May 5, 2011

Dealing with lightning

A READER IN FLORIDA wants to know how to protect his boat against a lightning strike. “Worried,” of Fort Lauderdale, says a boat in his marina was struck the other afternoon and all its electronics were fried.

“I have a Catalina 27 and I’m concerned that I will be caught out during a day sail. What can I do to prevent damage to my boat or, lord forbid, me?”

Well, the oft-repeated advice is to get close to some boat whose mast is taller than yours, but that won’t work at sea, of course. I personally dangle a length of anchor chain from the backstay a foot or two into the water, more as a sop to the gods than anything based on scientific precepts. It just makes me feel better.

I can tell you that the general principle of lightning protection appears to involve creating a simple pathway for lighting to traverse if it does strike you, thus guiding it harmlessly to “ground” — which, in this case, is the water you’re floating in.

A grounded vertical metal conductor 10 feet high for every 17 feet of boat length will attract and divert lightning flashes, thus providing a cone of protection angled downward at 120 degrees from the top.

A sailboat’s metal mast makes a good conductor, but a wooden mast will need at least #4 AWG stranded copper wire, or a copper strip at least 1/32 inch thick, projecting at least 6 inches above the mast.

This conductor, or the metal mast must be connected as directly as possible to a lightning ground connection — a submerged ground plate at least 1 square foot in area. The whole of the conducting pathway should be as straight as possible: no sharp bends, or else the surging current will be tempted to take shortcuts, and perhaps blast through the hull.

Standing rigging, winches, guardrails, and pulpits, in fact all large metallic objects that are not tied into a bonding system, should be joined to the ground plate with stranded wire of at least #8 AWG.

As for yourself, keep your hands away from anything made of metal during a thunderstorm. If you have a metal wheel, wear rubber gloves. If it’s safe to do so, douse all sail and send all hands down below until it blows over.

Put handheld radios, phones, GPS units and any other sensitive electronics in an enclosed metal box (a primitive Faraday cage) to avoid damage.

Finally, it has to be said that lightning is very tricky stuff to deal with. It doesn’t read the human rule books, and doesn’t always behave as we think it should. But on the principle that some preparation is better than nothing, and that it will at least earn you points in the black box, the advice given above should prove helpful.

Today’s Thought
I saw the lightning’s gleaming rod
Reach forth and write upon the sky
The awful autograph of God.
— Joaquin Miller, The Ship in the Desert.

Boaters’ Rules of Thumb, #195
Spreaders should be higher at their outer tips than they are where they touch the mast. This not only looks more pleasing, it is also structurally stronger. The spreader should bisect the angle formed by the wire shroud as it passes over the spreader tip.

“I can’t quite diagnose your complaint Mr. Brown, but I think it’s drink.”
“That’s okay, doc, I’ll just come back when you’re sober.”

(Drop by every Monday, Wednesday, Friday, for a new Mainly about Boats column.)

No comments: