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Old 08-02-2009, 01:20 PM   #15
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This subject has popped up recently on several different cruising/sail forums. Here is a reply I made concerning boats and lightning. . .

Actually, lightning is very well understood in the scientific community. How it forms and how it reacts is also understood. The University of Illinois and others have many detailed scientific papers available on the subject. Exactly where and when lightning will strike is a variable as it is part of Mother Nature and there is a reason she is called "Mother Nature" and not "Father Nature."

As to lightning and boats, the issue comes in two parts - 1. Prior to the strike; and 2. During the strike.

Part 1. concerns how to reduce the probabilities of lightning striking your boat. Lightning is a two part event. The ionized "leader" sweeps an area below the generating cloud and an opposite "leader" sweeps back and forth up from the surface of the earth. When they connect a pathway is opened and the energy is discharged. To a boat the ground/earth "leader" is of most interest. As this "leader" oscillates around it reaches up "x" distance into the atmosphere. Whenever this leader encounters a vertical object like a tree, house, telephone/power pole, or the mast of a boat it can reach further up into the atmosphere and has a better chance of making a "connection." [same thing as tall men in a bar full of good looking women].

So, reducing the "electrical" apparent height of your boat is good. This can be done with static dissipators such as the Forespar Lightning Master. (Which is a copy of the static wick principle used on airplanes and airliners). The sharp small "spikes" of the device "bleeds" off ions as they build up and electrically reduces your mast height to equal that of the ocean. However, to do this it must have a "significant" and good ground to the ocean. That translates to 4 square feet of flat plate copper in contact with the ocean and 2/0 welding cable between the mast and the plates submerged in the ocean. A static dissipator will not dissipate if it is not connected with the ocean.

So the result of part 1. is to electrically make your boat height equal to the ocean. If you are anchored close to shore or in the close vicinity of other "unprotected" masts/boats your probability of being hit is significantly lower than theirs. If you are out in the middle of no-where/ocean by yourself you are now "even-steven" with the ocean as to getting a hit.

Part 2. is what can you do to prevent/minimize damage to the boat and its contents during a strike. Again, the 4 sq ft. of copper joined with significant sized welding cable to the mast(s) will provide a highly desirable pathway for the lightning's energy to get directly to its only objective - earth ground. If the mast(s) are not sufficiently well-grounded then the lightning energy will try to find an alternate path to the ocean. If the mast is not available to the lightning, then it will travel down the shrouds/stays to the bonding system and set up a "field" inside the boat that will "fry" most electronics and has been known to heat metal thru-hulls sufficiently enough to melt them out of the hull and you are left with 1.5" holes for the ocean to enter. Lack of any grounding to the ocean and you can end up with holes blown through the hull.

Side note: while your boat is floating in the water it is grounded. When you boat is "on the hard" (out of the water) it is not grounded and any lightning strike to your boat or to a neighbors boat will fry your electronics or other fine metal objects or more serious damage. It is advisable if you are going to leave your boat on the hard, in an area with probable lightning, to drive a copper or steel grounding stake into the ground beneath your boat and hook up a significant sized wire from it to your masts or boat grounding system.

There are many other esoteric factors available to folks wanting to get the "whole story" such as positive versus negative forms of lightning, high frequency vs low frequency lightning, etc. but for the boater I think the primary interest is minimizing damage to the boat, contents, and not curling the crew's hair. This is done by dealing with the before and during aspects of protecting/guarding your boat from the energy in lightning.

Exactly where and when a strike will occurs is not determinable - the same as the actual path of a hurricane or tropical storm is not totally predicable as there are too many variables in nature for even the powerful human built computer to input and resolve. So you can only do what your think is cost-effective to "lower the odds" that you will the target.

- - On the subject of why to use welding cable versus flat copper foil - The "flat" copper strips/foil is used for RF grounding. HF/SSB radios use the flat copper rather than round wire as they are concerned with bleeding RF energies to ground. However in Lightning situations, only some of the lightning is "hi-freq" lightning, the rest is "low freq" lightning. The Hi-Freq lightning travels on the "outside" of things. This the type of lightning that can strike a human and he can live through it as the lightning travels down the outside of his skin to earth ground. Of, course he ends up "hairless" and with some surface burns but this form of lightning does not usually kill. Low Freq lightning on the other hand, does kill as the energy travels straight through the body to earth ground, frying, cooking and short-circuiting all the human electrical "circuits" resulting in death. It is also the lightning that splits trees and punches holes in things. Luckily most lightning is statistically of the Hi-freq variety.

But on a boat you need to protect against both which is why you use "welding cable" and not "boat cable." Boat cable is stranded, but only with a low multiple of household stranded wire. Flat copper ribbon which does well for RF does not handle high amperage unless it is in thick "bar" format. Welding cable on the other hand has an order of magnitude more strands and it can also handle large amperage loads. So it is the best compromise and is also flexible which is an absolute necessity in boat wiring. The size of the welding cable must be substantial as any electrical "choke" points can lead to melting and interruption of the pathway to earth ground. Considering the thickness and surface area of your mast(s), you should choose a size of welding cable that effectively matches the electrical surface area. You can find - with much difficulty - the bi-metalic cable clamps that have an aluminum mounting pad for bolting to the mast and a copper/bronze cable clamp to holding the end of the welding cable. The welding cable to in-water copper plates must not have any radical bends or right angle bends between the mast and grounding plates. If you try to make the lightning discharge turn a sharp corner it will simply leave the wire and "flash" over to earth/ocean ground possibly punching a hole in your hull.

The basic principle is engineer a nice, as straight as possible, easy electrical path for the lightning that enters at the top of your mast(s) down through a virtual electrical "tube/pipe" to get to the ocean below with no blockages or constrictions along the way.
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Old 08-03-2009, 02:14 AM   #16
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For years I worried about lightning strikes on the boat, yet there weren't a lot of places where we experienced electrical storms, though when we did, they were terrible. *I've just found a report that is most interesting to me: NASA's plotting the areas of the globe with the highest lightning activity. *There are lots of sites on the Internet reproducing this information, one*HERE

Here's the information in one graphic: *

Florida is one of the most active parts of the U.S. for lightning - hmmph! *No wonder US boats seem to be so paranoid about lightning. *And as all the comments regarding NASA's data point out, it's on or near land that electrical storms happen, it's clear to me that this is one more reason to head offshore and enjoy the blue water passages. *And for Brenda, aka Redbopeep, you'll notice that in general, lightning is not a problem in high latitude sailing. *Go for it!

Here's another tidbit of information I recently learned about lightning and lightning rods: *pointy is not as good as blunt for ATTRACTING lightning. *So now I'm reconsidering the Forespar Lightning Master mentioned by Osirissailing. *Maybe that would work after all. *

Because the link won't work, I'm pasting the article here:

Lightning Rods

Article in New Scientist Magazine, 27 May, 2000, by Jeff Hecht

"KING George III was right and Benjamin Franklin was wrong, at least when it came to the tips of lightning rods. The American scientist and diplomat believed lightning rods, which he invented in 1749, should have pointed tips, and his design has been used for over two centuries. But tests on a New Mexico mountain top show that blunt lightning rods, which George III decreed be used on royal buildings, are actually more effective at attracting lightning.

Electric charge builds up during thunderstorms, and Franklin's original aim was to prevent lightning by dissipating this charge, says Charles Moore, a retired atmospheric physicist formerly at the New Mexico Institute of Mining and Technology in Socorro. Franklin knew from his indoor experiments that earthed, pointed rods could discharge a nearby charged object without a spark. But when he placed a rod outdoors, he found that instead of preventing lightning strikes, it attracted them. Undaunted, he realised that attaching a conductor to the rod could divert the strike harmlessly to earth. This remains the basis of lightning rods today, although a perfect design has never been agreed.

In the 1930s, researchers showed that when a sharply pointed conductor is in an electric field, it is "protected" by a surrounding cloud of ions that reduces the local field strength. Moore wondered if rounded tips would better attract lightning because they lack these ions.

"We ran a competition between sharp and blunt rods," with tips from 0.1 to 50 millimetres in diameter, he told New Scientist. During seven summer thunderstorm seasons, he found that the ones struck most frequently had tips around 19 millimetres in diameter. The rounded rods invariably attracted lightning away from the pointed rods, none of which were hit. Moore calculates that the electric field strength above a 19-millimetre blunt rod is higher than that over an otherwise similar sharper rod, making it better at attracting lightning.

George III had no great insight when he insisted on blunt rods, Moore notes. "He did it out of political pique," because he was angry at Franklin for his support of American independence. "Franklin was right in putting up a conductor," he adds. "We just fine-tuned him."

Source: Geophysical Research Letters (vol 27, p 1487)"
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Old 08-03-2009, 02:55 AM   #17
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If you are considering the Forespar Lightning Master there is one major problem with the device. If you have a VHF antenna or anything else on the top of your mast - you must mount the Lighting Master so the top is at least 6 inches above the highest thing on top of your mast. This is easily accomplished by taking the Lightning mast to a ss welding shop and having them cut it in half and weld in a ss bar of sufficient length to get the "spray" at least 6 inches above the highest thing on top of your mast.

And I must repeat any of these devices are a waste of money if you do not have a sufficient large copper/bronze ground under the boat in the water. Dynaplates do not work as after only a few months, the "equivalent" whatever square feet of the sintered bronze is filled with sea creatures and barnacles reducing the real square feet of conductor to the physical dimensions of the dynaplate.

What all this ends up with is that it is a royal pain in the *** to properly protect your boat. And the occurrences, statistically, of boats getting hit by lightning is extremely low which suggests that the effort is not really cost-effective. There are other hazards more immediate to a cruising boat that spending money on would be more cost-effective. In my case I have been in the marine supply and repair business for almost 20 years and got everything free or at fractions of the cost so I could put all the "extras" on my boat for minimal cost and write them off as advertising/business expenses.
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Old 08-03-2009, 12:29 PM   #18
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Originally Posted by osirissailing View Post
What all this ends up with is that it is a royal pain in the *** to properly protect your boat. And the occurrences, statistically, of boats getting hit by lightning is extremely low which suggests that the effort is not really cost-effective. There are other hazards more immediate to a cruising boat that spending money on would be more cost-effective.
I agree with that sentiment. *As I mentioned, considering all the high lightning risk places we've been, only once being hit by lightning implies a low probability of a strike, UNLESS you spend most of your boat life in Florida or along the coasts of some of the other high risk locations. *
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Old 08-07-2009, 12:47 PM   #19
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I would be interested to know how you could construct and attach a bronze copper plate to the bottom of a fibreglass yacht and connect the welding cable. As my mast sits about 4 inches above a lead keel, I cant see how I could install the cable without putting a reasonable bend in it.

The plate seems it would take up an large area underneath, how would you attach it ? would there be enough copper content to prevent it from fouling?

. I thought of installing one of these despersers, but there is a lot more to it than just bolting it at the top of the mast.

My other theory is if you dont provide a path fot the current it wont worry you.

While on a passage in the coral sea I had a bolt of lightening hit the water about a meter from the boat and it melted the inside of a spare bronze sea cock, normally just above the water line, but it probably was under the water at the time of the strike. It made a very loud bang, but took 6 months to discover where the bang came from.

Finally to add something I heard from somewhere is while the boat is at sea and moving around the mast is less likely to be able to make contact, so we are at more risk while 'safely' moored.
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Old 08-19-2009, 11:56 AM   #20
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I would be interested to know how you could construct and attach a bronze copper plate to the bottom of a fibreglass yacht and connect the welding cable. As my mast sits about 4 inches above a lead keel, I cant see how I could install the cable without putting a reasonable bend in it.

The plate seems it would take up an large area underneath, how would you attach it ? would there be enough copper content to prevent it from fouling?
Our mast is not a conductor (it is wood) but of course can still be hit by lightening. Some other owners with wood masts and wood hulls have used a shroud to chainplate to cable to ground at lead keel. We know one fellow who doesn't worry about it while underway but when anchored he attached a 6' x 1" copper grounding rod (like you pound into the ground adjacent your house!) to the boat via a cable that is clamped to a shroud.

Hubby, an engineering PhD and tinkerer to boot, was just the other day saying he'd like to run a cable from chain plate to lead keel but would use a neon filled tube as a "gap" to prevent conduction (and damage to adjacent wood) unless there's a lightening strike. I wonder what he's thinking about but doesn't tell me sometimes...
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Old 08-19-2009, 12:23 PM   #21
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