Join Date: Jan 2011
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Re: Use a metal swing keel for lightning sink
Dynaplates and "Grounding shoes" WILL NOT WORK AS LIGHTNING GROUNDS.
The reason, Remember basic Electricity and Magnetism in physics? The electric field within a closed conductor is zero. This means that at high values of E field (Lightning), the field simply bridges the pores of the plate giving only the external area to dissipate the current.
At low values of E (low current typical of radios etc), the field does NOT bridge the pores so it acts as if it has a huge surface area.
For a lightning dissipating ground, you really do want a large external area and as much "external" edge as possible. The field is highest at the edge and that is where the current will go.
Yesterday, somebody posted a pic of what I think is a good lightning ground, it was a narrow long strip on their outside hull. However, it looked as if it was painted. Be careful, some anti-fouling paints do not conduct and you want it to readily conduct
So, as long as you keep your metal centerboard painted with CONDUCTING paint, it'll work cuz it has lots of surface area. I'd worry about using the CB pin as your contact though.
The reason for wanting your lightning ground close to the surface is for two reasons and I am not sure they make sense :
1. Analogy to the E field within a conductor (Yeah seawater is a good conductor for most low frequencies) says the field there should be zero so it seems like no current would flow. Not true, remember that in a VAn de Graff generator the charge is brought to the INSIDE of the big hollow HV terminal and then it simply goes to the surface. I do not think this reason makes sense.
2. High frequency "skin effect" whereby high freq components of the lightning pulse travel best on the surfaces of conductors. Maybe, maybe, maybe.
My lightning ground on my sailboat (up till now) was a 2' X 2' copper sheet (flashing) with a copper bolt/clamp/soldered to a tinned #00 insulated Ancor cable 15' long with a good welders clamp to attach as high as I can on the mast to avoid bends of the wire. During a storm, the plate gets dropped overboard and the clamp goes on the mast. A Nylon rope keeps the plate from being dragged backwards to ward the prop and I maintain only steerage speed. Yes, I am paranoid of lightning.