Making two wires do the work of three
Issue 110 : Sept/Oct 2016
While I have never seen it in a physics textbook, I think there is an electrical law particular to good old boats. I call it Churchill’s Law and, in short, it states that the number of wires led to any sailboat masthead will always be one fewer than needed.
Cp = Cn – 1(where Cp = conductors present and Cn = conductors needed)
I first encountered this problem when outfitting a boat for offshore sailing. She had only an anchor light at the masthead and I wanted to add a tricolor navigation light that would be more visible at sea and draw less current than three separate deck-level lights. The only conductors in the mast, of course, were the two used to connect the anchor light. While adding a third conductor was an option, the mast was stepped and I had no plans to pull it in the foreseeable future. I’ve seen it suggested that the mast can be used as a ground, but this is a poor practice as it promotes corrosion. Fortunately, for just a few dollars and some simple electrical parts available locally, I was able to run the two lights in the combination fixture independently using the two existing wires.
Part of the trick is a double-pole, double-throw, center-off switch (available from RadioShack) wired so it can be used to selectively reverse the polarity of the wires. The other part is a pair of diodes (also available from RadioShack) connected to the masthead fixture to allow current to flow to the selected bulb while blocking it from the other.
The diodes are small enough to be mounted inside the common AquaSignal lamp fixture without soldering. If necessary, or for a different make of lamp, it’s possible to fabricate a small plastic mounting board and pot it in epoxy so it can be installed externally.
I mounted the switch by the breaker panel, which is the source of its power. Turning on either light is as easy as flipping the switch to select the one I want illuminated. (A side bonus of this arrangement is that I can’t operate the anchor light and the tricolor light simultaneously.)
Proof of the law
When the mast on my current boat, Nurdle, was last down, I had the foresight to replace the existing in-mast cable with a 3-conductor cable in anticipation of adding a tricolor. I later came across a too-good-to-pass-up deal on a combination anchor/tricolor that also included a strobe. I am back in the same boat. QED: Churchill’s Law holds true.
John Churchill grew up a boat-crazy kid in Indiana. He built a raft at age 6, sailed Snipes as a teenager, and worked his way toward salt water and bigger boats as an adult. He has sailed a Cape Dory 26 singlehanded to Bermuda and back, and a Bristol Channel Cutter transatlantic with his father. Now in Florida, John races and daysails Nurdle, a Bristol 35.5 (and former repo) that he’s rehabbing for extended post-retirement cruising.
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