Loss of GPS on a night passage from Paros to Syros
At 21:00 on 14 September the 12.5 m sailing yacht Kymothoe clears the southern cape of Paros, bound for Ermoupoli on Syros. The last confirmed fix, plotted on paper at 21:20, is 2.0 NM due south of the cape. At 21:20 the chartplotter's GPS antenna cable fails and the unit displays 'no fix'. The skipper still has a steering compass, a hand-bearing compass, a trailing log reading 6.0 knots through the water, and a working echo sounder. Course steered is 330°T; the wind is NNE 5 on the starboard bow, giving about 5° of leeway to port, which the helmsman has not been correcting. The pilot book warns of a current in the strait setting 060°T at about 1.0 knot. The only paper chart on board is small-scale; the harbour entrance is marked by a Fl(3) W 15s light with a nominal range of 15 NM, and the approach is used by ferries at night. At 23:00 the echo sounder reads 26 m and the light is still not visible.
1. Starting from the 21:20 fix, construct the estimated position at 23:00 and explain the checks you would use to confirm it.
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Step 1 – run time and water distance. 21:20 to 23:00 is 100 minutes = 1.67 h. At 6.0 kn through the water the yacht has run 6.0 × 1.67 ≈ 10.0 NM.
Step 2 – course made good through the water. Course steered 330°T, but the wind on the starboard bow has pushed the yacht 5° to port, so the water track is about 325°T.
Step 3 – set and drift. Current 060°T at 1.0 kn for 1.67 h = 1.7 NM.
Step 4 – vector triangle. From the fix, draw 325°T / 10.0 NM; from the end of that line draw 060°T / 1.7 NM. The resultant is roughly 334°T, 10.0 NM from the 21:20 fix. Note that the leeway and the current very nearly cancel in this particular geometry – that is coincidence, not safety. A small error in current strength, log calibration or helm work moves the answer by up to a mile.
Checks. (a) Soundings: 26 m at 23:00 must fit the charted contours between the fix and the harbour; a contour that does not match means the plot is wrong, not the sounder. (b) Hand bearings: any two charted lights give a fix; one light plus log distance gives a running fix. (c) Radar range on a headland or the cape light. (d) Log calibration and steering error. Plot an uncertainty circle of at least 1 NM and log the EP with its basis.
2. At 23:00 the entrance light is still not visible, although its nominal range is 15 NM. What does that tell you, and what do you do?
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Absence of the light is not proof of being off track. Nominal range assumes 10 NM meteorological visibility and an observer's eye 5 m above the water; from the cockpit of a 12.5 m yacht the eye is roughly 2.5–3 m, so the geographical range is reduced by several miles (horizon distance ≈ 2.08 × √h in metres, about 3.5 NM from the eye alone). Add haze, a sector obscured by land, or the light simply not yet being within its sector, and a perfectly correct position produces no light.
The real problem is the error budget: a DR position with 1–2 NM uncertainty, an unfamiliar harbour, a charted reef and ferry traffic, at night. So:
- Reduce to a safe speed (about 5 kn) and keep well clear of the ferry track.
- Put a dedicated lookout forward with binoculars; station someone on the hand-bearing compass.
- Take bearings of any other charted light or prominent headland and run a fix; plot soundings every 5 minutes against the contours.
- Do not sound fog signals – visibility is good – but do call the port on VHF if you intend to enter.
- If no positive fix has been obtained by about 2 NM from the charted entrance, stand off on a safe heading and wait for dawn or for a confirmed fix. Entering a ferry harbour on DR alone is the error, not the delay. Log the decision and its reason.
3. The mate suggests switching on a phone navigation app with cached charts. Assess this proposal.
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It is useful redundancy, not a fix. A phone gives an independent position source, which is genuinely valuable after an antenna failure, and offline charts let you sanity-check your plot. But:
- The charts in consumer apps are often unofficial, simplified or out of date; buoyage, harbour works and depths may not match the paper chart or reality.
- Battery life, water ingress, screen brightness at night and the risk of losing the device overboard all make it fragile.
- It gives position, not depth, and it does not integrate with the echo sounder.
- It creates a temptation to stop plotting – the failure mode that got you here in the first place.
Correct use: treat the phone as a cross-check on the paper plot, not a replacement. Take the position, transfer it to the paper chart, compare it with the DR and the soundings, and if the two disagree find out why before acting. Never navigate into an unfamiliar harbour on an unverified app position at night.
Prudent practice afterwards: carry a handheld GPS with spare batteries and the appropriate paper chart for the area before departure, and have a written fallback procedure. Also check the antenna cable – a deck-level connector is a common failure point and is usually repairable.