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Old 24-07-2012, 12:52   #181
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Re: Lightning Strikes

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Originally Posted by Adelie View Post
A steel boat will not be a perfect faraday cage or even a very good one unless all the portholes are covered with steel plates and the hatches likewise.

Understand that an aluminum boat will not provide the same overall protection as a steel boat. While the aluminum boat will provide the same protection against direct electrical currents, there will be no protection from magnetic fields which can induce electrical currents in conductors already inside the faraday cage.
How big the holes can be in a Faraday cage is dependent on the frequency. A microwave oven has those little holes in the door, some cages are made of chicken wire, and lightning rods twenty feet apart are considered a Faraday cage of a crude sort because the current is spilt between several down conductors and the electromagnetic pulse tends to be canceled out because of the several down connectors. The distance between gaps in a Faraday cage is called the slot length. At a slot equal to one half the design frequency for a Faraday cage, falloff will commence. The attenuation of the electromagnetic pulse (power) will be one tenth the original amount if the frequency is increased ten times for a given Faraday cage slot width. In the case of lightning the highest frequency with any energy to it is about 1000 kilohertz, the middle of the AM broadcast dial. If you tune to the lowest frequency on the AM broadcast radio receiver and listen for static it makes a good lightning detector. For design purposes for lightning, 1000 kilohertz is considered conservative and is the frequency that is used.

The wavelength of an electromagnetic pulse is equal to the speed of light divided by the frequency. For lightning, take 300,000,000 meters per second for the speed of light and divide by 1000 kilohertz or 1,000,000 and get a wavelength of 300 meters. This means that if you use a slot size of 30 meters in a Faraday cage intended for lightning, the intensity of the electromagnet pulse will be reduced by ten times. Now 30 meters is considerably larger than a port hole; therefore, let’s use a size of 0.3 meters or slightly less than a foot, which would make for a power reduction of 1585 times less or 0.0006309 times the original power of the lightning electromagnetic pulse. If we want to use a steel enclosure to reduce the magnetic part of the electromagnetic pulse for very low frequencies, then a formula for absorption loss is used. Absorption loss is the amount of energy strength dissipated when a wave goes through a shield. Reflection losses, and correction factor for multiple reflections in thin shields, are also considered part of the calculation. These calculations are more complicated than that slot length. Measurement of power for slot length is based on a log scale with units that are in decibels. If you are curious about decibels, Wikipedia has a description.
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Old 24-07-2012, 13:14   #182
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Re: Lightning Strikes

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How big the holes can be in a Faraday cage is dependent on the frequency. A microwave oven has those little holes in the door, some cages are made of chicken wire, and lightning rods twenty feet apart are considered a Faraday cage of a crude sort because the current is spilt between several down conductors and the electromagnetic pulse tends to be canceled out because of the several down connectors.................

Westwinds,
Sounds like you have an engineering understanding of Faraday cages. Help me make something practical of it. Let's say you want to protect from lightening induced EMP an unconnected, backup, autopilot computer. Can I wrap it in copper mesh and ground the mesh? Stick in the oven? Microwave?
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Old 24-07-2012, 13:32   #183
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Re: Lightning Strikes

Brace yourselves, a micah-brainfart just occurred.

How about trailing a buouy on the end of an insulating line, with a kite/balloon flown (higher than your mast height) from the buouy on the end of a conducting line? If anything's about to get hit in in the local area, it would be the decoy, wouldn't it? Wait....probably only useful in conditions where the kite would not be blown flat to the sea. Ok, how about a rocket trailing a wire, launched when static charge builds up to critical levels?
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Old 24-07-2012, 15:39   #184
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Re: Lightning Strikes

Great idea micah. Might save you from a small strike. However what if Zee is right and the odds of the big one striking you are very remote but now you have given him a reason to come a bit closer?
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Old 24-07-2012, 19:57   #185
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Re: Lightning Strikes

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Originally Posted by Paul L View Post
Westwinds,
Sounds like you have an engineering understanding of Faraday cages. Help me make something practical of it. Let's say you want to protect from lightening induced EMP an unconnected, backup, autopilot computer. Can I wrap it in copper mesh and ground the mesh? Stick in the oven? Microwave?
I am a civil engineer and radio amateur, but feel there are radio amateur folks here on cruisersforum that have a better background and more experience. As for protection, here is a cut and paste with practical experience on this issue:
annk 14-08-2010, 19:41 #4
One tip( we have been hit twice), put handheld gps, vhf, phone, computer in the oven. It acts like a faraday cage and has saved our equipment twice now when everything else was fried!
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Old 24-07-2012, 20:31   #186
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Re: Lightning Strikes

GRRRRR Big Sparks From The Sky!

I cant read all 13 pages of this thread. But would like to share a recent strike story with all of you. Please forgive me if I am jumping protocall.

8 weeks ago during a storm here in Miami, FL my anchor alarm went off. As I headed out to deck we were hit! At this point I am unable to see clearly and had complete loss of my 12v system. Not a deal breaker if you have a diesel. My atomic 4 gas lost its alernator, electronic ignition, coil, fuel pump, rotor and cap. Crud I was screwed at this point. Clocking 70 knot gusts hanging on a stern anchor is not fun! Especially when you are looking at a really hard seawall.

TO keep this short I lost nothing but my ignition system. Everything else was bonded to the keel. The bonding wire from the engine block to the keel was broke. So please check that wire people.
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