The Importance of DR - SailNet Community
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The Importance of DR

Contemporary technology has its place and its allure, but proper navigation—whether you're heading offshore for long distances, or just sailing across the bay—requires redundant backup systems, beginning with dead reckoning.
A couple of years ago I thought making a 2:00 a.m. approach to an unmarked, unlit cut between a couple of the smaller islands flanking Tortola in the British Virgin Islands would have been uneventful, particularly given modern satellite positioning systems. But today, I'm beginning to realize just how wrong I was. Maybe a few anecdotes will help you understand my new outlook.

In the final stages of a delivery from New England to the Caribbean, I knew that we would be able to get a bearing from the lights on low-lying Anegada, the northernmost of the British Virgin Islands. And failing that, we expected our GPS, radar, depth sounder, and the high-powered spotlight to be able to get us through the darkness and safely past a couple of low-lying islands that were separated by only a mile and a half of Caribbean Sea.

To be on the safe side, we decided that Jim Barker (a qualified delivery skipper in his own right) would maintain a "conventional" DR plot using lights, depth, and radar fixes, all manually plotted on a paper chart of the islands. Meanwhile, I would use the digital charts on my laptop with our position displayed on the screen, thanks to our trusty GPS and the KiwiTech software I'd loaded on the computer. As we sailed on, the little yellow boat icon on my screen slowly got closer to the islands.

All good navigators know that proper navigation requires not only a fix, but a confirmation of that fix, and preferably an additional confirmation beyond that. Technology doesn't absolve us of these requirements. A mile and a half from the narrow passage between the islands, our trusty GPS decided to go on holiday. "No GPS Signal" was proudly displayed on the computer screen, with no time provided for the last GPS fix. The KiwiTech software froze the icon in place on the computer screen as our boat plunged headlong into the darkness.

"All good navigators know that proper navigation requires not only a fix, but a confirmation of that fix."

A quick check of the GPS unit confirmed the obvious: Its own screen displayed a bold "No Position." It was 2:00 a.m., there was no moon, and the sky was completely cloud-covered. Fortunately, a conservative approach to safety issues and decades of redundancy in the nav station paid off for us in the middle of that dark night. Jim was on the job, and even if we had been in the middle of the channel, we could have continued on his DR alone to sail safely through.

When a GPS unit shuts down, if you're not keeping a written log or penciling a DR plot with time indications, you won't know how far off your last-known position you may actually be. Was the boat in that spot on the computer screen five minutes ago or 20 minutes ago? Have you been averaging five knots or seven-and-a-half knots? The difference here might determine whether you tie the boat up in the morning or swim to the beach at night. That night, I silently blessed all of the people who had helped me to learn bits and pieces of navigation, particularly those who had refused to allow me to get away with only taking one fix without a confirmation.

Do you know how your charting software handles a lost signal? This can be critical information, and you'll likely need to know it when you least expect to.
Even though Jim's DR told us that were still a mile and a half from the slot between the islands—and we had plenty of water on three sides—we decided to take the mainsail down, spin the boat around onto a reciprocal course, and get to the bottom of the GPS problem before continuing. The middle of the night, we reasoned, was no time to let an unknown current take us off our DR course and onto the bricks. Within 15 minutes the GPS decided to return from its holiday, and our backup handheld unit confirmed our position. We again spun the boat around and motored safely through the channel.

Nine months later my merely cautious faith in the GPS would be confirmed again—this time it was back in New England on a different boat. Upon finishing the Vineyard Race aboard the 50-foot IMS speedster Idler, we crossed the line at the Stamford, CT, breakwater under spinnaker. We knew that we would need to bear away, out of the channel momentarily and toward some very shallow water, so that we could drop the chute. After that we'd immediately head up to get back into the channel before running out of water. I had been moving between the deck and the nav station, taking times, checking water depths, and monitoring the radio when I noticed on the laptop screen that we were coming dangerously close to shallow water. I called to the helmsman to make sure he was headed back into the channel. He was, and I confirmed that as well as our position with a quick glance on deck. We were in safe water, but back down below, things appeared quite differently on the computer.

The GPS—a very reliable, permanently mounted unit—had lost its signal at a critical moment. Our Nobeltec charting software kept running a DR plot based on our earlier speed and heading rather than "freezing" the icon. Before my eyes, the icon resolutely sailed on, crossing through a couple of feet of water, over the beach and into downtown Stamford—a feat I was glad that we weren't trying to duplicate in reality!

I experienced the dark side of the GPS again as we raced Bob McNeil's Zephyrus IV to a new record in the Middle Sea Race off Malta in the Mediterranean. We had been experiencing electrical problems aboard the boat that caused the sailing instruments to short out. The Northstar 951 was still functioning well, but with the instruments shut off, the instrument bus was no longer capable of carrying the signal to the computer. I hooked up my battery operated Garmin 48, taped the remote antenna to the hatch, and plugged the unit's NMEA 0183 cable into the back of the laptop. We were good to go with two completely independent units. That is, we were "good to go" until they both shut down simultaneously!

Navigators need to have a backup plan for their GPS units, and celestial navigation certainly could be one useful technique to have onboard.
As we passed the island of Stromboli and its active volcano north of Sicily, both units temporarily ceased operation. Within a half an hour both GPSs were back in operation after automatically re-acquiring the satellite signals. Perhaps it was electrical or magnetic interference from the volcano, I can't really say, but what I do know is this: navigators need to have a backup plan for their GPS units. Parallel rulers, dividers, hand-bearing compasses, paper charts and pencils can still go a long way toward getting you through an otherwise very tough situation.

Addtionally, it's important for navigators to be thoroughly familiar with how their charting software will handle a lost signal and how that signal will be indicated on the computer's screen. Will the icon freeze in place or will an automatic DR take over re-positioning the boat's icon? Will a big, bright red notice or a smaller, more discreet, gray note indicate the lost signal? Regardless of the tools we use, the navigator is still well-advised to confirm his position using several different methods. Those time-honored techniques apply, even with today's often-spectacular technology. Three separate GPS failures on three different boats, using different GPS units and different computer software in widely separated parts of the world has convinced me of that.

Bill Biewenga is offline  
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