What Happens If Lightning Strikes a Sailboat? A Skipper's Guide

Ask most people what happens when lightning hits a yacht and they imagine an explosion, or the boat going down in seconds. That's rarely what happens. The hull usually survives. The electronics usually don't. And in a few cases, the real danger only shows up hours later, below the waterline.

How likely is it?

For a monohull, the odds are roughly 1 in 1,000 per year. In the worst areas, like Florida, it's around 3.3 boats in 1,000. That's low, but over a long sailing life it's not a number to ignore.

Now the part catamaran sailors don't like to hear: multihulls are two to three times more likely to be struck than monohulls. They have a bigger footprint, and without a heavy keel it's harder to give the lightning a good path into the sea.

As a skipper who spends most of the season on Lagoon catamarans, this is the part I take personally. In the Adriatic, summer thunderstorms can build surprisingly fast.

What actually happens in a strike

On a sailing yacht, the mast is almost always the highest point for miles. Lightning hits it first.

From there, the energy wants one thing: to reach the water by the fastest route. If the boat gives it a clear path, most of it goes that way. If not, it finds its own, jumping between wires, rigging, engines and fittings. That jumping is called side flash, and it's where most damage happens.

A lightning bolt can reach around 30,000°C, roughly five times hotter than the surface of the Sun. It lasts a fraction of a second, but that's enough.

The electronics take the hit

The most common damage is to anything electronic: navigation displays, sensors, radios, autopilots, chargers and networked equipment. The surge travels through cables and wiring and burns out circuits in a fraction of a second.

The tricky part is that damage isn't always obvious. Skippers have found equipment that seemed fine until it was switched on days later. So after a strike, assume every electronic system is suspect until it's tested.

The hidden danger: below the waterline

The hull usually survives, but not always untouched. When lightning leaves the boat, it can exit through through-hull fittings, transducers, the keel or the rudder. Strikes have damaged rudders, stripped keel fairing and burned bottom paint.

In rare cases, an exit point can leave a small hole or a damaged fitting that leaks. That's why, once everyone on board is OK, the first thing to check is not the chartplotter. It's the bilge.

How lightning protection works

The idea is simple: give the lightning a preferred route to the water before it finds its own.

• Down conductor. A heavy cable from the base of the mast down to the water.
• Grounding plate. A copper plate, or metal fittings below the waterline, that spreads the energy into the sea.
• Bonding. Major metal parts on board, like engines, tanks and rigging, are connected so the current doesn't jump between them.
• Surge protection. Devices that protect electronics from the spike, especially on boats plugged into shore power.

No system guarantees zero damage. A good one lowers the risk of hull damage and fire, and reduces how far the surge spreads.

What to do when a storm is coming

1. Check the forecast and radar before you leave. Summer storms can build in under an hour.
2. If you can, get into harbour or a sheltered anchorage early.
3. Unplug what you can and disconnect aerials if you have time.
4. Put handheld GPS, VHF and phones in a metal box or an unplugged microwave. It works as a shield.
5. Everyone stays inside or low in the cockpit, away from the mast, shrouds and metal fittings.
6. Don't touch two metal objects at once, like the wheel and a stanchion.

What to check after a strike

1. People first. Check everyone on board. Anyone hit or near a flash needs medical attention, even if they feel fine.
2. Then the bilge. Check for water and inspect every through-hull and transducer.
3. Steering and rudder. Make sure everything moves freely.
4. Compass. A strike can change its magnetism. Compare it against GPS heading.
5. Electronics. Test every system. Use handheld backups if the main ones are gone.
6. Haul out and survey. Even if everything looks fine, get the hull, keel and rudder checked out of the water.
7. Call your insurer early, with photos and a list of what isn't working.

Captain's note

Lightning isn't something you can control at sea. What you can control is the forecast you check, the harbour you choose, and how well you check your boat after the storm has passed.

See you on the water,
Captain Phil

Sources: Yachting World, BoatUS.