Question 1

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 2

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.

Question 3

A junior officer asks you to explain the difference between spring and neap tides, why they occur, and why it matters for watchkeeping when approaching a port with a shallow bar.

Accepted Answer

SPRING TIDES: occur approximately every 14 days — at new moon and full moon, when the Sun, Earth, and Moon are in syzygy (aligned). Gravitational pull of Sun and Moon act in the same direction (or opposite directions equally increasing tidal range). Spring tides give the GREATEST RANGE (highest HW, lowest LW). Mnemonic: SPRING = Sun Pulls Rapidly, Important Notable Greater. NEAP TIDES: at first quarter and third quarter moon — Sun and Moon are approximately 90° to each other; gravitational forces partially cancel; SMALLEST RANGE (lower HW, higher LW). Tidal coefficient/range varies by port — MHWS/MHWN ratio on UK charts. Watchkeeping significance for a shallow bar: (1) Spring HW may be 1.5-2.0m higher than neap HW — this is the WINDOW in which a deeply laden vessel can cross the bar; (2) A vessel that crossed safely on springs may ground on neaps; (3) Always check whether current tides are springs or neaps from the tide table, and compare HW height with the required depth; (4) Spring LW may expose sandbanks and wrecks not visible at neap LW — alter passage plan hazard assessment. Spring and neap tides are examined at OOW level because they directly impact passage planning decisions for shallow-water ports, anchorages, and bar crossings. The lunar cycle: new moon and full moon = springs; first and third quarter = neaps. This 14-day cycle means springs occur approximately every 14 days. In UK waters (semi-diurnal): Liverpool springs MHWS ≈ 8.5m, MHWN ≈ 6.9m — a difference of 1.6m — significant for a vessel needing 7.0m+ of water. The rule of practice: for bar crossings and shallow draft approaches, always calculate the available depth at HW on the specific day — not simply "springs is safe." A neap HW may still give adequate depth; conversely, anomalous meteorological effects (storm surge, barometric pressure, sustained offshore winds) can reduce the actual HW height below prediction even on springs. Negative surges: barometric pressure 1 hPa above mean reduces sea level by ~1cm; sustained offshore winds can lower coastal water levels 0.3-0.5m below predicted — compound this on a neap tide approaching a shallow bar and the consequences are serious.