Can tide height affect an object’s visibility from a yacht?

James Stevens answers your questions of seamanship

Question:

The skipper and crew of the Bowman 46, Flying Colours, are anticipating their arrival in Falmouth after crossing the Atlantic.

They are currently about 30 miles off the Isles of Scilly, sailing through the night in clear visibility.

John, the skipper, tells his crew to look out for their first landfall, Bishop Rock lighthouse situated off the southwest of the Isles of Scilly.

The lighthouse has a charted height of 44m and a white light that flashes twice every 15 seconds.

John reckons they should see the light at 17 miles out. It will be low water at about the time the light will become visible.

The mate, Bill, thinks that because the yacht is lower down than at high water they will not see the light until they are closer; he believes that a yacht is higher up at high water so you can see further, just as you can if you climb the mast.

John says at low water the light is higher out of the water so you can see it from further away.

Alan, one of the other crew, says the tidal height makes no difference to how far you can see the light because the yacht being higher at high water is cancelled out by the height of the light being less above the water level.

There is a spring tide that night and the range is 5m.

Is the height of the tide going to make any difference to the distance the light is visible from the yacht?

James Stevens answers:

The light will be visible from further away at low water.

The higher an object is, the further away you can observe it at sea.

The almanac has a table of the rising or dipping distances of lights which tells navigators how far off they are from a charted light.

The charted heights of lights are given from Mean High Water Springs, MHWS, so in this case the height of the light is 44m plus the fall of the tide, 49m.

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Assuming the height of the eye of the observer in the cockpit is 3m, the tables will show the distance that the light will appear on the horizon at low water as 18.2 miles.

At Mean High Water Springs the distance is 17.4 miles.

Before electronic navigation this was a really useful way of fixing a yacht’s position with a bearing and distance off.

When sailors approach light, they see the loom first.

This is the light from below the horizon which becomes visible before the actual light.

In a heavy sea, the light will appear and disappear so an accurate distance off is more difficult, but seeing it after an ocean passage is a great moment.