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Old 15-06-2012, 10:54   #31
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Re: Lightning strikes

Tom,
Good to see you here!!! (and we don't have to worry about changing the subject/title...inside joke!!!)

I can't speak definitively whether every "direct strike" is in fact a direct strike, or a side flash....
And, yes the ocean is a better ground than a fiberglass boat, but that alum mast is a better conductor than the air!!!!
And that is in fact the real problem...not whether the boat is a "good ground", but that the mast is a better conductor than the air!!!

Although the strike to my Annie Laurie in 2006, melted one vhf antenna, blew the other one to bits and left a scortch mark on the masthead....so, I'm assuming it was "direct", but you may be correct that it could've been a side-flash...
(Oh, and one thing I forgot to write was that it blew a section of the epoxy off my keel...an oval shaped section on the bottom of the keel/wing about 6" x 10", leaving bare lead.....)

BTW, I can say with 100% certainty that I've persoanlly seen dozens of direct strikes to my repeater tower...

And, while I did have some electronics (a couple handheld Garmin GPS 76's, etc.) that were "okay", I didn't have any faith in them staying that way, so I replaced them all!!!!





Quote:
Second, if a boat took even a 10 million volt strike, the chance of nothing more than losing your electrical systems seems remote.
Tom, I understand your opinion here, as this is the opinion of just about every layperson out there....
But, the fact is that some antenna masts/towers (mostly galv. steel) are struck 1000's of times each year with 10 million+ volt strikes, and even though they have 10's of thousands of dollars of electronics, in a building at the base of the tower, connected directly to them / dozens of coaxes, etc..they suffer NO damage at all!!!
My own personal, first-hand experience with this backs up the engineering/science of what "lightning protection" actually does....
It DOES work and DOES work well....
{My home/office is in north central FL, and my ham radio repeaters are at 1000' above ground level, on a 1368' tall tower, just a few miles from my house.....and they get struck all the time....I've personally been sitting in my car (waiting for the rain to stop) and watched the tower take multiple direct strikes.....and I've been inside the building when the tower has taken strikes (bare concrete floor and block walls makes for a "thundering experience"!!) }

As, I wrote earlier, lightning topics do tend to spur controversy.....especially since the science and real-world experiences are so different from what "common sense" would tell 'ya!!!


Anyway, take care and fair winds...

John
s/v Annie Laurie
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Old 15-06-2012, 12:24   #32
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Re: Lightning strikes

No facts to back it up, but I would imagine that cats get hit more often as they don't have an easy way to bond their mast to an underwater bonding directly under the mast step. It seems most of the lightening bonding writeups require that the bond be almost directly below the mast step and have no sharp turns on the wiring.
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Old 15-06-2012, 14:43   #33
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Re: Lightning strikes

This has been an interesting thread.

One could go so far as to say, "En-Lightning!"
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Old 15-06-2012, 16:29   #34
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Re: Lightning strikes

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Originally Posted by Paul L View Post
So what damage occurred to the boat that it was a write-off?
Where do I start? Five holes at least blown through each hull. All f/glass scorched and delaminated off the cedar core, inside the boat. Mast has discolouration spots all over it. Tank lids (the soles) blown off. A fire ball had apparently gone through the boat. The insurance company wrote it off as unrepairable. It is the most unusual lightning strike we've heard off, different to the strike that wipes our your electronics and maybe puts a hole in the hull. This was an extreme strike.
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Old 15-06-2012, 16:37   #35
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Re: Lightning strikes

This was a catamaran - we have known of at least two other multis that have been hit so perhaps there is validity in the comment about the mast. It was a beautiful vessel, well-built, immaculate in and out, worth a lot of money, so it wasn't a cheapie being written off.
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Old 15-06-2012, 17:59   #36
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Re: Lightning strikes

[QUOTE=ka4wja;971150]Tom,
BTW, I can say with 100% certainty that I've persoanlly seen dozens of direct strikes to my repeater tower...
But, the fact is that some antenna masts/towers (mostly galv. steel) are struck 1000's of times each year with 10 million+ volt strikes, and even though they have 10's of thousands of dollars of electronics, in a building at the base of the tower, connected directly to them / dozens of coaxes, etc..they suffer NO damage at all!!!
My own personal, first-hand experience with this backs up the engineering/science of what "lightning protection" actually does....
It DOES work and DOES work well....
{My home/office is in north central FL, and my ham radio repeaters are at 1000' above ground level, on a 1368' tall tower, just a few miles from my house.....and they get struck all the time....I've personally been sitting in my car (waiting for the rain to stop) and watched the tower take multiple direct strikes.....and I've been inside the building when the tower has taken strikes (bare concrete floor and block walls makes for a "thundering experience"!!) }

Hi John, thanks for the input but in your experience noted above, are the electronics being protected by some kind of Faraday's cage and that's why they suffer no damage? A friend of ours suffered a heart attack while building his steel boat and wanting to continue, he built a type of Faraday's cage to wear around his chest!!!! while he welded his boat. If we could wrap our boats up in a Faraday's cage then perhaps we'd be okay!!!! We know lots of people who put their marine radios etc. inside their ovens during an electrical storm.
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Old 15-06-2012, 18:46   #37
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Re: Lightning strikes

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Originally Posted by sokari View Post
.......................

Hi John, thanks for the input but in your experience noted above, are the electronics being protected by some kind of Faraday's cage and that's why they suffer no damage? A friend of ours suffered a heart attack while building his steel boat and wanting to continue, he built a type of Faraday's cage to wear around his chest!!!! while he welded his boat. If we could wrap our boats up in a Faraday's cage then perhaps we'd be okay!!!! We know lots of people who put their marine radios etc. inside their ovens during an electrical storm.
No, they are mainly protected by good bonding and earthing techniques and the fitting of surge protectors.

As a generalisation, the tower is one metal electrical mass well earthed via earth stakes and earth leads. The outer conductor of the coax cable is bonded to the tower at both the antenna end and where the cable leaves the tower near the ground. Some antennas are designed to be grounded at their mounting point. The minimum radius of the coax bends and the earthing leads are controlled. All metal cable trays and cable supports are earthed as it the metalwork structure and cabinets inside the building. Surge suppressers are fitted in line with the coax cables after they enter the building. The sizing of earth leads and final resistance values of each part of the system is measured and controlled.

A faraday cage is designed more to deal EMP but unless completely contiguous (hard to achieve),it must be designed for a particular part of the radio spectrum. From what I have read (and there was a post about it some months back), most of the EMR from a lightning current is in the lower part of the spectrum (KHz & MHz, not GHz).

An oven might be suitable but would depend very much on the metal, size of any opening, gaps, glass panels etc and the bonding between the various metal components. I would not rely on it. Still it can't hurt

Interesting story about your friend and his welding and heart. Do you know if this was science based or just "suck it and see".
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Old 15-06-2012, 18:48   #38
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Re: Lightning strikes

After reading extensively on the subject, my conclusion for lightning protection is: INSURANCE AND KARMA

You can buy insurance, but have to earn Karma!
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Old 15-06-2012, 19:25   #39
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Re: Lightning strikes

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Actually the insurance figures show that catamarans moored, docked or at anchor are struck almost twice as often as mono likewise moored.

snip

OK, I made all this up, its not true.
Your last sentence is true, and part of your first sentence. The part of your first sentence that I changed to red is not true.

I read the same info (I believe it was from Boat US) and they postulated that the reason multihulls are struck more often is either 1) they are more often anchored or on moorings, and/or 2) they are more often situated on "face docks" (outer docks at the edge of the marine -- not finger slips).

Here is a reference with those stats:
Catamarans

Partial quote: "We can explain why catamarans are struck twice as often, which has nothing to do with having twice as many hulls or elevated bridge decks. However, it has a lot to do with being twice as wide, which means they get less shielding from nearby boats when they are in a marina. Also, multihulls, because of their wider beam, are more likely to be docked at the end of the dock, in which case they have no shielding at all on the open-water side."...
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Old 15-06-2012, 19:32   #40
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Re: Lightning strikes

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Originally Posted by SailFastTri View Post
Your last sentence is true, and part of your first sentence. The part of your first sentence that I changed to red is not true.

I read the same info (I believe it was from Boat US) and they postulated that the reason multihulls are struck more often is either 1) they are more often anchored or on moorings, and/or 2) they are more often situated on "face docks" (outer docks at the edge of the marine -- not finger slips).

Here is a reference with those stats:
Catamarans

Partial quote: "We can explain why catamarans are struck twice as often, which has nothing to do with having twice as many hulls or elevated bridge decks. However, it has a lot to do with being twice as wide, which means they get less shielding from nearby boats when they are in a marina. Also, multihulls, because of their wider beam, are more likely to be docked at the end of the dock, in which case they have no shielding at all on the open-water side."...
Thanks for the input , I thought I was making it all up Looks like I got a very small part of it true
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Old 15-06-2012, 19:35   #41
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Re: Lightning strikes

All boats I know of that took direct hits were Swedish. Just keep away from them and you may be actually pretty safe. ;-)

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Old 15-06-2012, 20:11   #42
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Re: Lightning strikes

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Interesting story about your friend and his welding and heart. Do you know if this was science based or just "suck it and see".
The guy building the boat was a very experienced electrician so it definitely wasn't just hit or miss. He really knew what he was doing. His heart attack was caused when he was checking work done at a local show - it had been faulty work done by the carnies and he was electrocuted but saved by his offsider. When he returned home from hospital he sat in his backyard in his pjs looking at his half finished yacht and figured out that he could build a faradays cage to wear - earthed to the welding gear. He finished the job and sailed the boat with his wife for many years after. He had to be dedicated, eh? The hospital have kept all the records of his project and even put a photo of him at work up for other heart patients to see. I believe it happened in Newcastle, NSW.
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Old 15-06-2012, 20:34   #43
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Forgive my naiveté, but I grew up in an area where thunderstorms did not exist. And for the past 30 years, I've lived in an area where we have about 3 "thunderstorms" per year, which means maybe two dozen lightening bolts within 5 miles of me each year.

It seems a grounded mast is obviously a shorter and quicker path to ground than air. If I were an electron, I'm pretty sure I'd jump on that.

What about an ungrounded conductor suspended in the air? Would I be equally likely to jump on that as a brief express lane that dumps me out short of my destination? If so, how willing am I to travel laterally to hit that brief low resistance thrill, only to be dumped back into the air? I'm sure this has been tested, and I'd love to see what the results are.

What I'm wondering, is whether a grounded mast is more likely to be stricken than an ungrounded mast. I understand grounding and bonding to minimize damage. But what is the research? Is there any correlation between frequency of strikes on grounded/bonded versus insulated?
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Old 15-06-2012, 22:38   #44
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Re: Lightning strikes

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sokari,

To sum up....
READ Dr. Thomson's paper, follow his recommndations (as I did), and add some lightning arrestors on your antenna coax cables, wind instr. etc....and disconnect things when not in use....
And, you'll be in good shape!!!

Oh, and read noelex77's posts here as well....

sokari, I do hope you find these suggestions helpful....

Fair winds...
John
s/v Annie Laurie
Thanks, John. I do appreciate your time and effort to give me all that information. And also to everyone else for their 2 cents worth - thanks all. Despite having sailed through areas with daily electrical storms we have never been hit - touch wood - but we have had boats anchored nearby struck and some of our electrics have gone haywire even though we haven't been hit. Also my husband, who always unplugged all electronics when we saw the storms appearing on the horizon, (particularly up in Malaysia and Thailand) was one day "zapped" by the residual charge in a lead. The storm had gone through and he picked up the coax to plug back in - didn't touch the end, but whatever charge was there decided to jump across to his finger. I just heard "zzzzzt" when it happened. He didn't make the same mistake twice. We waited until the storms were well and truly in the distance.
Thanks again
Rosemary
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Old 15-06-2012, 23:19   #45
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Re: Lightning strikes

Lightning is not random or mysterious. It is a governed by the laws of physics and is generally well understood.

What makes lightning seem mysterious is that it occurs in an electrostatic environment that humans are not even aware of until their hair stands on end or their is a bright flash in the sky. We are so ill equipped to sense this environment that we take shelter under a tree in a rain storm. If we could see in this environment the tree would glow as it leaves striped away electrons from the passing air and water molecules. And, we could watch these ions rise toward the sky as a charged cloud passes overhead. Then as this process builds a local charge difference between the tree and the air a small "leader discharge" occurs creating a highly conductive ionized path of superheated air reaching upward towards the cloud. Suddenly, the negatively charged bottom portion of the cloud has a discharge path to ground. WHAM! Lightning.

This process is not limited to just trees, it also happens to air currents that get deflected upwards, and to boats that are not properly bonded. However, if the boat is properly bonded then any charge buildup is conducted to ground and no lightning. This sounds good, but there is a problem. You see, there are two form of lightning. What was described above is the most common form of lightning. There is another form of lightning that goes between the negatively charged ground and the positive ions in the cloud. Two interesting things about this form of lightning is that it's comparatively rare, and it is a lot more powerful then the common type. It turns out that a well bonded boat acts as a skyward extension of this negatively charged ground and slightly improves the chance that lightning will occur.

To complicate this picture further there are two ways in which clouds can be charged. The most common is with the top positively charged and the bottom negatively charged. However, the reverse is also possible. This is of course not helpful in preventing a lightning strike when the boat is bonded. Fortunately, this cloud configuration is quite rare.

The above explains why bonded boats are less likely to be struck by lightning but never the less still do get hit. Also, it is the invisible paths that are forming that result in the near misses that are so often reported. So, no mystery here. Just good old physics.

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