Position Fix Off Cape Recife in a Cross-Swell
You are skipper of a 12 m motor vessel on passage from Port Elizabeth to Knysna, running about 1.5 nm off the coast near Cape Recife. It is 09:40 local time. The sea is moderate with a 2 m SW swell and a confused cross-sea rebounding off the point. Your GPS has been intermittent for the last 20 minutes, showing a 'no fix' warning and then jumping position by up to 0.8 nm. The magnetic compass is steering steadily. You have a paper chart, parallel rules and a hand-bearing compass. Two identifiable charted features are visible: the Cape Recife lighthouse tower and a prominent radio mast inland. The tidal stream is flooding west at about 1.2 knots. Your boat speed through the water is 7 knots. Visibility is good, about 8 nm.
1. Describe how you would obtain a reliable position fix within the next few minutes, and state what you would do about the unreliable GPS.
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Immediate action: Treat the GPS as suspect and revert to terrestrial fixing. The jumping position and 'no fix' warning mean you cannot rely on it for navigation close to a lee shore with a rebound sea.
Fix method: Take compass bearings on the two charted objects (lighthouse tower and radio mast) using the hand-bearing compass. Because the objects are close together in bearing from your track, a two-bearing fix will have a poor angle of cut; take a third bearing on any additional identifiable feature (a headland, beacon or charted wreck) to give a cocked hat. Convert compass bearings to true (apply variation and deviation from the deviation card), then plot with parallel rules. Check the cocked hat: if it is small relative to the scale, accept the fix; if not, re-shoot.
Use of the fix: Plot the fix, note the time, and lay off a DR/EP from it using boat speed 7 kn and the flooding stream setting west at 1.2 kn to give a course and speed over ground. Compare this with the charted safe water and the 1.5 nm offing.
GPS: Do not switch it off entirely - keep it running to see if it recovers, but cross-check every GPS position against a plotted visual fix before acting on it. If it does not recover, navigate by DR and visual fixes and consider standing further offshore until it does.
2. You are asked to give a safe course to steer to remain at least 1.5 nm off the Cape Recife headland while the GPS is down. Explain the reasoning you would use.
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Reasoning steps:
- From the plotted visual fix, measure the bearing and distance to the headland on the chart.
- Lay off a 'danger circle' of 1.5 nm around the headland and any off-lying rocks shown on the chart.
- Draw the intended ground track to pass tangentially to that circle. This is the course over ground (COG) you want.
- Convert COG to course to steer (CTS) by allowing for the tidal stream. The stream is flooding west at 1.2 kn; your speed through the water is 7 kn. Using a vector triangle (or the standard 'course to steer' construction), the CTS will be offset to the east of the COG by the angle whose sine is 1.2/7, roughly 10°, adjusted for the actual relative angle of the stream.
- Apply variation and deviation to convert true CTS to compass course.
Practical check: Because the stream and swell are pushing you west and the sea is confused, err on the side of a larger offing - aim for 2 nm rather than exactly 1.5 nm - and re-fix by bearings every 10–15 minutes. Keep a written DR plot with times so that if visibility deteriorates you still know where you are.