Lightning is a spark that can reach over 5 miles in length, attain a temperature of approximately 50,000°F, and contain over 100 million electrical volts.
Sometimes a damaged or missing VHF antenna is the only clue that an unattended boat has been struck. Some vessels have little or no damage after a strike, but an immediate short-haul is a must. When lightning exits your boat, it can go through the hull itself or via a through-hull fitting. This may cause a gradual leak that could go unnoticed.
No one keeps track of how much damage boats sustain from lightning each year, but costs likely total millions of dollars. Much of the damage results from ruined navigation, radio and other electronic equipment. On sailboats, grounding the mast is the best way to prevent lightning from sparking around the boat as it seeks a conducting path.
Sailors should ground the masts on their boats using a No. 4 gauge copper wire connected to conductors in the water. Freshwater boats require as a minimum a long metal strip along the bottom of the hull. Sailors should also ground the wire to other metal conductors in the water, such as the prop.