MARINE LIGHTNING PROTECTION: Getting Z-Z-Z-Zapped on a Sailboat

Lightning strike

I have to admit I don’t normally think about this too much. As is true of many sailors I suspect, I have subscribed to the philosophy that lightning and its effects are so random and poorly understood that you can get royally screwed no matter what you try to do about it. Which is a great predicate, of course, to going into denial and doing nothing at all. But the death in Florida last summer of Noah Cullen, a most promising young man who presumably was killed in a lightning strike while out singlehanding on his pocket cruiser, got me pondering this in a more deliberate manner. On doing some research, I found there are some hard facts out there that are well worth knowing.

Much of what we tend to learn about lightning is anecdotal, which mostly serves to make it seem more mysterious. I, for example, have never been struck by lightning, but I did once cut through some severe thunder squalls in the Gulf Stream in a grounded fiberglass boat and saw a bolt of lightning the size of a large tree trunk flash straight into the water just a few yards behind us. I can’t begin to tell you why it didn’t hit our nice 55-foot aluminum mast, and ever since then I’ve believed a strike is pretty much an act of God. It’s either going to get you, or not, and there’s nothing you can really do about it.

I have met a number of sailors who have been struck by lightning, mostly in grounded boats, and in every case they told me they lost all their electronics. So I have also always assumed there is nothing you can really do to protect installed electronics from a lightning strike.

But you should forget all the anecdotes you ever heard, at least temporarily, and think about the following:

Likelihood of a strike: It’s probably much higher than you like to think. One source states that a sailboat with a 50-foot mast will on average be struck once every 11.2 years. According to insurance data, the general average for all boats is about 1.2 strikes per 1,000 boats each year.  The average bill for damage is around $20,000. Most strikes are on sailboats (4 strikes per 1,000 sailboats each year). And these are likely lowball numbers, as it seems many lightning-strike victims are not insured or do not report the strikes to their insurers. According to one independent survey, unreported strikes could be as high as 50 percent of the total.

Location is also a big factor. Some areas, including very popular cruising grounds like Florida or Chesapeake Bay, are much more lightning-prone than others, and you are obviously much more likely to get struck when sailing within them. The overall average for reported lightning strikes on boats in Florida, for example, is 3.3 strikes per 1,000 boats each year, nearly three times the national average.

Lightning probability map

Map showing lightning strike probabilities around the world. The higher the number, the higher the probability

Interestingly, catamarans overall apparently are struck twice as often as monohulls. Could this be because they are effectively twice as much boat???

Preventing a strike: It really isn’t possible. There is no technology that can positively keep your boat from being hit. There’s seems to be little evidence, for example, that those silly little masthead bottle brushes some people put up are good for anything.

Sydney strike

Spectacular image of a sailboat getting hit in Rushcutter’s Bay in Sydney Harbor, Australia, with inset images showing damage to the mast. Lots of other targets with masts around, so why did the bolt hit this one boat?

Limiting damage: This is where the action is. To paraphrase one writer: it is a fallacy to think in terms of “lightning protection.” What you want is “lightning control.” Which definitely means grounding your boat! An ungrounded boat is much more likely to suffer potentially disastrous damage when struck (i.e., holes in the hull, dead crew, etc.). A boat in fresh water is also much more vulnerable, because fresh water doesn’t conduct electricity as well as salt water. An ungrounded boat in fresh water is most vulnerable of all. If you’re on one of these during a strike, you may as well just forget about it and put a cap in your head.

Hull damage

Typical exit damage around an anchor well drain on a fiberglass boat. Hull damage just above the waterline is not at all unusual

Grounding your boat: The old school notion of leading a big copper strip from the base of your mast in a straight line to a single grounding plate on your hull is the process of being discarded in favor of a more sophisticated technique that connects the mast as primary conductor to a network of dissipating electrodes installed just above a boat’s waterline, the idea being in effect to make all of the boat’s hull something like a Faraday cage, so that the equipment and people within will be safer.

Grounding system

Example of a more modern grounding system

Note (I was particularly gratified to learn this): a metal hull is indeed a great ground, and the fact that it is painted, or coated in epoxy, or whatever, doesn’t change this. But you can still suffer significant damage on a metal boat!

Bonding: You and the gear on your boat are more likely to survive a strike without damage if the major bits of metal on your boat are bonded to the grounding system. This reduces the likelihood of dangerous side flashes. (It does, however, create complications with respect to the potential for galvanic corrosion on a boat.)

Saving electronics: First of all, stowing handheld electronics (or any disconnected electronics) in your oven will protect them during a strike. Just remember to take them out again before using the oven!

More importantly, you can protect installed electronics using various individual surge protectors, fancy spiral wiring, and other techniques I’m not going to pretend to understand, much less explain. See the sources below for more details.

Your personal safety: This should be most important, right? You want to stay off the helm if possible, stay below, stay dry, and don’t touch any big pieces of metal. All of which are easier said than done when you’re in the middle of a big squall! It would seem the most prudent tactic is severely reduce sail, or take it all down, pop the boat on autopilot, and get below well in advance of and after a thunderstorm.


Lightning and Sailboats: Academic paper published by Ewen M. Thomson, currently recognized as the most well-informed go-to guy on this subject.

Marine Lightning Protection: Website for a business run by Ewen Thomson (see above), who is a pioneer in modern cage-style boat-grounding techniques. Thomson will ground and bond your boat for you, if you like, but there’s also lots of useful raw info in here.

Lightning Survey Results: Discussion re results of a small independent online lightning-strike survey conducted by a cruiser who owns a power-cat named Domino. Very informative.

Considerations for Lightning Protection: Conclusions reached post-survey by the owner of Domino, referenced above.

Lessons in Lightning: Ocean Navigator article by a cruiser in an aluminum boat who was struck by lightning in the Baltic. Of particular interest to those (like myself) who own aluminum boats.

There are lots of other resources out there, but these four links are a very good place to start. You’ll find many other valuable sources just by reading through these articles and following the links within.

Related Posts

9 Responses
  1. Robert Bartlett

    We were hit by lightning in a fast moving front off Newfoundland many years ago (before gps). All the electronics were fried! The binnacle must have been demagnetized as it hopelessly spun in circles, giving us only a hand sighting compass to steer by. The smell of burned wire insulation in boat was overpowering. Luckily this is a rare occurrence and for the most part just bad luck!

  2. Charlie

    @Robert: Interesting. In Bermuda once I met a tall ship, steel hull, that had been struck by lightning, and as a result the whole ship was magnetized. Which also kept their compasses from working properly. They were on their way to Norfolk, Virginia, to get degaussed.

  3. Bryan Tuffnell

    I feel obligated to take issue with a fair bit of what’s been said above. Without writing a textbook, the following is best seen as “almost correct”. If you consider that the sky has a positive electrical charge and the sea a negative charge, grounding the boat and the mast gives them a negative charge. Hence as far as the lightning is concerned, the bonded mast raised the local sea level to mast top height. Lightning will tend to bridge the narrowest gap with the greatest electrical charge difference – so by grounding boat and mast, you have made them MORE vulnerable to lightning strikes, not less. In other words, NOT grounding the boat and mast will REDUCE your chances of being struck.

    Tying an earth system into the keel bolts is not likely to result in loss of the keel, but it sure does constitute trying your best to do so. If the bolts are electrically weak they may act as a fuse and “blow” during a strike.

    Making a Faraday shield form shown above does help mitigate the effects of the strike compared to a simple bonding of the mast to the keel in many situations, but it’s over-rated. In a big strike, lightning will try to follow a straight path and the energy contained in such a strike is often too great for a simple system to be effective. And it needs to be understood that either method makes the strike a whole lot more likely to occur.

    A grounded mast does offer a degree of protection to a non-bonded electrical system in the boat underneath. There is what’s termed a “cone of protection” extending downwards at 30 degrees from the top of the mast. This is the standard system used in telecommunications.

    The best protection you can have is to park your ungrounded wooden boat with a wooden mast and an electrical system isolated from the sea, right next to a grounded metal boat with a big aluminium mast. In the photo above depicting the Sydney harbour yacht being struck, the question was posed “why did the bolt hit this one?” The answer is that it was best grounded boat in that area.

  4. phillip kellar

    @Bryan Tuffnell while part of what you say is true that a grounded boat is more likely to be struck the catch is that it will do less damage if struck where as a boat not grounded is less likely to be struck if it ever is you will have significantly more damage

  5. Bryan Tuffnell

    @Bryan Tuffnell while part of what you say is true that a grounded boat is more likely to be struck the catch is that it will do less damage if struck where as a boat not grounded is less likely to be struck if it ever is you will have significantly more damage

    Maybe, but quite likely not. The only way to offer lightning protection is to place a grounded lightning target above the mast, but electrically isolated from the mast and every other part of the boat. The grounding is completely independent of the mast, rigging, interior, electrical system, and above waterline areas of the hull. The idea is that this attracts the lightning and provides a low impedance path to ground, without drawing the charge into any part of the boat or its contents. Using this strategy one does not ground the mast, hull, rigging, etc. This is the only strategy to apply if one insists on lightning protection.

  6. ErnestV

    the sky has a positive electrical charge and the sea a negative charge

    Its the other way round. When polarity builds up the negative charge is at the cloud base, and the positive at the sea surface.
    [quote=Bryan TuffnellNOT grounding the boat and mast will REDUCE your chances of being struck[/quote]
    Wrong – the enormous voltage actually doesn’t care if you’re grounded or not. Given the fact that the boats surface will always be wet or moist in some way it is “grounding” enough to raise the sea level polarity up to the mast top. The only thing proper grounding does is trying to guide the current of a charge in a way that does the least harm.

    1. Bryan

      Not necessarily in the first case, and generally not true in the second… the polarity of lightning is variable, and there are countless examples of nearby strikes to ungrounded boats. Obviously if lightning didn’t care of you were grounded or not, lightning conductors wouldn’t work.

      As far as doing the least harm goes, grounding the mast is about the worst thing you can do, particularly if you have grounded electrical items onboard.

  7. a

    Our boat (20′ cruiser) has no grounding system. Is it foolish to think that the method where a set of jumper cables is attached to mast and other end dropped overboard, might be a good emergency strategy if caught in elec storm?

  8. tony

    what a topic indeed. to protect or not to protect, that is the question. simply do you use a brush type or spike type diffuser on your mast, do you protect for side strikes, or stay central with mast bonding..
    im trying to find an answer like us all and so far , the answers all differ..

Leave a Reply



Please enable the javascript to submit this form

Facebook Pagelike Widget


Google Ads