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Dead reckoning: old-school navigation

I could pretend I’m not old enough to remember Loran-A or tuning in the beeps of those eerie radio direction finder bearings. I might even deny the effective old navigator’s trick of shutting down the engine to listen for buoys in the fog.

When was the last time you piloted without a plotter?

Today the market overflows with sophisticated navigational equipment, and most boaters swear by their trusted suite. (I do.) You can have a redundant GPS/chart plotter, radar and a backup battery-powered GPS — maybe even a chart plotter app on your smartphone. Call me old-fashioned, but occasionally I want to feel self-reliant without this gadgetry, not because of it. That’s when I resort to a few applied math exercises free from the black boxes.

What’s needed? Up-to-date paper charts, a set of distance-measuring dividers and a plotting instrument, along with a reliable depth sounder and a properly adjusted compass. You’ll also need a sense of your boat’s average speed, which you can develop by checking rpm and wake characteristics against your chart plotter, making a game of the question, “How fast do you think we’re going now?” The most valuable tools are the navigator’s pursuit of accuracy and curiosity and the willingness to double-check.

In simple terms, piloting without a plotter — dead reckoning — means confirming where you think you are, then leapfrogging to the next point and repeating. To start, mark your departure time and location at a fixed point — for example, a navigation aid or inlet. Know the expected set and drift of the current.

From there, estimate the course to reach your next waypoint. A true course is uncorrected for the vagaries of the Earth’s magnetic variation, straight off the chart’s true compass rose. A magnetic course is corrected for magnetic variation and also is shown on most roses. A compass course is corrected for variation and the boat’s individual compass error, known as deviation. This is the course by which the helmsman steers.

On my boat, I use magnetic courses and calibrate the GPS and autopilot to coincide. I check my paper charts to determine whether the labeled courses are true or magnetic. (While I’m at it, I also double-check the chart soundings. Feet, fathoms or meters?)

A basic formula determines the length of time it takes to travel from one known point to another. Distance equals speed multiplied by time. Distance is measured between charted points, then applied to the latitude scale. One minute of latitude equals 1 nautical mile. Speed is nautical miles per hour traveled.

Today’s helms represent technology at its highest, but good seamanship includes the ability to navigate using traditional methods, such as dead reckoning.

So at a speed of 8 knots, you will travel 8 nautical miles in one hour. And if 8 nautical miles are covered in 60 minutes, then 4 nautical miles are covered in 30 minutes, 2 nautical miles are covered in 15 minutes, and so forth. (The sidebar of tenths can be used to amuse yourself and your shipmates when you say, We should pass that buoy in 18 minutes. And you do. Who needs Sudoku when you can play a math game to entertain yourself on board?)

To confirm your “reckoning,” or performance against what you expected, you can use visual and audible aids, significant bottom contour changes and other means. Then set up the next leg and leapfrog from one confirmed point to the next.

Technology is in constant motion, but we are no different from the seamen who preceded us. We still watch for seagulls standing on the water or waves breaking where they shouldn’t. We still value accuracy, curiosity and the willingness to double-check all means to ensure we arrive safely and in time for dinner.

Pat Mundus is a retired merchant ship deck officer who lives in Greenport, New York. She contributes to boating magazines while cruising her F. Spaulding Dunbar ketch, Surprise, in the Bahamas and the western Caribbean. During the summer season she arranges crewed classic yacht charters through East End Charters.

This article originally appeared in the June 2017 issue.



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