Lightning! Flash, BANG! Your Boat's Been Hit —
Now What?

Published: August 2010

If you've ever been to your marina during a thunderstorm, you've probably wondered how likely it is that your boat will be struck by lightning. The answer is, fortunately, not very. According to the most recent (2000-2005) BoatUS Marine Insurance claim files, the odds of your boat being struck by lightning in any year are about 1.2 in 1000. In fact, the claim files show no lightning claims for 13 states such as Idaho and Nebraska (no surprise). But, for those of you with boats in Florida, nobody has to tell you the odds are greater — much greater. Thirty-three percent of all lightning claims are from the Sunshine State and the strike rate there is 3.3 boats per thousand. Surprisingly, the second most struck area in the country is the Chesapeake Bay (twenty-nine percent), and those who boat there in the summer can attest to the ferocity of the sudden thunderstorms. Not surprisingly, the majority of strikes are on sailboats (4 per 1000), but power boats get struck also (5 per 10,000); Trawlers have the highest rate for power boats (2 per 1000) and lightning has struck houseboats, bass boats, and even PWCs.

One surprise: Multihull sailboats are struck more than twice as often as monohulls. Even accounting for the fact that a large percentage of multihulls are in lightning-prone Florida, the odds of multihulls being struck are still statistically much greater. Ewen Thomson, a well-known lightning researcher has a theory. Based on BoatUS supplied data, Ewen did an analysis of the "shielding effect" of nearby yachts. He theorizes that multihulls have a higher probability of being struck in a marina because their greater beam requires a wider berth. The result is less shielding from adjacent boats. Ewen cautions that his numbers contain a large uncertainty, though they appear to correlate with the BoatUS Marine Insurance claims history.

You Can Run, But You Can't Hide

Volumes have been written about methods to mitigate damage or even avert a lightning strike. Lightning, however, doesn't seem to read them. As an example, one boat, fitted with a popular "fuzzy" static dissipater at the top of the mast was struck twice in one year; ironically, the second time the bolt hit the dissipater even though the VHF antenna right next to it was higher (claim #0308082). Dewey Ives, a surveyor in Florida and member of the BoatUS Catastrophe Team who has seen his share of lightning damaged boats, says that lightning is unpredictable. "I've seen a small sailboat docked between two larger ones get hit and sometimes a powerboat in the middle of a marina filled with sailboats gets it. If lightning wants your boat, there's not much you can do about it." Ewen Thomson agrees, "Current research shows promise in mitigating damage from a lightning strike, but there is nothing that is effective in preventing a strike." Though not everyone agrees with that statement, in this issue of Seaworthy, we'll leave behind the sometimes contradictory expert opinions on how to prevent a strike and focus on what to do if your boat is hit.

First Things First

Often, according to Carroll Robertson, vice president of claims for BoatUS Marine Insurance, the extent of the damage from a lightning strike is not immediately apparent. Carroll advises that the first thing that should be done if your boat is struck (after calling BoatUS claims — 800 937-1937) is to get it short-hauled as quickly as possible for a quick assessment of the hull. The reason, Carroll says, is that when lightning exits your boat, it can leave via a through-hull fitting or even through the hull itself. Even if the force of the bolt doesn't blow out a through-hull or cause hull damage, it may cause a gradual leak that could go unnoticed and sink your boat. As part of its sue and labor provision, BoatUS Marine Insurance will pay to have your boat short-hauled to check for damage — the short-haul is not subject to a deductible. Once it's determined that the hull has no leaks, the rest of the boat can be examined for damage.


The amount of damage a boat sustains is determined in part by how the strike exits. In a properly bonded system that follows American Boat and Yacht Council standards, the strike should follow a low resistance path to a boat's keel or an installed grounding plate, though few boats are equipped from the factory this way. While no two lightning strikes are exactly alike, examining a typical claim can shed some light on the possible damages your boat might have if it's ever struck, some of which you may not have thought of. Claim #0104985: Priority, a 33-foot sailboat was struck in North Carolina during a July thunderstorm. Sailboats — and this one is no exception — are nearly always struck on the mast and a damaged or missing VHF antenna is typically the first sign that an unattended boat was struck — sometimes bits of a melted antenna are found on the deck. It's no surprise that electrical devices are susceptible to strikes; NOAA estimates a strike contains around 30,000,000 volts and a quick zap to a 12-volt device will certainly destroy it. But Carroll Robertson says that lightning is like horseshoes — close counts. There can sometimes be collateral damage when a nearby boat gets hit, either the result of the lightning's powerful electro-magnetic field (EMF), or the current induced by the field running through the boat's shorepower cord. This can create strange problems.

In one instance, the owner of a 28-foot sailboat noticed an amber LED on his battery charger that he'd never seen lit before and his depth sounder had quit working. He couldn't figure out what had happened until his neighbor told him his boat had been struck recently (claim # 0107363). On another boat moored next to a struck boat, the compass readings were 50 degrees off and slowly returned to normal after a few weeks. But a direct hit usually causes more obvious — and substantial — damage.

When a boat gets struck, lightning is trying to find its way to the water. In a sailboat, like Priority, gets struck, one of the paths it takes is down the mast; typically anything that happens to be close by on the way down can be destroyed — wind instruments, TV antennas, radar, lights, etc. Fortunately, the BoatUS Marine Insurance claim files have not shown that aluminum masts themselves get damaged; aluminum is a very good conductor and allows the strike free passage. However, wood and carbon fiber masts can get damaged since neither one is a good conductor. In one claim, a wooden mast that was partially rotted was destroyed when the charge heated up the damp mast causing the moisture to suddenly expand (witnesses said it "exploded". Standing rigging is another path lightning takes and although stainless steel does conduct as well as aluminum, damage to the rigging is rare.

Though mast-mounted components are the most likely to be destroyed, anything on the boat that is electronic can be damaged. In the case of Priority, the wind, speed and depth instruments were destroyed as was the air conditioner controls, the battery charger, autopilot, mast wiring, the refrigeration controls, the stereo, and of course, the VHF. In other cases, battery selector switches, power panel breakers, volt/amp meters, alternators, and even cabin lights were damaged. As a general rule, if the equipment works OK after the boat was struck, it probably wasn't damaged — it's unusual for electronics to fail months later. Dewey Ives says that often the first sign owners have that their boat was struck is that some of the boat's electronics don't work. "Look for fuse failures," he says. "If you have more than a couple of blown fuses, look to lightning as a possible cause." Power boats, he says, though not struck as frequently, are just as likely to sustain electronic damage.

Powerboats are typically struck on the VHF antenna or bimini top. One member who took his new 23-foot runabout out near Tampa Bay, saw a storm coming and turned around too late to get back to the dock,. He heard lightning strike the fiberglass VHF antenna ("A sound I hope I never have to hear again"). All of the boat's electronics were destroyed, but worse, the engine electrical system was damaged and the passengers had to endure the storm until the owner could wave down a passing boat. Although lightning struck an antenna that was only a few feet away, the passengers suffered nothing worse than temporary ringing in the ears. (Note: the fact that a boat's electronics may be destroyed during a thunderstorm — including the VHF — underscores the need for non-electronic signaling devices such as flares in case your boat is struck at sea and is taking on water, or worse, if someone is injured.)

Hull Damage

As hard as lightning is on electronics, it can be just as brutal to fiberglass. In the case of Priority, the lightning traveled down the mast as well as through the VHF coaxial cable. The cable had been disconnected and was resting against the hull inside the boat. When the strike exited the cable, it had no easy way to get to the water. After traveling a quarter of a mile through air, lightning has no trouble going through a fiberglass hull, and this is exactly what it did, blowing a three-inch hole on the way. Fortunately, the hole was above the waterline and the boat was saved from sinking. (Note: If you disconnect your VHF cable from your radio during lightning season, like some boaters do, be aware that anything near the connector, including you, can get zapped during a strike.) Other boats have not been so lucky.

Giving the lightning a low-resistance path to the water is a good idea, but if it's not done right, the damage can be even worse. The owner of a 27-foot sailboat bonded his through hulls properly with heavy wire, but didn't realize that underneath one of the seacocks, the through-hull fitting was made of Marelon — plastic. When the boat was struck, the lightning dutifully followed the wire, but instead of continuing to the water as it would have through a bronze fitting, it jumped across the plastic one, destroying it and partially sinking the boat.

Powerboats are also susceptible to hull damage and are less likely to have been fitted with a lightning protection system. Fortunately, the strike usually exits the boat through the props and rudders and aside from damage to the bottom paint, the running gear is not often damaged (although electronic engine controls sometimes are). Need another good reason to replace a leaking fuel tank? A 25-foot fishing boat with a small amount of fuel in the bilge exploded at the dock when it was struck, sending the contents of the boat's cockpit nearly 100 feet away. Occasionally, lightning enters a boat's electrical system and creates enough havoc to start a fire (claim #0107832). Fortunately, these types of claims are rare.

Minor Damage

One component that was destroyed in Priority were two shore power ground fault circuit interrupters (GFCI). Marine surveyors say that they are nearly always destroyed during a strike and can easily be overlooked. Though they may still power appliances, the protection circuit is often non-functional; GFCIs can be easily checked by pushing the test button on the cover. Other small items to check are hand-held radios and GPS's, bilge pumps, inverters, lights, and fans. It should be noted that lightning is fickle and boat damage varies enormously — one owner saw his boat struck on the mast and yet none of the electronics were damaged, and in fact the only evidence the surveyor could find of the strike was a blackened area on the masthead.