Night entry with unlit leading marks
You are skipper of an 11 m yacht, draft 1.7 m, with four crew. One crew member is seasick. At 0230 you are 3 nm off an unfamiliar harbour entrance, motor-sailing at 5 kn. Visibility is 1.5 nm, wind SW 20 kn, sea 1.5 m. HW at the standard port was 0140, height 3.6 m; LW will be 0740, height 0.5 m. The charted depth on the bar is 1.0 m. You want 0.5 m under the keel. The harbour leading lights are reported unlit. The entrance has a strong cross-tide on the ebb. You have GPS, echo sounder, paper chart, hand-bearing compass, VHF and engine. The marina berth is booked. A waiting anchorage 1 nm off is exposed to the SW swell. There is no other harbour within 10 nm. The bar passage takes 15 minutes.
1. Calculate the required height of tide and the available height at 0230. Is depth the limiting factor?
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Required tide height = draft + under-keel clearance − charted depth = 1.7 + 0.5 − 1.0 = 1.2 m.
HW was 0140, so 0230 is 50 min after HW. Range = 3.6 − 0.5 = 3.1 m. In the first hour after HW the rule of twelfths gives a fall of 1/12 = 0.258 m; at 50 min, proportionally about 0.215 m. Height at 0230 ≈ 3.6 − 0.215 = 3.39 m.
So at 0230 there is far more than the required 1.2 m; depth is not the limiting factor at that time. The ebb becomes limiting later: around 0600 the height is approaching 1.2 m. The real constraints now are night, unlit leading marks, cross-tide and crew condition.
2. Draft a pilotage plan for the last 3 nm and entry, using GPS, echo sounder and bearings, given the unlit leading lights. Include at least three abort criteria.
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Before entry: brief the crew, lifejackets on, engine ready, VHF on the harbour channel, reduce speed to about 3–4 kn, post a bow lookout with a torch, and keep the echo sounder on. Do not rely on the leading lights; they are unlit.
Use GPS waypoints: 1.5 nm off the entrance, entrance buoy, mid-channel, marina. Monitor cross-track error and use the echo sounder as an independent check. Take hand bearings on any lit, positively identified mark, and use a safe bearing on the harbour wall only if you are certain of the object. Keep to the starboard side of the channel where safe.
Abort criteria: cross-track error greater than about 15 m; depth less than 2.2 m; visibility below 0.5 nm; any doubt about position; engine temperature rising; or a vessel blocking the entrance. If aborting, do it before committing to the narrowest part.
3. Recommend whether to enter now, wait, or anchor. Justify with the specific constraints.
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Do not enter now on a blind GPS approach unless you can positively identify the entrance and have a reliable transit or safe bearing. Depth is adequate at 0230, but the leading lights are unlit, the ebb sets a strong cross-tide, visibility is only 1.5 nm, one crew is seasick, and the harbour is unfamiliar. A GPS track alone is not enough in a narrow entrance at night.
Holding off under power or sail until daylight is the safer option. Anchoring at the exposed waiting anchorage in SW 20 kn and 1.5 m swell is unattractive and may become unsafe. If the seasick crew’s condition deteriorates, go to the alternative harbour 10 nm away rather than force the entry. If you must enter, do so slowly with continuous soundings and a clear abort point.