There is a lot of rumour about earthing masts on boats, most of it goes along the idea that if you get struck by lightening it’ll give it a path to follow. What a load of rubbish.
if you get struck by lightening, 300 million Volts and about 30,000 Amps (typical lightening strike) will find its own path as directly as it can, your earth wire won’t help, unless its the size of Kansas!!!
So what’s the purpose and why should you ABSOLUTELY MAKE SURE YOU MAST IS EARTHED TO THE SEA? We’ll as you move through the wind and the wind through your rigging it generates static electricity and a charge forms in your mast.
This charge is called Electrical potential and will literally attract a lightening strike if it remains, because the electrical potential of your mast will be higher than the surrounding sea.
However if you ground that mast to the sea that electrical potential will be drained off into the sea as it is generated.
Don’t worry too much about how high your mast is compared to the surrounding sea. Our airdraft for instance, is 24m and the average lightening strike will originate at an altitude of 5000-16000m meaning our mast is somewhere between 0.4% and 0.1% of the total distance the lightening will cover to get to the ground. So in most case mast height is not highly significant, but electrical potential is!!!