Question 1

You need the GHA and Declination of the Sun at 14h 32m 45s GMT on a specific date. Walk through how you use the Nautical Almanac to find these values.

Accepted Answer

Nautical Almanac procedure for Sun GHA and Dec: (1) Open the daily pages for the date — the Sun column shows GHA and Dec for each whole hour of GMT (from 00h to 23h); (2) Extract GHA at 14h GMT (the tabulated whole hour value); (3) Turn to the Increments and Corrections tables (yellow pages) — find the page for 32 minutes; read the SUN/PLANETS column at 45 seconds row; (4) INCREMENT = the additional GHA for the 32m 45s after the hour; (5) TOTAL GHA = tabulated GHA (14h) + increment; (6) DECLINATION: from daily pages extract Dec at 14h; check the declination change per hour ('d' correction at bottom of column); from the increment tables read the 'v' or 'd' correction for 32m 45s; apply to Dec (add or subtract based on whether Dec is increasing or decreasing as shown by sign convention in the tables); (7) RESULT: GHA and Dec at 14h 32m 45s GMT. Note: GHA is always INCREASING for the Sun (approximately 15°/hour). Dec changes slowly — the change per day for the Sun is typically less than 1°. The Nautical Almanac (NP314, published annually by the UK Hydrographic Office and the US Naval Observatory) is the authoritative source for celestial body positions. The tabulation: the daily pages give GHA and Declination for the Sun, Moon, planets (Venus, Mars, Jupiter, Saturn), and selected navigational stars at each whole hour of GMT. The format: GHA is tabulated to the nearest 0.1' of arc for each whole hour. Increments and Corrections (I&C tables): these yellow-edged pages tabulate the additional GHA accumulated in minutes and seconds past the whole hour. For the Sun, the GHA increment rate is 15° per hour = 0.25° per minute = 15' per minute. The v-correction: applied to the Moon, planets, and stars whose GHA rate differs slightly from the tabulated mean rate. The Sun has no v-correction because its GHA rate is incorporated in the tabulated hourly values. At OOW oral level, the examiner may hand the candidate an open Almanac and ask them to extract values — the candidate must know the structure immediately without having to search through the book.

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.