Art Versus Science - 2/5/04
By Little Gidding - Published February 05, 2004 - Viewed 749 times
Art Versus Science
February 5, 2004
David practicing the art of navigation in Elizabeth Harbour -- taking a bearing on the monument
Navigation is the process of directing the movements of a craft from one point to another. To do this safely is an art. In perhaps 6,000 years .... man has transformed this art almost into a science, and navigation today is so nearly a science that the inclination is to forget that it was ever anything else.
- Nathaniel Bowditch, American Practical Navigator
GPS has made it remarkably easy for boaters to determine where they are and where they should be heading. Probably more than any other recent invention, it's given many people the confidence to head out into the great unknown and join the cruising ranks. In some cases, this is a false sense of confidence. As Bowditch (quoted above) and just about every other authority on navigation would implore, the prudent mariner seeks as much information and uses as many methods as possible to navigate safely. GPS has greatly advanced the science of navigation, but it's only one tool at the cruiser's disposal. We're currently cruising in the Bahamas, where the need for multiple navigational tools and techniques is paramount. There are few better places to practice the art of navigation.
North American boaters who are accustomed to reliable navigation aids -- lights that flash when they should and buoys that are located where the chart indicates -- may be dismayed to discover that such aids are few and far between in countries like the Bahamas. With a handful of notable exceptions, like the Elbow Cay light mentioned in our January 22nd entry, the lights, buoys, and range markers you might expect to encounter elsewhere are either not provided or not reliable in the Bahamas.
Our initial experience navigating with GPS was during our first trip to the Bahamas in 1995. We had bought the unit a couple of months before in Annapolis at the US Sailboat show and didn't have the occasion to turn it on until we left the security of the Atlantic Intracoastal Waterway and struck out across the Gulf Stream. We made our landfall at Gun Cay and, to cross the Grand Bahamas Banks, entered the co-ordinates of the Northwest Channel beacon, some 60 miles to the east. It was 10:30 PM and pitch black out when we approached our waypoint; there wasn't a flicker of light anywhere. Eileen said nervously, "I think we should slow down."
"Nothing to worry about," David responded, "the GPS says we're right on course." He turned on our high powered spotlight. A tangle of twisted metal jumped out at us in the glare off the bow -- dead ahead, one boat length away, and closing at a speed of six knots. Eileen cranked the wheel hard over and killed the throttle. Later, we read the footnote in our cruising guide that explained that the beacon had been struck by a large boat two years before and may or may not be replaced in the foreseeable future. We had almost seconded the act. That was our first lesson in the accuracy of GPS and the inaccuracy of Bahamian navigational aids.
Although it's unquestionably accurate, our GPS doesn't see things like the wrecked Northwest Channel beacon or, for that matter, an unlit boat (of which there are many in the Bahamas) crossing our bow at night. It also assumes the chart we're using is accurate. Many of the charts in the Bahamas are based on old surveys and have not been completely updated; the charts for the remote Jumentos island chain, for example, are replete with cheery caveats like "unsurveyed rocks" and "scattered coral heads". Even the features that were accurately positioned when the charts were printed may no longer be where we think they are; in this storm prone area, sand shoals are constantly shifting. Add to this pilot error: our GPS is only as accurate as the data we enter into it. David has his dyslexic moments and occasionally transposes the numerals he's punching in, sending us on a course to Greenland. Finally, there's always the possibility the damn thing will turn itself off. Ours has a disturbing tendency to do this just when we need it the most (see our June 20, 2002 entry, "Where Are We?").
Close-up of the monument on Stocking Island, providing an alternative to GPS
Safely navigating in the Bahamas means using your eyes and, in shallow, crunchy areas, traveling only during the day under good light conditions. We followed this practice when we arrived last Friday in George Town on Great Exuma Island. George Town is located in Elizabeth Harbour, accessed from the north through Conch Cay Cut. It's a tricky entrance because there are two parallel reefs between which you must maneuver. This entails turning sharply past the outer reef, running alongside it, and then turning sharply again to clear the end of the inner reef. Our original Bahamas cruising guide devoted a full page and a half to describing the intricate procedure, punctuating the discussion with lots of exclamation marks and italicized warnings. The subtext seemed to be "attempt this entrance and you're going to die". We've now been through Conch Cay Cut many times without death or injury, but we were still nervous as we approached it a few days ago.
Our new chart kit for the area has a series of six GPS waypoints to lead the intrepid sailor into Elizabeth Harbour. There's also a floating barrel marking the end of the inner reef that wasn't there when we first visited George Town. Despite these improvements, David got out our hand bearing compass as we approached the cut. "I don't trust that stuff," he muttered. It was sunny and just before noon; the light conditions couldn't have been better. Eileen steered the boat according to the route displayed on our GPS. David took bearings to confirm our course and position. The initial range was on some cays in the middle of the harbour and a distinctive hill behind them. Then we turned to follow a bearing on the concrete monument located at the highest point on Stocking Island. We could clearly see the end of the brownish inner reef; the barrel was correctly positioned. We altered course to line up the pink houses and palm trees on Simon's Point, as spelled out in the cruising guide. "If those guys ever paint their houses another color or chop down their trees, there's going to be a real pile-up on this reef," David predicted.
The view from the monument looking towards Conch Cay Cut at the upper right; the whitish patch at centre right is a shoal that's invisible at water level when the light is poor
Half an hour later we were dropping the anchor in Kidd's Cove off George Town. Our GPS, compass, and our eyes had safely brought us in. Later that day, when David checked in with the harbour master Elvis Ferguson to report our arrival, he asked about the barrel marking the reef at Conch Cay Cut. Apparently it had been installed by cruisers with the permission of the Port Authority. Amiable Mr. Ferguson said, "I have a couple of boys who help me maintain it." Those "boys" are cruisers who regularly visit George Town, volunteering their time and materials to make the port a safer place in which to navigate. We appreciate their work. But we also hope the house owners on Simon's Point have a good supply of pink paint.
David & Eileen
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