I use 5 very thick (approx. 50 sq.mm) copper wires about 3 metres each. On one end the have big battery (crocodile) clips attached. The insulation is removed about 2 metres from the free end.
If I run into thunder I clip them on the forestay and shrouds (my yacht is a ketch) and let them hang into water.
I do not know (fortunatelly I never been struck) if this really works, but this was something I learned from an article in a magazine.
The idea sounds sensible and uncomplicated for me
Two sorts of problem. A strike which damages your boat, (conducted eg down mast or shrouds) and static electricity from a nearby discharge which ruins you instruments.
Not much you can do about the former. Some say shackle some chain tot he shrouds and dangle in water - possble with a light anchor to dissipate charge - others will tell you if you provide a dischrage route like this you increase the likelihodd of a strike!.
Your instruments you can do something about. Disconnect anything electronic. If it is removable, (VHF GPS and handhelds) put it in a biscuit tin or in the oven where it is surrounded by metal. Wrap it in aluminium foil - anything to dissipate the static field.
A steel boat is possibly more likely to be hit than one of GRP or wood, but dissipates the hit better. Even so, we lost a lot of electronics. Also had 40deg deviation in the steering compass afterwards.
Since then I've followed a lot of threads on lightning protection. Various masthead 'lightning protectors' are marketted, claimed to protect the boat against a strike, but which frankly are about as useful as a Christopher badge. The advice on bonding/grounding given in Derwood's Florida Univ. reference is sound, though there is controvosy over the area of grounding you need to provide. Attaching a chain or thick wire to the mast/shrouds and leading it overboard is often mentioned as an ad-hoc means of protection, but will not provide anything like sufficient grounding. Some advocate attaching a large sheet of mild steel on the end.
The best advice in a thunderstorm is to stay inside the cabin and keep away from metal objects.
It seems it is impossible to predict what the result of a strike will be. When we got hit the wind instrument tranducer was vapourized, every piece of electronic gear had its circuit boards fried, the external alternator regulator burst into flames providing an engine room fire just to add to the fun! The batteries were unharmed and my wooden spars survived. There seems to be no concensus on the best action to take - frankly, draping chains from your shrouds or putting a wire brush at the masthead just aren't going to do anything in the face of such awesome energy.
My view too. When watching a lightning strike from clouds to sea, perhaps several kilometres in length, it's difficult to imagine that a 2 metre length of chain will have even the slightest effect.
The boat of some friends, struck in the Intracoastal Waterway, had every single wire in the boat burnt out, every electronic device destroyed. They were not on board at the time. A moderate fire burned itself out. The boat had a keel stepped mast and external ground plate. Message seems to be that lightning is totally unpredictable. Fortunately their insurance company followed the rules and everything was replaced.
Best advice I ever heard to avoid being struck in your boat is DONT GO! But just as on land, you can do very little on board to avoid being hit. Lightning strikes where it will, when it will, and if it has your number on it....
Extensive research on this board, inlcuding the experiences of those who have 'enjoyed' this particular aspect of sailing concludes there is basically nothing much you can do about it - except stay at home, and be struck in bed instead (it has happened!).
Remedies listed include sophisticated 'conductors' designed to lead the charge outside the hull, or to reduce the chances of a strike, or hangin anchor and chain over the side in contact with a shroud to provide a conductive path. Other remedies from those who have 'been there' include the yellow welly on the masthead (fancy climbing your mast in a thunderstorm to put it there?), and wearing rubber boots, gloves and for the men a condom, to avoid electric shock to the extremeties through the metal parts of the boat - strike reports mention crew receiving shocks from metal fittings.
Basically - as I remind myself every time I get caught out in thunder - in spite of the vulnerability of sitting under a 30 foot plus metal spike during a storm the chances of being struck are the same as they would be anywhere else - and the likelihood of survival of both self and boat is the same.
My favourite lightning story appered in PBO about 15 -20 years ago in an account of a Hirondelle cat that was struck on its Poole mooring. The owner returned the fried remains of his echo sounder to Seafarer, who responded suggesting that he only connected it to his 12 volt battery next time, and NOT across the mains supply.....