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Old 13-05-2011, 03:12   #1
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Carbon Fibre Mast Lightning Protection

Have a carbon fibre mast with rod rigging. Can anyone comment on the effect that lightning strike would have on the carbon fibre mast? Is the carbon fibre conductive enough to resist damage from a strike? I would guess the rod rigging is adequate pathway to earth but does the mast need additional protection? The rod rigging passes through CF spreader fittings which could presumably be damaged by a strike? Any experience or analysis done on this?
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Old 13-05-2011, 06:21   #2
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Re: Carbon Fibre Mast Lightning Protection

I'm not sure how your mast is constructed, but aircraft carbon fiber structures have a wire mesh built in to protect them in case of lightning strikes. I once saw a picture of a non-protected panel versus a protected one that were subjected to simulated lightning strikes. The unprotected one essentially disintegrated. There was a large hole (about 50% of the 2ft square) surrounded with frayed carbon fibers. The protected one had only a small burn mark at the strike point. If the company that made your mast put in a conductive mesh you would probably be ok, with perhaps just a small amount of repairable damage near the strike and exit points. If not, I would guess you would be getting a new mast. Fortunately strikes are fairly rare.
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Old 13-05-2011, 07:03   #3
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Re: Carbon Fibre Mast Lightning Protection

The proper lightning ground procedure for a carbon mast is to have a large internal copper wire, (like #2 MIN, or preferably #00). This would be connected at the top to a pointed rod or a pointed rod with bottle brush bristles as well. (Ion dissipater) It is supposed to be the "tallest thing", but the VHF antennae always wins out on that count.

The other end of the wire should go as straight as possible to a copper plate, (NO sharp turns), mounted to the hull. (2 sq ft min for salt water, 4 sq ft min for fresh water) The accumulated liner inches of edge is most important, so a long rectangle is better than a square.

Do NOT use a Dynaplate... "Dynaplates" are useless unless soaked in acid at every haul out, to remove clogging growth, AND they can be dangerous in a lightning strike. They have been known to explode. Regardless, they make a very poor lightning ground.

ALSO, grounding that rod rigging, (through the inner chainplate bolts), as well as large interior metal, with a #6 green wire, is also a good idea, as it helps prevent "side flashes". IT DOES NOT REPLACE THE MAST'S COPPER WIRE. SS is a very poor conductor, only a SMALL fraction of the conductivity of copper. The only reason an aluminum mast itself can be it's own lightning conductor, in spite of it's poor conductivity, is that it's size is hundreds of times that of a copper wire.

This "lightning grounding issue" is one of the downsides of a carbon mast, and eats up some of the lighter weight that you paid so dearly for. Nevertheless, it is wise for ALL of us to have a grounded mast. The likelihood of taking a hit in lightning prone areas, is in the neighborhood of 50/50 over the life of the boat. Proper grounding will not necessarily prevent all damage, especially to electronics, but it will prevent holing the hull, or driving the mast through the bottom!

The one given, is that IF lightning hits your mast, it WILL proceed to find "ground", through one path or the other!

Good luck as well!

Mark
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Old 13-05-2011, 08:25   #4
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Thanks for those good analyses.

Assuming that an internal wire would melt any insulation and leave a mess inside the mast, would you plan for an uninsulated wire?
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Old 15-05-2011, 11:57   #5
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Re: Carbon Fibre Mast Lightning Protection

still, I would contact the maker of you mast. AFAIK if a cf boat mast takes a direct hit, it is usually condemned and replaced, rather than gambling on whether the material might have been damaged in any small spot. But whoever makes it should have some authoritive information to share with you.
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