Dead reckoning fondly recalled

To the editor: Once upon a time dead reckoning – the plotting of a yacht’s position by distance run and course steered – was standard procedure. If you were out of sight of land, the weather was overcast and it was impossible to get sights, it was your only means of knowing where you were – or where you thought you were. If you were close to an unlit shore at night you plotted your DR carefully. If there were off-lying dangers you gave them an extra wide berth just in case the DR position was a few miles in error due to some unknown current, some minor miscalculation in your reckoning or bad steering.

If you were approaching land at night after an ocean passage and had been unable to get sights you stopped at least 20 miles short and waited for dawn. Incredible as it may seem today, your DR plot, adjusted for currents, tidal streams and leeway, was often all you had to go on until you could get sights, take bearings or were in soundings. Most people logged the course and speed every hour or two and plotted the DR position at the change of watch. Some navigators became very skilled at judging the distance run (if there was no Walker log) and the average course steered. There were times on ocean passages when in thick weather an overcast sky prevented the taking of sights and when the sun, the moon, the stars, nor the planets were visible for several days on end. The DR position, which might be 50 miles in error, was still all you had to go on.

It didn’t really matter as long as you were not close to land. Captain William Bligh was a man with a short fuse, but he was also a peerless navigator. On his 3,500-mile passage from Tonga to Timor, crammed in one of Her Majesty’s Armed Vessels (HMAV), Bounty, with 24 supposedly loyal men, he had a sextant and the necessary tables, but no chronometer and no chart. Yet while en route he charted the position of several Fijian islands, and his passage through the Australian Great Barrier Reef is still known today as Bligh Passage. With his sextant he was able to get his latitude at local noon, but without a chronometer he was stumped for his longitude. He had to rely on DR. He recorded his course and his boat speed hourly (plus weather, wind and currents) in a notebook. The Fijian islands are where he plotted them; Bligh’s Passage is a nautical mile from where he put it. The American OSTAR yachtsman Tom Follett used to keep the reckoning in his head. When it came to plotting the DR position for the noon sight, he would say (when he had crew), “Make it 105 miles for the day’s run and put the course at (so and so).” And he was never far off. He knew his boat speed even when he was lying on his bunk. Of course, it was the average speed during the 24 hours that counted and the average course steered. Ocean-going yachts sometimes hailed a passing steamer, and asked for a position and to be reported to Lloyds.

Sometimes there was quite a wait before the officer on the bridge reappeared from the wheelhouse and bellowed the latitude and longitude through a megaphone, having hastily brought his plot up to date. Today, with GPS, the promise of more satellites to come and chartplotters navigation is almost childishly simple. No doubt one will soon be able to get one’s position from a mobile phone or send digital photos with a chartplotter. But is it prudent to give up chart work? Keeping the reckoning, getting your DR position or your estimated position requires skill and that skill depends on practice. What happens if your electronics go haywire and you forgot to put the batteries in your handheld, back-up GPS? If you haven’t kept up the practice of chart work, your DR position will be very wobbly. Anyway, it’s a skill that is fun to practice. All you need, apart from your chart, are parallel rulers, dividers, a soft eraser and a No. 2 pencil.

Give yourself a test: turn off your GPS (out of sight of land), keep the reckoning for a few hours or over night, and find out how good you are. At first, of course, it’s going to seem scary, but with practice you will have the satisfaction of getting the hang of it, which will bring more confidence. Try judging your boat speed by the look of the bow wave and the water passing the hull. Switch off the GPS and, over the period of an hour or two, test yourself. You should be able to guess your speed to within half a knot; you should be able to judge that at the end of four hours you have done, for example, somewhere between 16 and 20 miles; so measure it off on the chart as 18 miles. When you turn your GPS back on you’ll probably find – after a bit of practice – that you’re not far off. Same with the course: take the average over the four hours. Allow for currents and for leeway in strong winds, but don’t fuss with detail; don’t try to work out your DR position every half hour; the errors will multiply; you’ll be miles off.

Keep your courses and speed in a logbook. If things go wrong you may have to go back over the figures and hunt for your error. If you are trying to find a harbor entrance on a featureless coast, fall back on the “aim off” technique. Deliberately steer several degrees off your course so that when you make your landfall you will know which way to turn toward your destination. The pattern of charted depth contours can be compared with the readings of the echo sounder. If it does not even begin to fit, something is wrong with the DR; if it does fit, it shouldn’t be taken as gospel: you could be somewhere else with a similar pattern. The skippers of halibut fishing schooners out of Gloucester, Mass., used to sound their way with lead line across the Newfoundland banks in depths of 100 fathoms (600 feet) and find the spot where they had a good catch. Unfortunately, you cannot arm an echo sounder with tallow like you can the bottom of a sounding lead. But you can use it on a dark night to follow a contour, say the 20m line, along the coast and into a bay, which could be the very one you were looking for.

By Ocean Navigator