Question 1

Your vessel is due to cross a shallow bar (charted 3.0m) with a draught of 5.2m. HW predicted is 4.8m, giving a calculated depth of 7.8m and UKC of 2.6m. When you arrive at HW, your echo sounder reads only 6.5m. What has happened and what action do you take?

Accepted Answer

METEOROLOGICAL SURGE (negative surge): tidal heights can differ from predictions due to barometric pressure and wind effects. A barometric pressure 10 hPa above normal (HIGH PRESSURE) reduces sea level by approximately 10cm (the inverse barometer effect — 1 hPa = ~1cm change). More significantly, a persistent OFFSHORE WIND pushes coastal water seaward — reducing the height of HW by 0.3-1.0m compared to prediction. In this scenario: prediction 7.8m depth, actual reading 6.5m = the tide height is 1.3m LESS than predicted — a significant negative surge. Remaining UKC at echo sounder = 6.5 − 5.2m draught = 1.3m (still positive). ACTIONS: (1) Do NOT proceed further into the bar — conditions are worse than predicted; (2) Anchor or heave-to in safe water; (3) Check barometric pressure and wind records to confirm cause; (4) Consult port authority and pilot for their assessment; (5) Monitor echo sounder continuously; (6) Check next predicted HW (springs vs today's height); (7) Inform master immediately if not already on the bridge. The Admiralty Tide Tables carry a warning that predictions are astronomical (gravitational) tides only — they do not account for meteorological effects. Real water levels can differ from predictions by ±1.0m or more in extreme conditions. Inverse barometer effect: every 1 hPa of pressure above the standard mean sea level pressure (approximately 1013 hPa) causes sea level to fall by approximately 1cm. A high pressure system of 1033 hPa = 20 hPa above mean = approximately 20cm below predicted. Wind setup: more significant — sustained onshore winds (pushing water toward the coast) create a POSITIVE surge (water level above prediction). Sustained OFFSHORE winds push water away from the coast = NEGATIVE surge (water below prediction). Storm surges: the most extreme case — the 1953 North Sea flood caused positive surges of over 2.0m above prediction in the Thames Estuary. Negative surges of 0.5-1.0m are relatively common during persistent high-pressure/offshore-wind conditions and are the most insidious danger for shallow-bar passages because mariners assume predictions will be accurate. The correct action: never rely solely on tidal predictions for shallow critical passages — always verify the actual depth with the echo sounder and do not proceed if actual depth is less than minimum required.

Question 2

The charted clearance under a bridge at MHWS is 8.2m. Your vessel has an air draught of 7.4m. HW at the standard port today is 4.8m at 1400. LW before it was 0.6m. The bridge is at a secondary port; seasonal correction +0.1m. You need to pass at 1030. Is it safe?

Accepted Answer

Step-by-step tidal calculation using the Rule of Twelfths (or use Admiralty Tide Tables co-tidal and range factor): (1) Range at standard port = HW 4.8m − LW 0.6m = 4.2m; (2) Duration of rise: LW to HW — need LW time. Assume LW was 0730 (HW 1400 = roughly 6h 30m duration — typical semi-diurnal). Duration ≈ 6h 30m; (3) Time from LW to 1030 = 3 hours; (4) Using Rule of Twelfths — first hour rises 1/12, second 2/12, third 3/12: in 3h the tide has risen (1+2+3)/12 × 4.2m = 6/12 × 4.2m = 2.1m; (5) Height at 1030 = LW + rise = 0.6 + 2.1 = 2.7m; (6) Charted MHWS = 8.2m = height of bridge above Chart Datum (CD). Bridge clearance at 1030 = 8.2 − 2.7 = 5.5m (height of bridge above 1030 water level); (7) Apply seasonal correction: +0.1m raises water level slightly: clearance = 5.5 − 0.1 = 5.4m; (8) Air draught 7.4m vs clearance 5.4m — NOT SAFE by 2.0m. The vessel cannot safely pass at 1030. Tidal clearance calculations appear in virtually every OOW oral. The examiner specifically tests whether the candidate understands that MHWS is the MINIMUM clearance marked on the chart (because that is when the water level is highest, reducing the gap to the bridge). At non-MHWS times, the water is lower than MHWS and the clearance is GREATER. However, this question reverses the trap — the water at 1030 is NOT at MHWS but still high enough to reduce clearance below the air draught. Rule of Twelfths: the tidal rise/fall in a semi-diurnal tide follows the pattern 1:2:3:3:2:1 twelfths per hour. This is an approximation valid for most semi-diurnal ports. For critical calculations (air draught, UKC), always use the Admiralty Tide Tables (NP201 series) and the tidal prediction curves for the standard port, applying secondary port differences (time and height corrections). Seasonal corrections: applied to the tabulated heights to account for seasonal variation in mean sea level (meteorological effects, barometric pressure). In UK waters the seasonal correction to MHWS is typically ±0.1-0.2m and must not be ignored for critical clearances.

Question 3

Your vessel has a maximum draught of 7.8m. The charted depth of the entrance channel is 8.5m below Chart Datum. HW was at 0600 with a height of 4.2m. You enter at 0800. The port requires 0.6m minimum UKC. Is there sufficient water?

Accepted Answer

Depth of water calculation: (1) Time from HW to 0800 = 2 hours — the tide is FALLING; (2) Range: HW 4.2m, assume LW (next) approximately 0.8m (typical UK 3.4m range). Falling tide — using Rule of Twelfths from HW: first hour falls 1/12, second hour falls 2/12; in 2 hours tide has fallen (1+2)/12 × 3.4m = 3/12 × 3.4 = 0.85m; (3) Height at 0800 = HW − fall = 4.2 − 0.85 = 3.35m above CD; (4) Actual depth of water = charted depth + tidal height = 8.5 + 3.35 = 11.85m; (5) UKC = depth of water − draught = 11.85 − 7.8 = 4.05m; (6) Required UKC = 0.6m; available UKC = 4.05m — SAFE with considerable margin. Note: in a real tidal approach, the pilot would calculate on the minimum tidal window and dynamic UKC accounting for squat at approach speed. UKC calculation integrating tidal heights and charted depths is a core OOW competency. The fundamental formula: Actual depth = Charted depth (below CD) + Tidal height above CD. UKC = Actual depth − Vessel draught. The confusion in option B is a classic trap — the charted depth alone does not define whether the vessel can navigate safely; the charted depth at chart datum combined with the tidal height at the time gives the actual available water depth. Entering on a falling tide: option C is incorrect — entering on a falling tide is standard practice; the critical factor is whether there is sufficient UKC throughout the entry. What IS prohibited is entering at a state of tide where the minimum UKC cannot be maintained throughout the transit. Squat: in real port approaches, squat (the dynamic sinkage of a vessel underway, typically 0.1-0.4m depending on speed and canal effect) must be subtracted from the available UKC. A 15-knot vessel in a shallow channel may squat 0.3-0.5m — this must be accounted for in the tidal calculation. The minimum UKC (0.6m in this question) is a port-specific requirement often specified in port authority sailing directions or the pilot card.