The most serious effects of lightning on a pleasure boat, sailboat or motorboat are not the most visible... It is often the electrical and electronic systems, as well as the energy conservation and conversion systems, that suffer the most damage. 

Damage that leaves boats in a very bad position, with no propulsion, no means of communication, no means of positioning, no pumps, no lighting...

When lightning transmits its current to the boat's electrical or electronic circuits, this is known as a secondary arc. This communication can be direct via a device, typically a radio, or it can occur when two nearby conductors are subjected to an overcurrent such that the conditions for creating an electric arc are met.

Secondary arcing can also occur when lightning strikes the water near the boat. At the point of impact, the current takes the path of least resistance. The current dissipates on the surface of the water, which is a poor conductor. A metal through-hull or a probe makes an excellent conductor. An excellent metal conductor means a point of entry into the electrical network. If the boat does not have a full equipotential bond, a bond that grounds all the metal parts of the boat via a plate, our through-hull fitting will transmit a current inside the boat capable of generating a secondary arc with a nearby conductor.

This is what happened in the ACI marina in Split in September 2020 when a bolt of lightning struck the water near four Lagoon catamarans, destroying the electrical circuits and all the on-board electronics and causing hundreds of thousands of euros worth of damage.

The cables that supply the various DC electrical devices on pleasure boats are all protected against a short-circuit current in relation to that of the device they protect.

This protection is measured in a handful of amperes, a few hundred for batteries and thousands for lithium batteries.

But in the presence of a secondary arc, especially if the lightning has struck the boat's receiver directly, the currents involved are of the order of several tens of thousands of amperes.

These are overcurrents against which the equipment and its power supply are not protected, and which are fatal to them.

Without specific protection, most of the onboard electrical equipment is at risk. The ECU of engines, alternators, pumps, electronics, windlass, thruster and lighting can all be destroyed in an instant.

There is no shortage of examples in the literature, the latest dating from the beginning of this month: an Australian man was recovered after 3 months adrift in the Pacific on a catamaran struck by lightning and deprived of all its systems...