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Old 02-06-2000
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John Rousmaniere is on a distinguished road
Dead but Not Deceased

In the back of every navigator's mind is the temptation to treat GPS like the fat of the land off which she or he can lazily feast, day after day, without fear of famine. Most of us hedge our greed with the worry that these gadgets can be pretty fragile, relying as they do on batteries and a mysterious network of signals from on high and far, far away. But there's a more fundamental problem with electronic navigation: a GPS readout is static, but the boat is dynamic. It tells you exactly where you are, but only right now while the boat is moving on a continuum from the past to the future. A good navigator will want to know where he or she has been, where he or she are headed, and everything else about the boat's track that cannot be learned from a little black box.

To get back in touch, turn off the tiny flickering screen, stop being led around by the device like a dog on a leash, and try a little dead reckoning, or DR. If those two letters send a shiver down your spine, you've probably taken a navigation class from an ancient mariner bleating endlessly about the days of wooden ships, iron men, and real seamanship, dammit. Of course it's fun to learn some traditional tricks of the trade, yet nostalgia is hardly the only reason to practice dead reckoning. DR navigation makes you a more alert sailor, more in tune with your boat than the navigator squinting all day at the little LCD numbers.

First, some terminology. While dead reckoning surely has nothing to do with death, there is some disagreement about what "dead" actually does mean. Some authorities believe it is an abbreviation of "deduced" in that positions found by DR are logically deduced from a set of facts. I've recently been persuaded that the deduction actually lies in the "reckoning," and that "dead" means "exact." A boat sailing directly before the wind is on "a dead run," an object exactly alongside is "dead abeam" - and a position logically deduced from information right before the navigator's eyes is a dead reckoning position.

The information for DR comes in three types, all based on what you see on board: the boat's compass course, the time as shown on a clock, and the boat's speed. If you know where you started, how fast you've gone for a given period of time, and the direction you've sailed, you should be able to plot your position. Lay out the track and the distance, draw a little half circle, and write in the time of day. Twelve, 24, 30, or 48 minutes later - or at any interval of 6 minutes - plot another DR position and before long there's a nice necklace on your chart between where you started out and where you want to go. Six minutes, or any of its multiples, are handy because it's 1/10 of an hour which simplifies extrapolation to whole hours and knots, or nautical miles per hour.

Because keeping up a DR plot requires such ongoing alertness, the navigator is forced to be honest with him or herself and with the vessel. A careful navigator comes to sense the forces that might cause errors - waves pushing the boat to the side or a gust of wind pushing her ahead - and quickly makes an adjustment.

Three Types of Plots

Heading south, the navigator makes a DR plot at 1934 (half circle), then an EP at 1938 (square) when she or he can take one bearing. The DR is kept until she or he can check it with a fix using three bearings at 2106 (circle), all the time carefully noting the boat's course C and speed S.

Dead reckoning is most handy when there's nothing at which to look, or, more accurately, nothing on which to take bearings. Whether at night, in a dense fog, or off soundings, there are times when no shore is visible to curb the road and no aids to navigation to mark the turns. When lights and land do come into view, the larger discipline of coastal piloting becomes essential, always to be maintained as assiduously as a dead reckoning plot. Add one piece of information (a compass bearing to a buoy, say, or a depth sounding) and you have an EP - an estimated position, marked on the chart with a little square. Provide two bearings, and where they cross you have an even more reliable position called a fix - a circle. If the DR is a reckoning, an EP is a good estimate and a fix is a probability.

These disciplines can be crucial when sailing indirect courses. If the wind is blowing from your destination, you'll have to beat there on a series of legs at 90-degree angles. If the wind's astern and light, or so strong that the waves are steep, you'll want to tack down wind, jibing from one broad reach to another to keep the speed up or stop a wild rolling motion. Keeping track of your position on indirect courses demands a keen sensitivity to the boat and her environment that you'll learn by keeping up the DR.

Tacking Tactics

On an indirect course to windward (from A to B) with the wind just off to the side (wind 1), sail the longest leg first (course 1, on port tack). If the wind backs (shifts counterclockwise, wind 2), the boat will be lifted up to B (course 2). If the wind veers (shifts clockwise, wind 3), the boat will be headed and can tack toward B (course 3). But if there is poor visibility, take the course that takes you closest to lighted aids to navigation. Here that would be on the starboard tack toward False Ducks Lt. on Sweetman Island.

Here are three rules of thumb:

  • When the destination is dead (directly) upwind or downwind, sail an equal time or distance on each tack so you stay roughly the same distance from the rhumb line (the direct course from start to finish).

  • When the wind is a little off to one side and the course is not a dead beat or run, you'll have to sail on one tack longer than on the other. Sail the longest tack first. That's the one that aims you closest to your destination and if the wind shifts, you'll have more to gain and less to lose.

  • In poor visibility - at night, in fog or in a rainsquall - sail the course that takes you closest to aids to navigation (while keeping the boat in safe water, of course).

A higher priority in misty conditions is not to get lost. But the highest priority of all, which can be reinforced by dead reckoning navigation, is to stay in touch with the boat.

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