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Old 12-10-2010, 02:54   #16
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Originally Posted by noelex 77 View Post
I have an aluminium boat that was struck by lightning a couple of years ago.

The result was that all electronics with outside, or with major components outside, such as the VHF and GPS were fried.
All electronics that was inside survived undamaged, apart from some LED lights that were on at the time.
This suggests that there was some protection from the hull, at least in this case.

I have read a lot about lightning damage since and there is no doubt that a metal hull and its occupants, will survive a strike better than any other material.
Contrary to popular belief a metal hull (or a well grounded fiberglass hull) is actually slightly less likely to be hit than an ungrounded hull. This is because a major cause of strikes is the build up of a static charge at the top of the mast. Grounding the hull means the mast top is at the same potential as the surrounding water.
This is my inderstanding. Antennae will allow the charge to get inside the 'cage'.
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Old 12-10-2010, 03:00   #17
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FM aerial outside "Faraday Cage"...

Believing that a steel hull would make an excellent Faraday Cage I went to great trouble to put an FM aerial on the top of the metal cabin.

The results were abysmal, worst FM reception ever, in the middle of a big city.

Through pure laziness I am now using a length of coaxial cable inside the metal cabin as an aerial with excellent results.
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Old 12-10-2010, 04:17   #18
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Through pure laziness I am now using a length of coaxial cable inside the metal cabin as an aerial with excellent results.
The signal obviously comes in thru the hatch, just like the lightening will
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Old 12-10-2010, 06:29   #19
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Dr. Ewen M. Thomson, founder of Marine Lightning Protection Inc., wrote the book on "Lightning & Sailboats" (Sea Grant pamphlet SGEB-17*), and more:
See ➥ Marine Lightning Protection Inc.
And ➥ Articles
* And ➥ http://www.marinelightning.com/ECE/SGEB17.pdf

See also the many excellent Lightning information links at
Lightning Protection on Sailboats
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Old 12-10-2010, 13:54   #20
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Thank you!!

I must thank everyone who has contributed. The education process is has been amazing.

I've read the various links that have been provided. My personal award for a great down-to-earth explaination of lightning and reference to the Faraday Cage goes to John Paynne and his article "Lightning Strikes on board"

http://www.kp44.org/ftp/LightningStrikesOnboard.pdf

Extract from the article: ...........
"Steel/Alloy Vessels. Connection of the mast base with a large, low resistance bonding strap to the hull or as more practical the mast step is sufficient."

I'd like to believe this action is all that is required.
and 2nd extract;

"St.Elmo's Fire (Brush Discharge). This phenomenon is more common on steel vessels and when it occurs usually precedes a strike, although the effect does not occur all the time. The vessel in effect becomes a large ground mass. Ionized clouds and balls of white or green flashing light that polarizes at vessel extremities characterize the discharge. The discharge of negative ions reduces the potential
intensity of a strike. Damage to electrical systems is usually induced into mast wiring, as the steel hull itself acts as a large Faraday cage."

That's one hell of a light show that I gotta see that before I die.
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Old 13-10-2010, 12:07   #21
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According to my understanding of EMP and faraday cages, a steel or aluminum boat is a good start but is not 100% because of all the holes in it for hatches and ports. I've been told that one way to check if it works is to put you cell phone in the boat, button her up and then try to call it from outside. If your cell rings, then RF is getting through which means that EMP effects will also get through.

As for electronics, the antenna pass thrus, and electrical wires on the outside, IE mast lights etc will act as a conduit for EMP to get at the rest of your gear. People inside should be ok, if the interior insulates them from the skin.

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Old 13-10-2010, 12:38   #22
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lightning is an odd beast,it's not uncommon for it to be in the hundreds of thousans of volts,not to mention a megabolt,as for amperage ???probably not super high,but as the flash only last's a few microseconds.
You must have babybolts at your home

Some numbers taken off the SMHI website (Swedish Meteorological and Mydrological Institute) says that the lightnings are a bit more powerful than you wall socket:
  • Amps: 5000-100 000 Amperes, Avg 20 000 Amperes
  • Voltage: 10 million - 100 miillon Volts, Avg 30 million Volts )
  • Time primary discharge lasts: 0.0001 seconds
  • Number of discharges on the same channel: 1-10, avg 3
  • Energy in a typical 3 discharge lightning: 20 000*30 000 000*0.0001*3=180 000 000 Joule.
  • Main part of this energy turns into heat in the lightning channel and close to the point of impact.
Weeeee, need asbestos overalls instead of swimsuits and better hope your hair isnt newly styled
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Old 15-10-2010, 13:53   #23
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Any wires, bulkheads, holes, ports, etc... will conduct lightening to the inside of the hull. You must shield/ground all such openings. There are devices called lightening arrestors that can be attached to antenna cables where they pass through the hull to conduct any voltage over the normal RF from the radio to ground. Your body must be insulated from the hull. There are numerous lightening links on how to make your boat safer. It doesn't hurt to implement as many as you can.
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Old 15-10-2010, 21:38   #24
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NoeleX77. No I undestand it very well. I have used Faraday Cages extensively for calibrating electronic instruments and ultra sensitve receivers and transmitters. One cage we used was about ten ft by ten feet by 8 ft. Faraday cages are not solid structures. Ours we copper mesh cages. A solid metal box does not make a very effective Farady Cage.
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Old 15-10-2010, 21:45   #25
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A mesh is no better than solid metal. They use a mesh because at most frequencies that is all that is needed...why use more.

But making a boat into such a thing is difficult. All conductors must be bypassed for HV, probably with fancy electronic devices. And any largish hatch will have allow a electromagnetic field to enter.

Do what I do...and I'm in the lightning center of the world...have a beer and don't worry about it.
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Old 15-10-2010, 22:37   #26
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Do what I do...and I'm in the lightning center of the world...have a beer and don't worry about it.
Now that sounds a lot like common sense, which is all too uncommon now days.
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Old 16-10-2010, 02:38   #27
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Quote:
Originally Posted by Ike View Post
NoeleX77. No I undestand it very well. I have used Faraday Cages extensively for calibrating electronic instruments and ultra sensitve receivers and transmitters. One cage we used was about ten ft by ten feet by 8 ft. Faraday cages are not solid structures. Ours we copper mesh cages. A solid metal box does not make a very effective Farady Cage.
I hope it wasn’t marine transmitters you were calibrating

Daddle has already explained why a cage not solid metal is used.
Quote:
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A mesh is no better than solid metal. They use a mesh because at most frequencies that is all that is needed...why use more.
Solid metal is expensive, heavy etc. It will however work at least as well as a cage
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