Not a sliver of moon nor a single star could pierce the thick clouds. We were sailing, levitating, in total darkness. Keeping Brick House, our Valiant 40, just half a mile off the unlit rocky shore was the only way to stay out of the swift counter-current as we fought our way south along Mexicos Caribbean coast. It was important to sail all night and make good time so that we would arrive at the entrance through the barrier reef in Belize before the sun got too high the following morning.
This close to a keel-crunching mess, there was a chance of catching a helpful counter-current. Our speed on the GPS hovered at 5 knots, the same as on the knotmeter. When our speed over ground dropped below our speed through the water, I cautiously edged Brick House even closer to the rocks. It was 2200, my watch. My wife, Rebecca, was sleeping. I knew she would be very uncomfortable if she knew what I was doing and would likely favor the slower offshore passage. I decided to work through her watch and not wake her till the excitement was over.
I concentrated on navigating while the steady hand of our Raymarine autopilot kept Brick House on a straight course. This was no time for the Monitor windvane to be following wind shifts.
Leaning over Brick Houses dimly lit nav station reminded me of piloting a small aircraft in the middle of the night. The drone of wind in the rigging and gentle buffeting of the boat were easily ignored as I focused on the flipping numbers, moving lines, and muted colors of critical instruments. Our sonar fishfinder was my altimeter, tracing the rugged contours of the sea floor 1,800 feet below our hull. The sea off this coast is thousands of feet deep but then it quickly ascends, like a cliff face, to meet the shallow reefs and rocky shore.
Without a single light on shore for reference, the situation required total precision and called into play every bit of modern navigational equipment on board. Trust your instruments, not your instincts, is the aviators golden rule. But on a sailboat I knew better than to trust just one source of information.
At 2300, I moved to the cockpit with our old monocular ITT nightvision scope in hand. The scope allowed me to scare myself with a gray-scale view of just how close we really were to scrub brush and gently breaking waves.
The visual scans were just one check against the chartplotter. The more we use the plotter the less we trust it for blind navigation, as many electronic charts are based on surveys made well before the invention of GPS. We have sailed in areas where our plotter was out by a mile or more. We are most comfortable when we can verify an electronic plot visually and with radar or fishfinder.
When preparing for this circumnavigation, I overcame my prejudice for stand-alone electronics and installed an integrated Raymarine C80 radar/chartplotter. It is a powerful tool, and on this night I really appreciated its radar overlay. The plotter has an offset feature so that a navigator can shift the chart to negate inaccuracies. As we sailed, I was able to move the chart 150 feet west to match the radar echoes of the shoreline. Now I knew the plotter was as accurate as our local conditions required. The radar could also detect objects or local weather forward that I could not see with the night scope.
Up ahead, the plotter and our paper chart showed a shoal called Punta Herredia extending 6 miles along the coast and 2 miles out to sea. The shoals outer depth contour was 66 feet, and our fishfinder was still tracking the craggy bottom at a depth of 1,800 feet as we charged toward it.
In addition to giving numeric soundings, a sonar fishfinder also presents a color-graphic image of the bottom contour. The colors relate to bottom hardness; brighter colors like yellow indicate a hard bottom, darker colors like blue indicate soft layers. Our Lowrance LMS-525C DF sonar yields soundings and color graphics down to 2,500 feet. Beyond that it displays only a gray-tone image of the bottom, while soundings can be read from the screen ruler down to 4,000 feet. The screen ruler also serves as a check against the numeric read-out. The Lowrance also gives boatspeed and water temperature. Leading up to the shoal, the charted soundings for the immediate area showed a minimum of 328 feet to over 3,000 feet. Our regular depthsounder has a maximum range of only 150 feet, which made it useless along this stretch of shore.
The suspense of approaching the shoal intensified my concentration. I decided to turn slightly eastward and cut the outer corner of the shoal rather than swing wide around it. As Brick House angled away from shore, our boatspeed and speed over the ground both stayed at 5 knots, showing we were still out of the current. As another check against the plotter I had already turned on our laptop computer, which runs completely different cartography. Connected to its own GPS, the laptop screen also tracked Brick Houses slow turn to the south east. The laptops chart showed a slightly inaccurate position for want of an offset. Apparently both electronic charts were derived from the same paper chart. Both plotter screens now showed Brick House edging over the charts dotted line into 66-foot soundings. Perched on the edge of the nav seat, I was ready to jump into the cockpit and grab the wheel if the fishfinder showed any indication of a dramatic keel-crunching rise.
The radar was echoing, the plotters were plotting, but surprisingly, the colors on the fishfinder stayed remarkably steady with soundings hovering around 1,700 feet. Brick House had now moved into the northeast corner of the shoal on the plotter screens, but the yellow contour line was not rising. Our speed over the ground dropped as we approached the outer edge of the apparently erroneous 66-foot contour line. Evidently the shoal was not as shallow as charted. Steering with the autopilot, I slowly turned Brick House back on to the charted shoal and out of the current. Gradually, I became braver and steered toward the middle of the shoal.