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World Marine Aids to Navigation Day

So, 1 July is World Marine Aids to Navigation Day. Which is a bit of a mouthful. But if I asked you to list the navaids that are most important to you, we would all I think have GPS at the top of our lists. But for myself, a close second would be the humble lighthouse. Lighthouses are part of the coastal landscape. Impressive structures that have been part of my seaside holidays from childhood. And more recently as a presence on the margins of a chart. In fact, they’re so common - ashore and afloat - that we don’t always give lighthouses a second thought.

But on more than one occasion, the silent flash of a lighthouse over a dark sea has saved my bacon.

The thing is that it’s so tempting to let the little grey GPS box look after our navigation while we concentrate on other things around the boat. We all do it, particularly when we’re a little short-handed and, let’s be honest, the GPS does a great job. But at the end of the day, it’s just a box of electronic circuit boards, and we should never forget the old mariner’s adage that everything will eventually let you down, normally at the point when you need it the most.  

So would you be able to complete your passage safely if you lost GPS out at sea?  

It’s happened to me, and it can be a bit of a shock. You suddenly find yourself reverting to the time-honoured practices of classical navigation - which have served seafarers perfectly well over the centuries, but which may have become a little rusty in this age of electronic navigation. 

A few years ago, I was the navigator of a nuclear submarine. We had been out in the Atlantic on an operation, and the time had come to return to the naval base at Faslane on the west coast of Scotland. For British submarines, the passage from the North Atlantic, through the North Channel, round the coast of Northern Ireland, and into the River Clyde is a familiar stretch of water, and we are well used to making our way through it, both surfaced and dived. But on this particular evening the equipment gremlins decided to have a bit of fun at my expense.

It was just getting dark, and there was a growing westerly gale pushing into the North Channel from the Atlantic. The ebb stream was starting to set against us, flowing out from the Irish Sea and into the teeth of the gale. Needless to say, there was a new moon, and this was a spring tide. Over the shallow waters that lie between Northern Ireland to the south, and Islay to the north, the wind and the contrary tidal stream were throwing up a steep, irregular and uncomfortable sea. The sky was dark and overcast, and we were restricted to periscope depth so that, even though we were dived, the submarine was being thrown about quite violently at times, and accurate depth-keeping was difficult.

I was in the Control Room, keeping an eye on the navigation as we started into the more restricted waters northwest of Malin Head. We needed to keep in the deeper waters of the North Channel and avoid the shoals, so I thought that I would just help out a bit until we had everything under control. It all seemed to be going quite well until suddenly, out of the blue, a mechanic popped his Head round the curtain that was screening the chart table from the rest of the Control Room. ‘I’m sorry to tell you, sir,’ he said, ‘but the aerial feed to the GPS has flooded with sea water and there’s nothing I can do to bring it back up again until we get into harbour.’ And then he disappeared.

Sure enough, the GPS display had gone dark.

So there we were, 15 miles northwest of Malin Head, with a good fix on the chart 20 minutes ago, a pitch-black night with 8 hours of darkness remaining, in a freshening gale with spring tides, and some tricky navigation ahead as we tried to thread our way into the North Channel, round Rathlin Island, and past the Mull of Kintyre… And my principal navigation aid was duff.

As any navigator will know, even with a good fix on the chart, together with a DR and an EP, your ‘pool of errors’ grows steadily (and slightly alarmingly) with the passage of time, unless you can keep refining it with accurate position lines. The ‘pool of errors’ is the sum of many small inaccuracies in your position-keeping: the accuracy of your last fix; compass errors; inaccurate helming; log errors; variations in the tidal stream; the effect of the wind on the surface water; and so on. In my yacht, I would normally estimate the ‘pool of errors’ to be a circle, centred on the EP and expanding at a rate of 1.5-2 nautical miles every 3 hours. On the high seas, this is normally insignificant. In coastal waters, it matters more.

When you lose the ability to fix regularly, the first and most important thing to do is to go back to the last reliable fix and draw out an accurate DR and EP for the next 3, 6 and even 9 hours. Then you apply your expanding pool of errors, and check that it doesn’t hit any dangers or shoals over that period.

After that, you try to look calm and unflustered while you search for an accurate position line by any possible means. My first port of call is always the echo sounder. Sadly, however, the nature of the seabed in the North Channel wasn’t really conducive to a bottom contour fix, for which you need clear features with steep, well-defined contours. Beneath us, the seabed was rugged but predominantly flat, so the echo sounder was sitting smugly on the side of the chart table throwing out soundings that were almost totally unhelpful. Back to the drawing board.

I couldn’t use radar: submarines on operational patrol never use radar, because it tells the bad guys more quickly and more accurately than anything else who you are and what your position is. It would have been ironic (and not particularly helpful) for the Soviet Navy to have known the submarine’s position more accurately than I did. So radar was out.

Discretion is always the better part of valour, particularly when you are pulling a big and active nuclear reactor, not to mention a good few tons of high explosive, around the ocean, so I seriously considered suggesting to the Captain that we stay in open, deeper water overnight before surfacing at dawn to transit through the North Channel in daylight. But professionally I wouldn’t have been happy with this, and anyway the crew would have lynched me, and the Captain would have given me one of those looks. Besides, we weren’t yet in imminent danger of damaging the paintwork, so we pressed on to the east, making good a steady 5-6 knots.

I tried getting the submarine’s ancient LORAN set to work. This was a radio navaid that I had never yet persuaded to produce an accurate fix… and sure enough, after 15 minutes of tweaking, it came back with the unsurprising news that we were somewhere in the northeastern Atlantic Ocean.

So, we were left with visual fixing. The only problem being that there was nothing in sight through the periscope with its height of eye of about 60cm, attenuated even further by the heavy swell. But the submarine suddenly took a surging wave under the stern, and rode up about 2 metres from its normal periscope depth. The Officer of the Watch who was keeping a lookout on the periscope suddenly shouted out to ask whether I was expecting a lighthouse on the starboard bow. ‘Yes,’ I replied, trying not to sound as profoundly relieved as I was feeling, ‘Inishtrahull Lighthouse, just off Malin Head is about 15 miles away to the southeast, flashing 3 every 15 seconds.’

The characteristics checked out, so we took a bearing and plotted it on the chart. And then 10 minutes later, we got another bearing. And we had a rough running fix. Then we took another bearing about 20 minutes after that, and the fix started to refine in accuracy. 20 minutes later, it was better still, and we progressively built up an increasingly accurate running fix to keep us on track. A few hours later, we spotted a second lighthouse on Islay, and that in turn guided us into the chain of lights that line the coast of North Channel.

In the end, it was a walk in the park. But for a moment it had felt very lonely and very exposed. I suppose that the thing that I learnt from this episode is that, even in the days of GPS and computerised navigation systems, there is still a place for time-honoured navigation aids like lighthouses and buoys, and, to be honest, the things that saved my bacon that day were a really good visual lookout, and the ability to plot a running fix.  

Not to mention a tall, magnificent and very welcome lighthouse, perched high above the coast of Ireland


Paul Boissier, June 2022

Paul’s books can be viewed as follows:
Mastering Navigation at Sea
Understanding a Nautical Chart
Learn the Nautical Rules of the Road