lightning protection

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lightning protection

Postby chaertl on Sat Aug 28, 2010 5:11 am

Has anyone considered any type of lightning protection for the Pocketship and if so how did you do it? I'm hoping to be camping off one someday and thunderstorms in the middle of nowhere don't mix well.

Chris
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Re: lightning protection

Postby John C. Harris on Sat Aug 28, 2010 10:33 pm

Everything I've ever read in the professional literature is that lightning protection on sailboats is like the anti-elephant powder I've been sprinkling in my yard all these years. Doesn't do a thing. Might even attract lightning instead.

In 25 years of living around sailboats, I've only personally seen three struck by lightning. (No one was aboard in those cases.) I HAVE seen lightning touching down all around marinas full of sailboats with their inviting aluminum masts, yet inexplicably striking water or ground instead of masts. I'm sure I don't know much about the physics of lightning and sailboat masts, but I do know the odds: very very very low of getting nailed. At least you can easily rotate the mast horizontal if you're at anchor, which would improve your chances more than adding a lightning rod, I think.
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Re: lightning protection

Postby chaertl on Sun Aug 29, 2010 5:19 am

Thanks John,

That's kind of what I expected but having an aviation background I remember all the trouble that composite construction caused. Of course airliners are struck several times a year so it would be a far bigger problem there. Hope the tour is going well.

Chris
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Re: lightning protection

Postby DanaDCole on Tue Dec 09, 2014 12:40 am

I have been somewhat concerned about lightning strikes because I sail in fresh water and statistically boats that are struck in fresh water are about six times more likely to suffer damage than those struck in salt water. That's because salt water is a much better conductor and so it is easier to dissipate the charge away from a properly-grounded boat. Boats in fresh water are not more likely to get hit, just more likely to suffer damage if they are hit. (Electronics are "fried" about the same in salt or fresh-water boats.)

That being said, it would be almost impossible to properly protect a boat as small as the PocketShip because you need a copper grounding plate at least 24 feet long in addition to a lightning rod at the top of the mast and a 4 AWG wire from the rod to the grounding plate. I suppose the plate could be wrapped around both sides of the boat, but that doesn't seem very feasible.

As John points out, the chances of being hit are extremely low and there are some prudent things to do if a sudden storm hits. Lower the mast and try to motor or paddle close to some higher object (but not under a tree). If possible, get back to the dock and either tie up your boat and get off it and under a roof, or trailer the boat and get it out of the water if there is time. And believe it or not, you are actually safer in the water than in the boat--electricity follows the path of least resistance, which in this case is the boat, not your head sticking eight inches above the water--but I doubt anyone wants to try that remedy. You should be pretty safe in the cuddy, although if the mast is hit it can explode.

So, again the chances of being hit are extremely low, especially if you are careful. Beyond that, there's not a lot you can do, so don't worry about it. Or just stay home where its "safe." :lol:
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Re: lightning protection

Postby Bflat on Tue Dec 09, 2014 11:57 am

Long ago I knew a winning skipper (M20 national champ) who was also a meteorology professor. I asked him about why sailboats at moorings were seemingly never hit by lightning even though they're the tallest things around and sit through every storm. What he said defies intuition, but makes some sense. Essentially, what he said is that even the tallest sailboat mast is virtually insignificant relative to the immense size of a lightning bolt. A mast at sea, like a piece of dust on a table top, is hardly there at all.

Still, it is scary to be on the water with lightning all around. Best not to dwell on it :)
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Re: lightning protection

Postby DanaDCole on Tue Dec 09, 2014 10:34 pm

Well I didn't want this to turn into a lengthy exchange but I am befuddled by that last post from bflat. This is an interesting subject and it won't hurt to discuss it a bit more--might help in fact.

That quote from a meteorology professor is mighty strange. (I might mention that I happen to know an ex-meteorology professor who said that humans cannot possibly be the cause of global pollution and climate change--so go figure.) It is a statistical fact that boats are hit by lightning from time to time but it is very rare and not ever having witnessed it at a marina is merely anecdotal. It is a documented fact that boats have been struck by lightning--sometimes with extensive damage, sometimes with no damage at all. For example, that mention I made of boats being more likely to be damaged in fresh water was from a study by the Florida Sea Grant. Must have happened or they would have had nothing to study. :)

What I know about lightning is very limited, but I do know that it can be seemingly capricious and unpredictable. Scientist who study it are often perplexed, for example, who can explain ball lightning? What is known is that lightning is caused by a buildup of an immense static charge by friction in the clouds, and the charge can become so high that it arcs to the ground from miles up in the air. My own house was hit by lightning and suffered some damage (before I moved here) and lightning has been observed hitting lightning rods and antennas, and masts of sailboats, so I don't see what the "immense size of a lightning bolt" has to do with anything.

For what it's worth, I have a theory about why you don't see it that often around moorings with large numbers of boats. There are a lot of very expensive boats there and I am sure most or all of them have properly grounded masts. It is possible (and again this is just a guess) that the large number of grounded masts is able to dissipate the charge in smaller amounts that are not visible or noisy. (This sort of thing has been known to happen.) There is a thing called the "Cone of Protection" (not to be confused with the "Cone of Silence" :D ) that is provided by grounded masts, lightning rods and so on. The cone's apex is at the top of the mast or rod or antenna, and the radius of the bottom of the cone is equal to height of the mast ( or whatever). The taller-masted boats could be providing many of these cones that protect the other boats.

Or there could be a tallish building on shore whose cone extends out to the moorings. Or the sea is so vast that it is dissipating all the charge. Or maybe your professor is right. Who the hell knows? At any rate it is a very interesting subject but it's not going to keep me off the water unless thunderstorms are imminent (then I would have plenty of other reasons not to venture out). If I can ever get in some multi-day cruises and a storm comes up, I'll just have to take my chances.

By the way, I can remember one time when I was in a small, open aluminum boat on a lake in Alabama when a thunderstorm came over. There was lightning hitting the water within 200 or 300 yards of us. It was very scary, especially way back then when I didn't even know what little I know about it now. For example, I thought the electricity could travel through the water and come up and electrocute me through the metal hull. Anyway, I survived that unscathed, so relatively speaking I will feel much safer inside the cuddy of a PocketShip (with the mast down).
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Re: lightning protection

Postby craig on Tue Dec 09, 2014 10:54 pm

I hope to never discover whether pocketsship (or any boat) is easily struck by lightening, but out of curiosity, why did you say that electricity won't hit the water and travel up through the metal hull? I would be scarred of exactly that! Maybe the low relative conductivity of fresh water. I don't know.... Just curious though

Craig
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Re: lightning protection

Postby Bflat on Tue Dec 09, 2014 11:40 pm

I didn't mean to imply that boats never get hit by lighting (I've known one). I was exaggerating.

The mooring field reference is just a stage setter - a convenient way to watch boats in storms. The fact remains that even a single boat on a broad sea is pretty insignificant relative to the enormous size of that thing we call lighting.

What's more, since there's no practical solution, we might as well take heart in "good odds."They're all we've got when it comes to lightning. That and staying home ;)
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Re: lightning protection

Postby DanaDCole on Wed Dec 10, 2014 1:47 am

The reason the lightning hitting the water nearby would not shock me in a metal boat is the same reason birds can perch on a 50,000 volt transmission line and suffer no damage. There is no reason for the electricity to travel through them because they are not in it's path. If one were to somehow peck the grounded tower (impossible since the insulators are several feet long) it would be blown to bits because it would become a path to ground.

Once the lighting found a path to ground through the water, that's it. It has no reason to travel up through the boat and me--nowhere to go. You can be swimming in water and unless the lightning happens to travel through you to get to the water and from there on to the ground, it won't bother you. It can hit a foot away and theoretically it would not hurt you, although if it was that close you probably would be in its path.

By the way, there were 20 or thirty boats out on the water that day and lighting was hitting the water all over the place, but not one boat was struck.
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Re: lightning protection

Postby craig on Wed Dec 10, 2014 11:01 am

Ah, I see, that makes perfect sense. Your reply reminds me of a lesson ages ago in middle school: squirrels are fine on power lines until they spread out and touch two lines simultaneously (a common occurrence with that long bushy tail, apparently). This is why you find dead squirrels, but not dead birds, near power lines.
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