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Old 18-12-2007, 09:43   #31
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Originally Posted by Southern Star View Post
... Perhaps I am mistaken (and I am certainly not an electrical engineer), but I had thought that it was generally accepted that the risk of such hull damage is greatly reduced if an effort has been made to ground the mast/standing rigging to a copper plate below the water line. I have and will continue to operate under this assumption unless/until I receive some very solid proof that it does not... Brad
Originally Posted by Triton318 View Post
Benjamin Franklin did some experiments with kites and lightning. I wonder if boaters could apply any of that to lightning protection on their boats?
Brad: Exactly.
Jay: This is what a Lightning Mitigation System is - a Franklin Lightning Rod System.

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Old 18-12-2007, 13:04   #32
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Gordon is right in that copper is the best lightning plate, but it is a real problem on an alloy hull as it immediately sets up a half-cell and begins to corrode the boat or more usually, eats the zinc off your anchor chain first then begins to eat your boat. A thick copper plate is very heavy, and since the mast is a wider pipe than it needs to be for carrying the sails, I have used the mast whivch is alloy as part of the system and run the aluminium cable up the inside of the mast. If my boat had a wooden mast I wouold probably have gone with copper--

However--the weakness is not the specific resistance of the conductors but the size of the grounding plate that stuffs mopst systems. A copper plate of one square foot is fine for a radio or Radar earth where only a few hundred watts at most will be discharged. It is simply of insufficienbt area, a little over two square feet, to allow for a discharge likely to occur from the locally charged atmosphere, which is where all small strikes come from. The bigger this plate can be, the better will be the area in contact with the salt water, and it is this area that will determine how effective the system will be. If the resistance at the saltwater junction is twenty ohms, then there is no point in running heavy conductors with a resistance in total of less than this amount--and twenty ohms id pretty thin wire. IIf you want to be safe, run big wires and have big plates, and if one under the mast lowered into the water is alloy and has an area of two square metres it has to be a heap better than one of copper half the size. The good thing about copper is it does not encourage marine growth and will be easy to clean--but it is heavy and loves to corrode anything else it is connected to electrically--especially anchor chain galvanising and zinc anodes.

I use aluminium because it is lighter and a good conductor--just not as good as copper--but it has so many other advantages for me. I used copper in the past and most of my electrical plant systems I used copper--so I have nothing against it on boats except weight and its potential for galvanic action with alloy or zinc.

The only simple analogy I can give for lightning is a gas discharge tube lkike an electronic flash. At each end of the tube we have conductors, and between these conductors is a large electrical potential difference. Between thes conductors is a gas at low pressure. Around or alongside the tube is an electrode or piece of coiled thin wire. It takes only a small amount of currebt flowing in the small wire to ionise the gas--and whamo--a big discharge.

Now--it takes a bit of imagination to see the charged upper atmospher and the millions of volts of potential difference with the surface of the earth as one electrode and the earth plus your boat as the other--and the local ionising charges eletrostatically generated from cloud movements in the lower layers of atmosphere as the ionising current to fire ythe big discharge--but if your lightning protection can prevent that ionising current from turning the air between you vessel and the nearest area of high charge potential by earthing it before it can increasingly ionise the moving and partly ionised gasses moving around and above your vessel--then it can certainly go a long way yowards making your vessel proof against a large strike, which will certainly damage it severely and possibly injure or kill anyone aboard.

Now--my little theory might be rubbish--but it is the only one I can come up with which explains simply why big strikes do not hit protected plant. Small discharges the conductors can handle--but not the biggies--and it is the biggies we need to protect ourselves from.

If you have stainless steel or glavanised shrouds and stays connecting them at the masthead will do to get rid of charges, but if you feel they may be outside the area of protection afforded by the mainmast collector then you do need additional grounding. Alloy plates dropped over the side connected by alloy strapping (chain is a bit risky--the resistance at the link contact faces is comparatively high) is a good idea. If propellor shaft and propellor etc is a better earth than your earth plate it too will form a path to the sea for any current in your dissipation system, so it ought to be protected by having good connections to any discharge plates forming an alternative path. Connect everything except your electronic equipment. It requires a separate earth system unbonded to the lightning dissipation system.

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Old 18-12-2007, 16:38   #33

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Guys, I've asked the appropriate questions while the experts discussed the fine points of vessel lightning protection systems. It appears that when a lightning hit occurs that this thread is justifyably concerned with:

1.) crew safety
2.) Keeping the boat afloat

Since the thread we are in is called lightning protection for a catamaran and I own a catamaran (without bonding or lightning protection of any kind), and having been in the unfortunate position of being struck by lightning while under engine power. I can say that with reasonable certainty that with the exception of electric/electronic items, crew and vessel were never in any danger.

The charge came down the mast wires and found a path into the hulls and down the drivetrain. Soaking wet and in bare feet, I felt nothing at all at the Helm. Not even a tingle. I just had to duck as the components at the top of my mast came down on me. The Admiral was in the Salon at the time with no ill effects.

With most catamarans having twin inboard engines with prop shafts and propellers to act as a ground, why don't either Gord or Mike feel this is protection enough for a catamaran?

Cats have twice the drive train grounding that a monohull has and from my experience this evidently works fine. It certainly doesn't protect anything electric, but I don't believe the systems proposed above do either. My question is that for crew and vessel protection, why should I do anything more?
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Old 18-12-2007, 16:55   #34
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Although I have heard it being used as such, it sounds as if a lead keel on a monohull would have too much electrical resistance between it and the water to serve as the ground plate (covered with epoxy primer and anti-fouling)?

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Old 18-12-2007, 18:17   #35
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monohull with lead keel

A mast and rigging bonded to an external lead keel will not be affected by epoxy barrier layers or anti-fouling paint. The thickness of such layers will not even be "seen" by a lightning strike making a current path from the lead keel to the seawater.

I have observed many vessels hit by lightning and the external lead keel grounded ones fair very well regardless of paint.

IN addition, many strikes that fuse open a bonding wire (#8AWG in some cases) passed through ordinary bronze thru-hull fittings without damaging significantly the bronze fittings which makes the concept of having to place huge ground plates on the hull false for these strikes.
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Old 18-12-2007, 23:24   #36
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Rick--you experienced a minor strike that the most basic well installed protection would have handled with ease.

Using a lead keel as the discharge electrode would be OK if it was not part of any electronic system (assuming you wish to keep them working) but the paint on it means that the vessel will rise to quite a high potential, a thousand volts or more, before the paint is electrically broken down and the current enters the salt water. Use it as part of the dissipation system by all means but I would still like to have a really good earth plate of equivalent or greater area in sea to plate conductivity than the cable linking the collector rod to the plate itself.

Back to the catamaran. I would use the drop-down plate vertical in the water and mounted in a raise and lower frame, and I would have it as close to the mast and properly bonded to the cable connecting the plate to the riser cable which in turn jas to be properly bonded to the collector rod. This can be done with aluminium solder or special low temperature alloy welding rods designed for aluminium.

Catamarans are usually glass, glass sandwich or wood. There are some alloy cats though. All of these folk who talk about minor damage to boats are talking about minor strokes. One sholuld not generalise about these or the value of whatever system was in place. If the boat suffered damage at all from a minor stroke, something needed improvement. It makes about as much sense as saying--my nephew was struck by lightning while playing golf. His feet got a tad burned and his hand lost a bit of skin--but the damage was so slight that lightning protection seems a bit of a waste of time. This is a non-sequitur.

Because someone survived a minor stroke means they have the luck of the Irish if their vessel is unprotected. It could have been much worse. If the gentleman concerned does not see the value in discharge plates which can be lowered as required--just do not use them.

However, do not discourage others who may get a much greater zap than you did--and might lose their vessel or worse as a consequence of not using them. If I am wrong--they put up with a little inconvenience. If I am correct then they may save their vessel and more importantly--their lives.

As far as proximity effects of a strike are concerned, I saw a cow killed outright without a mark on its body while standing close to a major discharge. The cow received such a powerful shunt current and voltage potential from its front legs to back that its heart was stopped. The same could happen to anyone really close--a few feet in the case of a biggie--but in an unprotected vessel or even a protected one with a too small discharge plate--one would be lucky to stay afloat. Having said that--steel vessels are more likely to stay afloat than a glass boat where the current flashes through the hull to the lead keel or anchor chain from the mast or compression post.

I have never met anyone who survived a severe strike on a small boat with no protection. Quite a few vessels are lost at sea in tropical seas, and I suspect that some of these may be the result of lightning damage. There are other reasons too--but being run over and lightning damage are two that I would not find too hard to countenance. In some areas pirates might be another.

For the realtively small amount of money required--fit lightning protection. By all means include the screws and shaft in the system--but remember--the big area in contact with the ocean gives the essential low resistance path necessary for dissipating a stronger strike, and for this purpose, the more discharge plates will give a greater discharge area.

Incidentally, most minor lightning ebvents are a single stroke. A strike, especially a biggie, usually comprises more than one stroke. Often the first stroke in an unoprotected vessel is enough to privide an ionised path for the subsequent heavy dischages of a strike proper--
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Old 19-12-2007, 03:10   #37
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Wow. Good read.
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Old 19-12-2007, 03:23   #38
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Thanks for your input Mike. I will revise my plan to use a copper rod, and instead use alloy, with a plate at the end. If I shape it properly it should be able to "retract" up the the bridgedeck floor tidily. Unfortunately this measure is probably only going to be viable when the boat is stopped or moving very slowly, but if there is a lot of lightning about, it might be wise to stop and deploy it.
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Old 19-12-2007, 05:19   #39
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Cruisecat - I like your idea of deploying an alloy rod with endplate through an frp tube, although I am concerned about the stresses on the tube if an adequate sized plate is installed. Surely even at slow speeds it would be akin to having another rudder under the boat; heck, even lying ahull or to a sea anchor in storm conditions would create significant forces.

I am also trying to think of a grounding method on my cat and at this point nothing seems ideal. Even the routing of heavy guage wire from a deck-stepped mast through the interior poses difficulties if one wants to avoid a significant bend in the wire. I have hollow keels with access ports (they are not 'sacrificial') and have been thinking about adding aluminum shoes to the bottoms. Might thereby kill two birds with one stone, and the total area for both keels would significantly exceed 1 square foot. Fabrication cost would likely be significant, but not in comparison to losing the boat.

Surely there must be someone out there with a decent grounding system on their cat. I (and I suspect others) could use some help.

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Old 19-12-2007, 13:22   #40
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OK--use an alloy rod of about 20mm diameter for the collector rod and you are approximately equivalent to the usual copper collector--and the mast and conductor cable can be bonded for greater current carrying capacity.

For a large plate under way one can fit stays. My original idea was a wooden centreboard with the enc copper clad--but alloy plate can be welded and simplified everything. It will get coated with a thin layer of aluminium oxide but that will make very little difference electrically.

With this protection you should survive any minor local event and it should significantly reduce your chances of being damaged by a large strike.

However--heavy currents running to earth put out a circular magnetic field centred on the conductor--in your case the mast area--and this field is at its most concentrated close to the conductor. There is a rapidly expanding magnetic pulse during a discharge, which can damage any instruments connected to wires cutting this moving field. Sometimes the voltage induced can be suffucient to overwhelm fuses and other protective devices built in to the equipment. Sometimes the equipment wiring itself is damaged by being inside a moving magnetic field.

If most of your equipment is as far as possible from the mast and its earth is not commoned to the lightning discharge system it might survuive. If the essential stuff can be put inside a soft-iron annealed "ferrous" metal cabinet it stands a far better chance. The cabinet can be painted. As one poster recommended in a previous thread--Mu metal is a non-ferrous alloy specially designed to have magnetically soft "ferrous" properties in regards to shielding from magnetic fields. Expensive but worth looking out for--sometimes these things can be bought from scrap dealers or auctions for recovered plant--

I hope anyone fitting these protection systems understands that it is a matter of reducing the chances of fatal injury or damage to one's vessel. There is no absolute guarantee that complete protection is possible. My own experiences tell me it is very unlikely a severe strike will be experienced on a properly protected vessel--but one can not make such predictions with absolute certainty.

The usual disclaimer applies--fit it at your own risk.
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Old 19-12-2007, 16:41   #41
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Thanks Mike - and your disclaimer is understood. As to instruments/electronics, I fully appreciate the risk but am more concerned about hull damage. The centerboard idea is worth considering (although in my cat, it would have to be more of a 'leeboard' pivoting under the bridgedeck).

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