Just ground the mast. I have used and experimented with various spark gap lightening arrestor setups for Radio transmitters. I've found them fairly ineffective. To ground the bulk of the strike without grounding your RF signal, use a 1/4 wave jumper. FOr anything else by the time you get a big enough gap to reduce odds of getting hit, you've also lost the effectivness, (remember that bolt just arc'ed through a couple of miles of air, the 1 inch, 10 inch, or 10 feet/meters of spark gap will have no effect). Neither will the difference of the top to the bottom of a 30 foot mast. The entire thickness of your boat even if it was made of a solid 15 foot thick block of fiberglass is not enough to stop it. Your best bet is give it the easiest path to ground you can that doesn't go through you or your hull. See lightening threads to find suggested grounding.
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