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From the pilothouse

Jan 1, 2003

In the age of GPS, every sailor is a navigational king. But what if you find yourself without a GPS or just want to practice some traditional navigation (odd thoughts these days, but stay with me here) and you're approaching landfall at night? That’s when a powerful lighthouse ashore - marked on the chart and described in the Light List - can be a welcome sight.

By reading the light's characteristics, its height and range, you can resolve when the light should be visible. You determine the square root of the light's height and multiply it by 1.17. Then you determine the square root of your height of eye and multiply that by 1.17. By adding the two figures you come up with a range that can be transferred to the chart. At the point on the chart where the DR track and range line cross, the watch can begin to look for the light's flash on the horizon. This is called "bobbing the light," an age-old navigator's trick.

I've approached and passed by Monhegan Island, a cliff-ringed cap of rock 10 miles off the Maine coast, many times at night. Its light, which has a range of 21 miles, stands 178 feet above the sea, sitting at the very top of the island in a small clearing, and flashes once every 30 seconds - or at least it’s supposed to. On one particular passage, when we were headed up on an overnight crossing from Cape Ann, Mass., we had set up on the chart a scenario to bob the light at Monhegan. According to my calculations, which I worked out the night before, the light would begin to appear - if the weather remained clear - at a distance of 19.8 miles. I had struck an arc with my dividers from the light and projected our DR track ahead until the two lines met. I then turned into my rack, delighted at my cleverness.

A few hours later, of course, a light fog had rolled in and it had started to rain. The sodden overnight watch had long forgotten about my bobbing any light, and while we were about three miles away from the island, the light on Monhegan suddenly stopped flashing. Through the patchy fog we could barely see the loom of the island. We were proceeding under power, the southerly wind having departed with the arrival of fog. When we were within a mile of the island the fog grew denser, obscuring the island, and then the radar quit receiving a signal - the green glow of the heading flash was all that was visible. A few moments later the screen went completely black. (The best-laid plans!) We continued our way slowly, luckily picking up a buoy by sight, and eventually worked our way safely into the harbor and dropped anchor.

Such is life on boats; but, as a good friend told me early in my sailing career, keeping a back-up plan in your back pocket, so to speak, is often the fine line between success and failure, whether working over the charts, keeping a complete inventory of engine spares, or knowing how to fix or jury-rig every essential piece of equipment.


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