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Old 11-10-2010, 11:24   #1
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Faraday Cage Principle - Do Steel / Aluminium Hulls Qualify ?

My boat is a 44' ketch with a fully welded aluminium deck, hull and keel.

I was discussing the potential risks of lightning strike damage with a two mates the other evening. It was suggested that Faraday Cage Principle may apply to a sailing boat with a full metal hull and deck. The expectation being that some protection may be provided to the electronics and electrical systems that are housed within. The logic seems sound, but I prefer not to test theory as I am more interested in the reality of any situation and risk minimization measures.

It was also suggested that a metal boat may be more prone to lightning strikes and that I should therefore consider carrying a few lengths of heavy duty electrical welding cable. Prior to an electrical storm, these cables can be attached to strategic parts of the two masts and rigging and finally draped over the side to trail in the wet stuff (to provide a path for the big flash to follow).

There were some other brilliant suggestions that evening but I subsequently forgot those after we had a few more beers.

Does the forum have an educated view on the Faraday Cage Principal and how it may/ may not apply to small metal hull sailing ships???
[Spelling corrected for Australian members].
SY Jaden, Singapore

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Old 11-10-2010, 11:30   #2
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Frustrated the hell out of me when trying to ESP coat Aluminum channels, but will be interested to see the comments from our more knowledgable friends.
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Old 11-10-2010, 11:33   #3
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Does the forum have an educated view on the Faraday Cage Principal and
Yes, I do have some info: Dr Huang Hoy-Lung said that Aluminum boats were "Microwave Principle" - "those inside during lightening strike get ass fried".



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Old 11-10-2010, 11:43   #4
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Being inside an aluminum box is about a best case scenario. The downsides are. 1. make sure all electrical fittings are grounded at the passthrough. 2. Insulate yourself from the hull to prevent induced currents. The jumper cables are not a good solution, better use flexible copper braid to electrically bond the base of the mast to the hull. The hull itself is the path to ground although the paint interferes with good conduction so having exposed copper strips bonded to the hull and electrically connected to the mast is the best. There are several sites with a good discussion of lightening protection. AYBC has a good page on the subject. All boaters should read and follow as lightnening is one of the higher causes of boating deaths.
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Old 11-10-2010, 12:07   #5
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here is the link Science & Technology
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Old 11-10-2010, 13:01   #6
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Excerpted from Michael Kasten’s “Strike Attenuation Strategies For Boats”

“... With a metal boat, the hull itself makes an excellent protective Faraday cage as well as an excellent conductor. The ABYC states that aboard a metal boat, "If there is electrical continuity between metal hulls and masts, or other metallic superstructures of adequate height, then no further protection against lightning is necessary."

However on a metal boat there must still be one or more air terminals at the top, attached directly to an aluminum mast or to the metal superstructure. The best strategy aboard a metal boat will be to encourage the hull itself to be integral with the primary conductor. In other words, to create an effective Faraday cage it seems appropriate to have a #4 AWG or larger copper jumper attached from the primary down-conductors (mast, shrouds, etc.) to the deck our house top, to encourage the lightning to go around the outside of the hull, rather than through the boat's interior...

... In the case of a metal boat, the hull serves as the Faraday cage, an integral part of the primary conductor, an equalization bus and a bonding conductor all in one. The remaining objective is to dissipate a strike safely. There is good justification to use a grounding strip, as well as supplemental grounding electrodes located just above the waterline, and in this case attaching the electrodes on the inside directly to the boat's metal structure...”

Here ➥ Lightning Attenuation Onboard
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Old 11-10-2010, 13:39   #7
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Does the forum have an educated view on the Faraday Cage Principal and how it may/ may not apply to small metal hull sailing ships???

I can say for certain that an all aluminum boat does not provide absolute protection from lightening damage - we know two that have had internal equipment fried during a strike. But metal hulls are almost certainly much better/safer than non-metal hulls in lightening strikes.

My opinion is that putting wires/cables in the water will add essentially no further protection on a metal boat - the hull (particularly aluminum) is already a good conductor and already in the water. As mentioned above, its probably better to make sure the mast and stays are cleanly/permanently grounded to the hull.

My opinion (based on quite a bit of research) is that the little brushes on the masthead are placebos and do almost exactly nothing. A lighting strike is a really really major effect and will just ignore these little brushes.
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Old 11-10-2010, 14:29   #8
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The little brush don't do much about the strike itself. The discharge the local area and make the atmosphere around the masthead the same potential as the boat and sea. No potential difference makes for no current flow. No existing current flow reduces the possibility of a strike using the existing path of flow.
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Old 11-10-2010, 19:19   #9
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A metal hull does not make an effective Faraday Cage, because a Faraday Cage is not a metal box. It is a fine mesh of conductors,

- A Faraday cage is a metallic enclosure that prevents the entry or escape of an electromagnetic field (EM field). An ideal Faraday cage consists of an unbroken, perfectly conducting shell. This ideal cannot be achieved in practice, but can be approached by using fine-mesh copper screening. For best performance, the cage should be directly connected to an earth ground What is Faraday cage? - Definition from Whatis.com

Frankly if a hull were a Faraday Cage it would sink, However, a metal hull will provide some protection with the addition of a good lightning protection system as shown in one of the links from a previous post.
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Old 11-10-2010, 19:28   #10
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Best description I have seen I think it was Gord who posted this a little bit of a stretch what makes a good farday cage? Other then putting spares in the oven. i cook so that's a bit of a pain. How do you store your extra electronics? here is gord's link great read I thought. Lightning Attenuation Onboard
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Old 11-10-2010, 19:41   #11
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The little brush . . . discharge the local area and make the atmosphere around the masthead the same potential as the boat and sea.
Nice in theory. But the brushes are too small to make any real difference. They are placebos - can't hurt except for lightening your wallet.

There has been lots of testing by the USDA on US grain silos - conclusion is you need a whopping big device to make any noticeable difference.
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Old 11-10-2010, 19:55   #12
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The little brush don't do much about the strike itself. The discharge the local area and make the atmosphere around the masthead the same potential as the boat and sea. No potential difference makes for no current flow. No existing current flow reduces the possibility of a strike using the existing path of flow.
All wishful thinking and not proven. It's just a logical argument, and like religion or politics, there are arguers on both sides who have very credible credentials.

And if true, does that mean the lightning is more likely to strike at deck level, or the radar or arch, or the heads of crew?

Getting back to the central question of this thread -- a metal boat should be safer for crew inside, but unless electronics are disconnected they will still pick up EMP (electro-magnetic pulse) radiation that will fry sensitive semiconductors and possibly a static/inductive pulse that can do likewise.

Lightning can travel an antenna and jump through control/power wiring affecting all connected equipment, or just some of it. Ask me how I know.

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Old 11-10-2010, 20:00   #13
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faraday cage,doug gardner ibew#jo1959

lightning will always find a path to earth ground,without a proper path to ground a side flash to the person at the helm ,although in theory the people inside whould probably be safe,i whould install a traditional grounding system,due to alluminums low melting point,while it's true that youre hull will act as a giant grounding plate,if the lightning bolt decides to exit at one small point near youre mast,it will surely melt a hole,a good example,i once worked on a electric hot watter tank,thought the power was off,it melted a hole thrugh the pipe,that was at 110,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,i whould recomend going up one size above the maximum recomended size of number number one to number two,i whould also install the lightning disipator type lightning rod it looks like a bottle brush,hope this helps.
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Old 12-10-2010, 02:37   #14
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Originally Posted by Ike View Post
A metal hull does not make an effective Faraday Cage, because a Faraday Cage is not a metal box. It is a fine mesh of conductors,

- A Faraday cage is a metallic enclosure that prevents the entry or escape of an electromagnetic field (EM field). An ideal Faraday cage consists of an unbroken, perfectly conducting shell. This ideal cannot be achieved in practice, but can be approached by using fine-mesh copper screening. For best performance, the cage should be directly connected to an earth ground What is Faraday cage? - Definition from Whatis.com

Frankly if a hull were a Faraday Cage it would sink, However, a metal hull will provide some protection with the addition of a good lightning protection system as shown in one of the links from a previous post.
You are misunderstanding the physics.
A metal structure is an "infinitely" fine mesh of conductors. Filling in the meshwork (to stop water coming in) helps rather than hinders the protection.
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Old 12-10-2010, 02:39   #15
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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.
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