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  #1  
Old 09-24-2007
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lightning - grounding or isolation?

I was giving consideration as to whether or not to add lightning protection to my boat (seems like a reasonable concern even for a weekend cruiser if you are ever going to sleep on the boat) and if so how to do it, so I did some searching about. I came on two completely different schools of thought.

One seems to be more common and perhaps traditional - grounding the mast to an external plate on the bottom of the boat.

The second school of thought is to do nothing if you have a small boat like mine with a pull start outboard or to isolate the engine electronics if you have such. The thinking there is that the reason sailboat masts get hit is because they are grounded or have a grounded wire in or on them; if you have a mast top light that shares ground with the engine and the prop is in the water, you have a lightning attractor. I read an interesting explanation of the theory (even scientists don't seem to completely agree about lightning) that cited a lot of circumstantial data, like cars and trucks almost never getting hit even though they are often out in thunderstorms, but tractors with gear in the ground getting hit frequently. The lack of many reported lightning strikes on fiberglass boats without motors was also cited. Observed data seems to give the theory pretty strong support. The interesting thing about this line of thinking is that it isn't just a slightly different alternative; it paints the other approach in a very negative light, concluding that boats that add "lightning protection" actually attract lightning and bring damage on themselves.

I do plan to get a motor with some electronics; it makes sense to me to get a small outboard like the Nissan 6 that charges the battery when running. I guess one option would be to only hook it up to the battery when the engine is running, but that seems like a pain and it would probably get left hooked up a lot.

I am curious what the general consensus on this is...
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Last edited by arbarnhart; 09-24-2007 at 08:42 AM.
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Old 09-24-2007
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Can't wait to hear advice on this thread. Lightnig scares me more than anything else there is on the water. I'm terrified by it. My little Chrysler 26 swing keel has no protection - for now - but it will as soon as I figure out the best way to do it.


A little side note:
My mother (retired flight attendant) was on a 767 that was struck on the ground once. She said a lady looking throuh the window had her hair friz straight out and stay. She looked like Phyllis Diller on a bad day. She said it was the funniest thing she has ever seen in all her years of flying.
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Old 09-24-2007
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There are two schools of thought on this.

The first is that you should create a lightning protection system, that bonds the mast, shrouds, stays, and lifeline stanchions to form a protective "Faraday Cage" to protect the contents and people on-board the boat. This requires using a fairly heavy copper conductor to bond the mast in as straight a path to a heavy copper or bronze plate attached to the hull's exterior—with at least four linear feet of edge. A 2" x 4' strip is a good way to do it. This will, in theory, direct the force of a lightning strike as quickly into the water as possible, minimizing the possibilities of sideflashes and such.

The other school of thought is to have no path between the mast and the water. This is due to the idea that a grounded boat gets hit more often than an ungrounded boat. Statistically, I believe this has been proven. However, the converse is that ungrounded boats, when hit by lightning, tend to suffer far more damage than a properly grounded boat.

So, you either ground the boat and increase your chance of getting hit, while minimizing the possible damage....or you isolate and decrease your chance of getting hit, while increasing the probability of severe damage in the case of a hit.
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Old 09-24-2007
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Well...for boats with no built in grounding system, there's always the "clip a set of battery jumper cables to the shrouds and throw the other end in the water" option.
My personal preference it to get close to boats with taller masts!! (g)
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Old 09-24-2007
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Yep I agree with Sailingdog. I work in Electrical Engineering and have wondered about this for many years and done some research on it.
I do believe that if you are grounded you increase your chances of being hit, I have no idea by how much of course.
However if you are hit, and you have a very complete heavy duty bonding system on your vessel, connected to an adequate external grounding plate the damage will be reduced, not eliminated just reduced.
There was a discussion on here just a few weeks ago and I stated then that I am amazed at how few sailboats actually get hit. Basically lightning does not deflect very far from it's intended path so you have to be pretty unlucky to get hit.
Staying away from the standing rigging during a storm will greatly reduce your risk of personal injury if your luck runs out.

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Old 09-24-2007
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Quote:
Originally Posted by camaraderie View Post
Well...for boats with no built in grounding system, there's always the "clip a set of battery jumper cables to the shrouds and throw the other end in the water" option.
My personal preference it to get close to boats with taller masts!! (g)
The getting closer to other boats with taller masts is probably the better of the two options. The jumper cables have such a poor connection, and the shrouds, being made of stainless, are rather poor conductors, that you're really not doing anything effective.
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Old 09-24-2007
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The crazy thing about this is the conflicting info I find. I have found reports by experts backed up by data that say completely contradictory things. But everybody's data seems suspect. One study in FL concludes that every boat left on the water there stands a pretty good chance of being hit at some point and that grounding doesn't increase your chance of getting hit. But the data came from interviewing surveyors who had looked at damaged boats, the variances between surveyors were huge and it didn't seem to be cross referenced against the general boating population. Another one used much more data to try to prove the other point, but it was mostly not boating data. Wiring the mast to the bolt for the swing keel would be the simplest option I have. I have a small boat with only a 22' mast and an engineered arch in the cabin top, so my mast is very isolated. The bad news is the boom might offer a better path to ground.
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Old 09-24-2007
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The general consensus when I looked at this was either do it right or don't do it at all. Grounding will increase the chance of a hit but funnel it harmlessly away.

But if you can't get a straight path to the ground plate, bolted not soldered or squeeze connectors then you've increased the chance for more damage. The idea of the jumper cables are in this category.

The best compromise I read for plastic boats was to get a 1/2" cable, strip 6 foot of insulation off each end. You wrap a turn around the mast and put the ends in the water. You're not grounding the mast so not increasing the change of a strike, but if you do get hit the insulation of the cable is irrelevant to a billion volts and may divert the strike overboard.

My boat has been hit! A neighbor saw it. I was on the trailer and had the mast up. I couldn't see any damage afterwards.

Last edited by whroeder; 09-24-2007 at 10:48 AM.
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Old 09-24-2007
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Based on everything I've heard, read, and seen over the years, I'm inclined to believe the argument that grounding increases likelihood of being struck but minimizes potential damage; not grounding decreases likelihood of strike but increases damage if struck. Our boat is not grounded and I would not go to the effort of grounding it. It has survived almost two decades in heavy thunderstorm regions without being struck.

A sailboat getting struck by lightning is very rare, so from a safety point of view I'd spend more energy thinking about First Aid and MOB recovery. Of those boats that I am personally familiar with that were struck by lightning -- all were grounded. And, all were struck at dock or mooring while unoccupied. I think when you look at the chances of being struck, added to the chance that you will be aboard when the stike occurs, the risk is very low. Knock on wood.
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Old 09-24-2007
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My neighbor's boat was hit and I was out on the cockpit of my boat about 15 feet away... all I heard was this shrill sizzle noise and then a crack like no other. My PTSD from combat kicked in and I ducked thinking we were having incoming fire hahaha!

Anyhow, my mast on my Bene is 9 feet taller than his. I was not grounded (well, with exception for the masthead light which shares a ground with the engine) and I have a VHF/SSB/SW antenna that sticks up another 6-8 feet or so from the head.

He had a "long time" grounding system of a shroud chainplate with battery jumpercables dragging in the water off of each side (wires split, one on each side)

Damage to his boat - it was AMAZING to say the least. He lost 2 diagonal shroud lines, like it disappeared (think a cannon fuse) there was black spots all over his topside deck, and the mast had a big burn mark you could plainly see near the top 2 feet of the mast.

I've seen the aftermath of a strike in a powerboat in Nevada. That guy had a hole in his hull, and they barely got the boat grounded at the beach before it took on too much water to motor back to safety.

I would think that I'd rather loose a mast than a hull. If I decide to ground, it will be to the chainplate like my neighbor did. This should create a farraday cage around the rest of the hull (and people) I think.

I have suspected since this happened this summer, "What if I was grounded, would the strike have hit me instead with the taller mast?"

I guess I'll never know - I'm not into fooling with gazillion volts of mother nature.
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