed over the shoalest section; request a tug standing by; ask the port to confirm the latest survey and tide-gauge reading at the bar.
- Confirm the decision by observation, not prediction alone - a tide board or gauge reading at the bar, and the echo sounder with an offset correctly set.
- Note that the berth window is the agent's problem, not a reason to breach the ship's UKC policy. Under the ISM Code and STCW the master has overriding authority on safety matters; commercial pressure is not a defence after a grounding.
If the berth is lost, the cost is demurrage or a missed slot. If the ship touches, the cost is a salvage, a hull survey, possible pollution and a master's licence. The trade is not close.
3. What must the finished passage plan contain for this bar crossing, beyond the tide calculation?
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A bar crossing of this type needs a plan that a relieving officer could execute without the master in the room:
- Waypoints and no-go lines with a clearly marked abort point short of the shoalest water, and the turn-out course to the deep-water anchorage.
- Tidal window stated as an earliest and latest time over the bar, derived from the curve, not just HW, and cross-checked against the port's tide gauge.
- Speed and squat control - maximum speed over the bar, and the engine-room standing order for immediate response to a ring from the bridge.
- Under-keel clearance monitoring: echo sounder on and alarmed, offsets verified; a second independent depth source (forward echo sounder or a reported sounding from the pilot boat) if fitted.
- Environmental limits: maximum swell and wind for the crossing, with the harbour master's notice criteria and a defined point where the master cancels.
- Contingencies for engine failure, steering failure, loss of the pilot ladder or a fouled anchor, including the anchoring position and holding ground outside.
- Bridge team organisation: two officers and a helmsman, a dedicated lookout forward if possible, pilot briefing on squat and UKC, and no non-essential traffic on the bridge.
- Records: the calculation, the source of tide and survey data, and the master's decision, retained for the passage file.