We were on a mooring off Cap Ferrat when the storm came through, one of those August Mediterranean cells that builds over the warm sea in an afternoon and turns the sky the colour of a bruise. The flash and the bang arrived together, which means it was close, and the whole boat seemed to flinch. Nothing caught fire. Nothing obviously broke. The instruments still glowed and the engine still started. For about a week we told ourselves we had got away with it. We had not.
That is the cruel thing about a lightning strike on a boat: the dangerous damage is usually the damage you cannot see, and it shows up slowly, days or weeks later, often when you least want it to. If you cruise the Cote d'Azur in summer you are cruising prime thunderstorm territory, so it is worth knowing exactly what to check and in what order once the storm has passed.
In the moments after the bang
Before any checklist, two things. Make sure no one is hurt: a strike can throw crew off balance, and a direct hit can injure anyone touching metal connected to the rig. Then steady the boat and the situation the way you would after any sudden emergency at sea, from a rigging failure on a French passage to a flooding scare. Slow down, get the crew accounted for, and resist the urge to start flicking every switch on the panel, because a damaged circuit energised carelessly can finish off gear that survived the strike. Note the time and your position in the log; if anything fails later, the insurer will want the strike pinned to a moment.
First, the things that can sink or strand you
The glamorous fear is fried electronics. The real fear is a hole in the boat. A lightning strike looking for the path to earth will often exit through the hull below the waterline, and it can blow out a transducer, a through-hull fitting or punch an actual hole in the laminate. Before you worry about the chartplotter, check the bilge.
- Look at every through-hull and transducer. Lightning frequently exits at these points, and a cracked or blown fitting is a slow leak waiting to happen. If you find one weeping, this becomes a flooding problem, and the damage-control drills in our guide to a boat taking on water emergency apply directly: close the seacock, plug the fitting, pump and assess.
- Check the bilge water level over the next hours, not just once. A hairline exit wound can seep rather than gush.
- Inspect the keel bolts and the area around the mast step, the most common path for the current on a keel-stepped boat.
If the boat is taking water, you treat that first and worry about the electronics later.
Then the systems that keep you safe
Once you know the hull is sound, work through the systems you rely on to stay out of trouble, because a strike commonly disables exactly the gear you would use to call for help.
- VHF radio. Test transmit and receive, ideally with a radio check, because a dead set means you cannot reach CROSS in an emergency. The procedure for that call, if you ever need it, is in our guide to the French distress and safety call procedure.
- GPS and chartplotter. A strike, or even a near miss, can fry navigation electronics outright or leave them working now and failing later.
- Engine and its electronic control unit. Modern engines have an ECU that lightning can damage even when the engine starts today; intermittent faults are a classic late symptom.
- Compass. A strike can magnetise and permanently throw out a steering compass, so swing it against a known transit before you trust it.
- Navigation lights, autopilot, instruments, battery chargers and the shore-power system.
Why hidden damage is the real story
The reason you cannot relax after a strike is the way lightning damages electronics. Most of the harm comes from overcurrent racing through the wiring and arriving at whatever is plugged in at the end of it, and from the electromagnetic pulse a nearby strike throws out even without a direct hit. Boats have had their masthead unit destroyed while the rest of the electronics survived, and boats have suffered no visible damage at all yet found that nearly every electronic system had been quietly degraded, with failures appearing one by one over the following weeks.
That delay is why "it all still works" is not a clean bill of health. Diodes, sensors and circuit boards that took a surge may run fine until they are stressed, then quit. Turning everything on and watching it light up is not a survey; it is a hope.
The grounding question, after the fact
If you have been hit, it is worth understanding whether your boat had a lightning path at all, because it shapes what to inspect and what to fix. A protected mast has a low-resistance conductor, equivalent to a 25-square-millimetre copper cable, running from the mast base to a ground plate or the keel. The simplest arrangement bonds an aluminium mast to the keel bolts, which on a keel-stepped boat is straightforward because the heel and the bolts sit close together. On a deck-stepped mast the chainplates are bonded down to the keel instead. If your boat had none of this, the current improvised its own path to the sea, which is precisely how through-hulls and transducers get blown out. Knowing your boat's grounding tells the surveyor where to look hardest.
Survey and insurance: do it properly
Call your insurer promptly. Comprehensive policies typically cover fire, lightning, sinking, theft and vandalism, but some Mediterranean and warm-latitude policies carry specific lightning clauses, including deductibles set as a percentage of the agreed hull value rather than a flat figure, which can run to several percent and a substantial sum on a valuable boat. Read your wording before you assume the cost.
Insurers will usually instruct a surveyor, and a thorough lightning survey often means lifting the boat out and un-stepping the mast to inspect the wiring runs, the chainplates and the hull around the fittings. Let the specialists test the systems: electrical, electronic, engine and any air-conditioning technicians should each inspect, test and sign off their own kit, because a strike can damage safety-critical systems in ways an owner flicking switches will never detect. This is slow and it is frustrating when the boat looks fine, but it is the only way to be sure the next failure is not the one that matters at sea.
Reduce the odds next August
You cannot lightning-proof a boat, but you can shorten the odds and soften a strike. A proper bonded conductor from masthead to keel gives the current a path that bypasses your electronics and your hull. Anchoring or mooring rather than being the tallest thing on open water during a cell helps a little. Disconnecting and stowing handheld electronics, a spare GPS and a handheld VHF in the oven, which acts as a crude Faraday cage, means you keep a navigation and comms backup even if the fixed gear is cooked. After the strike off Cap Ferrat, the handheld VHF we had stuffed in a locker was the only radio that still transmitted reliably for the rest of that trip.
We eventually hauled the boat. The survey found a blown depth transducer that had been seeping, a half-dead chartplotter and a compass that read fifteen degrees out. None of it was visible the afternoon of the strike. If you take one thing from a Mediterranean lightning hit, take this: the boat being afloat and the screen being lit is the start of the inspection, not the end of it.
