Admit it. You take a certain pleasure when you hear about another boater’s misfortune. You feel smarter and somehow superior. And that’s ok; we all do. But remember that not learning from the mistakes of others is just plain dumb. Here are some screw-ups from the files of the Coast Guard. The names have been changed to protect the innocent — just make sure you don’t join their ranks one day!
Blinded by Science
San Juan Islands, Washington
“I’d just installed a new electronic chart, totally up to date; the chart plotter said we were far off and in deep water.”
Problem: Cold, rainy and almost no visibility. Just another night passage for Mark in his 42-foot motoryacht through waters littered with islands swept by strong currents. It’s too nasty to stand watch outside, plus he’s alone and can’t leave the helm, so Mark puts his faith in the chart plotter — which in turn puts him on the rocks. Afterwards it was found that he was unknowingly dealing with a navigational error of more than 450 feet.
Prevention: Navigating with GPS is not always as accurate as it seems. Set one down so it is not moving and just watch the readings keep changing. According to the U.S. Department of Defense, which maintains the system, the GPS signals we get should be accurate to within 50 feet 95 percent of the time, while the other 5 percent can be out as much as 300 feet. In practice, however, the average tends to be around 30 feet. To improve on this, WAAS (wide area augmentation system) brings it to an official 23 feet for 95 percent of the time but seems to average closer to 10 feet.
All very good, but what about the chart the GPS is putting you on? Until the mid 1990s, in pre-satellite times, NOAA’s general requirement was for position accuracy on a typical coastal chart to be around 30 yards. While charts are constantly being updated using modern electronics to provide greater accuracy, it’s a slow process.
In Mark’s case the position of the tiny island he hit was from an old survey, accurate to only 160 feet. Plus, three of the four visible satellites (ones above the horizon that the receiver can use) were almost in a straight line, giving a poor fix. For optimum accuracy you want the satellites to be spread out all around you. To check, look at the unit’s satellite page to see where the satellites are and the EPE (estimated position error) display.
When navigating in a channel the Coast Guard likes to have an accuracy of 15 feet or better, which is not always possible. That’s why when things get tight it’s time to take your eyes off the screen and start keeping a good lookout.