I figured we had six inches of water under the keel at best. It was a pristine keel attached to a brand-new charter boat, and I was determined not to be the first charterer to ding it, though we were in a place where that was easier said than done, a narrow channel leading to a Bahamas anchorage. As we inched along under engine power, I steered to follow the white sand path easily visible through the pellucid water, while the First Mate looked ahead for the stick the cruising guide said should be lined up with a building on shore to complete a safe entry into the harbor. It was eyeball navigation in its purest form, yet for some reason-habit, I suppose-I let my eyeballs wander to the wide-screen chartplotter mounted above the steering wheel, where I was appalled to see that we were hard aground 75 feet to the right of the channel.

Anyway, that's what the GPS said. I knew better, but it was all I could do to stop myself from making a hard turn to the left. I'll bet other sailors can empathize. Such is our faith in the satellite positioning system that makes even casual, recreational navigators feel like masters of the universe.

Later I learned from some local sailors that the GPS position was probably right-it was the old Bahamas charts that were wrong. Whew! Faith in technology restored.
It's comforting that this faith is nourished every day by evidence of the near perfection of GPS.

The U.S. military sends a missile through the windshield of a car speeding across the landscape of Afghanistan. My iPhone mapping app confirms that I'm sitting in my dentist's office. Surveyors mark precise points on the face of the earth in minutes with a small electronic device. Airplanes land in zero visibility. Police check computer screens to see if criminals under house arrest are in their houses. A driver is instructed by an English-accented feminine voice to turn left in two-tenths of a mile. All thanks to GPS.

But then there is this, also thanks to GPS: An 80-foot racing sailboat known as Shockwave5 (rather than its clumsy official name of PricewaterhouseCoopers) sails onto a rocky ledge at the edge of the well-charted Flinders Islet, which stands high enough to be seen from miles away, in the Tasman Sea off the southeast coast of Australia.            

The forces unleashed by the instant stopping of a 25-ton boat traveling at 15 knots are so extreme that skipper Andrew Short and the steering wheel he clings to are carried over the side. A much respected Australian racing sailor, Sally Gordon, though wearing a safety harness and tethered to the boat, is ejected from the cockpit and through the lifelines. The skipper's son is washed overboard. Crewmembers who were near the stern find themselves clinging to the headstay. Pounded by seas breaking on the rocks, the boat disintegrates in a matter of minutes.

The skipper and Gordon perish. The skipper's son survives, as do the 15 other members of the crew who, as the boat is driven farther onto the islet, are able to clamber onto the rocks.

An analysis of the shipwreck, which occurred in 2009, was issued by the Cruising Club of Australia last fall (detailed in the November/December issue of SAILING). It says a lot about GPS:

The boat was being guided only by GPS; no alternative method, such as dead reckoning or, apparently, even monitoring a depthsounder, was in use, nor had anyone in the crew been designated as a lookout to employ the most basic form of navigation-watching where you're going.

The boat had only basic, garden variety chartplotters with plain GPS and no provision to receive differential corrections.

GPS signal reception can be iffy in Australian waters and on the night of the grounding was particularly degraded with only three or four satellites usable. An error of more than 100 meters was possible, the report said.

The last point tells us how absurdly high our expectations for GPS have become. Only in the world of GPS would a 100-meter navigational error be worth mentioning. 
Even with low signal quality, GPS was the best navigation aid available to Shockwave5. Nothing else could have come close. Celestial navigators are thrilled to calculate positions within a half a mile of a vessel's actual position.

An error of more than 100 meters is nonetheless of interest because with the satellite coverage available in most places in the world, standard GPS can do much better. With differential, the margin of error is three or four meters. With the radio signal assist of WAAS (wide area augmentation system, developed to guide aircraft in instrument-flight-rule landings), accuracy is within a meter.

Shockwave's shocking end was not the result of a failure of technology. The science is sound-it's human nature that isn't. It wasn't that her sailors were not well prepared. They were an extremely experienced group, with thousands of miles of hard sailing in stormy latitudes to their credit. Of course, they knew how to navigate without GPS. But they succumbed to the siren's call of technology that infects us with hubris and invites us to sail at the edge of disaster.

Navigators without GPS would never knowingly allow themselves near a hazard like Flinders Islet. Knowing their position finding was fallible, they would have given the rocks a berth of many miles and posted lookouts in case their navigation was faulty.

GPS doesn't make us masters of the watery universe. It just makes us think we are.