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Dead Reckoning

2010 September 1

Don't rely solely on gadgets to know where you are going

I was down below checking our position on the ancient Loran C that came with our first boat, a 1976 Bristol 24. It was in the mid-1990s, and as a monetarily challenged writer, I'd invested nothing in additional electronics except for an Autohelm ST 2000 autopilot. We were off the coast of Maine between Casco Bay and Boothbay Harbor.

"Where did all this fog come from?" my wife, Liz, asked from the cockpit.

"It lives here," I said, concentrating on the Loran and the chart.

"Well, it's here big time," she said.

I stopped what I was doing. "It is?" I asked.

"Yeah, it's home," she said.

When I emerged from below a few seconds later, I was shocked to see nothing but gray mist. With only a highly suspect speedo, and no depthsounder, GPS or radar, and with big tides and plenty of rocks to hit, I started to sweat. I told Liz to adjust our heading to better compensate for the flood tide, adding about five degrees to the south just to be sure we'd clear the Cuckolds, a nasty ledge off Southport Island.

"Better to overshoot than find ha'd watah," I said, mimicking the accent of Down East Mainers.

I went below again, hoping that the Loran was working properly, but I didn't put much stock in it. It was almost always a half-mile off or more, and it sometimes wouldn't even work. I was pretty much on my own-in the fog.

Fortunately, I'd been keeping a DR plot, or dead reckoning, as well as an EP, or estimated position. I had a series of running fixes from sea buoy to sea buoy. I'd also gotten a couple fixes, which is by definition a crossing of two, three or more bearings on a chart. The bottom line is I knew where I was, even if I couldn't see where I was.

Nowadays, some sailors rely too much on GPS and electronic charts. In fact, I know a number of sailors who no longer carry paper charts. "Paper charts are too expensive," they say, choosing instead to rely on the "toys" to get them safely from point A to point B in a boat that costs as much as a nice house. GPS is fallible. So are electronic charts. A prudent mariner won't rely on any single source of data while on passage. Redundancy is the key, making DR handy to know.

The basics of DR are as follows. First, a DR differs from an EP because a DR is based on a plot from a known position-a harbor entrance, for example. It is then carried forward along a rhumbline from that position until the next fix. To do a DR, you need to know three things: the speed of the boat through the water, the time the boat has been under way since the last fix to determine the distance traveled, and the course you are steering.

That's an over-simplification, of course, because on a sailboat you're never going to maintain an exact speed. You're also going to be subject to (1) leeway, the push of wind against a boat that forces it to leeward; (2) set and drift, which translated into English means the set is the direction of a current (water moving in a given direction), and drift is the speed of the current (tidal or river); and (3) steering error. A DR doesn't take leeway and set and drift into account, but an EP does. That's why prudent navigators do both a DR and an EP. These folks are, alas, going the way of the pterodactyls.

Let's briefly talk about courses. You've got true and magnetic courses. True courses are the convention when doing chart work (the outer compass rose on a chart, as opposed to the inner one, which is magnetic). The navigator uses parallel rules or a protractor to plot a course (rhumbline), and then converts the course into magnetic, taking into account something called variation-compass errors due to changes in the earth's magnetic field. Variation differs from place to place, and it changes from year to year. Variation is marked on charts inside the compass rose, and you have to account for it when converting true courses to magnetic courses. Frankly, most sailors I know just use the magnetic compass rose and forget about the conversion.

However, sticking with the true course and variation, if you plot a true course of 90 degrees, and you have a variation of 10 degrees west, your magnetic course to steer will be 100 degrees (add west variation; subtract east variation). Also on the chart is the annual increase of variation for given locations, and that should be accounted for as well.

You also have to, or should, take deviation into account. Deviation simply means that a particular boat will have deviations in the compass because of metals in the boat that throw off the compass. A deviation table is necessary, and to get that you have to "swing the compass" to see how far off your compass is on any given heading.

To measure distances, use the latitude scale on the chart in the area where you're sailing. A degree of latitude equals 60 nautical miles. Each nautical mile is equal to one minute of latitude, and each nautical mile, or minute of latitude, is equal to 60 seconds. As to time, that's usually expressed on a 24-hour clock (military time).

To calculate distance, multiply the speed of the boat by the time traveled in minutes, and divide it by 60. Here's an easy example. If your boat travels at 6 knots in one hour (60 minutes), you get a figure of 360. Divide that by 60, and you get six nautical miles. Or put another way, you get this:
Distance = S x T

To calculate your speed, multiply 60 by the distance traveled, and divide it by the time. For example, if your boat has traveled six nautical miles in one hour, you will have been making good a speed of 6 knots.
Speed = 60 x D

To calculate the time it will take you to close with your next marks or for an entire passage, multiply 60 by the distance, and divide it by your speed. For example, if your passage is six nautical miles and your speed is 6 knots, you'll require an hour to make the passage.
Time = 60 x D

The temptation is great to rely solely on GPS and electronic charts, but some basic knowledge of DR will stand you in good stead if your electronics go on the fritz. Besides, a little brainwork with a paper chart is a good thing, an enrichment of the sailing experience through the practice of an ancient method of navigation.