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  #11  
Old 08-16-2006
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No sh*t, I was there

My Dad is a retired EE with a lot of experience with tracking antennas of all sizes. You know, those big dish shaped things mostly seen in the movies and documentaries. Lightning was a bug issue with them since they were large and usually in the middle of a field. Anyway, they used dissipators and grounding. The ground connections were welded to the assembly to prevent the possibility of corrosion degrading the grounding path. At that time he said NASA had barbed wire running the roof perimeter of the acres of the large two story buildings they had at Cape Kennedy. The purpose was as a dissipator with all of the little points oin the barbed wire.

We talked about sailboat strikes recently. He considers it foolish to not have a dissipator for on anchor or underway. That is when it will help you reduce the known induced potential that rolls along beneath the storm cloud. Your mast is a huge target without much else around. The higher the potential voltage induced on an object the greater the probability of a strike, so the dissipator will reduce your strike probability. On the other hand when in the crowded marina, he said that unless most of the boats in your marina have a dissipator, it won't matter if anyone does. The marina tends to become one huge target at the edge of land where the masts are the only tall objects around. In that case all the boats tend to have roughly equal chances of being hit since they are so close together.

My boat was struck while underway on the Chesapeake several years ago . I must say it was a fascinating experience but not recommended. After that I first talked to my Dad and decided to install the $80 bottle brush dissipator during the repairs. In my subsequent research I found a lot of controversy but there was some common ground. The US Navy has researched the subject extensively to the point of going offshore and actually attracting lightning strikes. I've seen up close pictures of a bolt striking the deck. From the Navy research the common conclusion is that during a lighning storm the very best boating place to be is on a metal boat. The surrounding metal tends to conduct the strike charge safely to ground. They suffer very little damage to vessel or crew. Based on that it would seem that more grounding is better than less, but maybe it doesn't extrapolate to the minimal case of a fiberglass boat.

At the other end of the safety spectrum is the small fiberglass power boat. Typically the strike will hit the cabin top but then it might pass through an individual standing underneath it.

In between the two extremes of the safey spectrum with the metal boat on the safest end and the fibeglass powerboat on the riskier end is the fiberglass sailboat with it's "cone of strike protection" extending down from the tip of the mast to the deck. It's no surprise that the mast takes the strike and you want to be away from it but not too far so you are inside the cone. I talked to a couple who were riding out a storm on anchor when the boat was struck. They were sitting on the settees and saw a bunch of small ball lightning spread across the cabin sole and dissappear into the cabinets beneath them. I saw a hit on a power pole once and a ball lighting the size of a basketball went down the street of a few hundred feet in a couple of seconds.

For lightning protection I'd prefer a metal hulled vessel. It is known that cars take lightning strikes and nobody is hurt as long as they are not touching metal attached to the car's body. Unfortunately metal boats seem to have serious long term maintenance issues. So a few years ago I came up with the idea of a fiberglass hull with an extra layup inside of a metal mesh. When I discussed it with my Dad, he thought it had the potential to give the vessel enough conductivity to protect it's occupants and structure well. I'd really like to know if practically speaking this can be done, or would any metal mesh cause structural problems inside the rest of the layup. Possibly the mesh weight could offset by less fiberglass due to added strength from the mesh. It certainly could make for an interesting yet expensive experiment. Another way to go is if you started with a project boat that you had to gut the interior anyway. Then you could line all of the exposed hull and cabin with a light metal mesh before reassembling the interior. In both of the mesh designs, the interior would be come a Faraday cage so your cell phone would no longer work. Oh well.

As for the onboard electronics, they are just at the mercy of the electromagnetic pulse. The strike itself is actually a subsecond series of extremely high voltages pulses. Any conductor laying radially from the strike can pick up huge induced voltages and that is all it takes to fry solid state equipment. Physically unplugging equipment will protect it from surges that enter the vessel power distribution wiring, but will not protect it from the induced voltages within the equipment itself. Even a strike in the water a hundred feet abeam has been known to wipe out all of the electronics from induced charges.

I'll finish this with one more point from a true story of my friend with the C&C 37-40+, a very nice performance cruiser. A few years ago he decided to buy that particular model and bought one on the hard in Florida. Before closing, it took a strike causing much damage including an extra hole below the water line. At least it wasn't in the water. Anyway, the seller wasn't up to making proper repairs so my friend walked on the deal. Being persistent, he soon found another one in the Cayman Islands. This one was intact without extra holes below so he quickly aquired it and for the last few years it has been berthed a few piers in front of my boat.

A few Saturday nights ago we had a really nasty storm sweep through our marina while we were in the slip. The next morning I found out that one of the close strikes that we heard that night was on his boat in the slip and they were onboard . The boat is on the hard now with the mast unstepped for rewiring. Attached is a picture of his vhf aerial which landed in a temporarily maleable state so that it wrapped around his slipmate's starboard safety rail. So far the repair estimates are about $15K. Several vessels surrounding his also have significant electrical damage.

My point from this story is that besides whatever else you do about the lightning issue, make sure to have good luck. In my friend's case having purchased the same model twice and having both of them hit within a few years is quite a statistic. His luck in the strike extended to the point that no one was hurt, and that of course is the most important.
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  #12  
Old 08-16-2006
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Obviously a metal vessel is going to fare a bit better, as the hull can act as a ground plane... the best type of metal vessel would be a copper-nickel alloy boat, which is exceptionally rare.

Some electronics can be protected by simple enclosure in an all metal box. The box acts as a faraday cage and removes the ability for the pulse to destroy the contained electronics... I have used a pressure cooker for the "box".

The real debate is whether an ungrounded fiberglass vessel is more or less likely to get struck. If grounding a fiberglass vessel makes it more likely to get struck, I see no point in doing so. Lightning strikes are relatively rare phenomenon, and I'd prefer they stay that way, with respect to me and my boat. However, if lightning does strike, the chances of serious damage seem to be much higher on an ungrounded boat, as the lightning has to create a path to ground, rather than follow an existing path.
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  #13  
Old 08-16-2006
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Thanks saildog...yep...I am just waiting till I return from Argentina to sail back from the keys to Fort Lauderdale where my dock is...

thanks again for the help...
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