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post #71 of 75 Old 09-16-2008
Learning the HARD way...
 
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I just stumbled across this thread and tought that I'dd add a couple more cents...

Faraday cages block static electrical fields. Like lightning. In Hartley's pic above, the fuselage acts as a faraday cage. Faraday cages can not block EMF or Electro Magnetic Feilds.

A good Faraday cage on your boat is the oven (if you have one), or the pressure cooker mentioned above. the CDI box probably IS a Faraday cage, but this one in meant to keep the static IN the box, rather than out, as part of a noise supression circuit (so people with nearby radios don't hear your sparkplugs every time they are fired).

Also - WOW! Waltsn's pic is unreal!
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post #72 of 75 Old 09-16-2008
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Yup, the designers of the outboard probably didn't worry about it working in a lightning storm. I think if I added a shield around the two wires going to the pickup coil and terminated the shield at the CDI control box, I would have not had any problem with the outboard - but also wouldn't have got any pictures.

Here is a pretty crazy idea (good that its burried way back in this thread) regarding how a lightning upwards leader is formed and possibly has some influence on a sailboats chances of getting hit.

A premise here is that in a lightning event, only air is ionized and water is not because it is at least 80 times as difficult to ionize than air (maybe even thousands of times more difficult to ionize). We could discuss this but I put all this mumbo jumbo in this web site trying to back this idea up ( analogengineering.com/lightning/surface.html - cut and paste again )

You can also read this paper wwwq.lightningsafety.com/nlsi_lhm/Radials.pdf (remove the q after the www) but the bottom line of this paper is that the water surface will charge up during a lightning event. The water surface charge is typically positive because electrons have been driven into the bulk of the water. The speed of this charging depends on the salt content in the water. The charging takes nano seconds for salt water and tens of microseconds for very fresh water. The charging of the water surface also completely "encloses" the electric field between the water surface and the charge in the sky.

So the negative charge in the sky or the lightning streamer coming down from the sky creates an electric field which "charges" the water surface.

The crazy theory is that the water at the surface is very difficult to ionize but the air just above the water is much easier to ionize so the air above the charged water surface gets ionized and creates channels for charge to flow. These channels meet up at some place (like under the downward coming streamer) and then head upwards to meet the downward coming streamer. Sort of just the opposite process of the "radial pictures" you can see in the referenced links. Generating a upwards streamer is of course the best way to receive a downwards comming streamer.

The interesting thing about this theory is that it is pretty much only dependent on structures above the water surface which can influence the electric field such as a mast. Or the indentation in the water surface left by a boat hull. Anything under water (such as a grounded keel) has no influence.

Cant back this up with references.. just throwing out something for discussion.
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post #73 of 75 Old 09-16-2008
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from my blog here awhile back:

Lightning is a series of several strokes the first being a stepped leader stroke which develops a path from the cloud to the ground sometimes the channels lead nowhere which explains why lightning forks eventually a path is developed from the cloud to the ground and lightning happens..
Once the stepped leader stroke develops a conducting channel to the ground or you get the "Return Streamer" which is a discharge of electricity from the ground to the cloud. This stroke is more brilliant than the leader stroke and is the observed stroke we see..an average of three leader strokes and three return stokes occur in one conducting channel..

SO

the better the channel the more chance of the strike
I'd say not grounded would be safer and moving is even safer than being at dock or anchor.....
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post #74 of 75 Old 09-16-2008
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Quote from this link edis.ifas.ufl.edu/pdffiles/SG/SG07100.pdf (some day I may be able to post a link)

As the negatively- charged stepped leader moves downwards, it induces a positive charge on the ground below. When the top of the leader is about 30-100 yards above ground level, the induced positive charge becomes so concentrated that a new spark forms, as shown in Figure 2. This positively charged spark is the crucial process as far as the attachment to a boat is concerned. If it starts at the top of a boat mast, then lightning strikes the mast. Unfortunately, there is no scientifically accepted technique to prevent this spark from forming. Even if a device were effective in diverting the attachment spark, it would not be a good idea to mount it on the masthead as the attachment spark may start elsewhere on the boat or crew. The likelihood of lightning attaching to the masthead is a safety feature as far as crew is concerned.
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post #75 of 75 Old 09-17-2008
1980 Nonsuch 30C #77
 
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Bottom line is that you could have the most sophisticated grounding/bonding system available. But, if the system isn't independent of the boat (that is, insulated FROM the boat as it is on a house), or if the strike exceeds the capacity of the conductor...you're screwed.

Cheers,

Bob

You can polish a turd, but you just end up with a shiny turd.
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