Lightning strikes must be conducted to ground as directly and safely as possible to keep parts of the boat from becoming active in the strike circuit.
Depending on the boating location, the chances of a lightning strike can vary immensely. Marine surveyors in Florida, for instance, have far more strikes to inspect than those in the Northeast, since thunderstorm activity is much higher in the South. The science of strikes has given us ways to minimize damage and keep people on board safe, but as materials change and science evolves, the technology will keep changing, too.
For instance, carbon-fiber masts and hulls have their own peculiar characteristics as conductors. Parts of the boat that project above the superstructure, such as masts and outriggers, can easily become part of the lightning circuit, and we have to conduct the strike to ground as safely and directly as possible.
To do this, we provide an air terminal at the highest point and suitable conductors down to a ground plate or strip, allowing the strike to dissipate. Air terminals used to be pointed, but they can now have a radi-
us at the tip and be chrome-plated bronze or aluminum. Using them with conductors to a ground plate eliminates the often-catastrophic side flashes that can occur if lightning has trouble passing through or around a boat.
An aluminum mast is a fine conductor, but wood and carbon masts should have a down conductor of at least 4 AWG (American Wire Gauge). The conductor connects directly to a ground plate that is a minimum of 1 square foot of copper for saltwater dissipation, or two strips of copper for fresh water. (Resistance is higher with fresh water, so more edge contact is required.)
In the boat, all chainplates and large metal objects, such as stanchions and handrails, must have side conductors of at least 6 AWG that lead as directly as possible to the ground contact. The bonding system’s conductors can be increased from 8 AWG to 6 AWG to bring as many fittings to the same potential and assist in controlling side flashes.
During a potential thunderstorm, even with all of these precautions in place, keep all people on board as low as possible in the boat, and avoid contact with metal objects so as not to become part of the circuit.