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haven't brought a sextant on an ocean race in years, thinking that a handheld GPS or three, and a bag of extra alkalines, was all the backup any boat needed. But my skipper felt differently, citing seemingly far-fetched scenarios involving hostile satellite killers, ground-based hackers, electromagnetic pulse attacks, and even coronal mass ejections from the sun. But my sextant is a heavy old thing, and does not belong on a racing sailboat. I recalled Lee Helm's formula for the effect of extra weight on finish time, if the weight of the boat, the waterline length and the distance of the race are known. It's based on a revised "hull speed" formula for lightdisplacement hulls, from Dave Gerr's book about propellers: V = 1.24 * LWL^1.433 / Disp^0.322 Our sailing displacement was going to average about 13,000 pounds, waterline 31 feet, and the race distance 2,070 miles. After a little arithmetic, I determined that each pound of added weight would cost us 20 seconds at the finish line. Now, I love my old sextant, and my aging eyes love its 7x50 scope. But in the box it weighs 9 pounds. "It will cost us three minutes at the finish line," I emailed the skipper, with a cc: to Lee Helm so she could confirm my numbers. "Are you sure you want to carry all that extra weight?"
"I've had closer finishes than that," he replied. "Let's go plastic on the sextant." "Roger," I typed back. "Plastic it is." But which plastic sextant to buy? Jake van Heeckeren navigated backto-back Transpac wins in the '70s using nothing more than a plain old Davis Mark 3. And he said he threw away the box to save even more weight. But with my aging eyes, I'd need reading glasses and a magnifying glass to get anything out of that vernier scale, not to mention that there's no telescope. So it's a micrometer model for me. There are two choices: the Davis Mark 15 or the Mark 25. I ordered the top-of-the-line Mark 25, and brought it to the next race prep seminar to show it off. I was surprised to see Lee Helm sitting behind the registration desk at the entrance to the event. "When a last-minute crew spot opens up," she explained, "they, like, always ask the volunteers first!" "Good strategy," I said, "the odds are in your favor." "Show me your new toy," Lee said when she noticed the sextant box. I popped open the plastic box, but Lee was not at all positive about it. "Oh no!" she exclaimed disapprovingly. "You got, like, the one with the beam converger. Check out the Burch book, How To Use Plastic Sextants. He
makes a good case against this model, favoring the Mark 15 with the conventional split mirror. Burch says the full-field mirror 'makes the easy shots easier but the hard shots harder.' Also makes index error checks more difficult, and that's, like, a big deal with plastic because: thermal expansion." "Now you tell me," I mumbled. "I'm aware of the index error thing, so I thought the 25's gray plastic would be less affected by sunlight heating it up than the black plastic the 15 is made of." "All is not lost," the next person in line at the registration table assured me. "For about $20 you can buy the parts kit with new replacement mir rors. Swap out the full-field mirror for the half-silvered mirror, like in the 15, and you still have the lighter color plastic. Best of both worlds." "But that cheap plastic telescope with the plastic lenses," said the next person in the check-in line, which had now come to a standstill. "I have rather bad astigmatism, and even my good eye sees multiple images, the strongest extra image offset by about a third of a degree. So I have trouble getting within 10 miles with that Davis scope. The solution was to retrofit a better scope. After a few tries, the one I like is a 7-to-21 zoom, so at full zoom that 20-mile double image error is reduced by a factor of 21, and I can consistently get lines of position
How to Upgrade yoUr davis plastic sextant 1) Swap out the plastic telescope for one with better optics and much more magnification, especially if your vision is poor. This one zooms from 7 to 21 power. It has a ¼"-20 threaded socket for a tripod mount, so it's easy to attach after some simple surgery to the sextant with a hacksaw and file. Install with threaded rod that fits the tripod mount on the scope, a spacer or fender washer, and a wing nut for quick removal and attachment. See: www.amazon.com/ dp/B015MSDA60 2) Add a rubber eye cup to keep the telescope eyepiece the correct distance from your eye. You'll want to press the instrument against your face to keep it steady. See: www.amazon.com/dp/B07PNX5FCG 3) If you have a Mark 25, you'll probably want to replace the Mark 25's full-field "beam converger" mirror with the more basic split mirror for the Mark 15, for easier index error checks and other advantages. Davis sells a parts kit for the Mark 15, with replacement mirrors and assorted hardware. Also a fix for an older used instrument with tarnished mirrors. See: www.davisinstruments.com/collections/marine-1/products/index-horizon-mirror-springsscrewsnuts-for-mark-15-sextant 4) The box is flimsy and not even watertight. Throw it away and pick up a Sistema "Klip It" 5-liter food storage box (model 1850). It's a perfect fit for the Davis Mark 15 or 25 sextant. Available from many outlets.
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