How to navigate when a navigation light is obscured

James Stevens considers a problem sent in by a Yachting Monthly reader who wants to know whether to proceed when a navigation light is obscured

Sue is cruising around the small ports of north Brittany singlehanded on board a 9m cruising yacht. She is intending to enter a small harbour with a visitor’s buoy for an overnight stay. The harbour has leading marks which are lit at night to guide yachts between the drying rocks which extend either side of the channel for about half a mile.

The sea conditions are flat calm and the forecast is for light offshore winds overnight. The tidal height is 6m.

Light winds and strong tides have delayed Sue and it is now dark. Visibility is good but there is no moon and there is a two-knot cross tide. Sue can see the lights
of the village behind the harbour and is looking for the leading lights which both have the character QR.Sue’s yacht has a chartplotter at the chart table but no repeater at the steering position.

From about a mile out Sue can see one of the QR lights, which she thinks is the nearer of the two. But in spite of searching hard with the binoculars, she is unable to identify the other one. All she can see are the lights of the village on the hillside behind the harbour.

Sue is very conscious of the hidden but dangerous rocks either side of the channel. How does she pilot her way in?

Approaching a rocky and tide-swept coastline at night is a daunting prospect, especially when you can’t find the leading lights

How to navigate when a navigation light is obscured

There are two ways to approach this problem without continually running up and down the companionway.

Most chartplotters have a Cross Track Distance (also called Cross Track Error) function which allows the navigator to create a zone bounded by lines parallel with the transit either side. If the yacht crosses outside the zone an alarm sounds. On some plotters the alarm is different depending on which side the yacht strays. The cross track distance can be set up while Sue is still in safe water before closing the rocks.

The other method which can be used at the same time is to line up the known light, QR, with a street light or other fixed light on the correct bearing. To achieve this Sue needs to position the yacht on the transit. Traditionally this is done with the aid of a hand-bearing compass but it’s easier to motor gently into the tidal stream in safe water using the autopilot until the plotter shows the correct bearing of the QR, come up on deck and find a suitable light in the right place. A check with the hand bearing compass would be sensible.

A transit is going to be more accurate and easier to steer than keeping within cross track error lines and importantly it keeps the skipper constantly adjusting the course to account for the tide while staying on track.

At the breakwater it will be a case of using a powerful torch to identify the visitor’s buoy.

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