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Lightning Protection questions
On Sat, 08 Nov 2008 23:35:13 -0500, Marty wrote:
Richard Casady wrote: Sounds like BS. My house barn and corn crib all had half inch or so braided copper cable connecting 1/2 inch pointed rods. The ground rods are presumed to be the standard ones the electrical places all sell. They were struck hundreds of times, they were on top of a hill. I heard it was 20 000 amps, at a million volts. Guess I should have included a tongue and a cheek,,, However my point was rather that lightening is inherently unpredictable. Our barn is "protected" the same as yours, still has the cool glass globes at the base of the pointed rods, what the hell they are supposed to do I don't know. As far as I know it's never been hit. I've seen a neighbor's lightening system vaporized, fortunately it was raining hard enough, and his roof was leaky enough that damage was limited to a some singed hay and a couple of scorched boards. I've been involved with ground protection systems for airports, (counterpoises in the trade) and seen some absolutely whacky things, like edge lighting halogen bulbs blown up, even though the feeder was buried three feet below a 4 gauge bare copper ground wire just a few inches below ground..... I've seen constant current power supplies that feed the lighting fried but the lighting fixtures and transformers remain unharmed.... lighting is funny stuff.. Pretty much my take based on fairly extensive reading. Think I came to the conclusion that: Bonding can pretty much provide a safe area so long as you keep you hands off metal. An electrical disconnect that won't likely be jumped might be worth exploring to cut down the expense of damaged electronics. Basically like pulling the plugs in the house, which I usually do during electrical storms. Keep some portable radio/gps gear in an insulated container. Since you might get holed, have a good collection of damage control plugs. Dry weather lightning can still catch you off-guard. --Vic |
Lightning Protection questions
"Roger Long" wrote in
: s the standard metal rod VHF antenna at the top of the mast with the typical metal can on a bracket riveted to the mast a sufficient air terminal or should I add a dedicated rod? I have no illusions about having any electronics working after a strike on a 32 foot boat but replacement of my minimalist outfit wouldn't break me financially. I'd just like to be alive with a working engine and watertight boat. -- Roger Long http://www.thomson.ece.ufl.edu/lightning/ This sailor has done extensive research at ufl..... |
Lightning Protection questions
On Sat, 8 Nov 2008 14:48:10 -0500, "Roger Long"
wrote: I'm also thinking of some fresh water tripsafter my recent Hudson experience and would like to take the boat south for at least one winter. There are almost no thunderstorms in south Florida from November through May. We call it the dry season here for good reason. |
Lightning Protection questions
Larry wrote:
http://www.thomson.ece.ufl.edu/lightning/ This sailor has done extensive research at ufl..... Thanks, that was one of the first things I turned up. This quote is especially applicable to this thread in view of Wilbur's comment that an ungrounded boat is less likely to be struck: "While the individual estimates varied widely between surveyors, there is no support for the argument presented by some sailors that they should not ground 'their sailboat since it will increase the chances of it being struck by lightning." The statistics presented in this article make a good case for just forgetting about it unless I plan to sail south, which I do hope to do at some point. My original post was actually prompted by this and a couple of other web articles. Here is my situation: http://home.maine.rr.com/rlma/Keel.jpg The mast stanchion is essentially equal to a keel stepped mast. Since my boat was originally a keel / centerboarder, the keel is large volume and I doubt that Endeavour spent the money for a keel casting. I'm quite sure the ballast is just stacked lead pigs in resin. Lighting current going through that stuff would be like a bomb and the high resistance at the bottom of the main conductor would create extensive side flashing. For reasons not evident on the crude drawing, any grounding plates have to be outboard of the cabin sole. The Thomson paper says not to let grounding conductors contact the hull but I have no choice if I am to maintain the maximum radius recommended by other sources. The reason for overkill on conductor and ground plate size is to compensate for the tight conductor radius and need to run the conductors close to the hull skin. -- Roger Long -- Roger Long |
Lightning Protection questions
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Lightning Protection questions
wrote in message ... Protecting a sailboat from lightning strikes is pretty much a fools errand. Unless you have witnessed lightning strikes up close and personal, you have no idea of the forces involved. It beats any "fantasy explosions" or other special effects mayhem you have ever seen in the movies. A 1/2 inch copper wire would vaporize before it could carry the strike anywhere. You aren't going to steer a lightning bolt, either. It has no brains and will often jump a long distance, even if already following what seems to be a path. The best thing you can do for lightning protection on a sailboat is anchor near other boats with taller masts than yours, and THAT is also foolishness and completely useless. It's only benefit is a little humor. Some years ago I was caught in a lightning storm while well offshore. My boat had a metal mast with a pointy VHF aerial on top. The mast was keel stepped, but the step seemed to be just glassed into the hull and I could see no sign that it was bonded to the lead keel. Lightning bolts were hitting the water so close to us that I could distinctly hear the sizzling noise of water boiling and this happened several times. Despite the fact that my mast was the only thing sticking up above sea level in the vicinity we were not struck although it was a very frightening experience. I am afraid that this does not assist in deciding whether earthing the mast is a good idea or not.. |
Lightning Protection questions
Roger Long wrote:
Larry wrote: http://www.thomson.ece.ufl.edu/lightning/ This sailor has done extensive research at ufl..... Thanks, that was one of the first things I turned up. This quote is especially applicable to this thread in view of Wilbur's comment that an ungrounded boat is less likely to be struck: "While the individual estimates varied widely between surveyors, there is no support for the argument presented by some sailors that they should not ground 'their sailboat since it will increase the chances of it being struck by lightning." The statistics presented in this article make a good case for just forgetting about it unless I plan to sail south, which I do hope to do at some point. My original post was actually prompted by this and a couple of other web articles. Here is my situation: http://home.maine.rr.com/rlma/Keel.jpg The mast stanchion is essentially equal to a keel stepped mast. Since my boat was originally a keel / centerboarder, the keel is large volume and I doubt that Endeavour spent the money for a keel casting. I'm quite sure the ballast is just stacked lead pigs in resin. Lighting current going through that stuff would be like a bomb and the high resistance at the bottom of the main conductor would create extensive side flashing. For reasons not evident on the crude drawing, any grounding plates have to be outboard of the cabin sole. The Thomson paper says not to let grounding conductors contact the hull but I have no choice if I am to maintain the maximum radius recommended by other sources. The reason for overkill on conductor and ground plate size is to compensate for the tight conductor radius and need to run the conductors close to the hull skin. -- Roger Long The problem is, lightning doesn't like going round corners and *will* jump back out of the lightning conductor at the bend. E.M. Thompson states: At this time the peak lightning current is generated, during the 'return stroke'. Although cresting at ten thousand to hundreds of thousands of amps, it only lasts for about a millionth of a second. This 1 us pulse considered in the frequancy domain has a fundamental at 1 MHz and various unspecified (we dont really know the pulse shape) strong harmonics going *way* up. As a mimimum, the grounding system *MUST* have a low impedance from DC to 10 MHz which essentially prohibits anything except large radius bends through less than 90 degrees. Due to the 10 KA current pulse, an extremely high voltage will be developed accross any bend, so there is an extreme risk of arcing to the hull if the bent strap is in the bilge. If its touching the hull, you basically needn't have bothered fitting it from the point it touches the hull onwards to the grounding plate. The other nice little problem is that multiple KA currents *will* straighten out any sharp bends in the conductor carrying them and a lightning strike consists of multiple strokes so the grounding system has to survive in usable condition to be effective. The main lightning conductors need to be joined to the mast support post at least as high above the grounding plates as the lateral distance and led in a smoothly swept curve. Unless your pillar is very close to a bulkhead this may not be practically achivable. Once you've led the lightning below decks, you are basically screwed unless you can lead it almost streight down and out. Once you've led it below the waterline you are totally ****ed unless you can get it out to large area grounding plates. From your description, I dont see how anything except boring the top of the keel downwards and outwards for thick pure copper conducters to ground plates extending from the sides of the keel to the turn of the bilge could help much. There are problems with this approach as well, as the conducters will try to move quite a bit duing the strike and if undersised and they go open circuit during the strike could cause major explosive damage to your keel root. |
Lightning Protection questions
wrote:
Protecting a sailboat from lightning strikes is pretty much a fools errand. This document, which Larry provided the link to, contriticts you with actual facts by researchers. http://www.thomson.ece.ufl.edu/lightning/ See damage distribution graphs. There will usually be some damage and some strikes will overwhelm any system. However, saying protection is pointless is like saying it's a fools errand to wear seatbelts because some car crashes will be fatal anyway. Everything on the water is about odds. Stack the deck in your favor when you can. -- Roger Long |
Lightning Protection questions
Sounds like I'd better stay north of Cape Cod:)
I've thought of drilling the keel since lead is fairly easy to drill. I'd want to be sure there wasn't any scrap iron in the ballast mix though. How thick a copper conductor do you think would be safe? The mast support is only about 1 1/2" stainless steel pipe so it would be reasonable to exceed it's cross section area and current carrying capacity. OTOH if the mast support itself is insufficient, maybe I should just forget the whole thing and put my faith in prayer. I ran a copper wire of the kind used to ground electrical meters from one of the shroud chainplates to an unused through hull in a fairly gentle sweep when I first got the boat just to have something to lead a strike a way from people inside. I still expect a major strike would sink the boat but thought it might increase the chances of being alive to get into the dinghy. Now I'm thinking I might be better off without this connection. What do you think? What would be the probable effectiveness of a portable system? Say, four 1 sq foot copper plates attached to 4 GA wires shackled to headstay, backstay, and shrouds before anchoring or drifting for a major electrical storm? -- Roger Long |
Lightning Protection questions
On Nov 9, 8:24*am, "Roger Long" wrote:
Sounds like I'd better stay north of Cape Cod:) I've thought of drilling the keel since lead is fairly easy to drill. *I'd want to be sure there wasn't any scrap iron in the ballast mix though. How thick a copper conductor do you think would be safe? *The mast support is only about 1 1/2" stainless steel pipe so it would be reasonable to exceed it's cross section area and current carrying capacity. *OTOH if the mast support itself is insufficient, maybe I should just forget the whole thing and put my faith in prayer. I ran a copper wire of the kind used to ground electrical meters from one of the shroud chainplates to an unused through hull in a fairly gentle sweep when I first got the boat just to have something to lead a strike a way from people inside. *I still expect a major strike would sink the boat but thought it might increase the chances of being alive to get into the dinghy. Now I'm thinking I might be better off without this connection. *What do you think? What would be the probable effectiveness of a portable system? *Say, four 1 sq foot copper plates attached to 4 GA wires shackled to headstay, backstay, and shrouds before anchoring or drifting for a major electrical storm? -- Roger Long 4 gauge may be a bit small. A fellow in my old marina took a scrap welders lead and cut 6 ft sections and had them saddled clamped to the shrouds on each side, had them coiled and zip tied to each side. In a storm he just cut them loose and figured that would be the least path of resistance right down into the water. As Neal's example proved when the lighting jumped in an air arch to his skull lightning seeks the path of least resistance. I dont think he's been struck yet to test his system. He has many taller boats all around him. Joe |
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