What to do if a thunderstorm catches you on a boat

01

Why are boats especially exposed?[1][2]

Because height, isolation and a flat surrounding surface are exactly what determines where lightning connects. A boat on open water is all three at once. A sailboat mast makes it worse by adding a tall, pointed, grounded conductor.

On land, a person is rarely the tallest thing in view. On open water, a boat almost always is, and that changes the odds far more than the size of the vessel does. A small open dinghy and a cruiser share the same basic problem.

Masts are a mixed story. A properly bonded mast on a sailboat gives current a defined path down to the water, which protects the hull and the people better than an unbonded one. It also guarantees that the boat is the connection point for any strike in the area.

02

What should you do when a storm builds?[1]

Head in early. The decision point is when you first see building cumulus or hear distant thunder, not when the storm arrives. Boats are slow relative to storms, so waiting until the weather looks bad usually means you cannot outrun it.

Leave early

Start back at the first thunder or the first tall cumulus tower, not at the first raindrop

Get everyone inboard

No swimming, no towing, no one trailing hands or feet in the water

Stay low and central

Away from the mast, rails, rigging and any metal that connects to the water

Drop antennas and rods

Anything that adds height and points upward comes down

Wear life jackets

A strike can knock people overboard or unconscious; the jacket is the backup

03

Is a cabin safer than an open deck?[3]

A closed cabin on a boat with a metal hull or a bonded lightning path behaves somewhat like a car, giving current a route around you. An open cockpit or a flybridge does not. Fiberglass hulls provide no shielding on their own.

The comparison to a car holds only where the conductive shell holds. Aluminium and steel hulls give a genuine enclosure. Fiberglass is a good insulator, which sounds protective and is not: without a bonded path, current finds its own way to the water, sometimes straight through the hull.

Sit in the middle of the cabin, keep away from the mast step, the engine, the wheel and any through-hull fittings, and do not touch two pieces of metal at once. Handheld VHF and phones on battery are fine; the fixed radio wired to a masthead antenna is not.

04

What gets damaged when a boat is struck?[4]

Electronics almost always, and the hull sometimes. A typical flash carries roughly 30,000 amps at around 300 million volts, which destroys chartplotters, radios and instrument networks, and can blow through-hull fittings on its way to the water.

The repair pattern is well known to marine surveyors: the visible strike damage is minor and the electrical damage is total. Every device on the network is suspect afterwards, including ones that still power on, because degraded components fail weeks later.

The safety item that matters most after a strike is the hull. Current exiting below the waterline can crack or blow out fittings, so check the bilge before assuming the boat is sound.