Each lighthouse has its own individual light characteristics but there are some common definitions.

Occulting (Oc) the total duration of light is longer than dark

Isophase (Iso) the duration of light and dark is equal

Flashing (Fl) the total duration of light is shorter than dark.

Examples from our local chart “Channel Islands and Adjacent Coasts of France” include

Corbiere Iso.WR.10s36m18/16M

Roches Douvres Fl.5s60m28M

La Croix Oc.15m19M

Other information, which is given on the charts, is the height of the light in metres above Mean High Water Springs. In the case of the Roches Douvres it is 60m (metres). It is a big lighthouse. Clearly on Spring Tides, towards low water it is possible that the light will be 70 metres above sea level, due to the tidal range.

It is also possible to work out the distance that the light can be seen from. Again in relation to the Roches Douvres it has a visible range of 28 nautical miles (it is a bright light).

Corbiere

It is important to remember though that as sea kayakers are so low in the water, the light may be beyond the range that we can see. Using distance off when rising or dipping tables it is possible to work out how far we can see. Assuming that our eyes are 1 metre above sea level (it is likely to be less) then a light, which is 60 metres high, only comes into viewing range when it is 18.2 nautical miles away. If we were on top of a 2 metre swell though the light is theoretically visible from 19.7 nautical miles away. The moral being that if you are searching for a distant object always try and look when on top of wave, unfortunately this isn’t always the most stable position to be in when scanning the horizon.

It is important to get some practice in identifying lights so a good exercise is sit on a headland on a clear night with a chart and spend time ensuring identification becomes second nature, it is much easier than trying to acquire the skills whilst bobbing about in the dark of the North Atlantic.

La Croix

Navigation when completed accurately is not only an immensely satisfying experience but it will also help to ensure that you are safe when out on the water.

Kevin Mansell

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