Lightning and Your Boat: Protection, Storm Safety and After-Strike Inspection

Lightning is trying to reach the water. On a sailboat it usually hits the mast; on a powerboat, the VHF antenna, a T-top or a bimini frame. From there it takes every path it can find to the water: down the mast and rigging, through wiring and electronics, and out through through-hulls, props, the keel or even straight through a fiberglass hull. NOAA estimates a strike carries around 30 million volts, so a direct hit on any 12-volt device destroys it.

A nearby strike can also cause damage. BoatUS has claims where boats next to a struck boat had damaged electronics or a compass that was off by 50 degrees or more.

A lightning protection system does not prevent strikes. Its job is to give the strike a direct, low-resistance path to the water so the current goes around the boat and the people aboard instead of through them. The basic parts, as described in a BoatUS article on NFPA 780 by marine electrical engineer James Coté:

• Air terminal: a lightning rod at the highest point, taller than anything it protects, including the VHF antenna.
• Down conductor: a heavy, dedicated conductor from the air terminal to the water ground, run as straight as possible and kept apart from other wiring. An aluminum mast or tower can serve as the down conductor if it is properly connected.
• Grounding plate or terminal: a metal plate or a bare metal keel, strut or rudder in contact with the water.
• Surge protection devices: fitted to engine computers, chartplotters, autopilots and other critical electronics.

Most boats are not built with a full system from the factory, and BoatUS notes that the recreational boating industry has been slow to adopt the newer NFPA 780 recommendations.

Lightning protection is a design job, not a weekend project. A badly done system can make damage worse. BoatUS describes common installation mistakes: an air terminal lower than the antenna next to it, a down conductor tied into the bonding system instead of going straight to the ground plate, and lightning conductors bundled with other wiring, which induces surges in that wiring. In one claim, an owner bonded his through-hulls with heavy wire but one fitting was plastic; the strike jumped across it, destroyed it and partly sank the boat.

If you keep a larger boat, especially a sailboat, in a lightning-prone area like Florida or the Chesapeake Bay, ask an ABYC-certified marine electrician who knows NFPA 780 and the ABYC lightning protection guidance to assess your boat. Ask specifically about surge protection for the engine electronics.

"Fuzzy" or brush-style dissipaters and other devices sold to prevent strikes have no independent laboratory evidence that they work, according to the BoatUS article. BoatUS even records a boat with a dissipater at the masthead that was struck twice in one year. Spend the money on a properly designed protection system and surge protection instead.

Boating safety in storms starts at the dock. The National Weather Service advises:

• Check the forecast. If thunderstorms are possible, do not go out.
• Keep watching the sky and the radar while you are out, and listen to NOAA Weather Radio on your VHF.
• Have an escape plan: know the nearest safe harbor. Thunderstorms are usually brief, so waiting one out ashore beats riding it out on the water.
• Carry a handheld VHF and non-electronic signals (flares) in case a strike knocks out the boat's electronics.

Head for port or shelter as soon as you see a storm building. If you cannot get in:

• Put on life jackets. Storms bring sudden wind shifts, waves and poor visibility as well as lightning.
• Boat with a cabin: go inside and stay away from metal and electrical devices. NWS notes that large boats with cabins, especially those with properly installed protection systems, and metal boats are relatively safe.
• Open boat: get as low as possible in the boat. If you cannot reach land, drop anchor and get low.
• Keep away from the mast, shrouds, T-top legs, rails and other tall metal. The NWS explains that side flashes jump to people within a foot or two of a struck object.
• Keep arms and legs out of the water, and stay off the radio unless it is an emergency.
• Don't waste time pulling off watches, rings or phones. The NWS lightning myths page explains that metal doesn't attract lightning; height, a pointed shape and being isolated do, which is exactly what a boat on open water is. Metal does conduct a strike, though, which is why you stay off rails and rigging.
• Ashore, NWS advises waiting 30 minutes after the last rumble of thunder before going back out.

Call for help at once: VHF channel 16 or the DSC distress button, or 911 near shore. NWS stresses that calling 911, starting CPR and using an AED quickly can keep a victim alive. Victims do not carry an electrical charge, so you can touch them safely. Then get to shore as fast as you safely can.

The most urgent risk after a strike is a hidden leak. Lightning can exit through a through-hull fitting or blow a hole through the hull, sometimes at or below the waterline.

• Check the bilge immediately and keep checking. Run the bilge pumps and look at every through-hull and seacock.
• If water is coming in faster than the pumps can handle, call the Coast Guard on VHF 16 and see the taking on water guide.
• If you smell fuel or burning wiring, shut down the engine and electrical system, ventilate and get ready to leave the boat. BoatUS records a 25-foot boat with a little fuel in the bilge that exploded when struck at the dock.

BoatUS advises calling your insurance company first and getting the boat short-hauled (lifted briefly out of the water) as soon as possible so the hull and through-hulls can be inspected. A strike can leave a slow leak that sinks the boat days later. BoatUS Marine Insurance pays for this post-strike short haul without a deductible; ask your own insurer what they cover. Take photos of everything before anything is repaired.

On the haul-out, look for:

• Small holes, burn marks or blisters near the waterline, especially near through-hulls, the keel and chainplates
• Cracked, burned or loose through-hull fittings and seacocks (plastic fittings are especially vulnerable)
• Damaged bottom paint around props, struts, rudders and metal keels, where strikes often exit

Have a marine surveyor do this if you are not sure what you are looking at. Hidden damage to the hull laminate or a keel joint needs a professional.

A missing or damaged VHF antenna is often the first sign that an unattended boat was struck. Look for melted bits on deck, burn marks at the masthead, damaged wind instruments, lights and radar. Aluminum masts are rarely damaged, but BoatUS notes wood and carbon masts can be. Have a rigger inspect the rig and chainplates (see rigging inspection).

Turn things on one at a time and note what does not work.

• Fuses and breakers: surveyor Dewey Ives told BoatUS that more than a couple of blown fuses points to lightning.
• GFCI outlets: they are often destroyed but may still pass power without protecting you. Press the test button on each one; if it does not trip, replace it.
• Electronics: VHF, chartplotter, autopilot, depth and wind instruments, stereo, battery charger, inverter, bilge pumps, lights and fans.
• Engine: start it and check the gauges and alarms. Electronic engine controls are sometimes damaged.

BoatUS notes that equipment that works after a strike usually was not damaged, and that it is unusual for electronics to fail months later. Still, list everything for the insurer.

Lightning's magnetic field can throw a compass far off, sometimes by 50 to 100 degrees. Compare your compass with a GPS course on a few known headings. If it is off, it may slowly return to normal, but do not rely on it until it has been checked and, if needed, adjusted or replaced by a compass adjuster.

Keep every photo, invoice and survey from the strike. Have a qualified marine electrician repair wiring, and replace damaged electronics with surge protection where it makes sense. Add the strike and repairs to your boat records: insurers, surveyors and future buyers will ask.