Protecting Against Lightning Strikes

Thunderstorm Lightning Over Thea Foss Waterway Boats Tacoma Wash

With our increasing reliance on electronics for navigation, communication and general operation of our boats, lightning is a subject of rather deep concern. In addition to the potential immediate dangers—fire, holes blown through the hull, crew injury—we are now, in the aftermath of a lightning strike, left with a boat that may have no power, no navigation equipment and no means of propulsion.

Today, then, we’ll give some thought to how lightning interacts with a boat and its equipment, and what we can do to mitigate the damage if it does hit.

Anatomy of a Strike

It’s important to note that, while we have a reasonably good idea of how lightning works, our understanding is far from complete and no one has yet developed a reliable predictive model for its behaviour. Here’s a (very) short summary of what we do know about cloud-to-ground strikes, the kind of lightning that we’re worried about when we’re caught out in a storm.

Inside the storm cloud, electric charges become separated—negative near the bottom, and positive at higher altitudes. (The exact mechanism by which this happens is still unclear.) The potential involved is a few tens of millions of volts to about a hundred million volts—impressive, but no more than a few percent of what would be needed to create a spark through several kilometres of air.


Now and then, a bunch of electrons are repelled from the bottom of the cloud, forming a “stepped leader”. These first electrons act as scouts, heating and ionizing the air (and therefore rendering it conductive) as they find their way downward in jumps of about 60 metres at a time. There are plenty of electrons waiting in the cloud to follow the newly ionized trail, keeping it hot and conductive as well as keeping the step leader charged.

At the same time, the charge separation in the cloud has induced a charge separation in the surface, with electrons being pushed down into the Earth, leaving the surface (and, in particular, tall objects) positively charged. Positive “streamers” start to form around convenient attachment points—the preferred ones being high places with sharp tips. (The classic spiked lightning rod is designed to create a very strong streamer.)


When a stepped leader meets a streamer, we have an ionized (therefore, conductive) channel through the air, and the return stroke begins. Propagating upward almost instantly, the return stroke carries tens of thousands of amps across the cloud-to-ground potential, which is many millions of volts. In most cloud-to-ground strikes, electrons are emitted by the cloud towards the surface; using standard sign conventions, the direction of (positive) current flow is from surface to cloud.

Prevention (Maybe) or Amelioration

From this, we can infer two possible defences in our anti-lightning strategy:

  • Force a streamer to form at a specific point that we control; this is what a conventional lightning rod does.
  • Prevent the streamers from attaching to your boat. This is the idea behind lightning dissipators. If, at what should be the most convenient attachment point for a streamer (the top of the mast), we prevent the electric field from concentrating at a point, streamers—and therefore lightning strikes—should be less likely to form there.

Air Terminals

This would be a good time to explore air terminals—the chunk of metal sticking out the top of the mast—in more detail.

Lightning Rods

The traditional air terminal, “officially” invented by Ben Franklin in 1749 but likely several thousand years older than that, is the lightning rod. It is shaped to concentrate the electric field at a pointed tip, forcing a very strong streamer to form at that point. This streamer is so much more intense than the ones given off by other nearby objects that the stepped leader will almost always choose it, ensuring that the lightning strike is safely conducted through a heavy copper cable between the lightning rod and the sea.

Lightning Dissipators

Once we learned how to calculate electric fields around objects, various lightning dissipators were invented. These try to do the exact opposite of a lightning rod. They spread out the electric field, allowing many small currents to flow and dissipate electric charge without producing a streamer. In effect, they try to render the object they’re attached to invisible to the stepped leader, thereby preventing the lightning from striking. Obviously, a lightning dissipator on the mast head will only protect the boat if there are no sharp pointy metal things sticking up that will generate a streamer anyway.


All air terminals need a grounding system that can safely carry the full current of a lightning bolt if they are hit. The choice of which type to fit is up to you. One class says “Hey lightning, go elsewhere, you can’t see me” (it can still be hit, but might at least reduce the probability of a strike). The other class is designed to yell “Here I am, great target, hit me!” and will almost always succeed in that goal.

Path to Ground

The combination of enormous voltage and enormous current leads to some interesting electrical effects that, in more mundane circumstances, we tend to ignore.

In an ordinary electric circuit, the physical layout of the wiring is largely irrelevant; a wire that takes a straight path is pretty much equivalent to one that meanders around like a drugged-up snake. What makes this approximation work is that ordinary wiring has very little voltage drop along its length; any two points on the wire are at approximately the same potential.

That’s not the case in a lightning strike. When we are talking about a lightning discharge of kiloamperes and megavolts, it’s quite possible to have a gradient on the order of a hundred thousand volts per metre in the conductors carrying that current.

If the voltage were provided by an ordinary high-voltage low-current source (like a sheep fence charger) the source would almost instantly short to nearly zero volts. A storm cloud, though, is not so easily depleted, and the high gradient is sustained as fifty thousand amps flow through the boat.

At a hundred thousand volts per metre, a right-angle turn in a wire is a real obstacle. Electrons in one leg of the right angle will feel the much lower voltage in the other leg, and—if conditions permit—some of them might find that jumping through the air is easier than sharing the congested route inside the cable. Once a few electrons try it, the air along their path starts to ionize, and now you have a nice, conductive plasma arc that all that current will find awfully appealing.

Fibreglass, wood, plastic and other insulating materials mean little to a lightning discharge. What’s a thousand or two volts to break down a few millimetres of insulation when you’re a fifty-kiloamp plasma arc with millions of volts to spare? (Pressurized sulphur hexafluoride might stay insulating even under all this, but you really don’t want that stuff in your bilge.)


Lightning wants to take the most direct route to ground, and it packs the punch to make its own way if it has to. Your 4-gauge copper cable will politely ask it “Please consider going this way”, and if the cable follows a nice easy path, the lightning will usually follow it. If you don’t give it a clean, straight route to the sea, though, it’ll take the straight route anyway—even if that means blowing a hole through the mast step in the process.

(Note that if your mast isn’t metal, you’ll likely need a 4-gauge grounding cable running all the way up it.)

So, here’s our next line of defence: Give the lightning a straight, low-resistance path to ground. 

Fields and Induced Voltages

The direct discharge is the most dramatic damage mechanism, but it’s not the only one.

There’s an enormous amount of electric charge in a lightning bolt; all that charge creates a strong electric field. That field, in turn, induces electric fields in all sorts of nearby objects, some of them strong enough to generate arcing currents between metal fixtures if the potential between the objects isn’t equalized by bonding them to a common ground.

A large, rapidly changing current also creates a strong magnetic field. The changing magnetic field, in turn, induces a voltage in any conductor that crosses it. Induced voltages are proportional to the rate of change of the magnetic field, which is itself proportional to the rate of change of the current that’s creating that magnetic field.

The rate of change of the AC current that drives your isolation transformer is on the order of a thousand amps per second. The rate of change of current in a lightning bolt is more like fifty million amps per second. Without the multiple windings and iron core of the transformer, the coupling is much less efficient, but it’s still quite adequate to induce a few hundred to a few thousand volts in nearby conductors.

Those conductors include not only the power wires to every device on the ship, but also the data cables, antenna cables, even the traces on the circuit boards.


Thus, we come to the result we all know so well: Any and all devices that include sensitive electronic components are, even if the direct lightning strike misses them, likely to be fried by induced voltages. In addition, any ungrounded metal objects could become part of “side flashes” that arc across the boat.

Our next major rule of lightning protection is: Bond all metal fixtures of appreciable size to a common ground. This helps to discourage side flashes, and is called for by every marine electrical code I know of. That won’t help the electronics, though.

How do we reduce the odds of frying the electronics?

  • Surge protectors can help a bit, if they’re very close to the power inlet terminals of each device, but they offer no guarantee of protection.
  • Keep all wiring as short as possible, since induced voltage is proportional to length. It’s also not a bad idea (thanks Erik de Jong) to unplug cables from expensive devices when you’re away or when the risk of a strike is high.
  • Keep each positive/negative wire pair as close together as possible (ideally, bundled or twisted together) so that any induced voltage is the same on both wires. Electronics are generally more sensitive to the voltage difference between the wires than to the absolute voltage relative to ground.
  • Avoid running wires near, or parallel to, bonding/grounding cables. The closer something is to the lightning current, the stronger the fields will be.

Even so, expect at least some of your electrical systems to die in a strike.

Faraday Cage

If you solve Maxwell’s equations for the inside of a closed conductor that is being bombarded by external electric and magnetic fields, you get zero.

This somewhat odd result gives us a beautiful, elegant defence against lightning: the Faraday cage, essentially just a closed, grounded metal box. Anything inside it won’t feel the fields induced by the lightning. For this to work, the entire surface of the box should be a continuous conductor, unbroken by insulating materials. A gasketed ammunition box won’t do (the lid is electrically isolated from the box by the gasket). A microwave oven can be pretty good – but, as some readers have pointed out, many (or most) recent microwaves will still allow some radio-frequency EM fields to enter. A simple closed box made from aluminum or copper sheet metal is darned close to perfect.

We can, therefore, hide some backup gear—a handheld GPS, a tablet computer loaded with charts, a portable VHF, an autopilot head—in a Faraday cage, and have at least a fighting chance that it will be usable after a lightning strike.


Our lightning defence strategy now consists of:

  • Good grounding from mast head to sea, with lightning dissipators up high to try to discourage streamers from forming on the mast.
  • A straight, low-resistance path to ground for any lightning that does strike. Conveniently, this can be the same bonding system used to equalize voltage in the previous point.
  • Electrical bonding from all metal fixtures to a common ground point, which helps neutralize the potential differences that create side flashes. Conveniently, one bonding system can perform both this role and the corrosion protection role.
  • Backup electronics stashed in a Faraday cage so that they’ll have a chance of surviving a strike.

Further Reading

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Meet the Author


Matt, Engineering Correspondent, is a Professional Engineer and true renaissance man, with a wide range of expertise including photography and all things boat design. He has a unique ability to make complex subjects easy to understand and he keeps an eye on the rest of us to make sure that we don’t make any technical mistakes. Working as M. B. Marsh Marine Design, Matt designs innovative powerboats of all shapes and sizes.

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58 comments … add one
  • John Feb 23, 2014, 7:20 pm

    Hi Matt,

    A great post that explained a lot of stuff about lightning that I was not clear on.

    One question: Over the years I have seen lightning strike several times into the water very close to “Morgan’s Cloud”. In one case, so close that I saw steam rise from the water at the strike location. In each of these cases MC’s mast was clearly the highest thing around, and yet we escaped unscathed.

    My theory has always been that because our boat is metal and therefore an extremely good ground plate, that we were discharging the air above the boat and therefore creating a “zone of protection” around us. (I have also talked to other metal boat owners who have experienced the same thing.)

    What makes it even more interesting is that up until eight years ago, when we replaced our aging and cracked aluminium mast with one made of Carbon, we had no terminals at all. Since then we have had a lighting rod connected to the hull with #4 cable just as you recommend above. As I understand it from your post, this should make us more likely to get hit (although less likely to suffer damage) but once again we have been in at least one truly spectacular lightning storm off Florida with strikes all around us, but no hits.

    Do you think we have just been lucky, or do you think that there may be some validity to my metal boat protection theory?

    • Erik de Jong Feb 23, 2014, 8:00 pm

      Hi John,

      I was explained by a prof at the university, that lightning can hit any spot that can be touched by a large imaginary beach ball of 20m (65 foot) diameter. So imagine a large ball like that, and roll it against your boat. Any point that can be touched by that ball, can suffer a direct hit.

      The prof explained that lightning travels so fast, that the reduced resistance through a metal instead of air does not make up for the extra distance it has to travel to reach that specific point.

      So lightning never “searches” for the highest point, but always for the path of the fastest travel time. Usually this is a straight line to the earths surface.

    • Matt Feb 23, 2014, 8:43 pm

      I would say there are two big advantages, if you’re in a lightning storm, to having a metal boat.

      One is that almost everything on board is conductive and electrically connected; it’s therefore very difficult for voltage differences to build up. You can think of the step leader as “sniffing out”, in each step, the most strongly positive electric field within about twenty metres of its current point (this is the origin of the very useful “beach ball” geometry Erik mentioned). Most points on a metal boat are at essentially the same potential (voltage) as the sea surface; the step leader can’t really feel the boat unless it gets within 20 m or so. A boat without low-resistance paths to ground could support a relatively strong induced positive charge up high, which the step leader could feel from farther away.

      The other reason is that, with a metal boat, any path through the boat to the sea is a valid one. The lightning current will simply spread out wherever it wants; we’re not trying to force it to follow a single path. Such a boat would, I think, have a lower chance of sustaining severe damage in a strike than might be the case with a non-metallic boat.

      That said, lightning is chaotic, unpredictable and not terribly well understood. And consider statistics: if you can see every lightning flash within three miles, the odds that any given strike will be within the 20-metre range that your boat will attract are approximately 0.01%.

      • John Feb 25, 2014, 1:32 pm

        Hi Matt and Erik,

        Thanks very much, great answers to my question that have upped my understanding several notches.

    • Marc Dacey Mar 1, 2014, 7:34 pm

      As the owner of 16 tonnes of steel sailboat, we sure *hope* there’s something to your theory, John. I have heard of the “cone of protection” imagery prior to this, even though it sounds a touch magical.

      Any time I have been out in lightning, whether Lake Ontario or the Atlantic, I’ve unhooked the electronics (going to just a handheld in the cockpit) and put on thick rubber gloves, in addition to seaboots. The latter is because not one, but two guys at my club have suffered “side strikes” that shocked them through the wheel when their masts were hit or there was a nearby strike. Both were knocked down, although both recovered enough to sail back.

  • Erik de Jong Feb 23, 2014, 7:54 pm

    Hi Matt,

    What an awesome article again!

    We have been in the unfortunate situation to get hit by lightning with our boat. We are however lucky with our metal boat and metal rigging. The lightning dissipated in the water no problem without us taking any real precautions during the build of the boat.

    In order to protect our sensitive electronics, we always disconnect all our antenna’s and power supplies from the computer units when we leave the boat for a longer period of time, or when we see a thunderstorm approaching. This really saved us a lot of grieve when we actually suffered a direct strike. The only piece of electronics that was actually fried was the $100 car radio/cd player and some antenna’s/sensors.

    Some stainless steel mounting bolts that looked liked they were cut of with a saw, were the only remnants of what used to be a tricolor, wind sensor and VHF antenna. There were no burn marks, the things just disappeared.
    We now have our antenna park re-arranged so that the main current will not be led through the sensors. This basically means that nothing but a tricolor is left on the top of the mast.

    After the hit, the light bulbs in the lamps that were not on the top of the mast, were still working and are still working more than a year after the hit. This is enough proof to me that lightning energy will indeed take the shortest route down and does not influence parts that are not directly in the way.

    I’m pretty confident that, with the new setup, we have a good change of not having any damage except for the tricolor, as long as we disconnect the antenna’s and power supply cables to all electronics when we see a thunder storm approaching.

    The reason the car radio was fried, was because it was mounted very close to the power cable to the tricolor/anchor light. That was the only cable we forgot to disconnect. The front of the radio literally exploded and was shot against the collision bulkhead 7 meters away, which was to us the scariest thing that happened in that split second since if flew close past our heads.

    • Matt Feb 23, 2014, 8:45 pm

      Erik, that’s a good point about disconnecting power and signal cables when you’re away. The induced voltages that blow up circuit boards tend to originate in these wires; disconnecting them when you’re away should considerably reduce the odds of frying expensive machines.

      I think I will add quick-disconnect couplers on the instrument power cables as recommended equipment in the future.

      • Eric Klem Mar 1, 2014, 8:12 pm

        Erik and Matt,

        In an article in August 2010, Boat US briefly made a caution about having unconnected antenna cables. They pointed out that when they are disconnected, the path of the lightning inside the cabin is less predictable and more likely to zap other things. The mention is really brief and I don’t know if it is backed up by their insurance data or not. If anyone has a good database to work from, it should be Boat US.

        Ham radio operators routinely install lightning arrestors in their antenna cables which are supposed to protect the radio. I know of numerous strikes to land based antennas with these and I am unaware of any radio damage (this includes 1 strike than an antenna of mine took). I have never seen one of these installed on a boat but it would seem like a logical thing to do. I would be curious to know if anyone knows why?


        • Matt Mar 24, 2014, 6:01 pm

          I’m honestly not sure what to think about lightning arrestors in antenna cables. I haven’t seen enough good data about them to make up my mind.

          If you have a proper down-conductor (4 AWG or heavier cable, running straight from the highest point on the boat to the keel), then I wouldn’t think a disconnected antenna cable would be a particularly great risk.

          • Andy G May 5, 2014, 5:15 pm

            I have replaced antennas and antenna cable on ships after lightning strikes. I have never seen damage to the system on the other side of a lightning arrestor, thought I have seen the lightning arrestor damaged.

  • JCFlander Feb 24, 2014, 10:48 am

    Hello Matt,

    Very good article. On the subject of efficient lighting protection, there’s some further discussion on Ewen M. Thomson’s studies. Here’s an article:

    He finds that lightning usually seeks the surface charge, and thus strikes sideways from boat. To ensure that to happen efficiently, there’s now tailor-made side terminals available. Like these:

    Also, ABYC Technical Report TE-4 “Lightning protection” has been revised to take account of this new research. If interested, it can be purchased here for 50$:
    or, downloaded from here, on pinch…


  • Chris Feb 24, 2014, 10:58 am

    As to Faraday cages. Several of us did some bench testing and found that heavy duty aluminum foil wrapped around small electronics as if they were a lunch to keep fresh produced the same positive results as the $$$ copper mesh box we used as a control.

    • Matt Feb 24, 2014, 5:45 pm

      I’ve seen the same thing in some high-vacuum physics facilities…. a $500,000 piece of precision machinery with the important bits wrapped in crumpled kitchen foil. (Their concern is mainly RF interference.) Looks really weird, but it seems to work.

      • Chris Mar 1, 2014, 5:58 pm

        Maybe to folks in tinfoil hats are on to something!

  • Hoftman Feb 24, 2014, 11:35 am

    And it all comes standard in the Adventure 40!


  • Harlan Carswell Feb 24, 2014, 11:38 am

    One small dissent: a microwave oven makes a very poor Faraday Cage. Even at the operating frequency near 2.4GHz, the microwave does very little to exclude electrical energy. You can try it yourself: place your cellphone in it and call it, or have a look at the signal strength indicator through the door.

    I have tested a variety of microwaves at various radio frequencies. None of them has ever indicated the slightest tendency to act as a Faraday Cage.

    The reasons the microwave radiation does not leak out at dangerous levels seems to be (a) the inverse-square law, (b) absorption by food, which is why you are not supposed to operate it empty, and (c) wave guides inside the oven.

    The only thing I have ever found around a boat that acts as a decent Faraday Cage is a metal-walled vacuum bottle with the metal lid in place. Most of those are too small to accomodate handheld electronics.

    • Matt Feb 24, 2014, 5:54 pm

      Thanks for doing those tests, Harlan, and for sharing the result. I guess the shielding in a lot of microwaves (relatively recent Chinese ones, perhaps?) is just not up to the standards of our massive old Panasonic.

      • Harlan Carswell Feb 25, 2014, 12:34 pm

        Admittedly, all my tests were on *relatively* new microwaves, but all of them had a metal skin everywhere except for the door, which was glass. A Faraday Cage requires a *complete* conductor—it cannot have any large openings, with large being defined by comparison with the wavelength of concern. At 2.4GHz, the wavelength is 12.5cm. Allegedly, there is a conductive pattern on the glass door that completes the cage for signals at the operating frequency. Presumably, the attenuation is enough for certification, given the conditions mentioned above, but not enough to create a good Faraday Cage.

        A Faraday Cage also requires good grounding, although an ungrounded cage works well at radio frequencies, presumably because a high-frequency alternating current reverses any charge buildup during the previous half cycle. How well an ungrounded cage will work for lightning protection is an open question, since the transient current appears to persist over many microseconds. See

        So far as I know, all microwaves have a glass door, even your old Panasonic. I would urge you to conduct a few simple tests—you may be very surprised by the results.

        The tool box Faraday Cage described elsewhere in the comments is probably the best alternative, possibly with the addition of a very good ground, with foil being a potential saving throw. Building a Faraday Cage is very difficult, because any small discontinuity in the conductor invalidates it completely.

        I use an Iphone/iPad app called Boltmeter to monitor the approach of lightning when the radar shows a storm approaching. If Boltmeter and my eyes/ears give me an indication of significant cloud-to-ground lightning strikes, I bundle the handheld gear into the best protection that I have. No idea if it makes any difference.

  • John S Feb 24, 2014, 3:03 pm

    Interesting article. I have a pretty good memory of sailing in our 39’ ketch in the Gulf Of Mexico off the coast of southwest Florida in 1976 and getting wacked by a line squall packing 55 knot winds (according to our mast head anemometer). Accompanying the squall was a huge and very scary electrical storm. Lightening was flashing everywhere and at one point it seemed almost constant. Just as John described, I have a clear image in my brain of several lightning bolts that hit the water near the boat with steam rising from the surface where the bolt struck. Our boat was never struck in that storm. Later that year our boat was hit by lightning while docked in the Marina. We did not have the tallest mast. It appeared the mast head VHF antenna was the entry point—pieces were strewn about the deck. The electrical panel was blown apart. The starboard cap shroud turnbuckle was fused together permanently. No other electronics were damaged. The VHF worked perfectly once we replaced the antenna. No other boat was hit. I don’t recall if the boat had a lightening ground system.
    Like many sailors, I have read everything I can on boats and lightening. My current understanding is that if your boat is grounded you may be more likely to be hit but have less damage. But, I am not sure what the statistical advantage of one is over the other. And more important to damage to equipment is damage to the boat itself or the crew. I have read about boats that were grounded and struck by lightning—some damaged and some not damaged, boats not grounded and hit by lightening—some damaged and some not damaged, boats not grounded and not hit by lightening and so on and so forth to include every possible combination thereof. I read one theory that suggested that an ungrounded boat can be hit by lightening and because wind and rain usually is part of the experience the deck and hull of the boat are wet and the lightening can come down the mast and leap to the deck and travel along the skin of the boat on the wet surface and leap to the water with no ill effect to the boat, its equipment, or crew. I have read a report that said the amount of metal and the thickness of the wire required to handle the voltage of a lightning strike far exceeds what most boat owners could or would ever consider installing in a boat. I have read an article written by a well know and highly regarding boating “expert” unequivocally state static dissipaters are nonsense. I corresponded once with a well known sailor that circumnavigated the globe twice encountering dozens of electrical storms near the equator and elsewhere and was never struck—his boat was ungrounded. I corresponded with a sailing couple that had similar experiences circumnavigating multiple times and were never hit—their boat was heavily grounded.

    I do not profess to know anything about how lightening works with regard to boats . . . and I suspect all we have are theories each supported by as much evidence as one is committed to gathering. Maybe one of the theories is correct. Maybe all are correct. I suppose the prudent thing to do is to ground one’s boat as the article, and many others suggest and hope for the best. Then again, I am for whatever works . . . .


  • ben garvey Feb 24, 2014, 4:06 pm

    Hi Matt- good article. Thanks for bringing it up.

    I understand that at the voltage potentials being discussed, most materials are conductive; but I wondered about a few things that might be at play in my specific case – and may be at play in others. On my boat (a 47′ steel cutter), we have the usual aluminum mast, with stainless rigging terminated through the usual turnbuckles to chainplates. The mast is deck stepped, and sits on a 1 1/2″ thick piece of teak, which supports the base casting, both of which are bolted thru the deck. There is a welded support post on the underside. I’m wondering really how much this teak block (and really all the epoxies and polyurethanes that make up typical coatings) are barriers to conduction of strikes to ground. The same question applies for hull coatings – really there is not very much actual metal exposed to seawater to make a big ground – and we work hard at keeping it that way! Does this matter, or at these potentials do all these materials really appear transparent to the electrons?



    • Matt Feb 24, 2014, 5:51 pm

      I’d suggest that if you actually have a lightning current (tens of thousands of amps, millions of volts) trying to go through it, then no, a few inches of wood or fibreglass won’t stop the current. That material will, however, be severely damaged in the process – fibreglass gets scorched, dry wood burns, wet wood can explode.

      The charges you’re trying to dissipate before a lightning strike might be a different matter; in this case, a good low-resistance path does make a difference. But the currents are also much less – milliamps instead of kiloamps.

  • richard e. stanard (s/v lakota) Feb 24, 2014, 5:17 pm

    this reminds me of the battery posts from several months ago…sort of squishy…not the posts…the subject matter…especially for those of us who are technologically challenged…for instance i know enough about battery care and maint to be dangerous…therefore i decided long ago that lightning is essentially unpredictable and will basically do whatever it wants to do including frying electronics even in a faraday apparatus if it wants to, and i think this post essentially confirms this view…my exposure to lightning, while infrequent, has been on the dramatic side including bolts that emerged suddenly with no warning along with thunder clapping that probably is one reason i need hearing aids today, but never with any damage to my boats at those times…one theory possibly explaining this is the rigging forming a bit of a natural shield that lightning tends to steer away from…another theory says that grounding attracts lightning so don’t have it especially on a boat…all i know is no boat of mine has ever been struck in spite of what seemed to be close calls…when you are on board either underway or anchored there is no way to escape the exposure although i will instinctively retreat to the enclosed galley area hoping and praying for at least bodily protection there knowing i might be swimming shortly…but this can happen ashore just as easily because of lightning’s essential unpredictable nature easily obviating natural laws if it so decides…as i said with the battery posts i do the best i can and then live with any resulting fallout because i refuse to let that deprive me of my water-borne pleasures…lastly i do take some comfort with the odds stated in this post of lightning being actually close enough to be a threat…i read those odds as 1/100th of a percent, although i would have said more like 2 or 3 percent based on my experience…the truth is probably somewhere in between, and i will start foil wrapping my handhelds in electrical storm conditions from now on based on this post…cheers from tampa bay

  • Matt Feb 24, 2014, 6:01 pm

    One further thought about air terminals.
    Before spending big money on one, think about how much a two-foot piece of 5/8″ copper bar costs. If you like standard lightning rods, grind a point on the end of that bar. If you like dissipators, crimp or solder a couple dozen scraps of 14-gauge bare copper wire to it instead.

    Given a limited budget for lightning protection, I’d spend most of it on good bonding/grounding of all the metal objects on board. There’s no magic in any of this; it’s mainly just a matter of following established good practices with respect to bonding, cable routing and providing some kind of valid path for any lightning that does strike.

  • Scott Feb 24, 2014, 8:23 pm

    Does the interior of a metal boat (ours is steel) act as a Faraday cage?

  • Evan Gatehouse Feb 24, 2014, 8:26 pm

    We were using our oven as a Faraday cage – until I noticed it had a glass door. Duhhh. I tried a Pelican box lined with aluminum foil. Put cell phone inside and it rings when dialed. Drat.

    So I went out and bought a large aluminum tool box. Thin (2mm?) plate and the lid did not fit tight enough to make a perfect seal. So I added a conductive metal mesh/foam gasket made especially for this purpose. And clamps to hold the lid tight.

    Now a cell phone placed inside will not ring when dialed. It is important to line the box with a non conductive material like corrugated plastic sign sheeting to make sure no electronics are in contact with the surface of the box that may have a charge. I don’t think a Faraday cage will need to be grounded – the charge just stays on the outside until it goes somewhere else.

    By the time we put in 2 DSLRs, lenses, compact cameras, 2 laptops, GPSes, Pactor modem, portable battery charger, e-books, portable hard drives, label maker, Nicad battery charger, camera battery charger, video camera, spare autopilots, handheld VHFs, i.e. anything with a microchip there sure isn’t a lot of room – buy the biggest box you can fit inside your boat if you are doing this yourself. We have a checklist on the box of everything that is supposed to fit inside. About the only thing that doesn’t fit is the Ham radio and the desktop computer – and we physically unplug these.

  • marie dufour Feb 25, 2014, 1:19 am

    Hi Matt… great article. Yes, protecting your yacht, crew and electronics against the damages of a lightning strike (you can’t PREVENT a strike) is paramount. For more info and reference, check!/2013/07/considerations-for-lightning-protection.html
    and the results of a lightning survey at!/2013/07/lightning-survey-results.html
    There is no definite answer! dominomarie

  • Bryce Winter Feb 25, 2014, 2:08 am

    Great article, thanks Matt. I’m fascinated by lightning and have recently been reading up a little on it. My knowledge about lightning is probably just enough to confuse myself often (like most of the things on my boat, unfortunately!).

    I’d like to second the question above about whether the hull of a metal boat would act as a Faraday’s cage, and also as a follow-up, what implications does that have for crew on deck during a lightning storm?

    In the past I have confined crew below-decks in lightning storms and only done any necessary deck-work myself under the assumption that they would be safe in the cabin as long as they avoided contact with the metal hull. I have assumed that I’d be fried in this instance if the boat was struck, so I’ve obviously tried to minimise deck-work during a lightning storm… but sometimes things just have to be done!

    Our boat has both an aluminium hull & mast with no lightning protection apart from the inherent properties of those materials. In your view, are my assumptions reasonable that anyone below-decks should be safe and anyone on deck is likely to be fried in the event of a lightning strike?



  • FAIVET DANIEL Feb 25, 2014, 4:39 am

    Excellent article , d actualité nous concernant, il y a 4 mois nous avons eu des dégats électriques suite a la foudre ( expertise) notre voilier étant insuffisament protégé , nous avons change les 3 batteries, et le transfo ( vectron) nous faisons actuellement révisé tout notre circuit électrique
    Cordialement D F

  • Matt Feb 25, 2014, 9:33 am

    A few folks have asked whether the hull and deck of a metal boat will form a Faraday cage.

    They do to some extent, much like the metal body of a car provides some degree of lightning protection to its occupants. But I wouldn’t count on this protection as far as electrical equipment is concerned.

    A Faraday cage excludes electromagnetic fields originating outside its borders. When components inside the metal surface – things like the mast (or its compression post) – are conducting large currents, the fields created by that current are free to bounce around inside the boat.

    I would be fairly comfortable holing up inside (or telling my family to hole up inside) a metal boat during a storm, just as I’m not too worried about lightning hitting my car. Out on deck is obviously a riskier place, but if everything’s properly bonded against side flashes and there’s a good down-conductor to channel the main strike to the sea, the risk of frying yourself is pretty miniscule – particularly when compared to the risk of capsizing due to not reefing down in time, or the risk of being knocked overboard by the boom due to failure to rig a preventer.

  • John Feb 25, 2014, 1:38 pm

    Hi All,
    A huge thank you to all of you who have commented and added so much to Matt’s post that set the base and standard for thoughtful discussion.

    I, for one, am way better informed about this complex subject than I was two days ago and will be making some changes on “Morgan’s Cloud” as a result of this post and the discussion. Most notable a proper Faraday cage, rather than the hit or miss “stick a hand held GPS in the oven and hope for the best” approach that I have pursued in the past.

  • Jack Woolfe Mar 10, 2014, 6:07 pm

    Can insulated aluminum power cable be substituted for the 4-gauge copper grounding cable? Even though an aluminum cable would undoubtedly have to be a larger gauge than copper, I would guess it would still be both lighter and less expensive than copper.

    • John Mar 12, 2014, 6:51 pm

      Hi Jack,

      Sorry, I simply don’t know. But given the importance of the function and the fact that copper is pretty much always specified, I guess I would suggest sticking with that.

    • Matt Jan 31, 2015, 11:27 pm

      The electrical conductivity of copper is only about 1.7 times better than that of aluminum, so from that standpoint, it’s good. (High-voltage transmission lines, for example, are usually aluminum.)
      The trouble comes when things start to corrode. Aluminum oxide is a damn good insulator, so good that we use films of it a few atoms thick to insulate between components on an integrated circuit chip. This becomes a real problem when you try to connect an aluminum conductor to just about any other metal.

  • John Kettlewell Mar 14, 2014, 6:32 pm

    I have never had a properly bonded or grounded boat and I have never been hit by lightning—does it mean anything—I don’t think so. I have also talked to lots of metal boat owners, and it seems like a lot of them have been hit by lightning (maybe the majority I have spoken to). From talking to many other cruisers, both who have been hit and who have not been hit, often in the same anchorage at the same time in the same storm, I can’t draw any conclusions as to what works and what doesn’t. Some who had no protection, like me, were struck and suffered damage, while others who had what sounded like great protection were struck and had damage. I think it would require a scientific study that will never be done to conclusively prove that one way or another is the safest.

  • John Mar 14, 2014, 6:38 pm

    One other thing. Down in the Southwest Caribbean I saw columns of lightning come straight down into the ocean nearby that looked like they would cause catastrophic damage to anything they hit, whether grounded or not. The water where these vertical columns of lightning hit looked like bombs were going off. Down there we had one close call when it hit near enough to the boat that it sounded like a steak sizzling on the grill, and electronics that were turned off on the boat suddenly popped on. Again, the sound was like a bomb going off nearby. Frankly, I can’t imagine any puny wire leading from the masthead would do one bit of good directing that much energy to ground.

  • Dick Stevenson Mar 30, 2014, 5:47 am

    Good day John,
    We are on the move again having left London: this year there was no sleet/snow going down the Thames and wx is typical for the season. This thread brings up spring commissioning thoughts and renewing the batteries etc in our grab bag (Abandon Ship Bag). It occurs to me, in light of AAC’s concerns/contributions of late on lightning protection, that wrapping the handheld and the GPS, stored in the ASB for long periods, in aluminum foil might be wise (how many layers? Any other suggestions?). This did not seem to come up in other threads (where this comment might be better placed) nor did the question of how well protected EPIRBS are on their own, or should they also reside in a Faraday cage. I do not like the idea of moving our EPIRB around when a storm approaches and have never heard of an EPIRB being lightening disabled. I am going to explore turning a drawer into a Faraday cage with some metal workers in the days to come. I will post anything of interest.
    My best to all, Dick Stevenson, s/v Alchemy

    • John Mar 31, 2014, 9:27 am

      Hi Dick,

      Interesting, we are going through the same thinking process as we plan commissioning MC, but I hadn’t thought of either the ASB or the EPIRB, so you are way ahead of me.

      I think your idea of wrapping the electronic items in the ASB in tin foil is a very good one that I will copy. On layers, as I understand it, the thickness is not what makes this work. Rather I think the key is to make sure that there is no breaks in the conductive surface. Therefor I would guess that two layers with the seams on opposite sides would provide at least some protection. In fact Chris, in an earlier comment said that he had done some testing and found foil a surprisingly effective Faraday Cage.

      Having said that, I, like you, am going to investigate making a proper purpose built metal box to hold a backup GPS and hand held VHF.

      On the EPIRB, that’s hard one. I think, in the absence of any really solid information, I would leave it in its normal storage position. Particularly if it is, like ours, self launching, since I have always believed that one of the advantages of an EPIRB is that, if properly set up, it will launch and activate even in a disaster that overwhelms the boat so quickly that the crew have little time to react—many a fishing boat crew have been saved after their EPIRB activated after a sudden down flood.

      One other thing. Given that I will leave the EPIRB in its bracket I think I might take to keeping the small personal locator beacon (mini EPIRB) that we carry for hiking in remote places in the Faraday Cage too.

      • Matt Sep 10, 2015, 12:31 pm

        I wouldn’t worry too much about the EPIRB. They are self-contained, heavily ruggedized, internally shielded, and have no electrical connection to the outside world.

  • Steve Guy Mar 30, 2014, 8:46 pm

    So, would you say for the best ground on a non-metallic hull, a big cable should be run from the base of the aluminum mast to an external lead ballast ? A big choice for the A40: internal or external ballast.

    Thanks for your good work and all comments,

    Steve Guy, s/v Pilgrim.

  • Torbjørn Mar 31, 2014, 4:39 pm

    Hi All,

    Good article! Good comments! Just became a member yesterday and have to say that makes it very difficult stay focused in front of my computer at work!

    On faraday cages. I am outfitting a boat for a Norway – Caribbean – Norway lap for the upcoming season. I am planning on purchasing some 50′ ammo boxes from my local Army Surplus store to use as safeboxes for a handheld VHF, PLB, EPIRB and a tablet with charts on it. The boxes are cheap, strong and, at least here, I can by water-proof zip-loc innerliners protecting the contents from water. One of these I will paint orange and store in the cockpit as a grab-box. To properly ground it I am thinking of a) streaming a wire into water aft of the boat directly from it or b) running a grounding wire to it from the engine.

    As a second question. Most of the discussion above has been concerning loss of navigational equipment and loss of radios etc. All electronics! However, what happens to more basic systems. Alternators, starters, batteries etc. All of the electronics I can more or less replace with handheld devices and a small solar panel. Even spare battery powered LED NAV-lights could be stored in a box. But if batteries, alternator, starter etc. are fried then that opens a whole new can of worms where the all use of engine is lost.

    All comments are highly appreciated!

    S/Y Eir

    • John Apr 1, 2014, 8:54 am

      Hi Torbjørn,

      Welcome to AAC, thanks for joining.

      Did you note Matt’s caution in the post against using ammunition boxes? The problem is that most have rubber gaskets on the lids and this breaks the conductivity and renders them less effective as Faraday Cages.

      I would ground whatever Faraday Cage you do use to the yachts bond system, not stream a wire over the side. Also, I’m not at all sure about using a metal box as a abandon ship grab bag. First off, it might not float and second the sharp hard corners could damage the life raft or hurt someone.

      On your second question, yes the entire electrical system including that required to use the engine is vulnerable in a lightning strike, although not as much as sensitive electronics. I think the best defence against losing the engine is a good lightning conductor and bond system, but nothing is guaranteed.

      One other point on this, those with computer controlled engines would probably be well advised to keep a spare control unit in the Faraday Cage. Our own Perkins M92-B requires a 12-24 volt converter and we keep a spare wrapped in tin foil. That will be going into the Faraday Cage we will be installing.

  • todd Apr 8, 2014, 5:29 pm

    So after reading fascinating article can we conclude that wrapping everything electronic that is not attached to the boats hardwire systems in aluminum foil would act as a faraday cage mechanism for these portable devices?


    • John Apr 9, 2014, 8:27 am

      Hi Todd,

      I think that might be a bit of a simplification. Yes aluminium foil may help, in fact probably will, but I don’t think that it is a substitute for a true grounded faraday cage for the mission critical items.

  • John S Apr 21, 2014, 10:05 pm

    All this discussion about lighting strikes got me thinking . . . how does a lighting strike affect a boat with synthetic rigging, e.g. dynex dux . . . or some smallish gaffers sometimes use vectran? Is synthetic rigging electrically conductive? Or is it conductive only because it is wet? If not, does one need to take the same precautions, i.e. should the chain plates be grounded with synthetic standing riggings or just the mast? Is there any data for lightening strikes and boats with synthetic rigging?

    • John Apr 22, 2014, 7:26 am

      Hi John S,

      I would think that any boat so equipped should have a really good separate lightning conductor running down the mast to carry the current of a strike. And I would still want the chain plates properly bonded because one simply can’t be sure where a strike will land. Erik’s beach ball analogy earlier in the comment stream makes that clear.

  • Dick Stevenson May 6, 2014, 3:00 am

    Could you say a little more about lightning arrestors? How do they go into antenna cabling? Do they affect SSB, VHF, etc.? Suggest manufacturers?
    Thanks, Dick Stevenson, s/v Alchemy

    • Andy G May 6, 2014, 12:17 pm


      All lightning arrestors have RF connectors on the in and out sides and are simply placed in line with the antenna cable. Every time a connection is made in an antenna cable some transmit power and receive sensitivity are lost. On paper the two extra connections would show up as a 3 decibel loss which the math says cuts the signal in half. In the real world when I have installed the same radio and antenna for voice communication capability I haven’t seen a truly measurable difference in either performance or reading on an RF power meter (I’d admit to a few percent difference noticeable on the meter sometimes). You would need to order one that matches the frequency range of the radio system (VHF, HF/SSB, etc…). The brand that I am the most familiar with is Polyphaser.

      That being said on a large steel ship there are significant distances between the lightning ground paths and the power equipment along with pretty religious grounding. A lightning arrestor may save the radio from damage through the antenna line, but won’t do much if the surge is introduced through the power lines. If you are going to go through the expense/hassle you might want to put surge protection in the power lines at the back of the equipment as well.


  • Bill Balme Aug 31, 2015, 1:01 pm

    I’m always reading things a little late… Last week my boat took a lightning strike. It’s fried pretty much everything electronic and at least some of the general electrical stuff. Only now am I working out that perhaps my lightning protection was not up to snuff! It’ll be interesting getting through the repair/replacement process to see what I can do differently going forward!


    • John Sep 1, 2015, 8:07 am

      Hi Bill,

      I’m so sorry to hear that, what a huge drag. I have been lucky enough to avoid that experience so I really don’t have much to offer, other than to say that Dick’s suggestions make a lot of sense.

  • Dick Stevenson Aug 31, 2015, 4:13 pm

    Hi Bill,
    I am very sorry for your lousy luck. I have done a fair amount of research and believe that lightning protection is an oxymoron roughly akin to military intelligence so luck may well be the operative word.
    A piece of advice from a surveyor friend of mine. Turn everything on that has electrical components and run it hard until the survey is complete or it declares itself damaged. Also, see if you can get a stipulation that there is likely to be more equipment exhibiting damage down the line so this first list of insurance items may not be the last.
    Good luck with the repairs. May be a good time to re-visit decisions that you were not satisfied with prior to the strike.
    My best, Dick Stevenson, s/v Alchemy

  • Petter ;-) Sep 7, 2015, 6:34 pm

    Brain picking challenge; aluminium yachts, nmea installations and avoidance of electrostatic discharge.

    John and AAC members,

    I have an issue that pains me and hope to find solutions by picking the brains of the AAC members – as it, to me, is a complex issue. Here is the case;

    During a recent refit of an aluminium sailing vessel, all the old navigation electronics were exchanged for new (B&G), and the units connected by a common NMEA 2000 backbone. Early summer the plotter/radar (Zeus2) stopped connecting to the network – and in the same go disabling many other components in the network.
    (There is already another post on the site that discusses the merits of a fully integrated system – and I got a taste of the merits. Did not taste good, at all.

    The supplier guessed it was a ruined NMEA port and exchanged the plotter/radar display. The network sprung back to its former glory and everything worked – till a few weeks ago. During a short sail three connected units, the plotter/radar, rudder angle indicator and autopilot computer stopped working, at the same time. (Refer to above link, for another taste of merits.)

    The network was then checked by an electronics installer and they can not find any particular defects. A new rudder indicator was fitted, and it immediately found its place in the network. Their diagnosis is now a set of failed NMEA interface port on the three units.

    The electronics installer, as well as second line support personnel seem to think that, if the result is blown NMEA ports, then this due to uncontrolled electrostatic discharge (ESD) that finds its way into the NMEA network. They have measured electric resistance between the components of the yacht. There is full connection between all of the following: mast, stays, hull, keel. There is however, 40 mega ohm resistance between all of the above and the sea, i.e. to me, the amateur, an indication that there is electrical insulation between the hull and the sea. (The underwater part of the hull is epoxy painted prior to adding anti fouling.) In terms of potential electrolysis, I would view this as positive. Am I right or wrong?

    Further the installers think there is static charge that accumulates in the mast and stays, particularly on days with thunder storms as the levels of energy in the air is higher. At some point the energy finds its way into the NMEA network and releases a killing spark.

    They have proposed some solutions and I would be more than really grateful if someone with experience and knowledge would like to add their view and comments to the below;

    1) what is the experience from unwanted release of electrostatic energy on board aluminium yachts?

    2) Solution 1; To ground the masts/stays on your vessels. What is you opinion on this and in case what methods is used to do this successfully without compromising the isolation of the hull?

    3) Solution 2; The installer think it is good practice to power the NMEA network with a separate DC/DC converter with galvanic isolation. A 24/12v Mastervolt converter is already in place. Next they would like to ground the network by connecting the negative lead of the NMEA network power to ground via the hull. They claim that since the network is powered by a separate DC/DC converter this negative lead should be electrically isolated from the power system of the vessel as such – and in this not compromising the hull. What is your view on this proposal?

    4) In relation to the above, what is the “best practice” methods for installing NMEA networks in aluminium yachts?

    Responses are highly appreciated, as till now there seems to be a lot of views around, but facts are more difficult to find

    • John Sep 8, 2015, 8:28 am

      Hi Petter,

      Hum, let me cogitate on it for a day or so (we are out cruising).

      To help with that, two questions?

      1). Do you have zincs attached to the hull, and if so how many and of what type and size?

      2). Exactly how did you measure that resistance between the hull and the water—what electrode did you use in the water and where did you connect to the hull? (I’m a bit sceptical of that reading.)

      • Petter ;-) Sep 8, 2015, 8:46 pm

        Thanks a lot for engaging. Below are answers to your questions.

        1. I have 4 magnesium-zincs anodes attached;
        a) 1 – rudder, 1 centerboard, both type MgDuff zd56 (1kg)
        b) 2 on each side of the aft part of hull in a circular recess, type mgduff zd55 (7kg)
        2. Reading of resistance was done with 1,5mm sq electrical wire in the water where 10cm of end was stripped and spread well. Other end of wire to one pole of multimeter, other pole of multimeter to either mast/stays/axel of centerboard.
        When multimeter connected to inside of hull anode fixing block, multimeter read resistance of 5 MegaOhm, rather than the 40 that was measured in above areas.
        – petter

    • John Sep 9, 2015, 4:55 pm

      Hi Petter,

      OK, cogitating done, here are my thoughts.

      • I think this measurement of resistance to the water is a red herring. There is no good and accurate way to make that measurement. The reason is that the boat’s zincs have a potential difference with ground (the water) and therefore when you make your measurement in the way you describe there is a current flowing through the meter which will fool the meter’s resistance measurement. If you want to test this, try reversing the polarity on the meter. I will bet that you get radically different resistance readings depending on polarity. If so, that proves my theory.
      • The idea of an aluminium boat immersed in a conductive solution (salt water) holding a static charge without leaking off to ground is pretty far fetched and, at least in my opinion, totally flawed. The point being that to hold a charge you need a capacitor, and I can’t imagine a worse and more leaky capacitor than said boat in sea water.
      • Even with no zincs the charge would leak off quickly through the water content of the paint, primer, etc. With zincs it will just leak off quicker.

      So, my opinion, this approach is fundamental flawed. That’s the negative. Now let’s look at what the cause might really be:

      • In the rush to get new products to market many marine electronics manufactures are not properly designing their gear to withstand the voltage spikes from other gear on the boat, most notably anything with an electric motor. We have seen this with the Lopo light and at least three other pieces of gear recently. A big contrast to older commercial gear from companies like Furuno that never had these problems.
      • I’m fairly certain that your problem is with inductive spikes originating in other gear on the boat, not anything to do with half-baked static build up theories.
      • The interesting thing is that in each case where I have had problems caused by new gear’s inability to live around electric motors, the manufacturer has always told me that I was the only one having the problem. And in each case that turned out not to be true.
      • It is worth noting that this problem may be exacerbated on a boat with a floating ground.

      Assuming that I’m right, what to do?

      • I think that running the electronic gear through a DC/DC converter may be a very good idea, as it will isolate said gear from gear like electric motors and high current switches (windlass for example) that produce inductive transients.
      • However, you don’t want to ground the negative to the hull. The installer is right that the converter will isolate the rest of the system from the hull. But that does not alter the fact that if current leaks from any of the electronic gear powered by the converter to the hull, you will have a galvanic problem.
      • And, by the way, of all the electrical leaks to hull that I have troubleshot over the years a good 75% of them came from electronics.
      • What you can do is connect several capacitors of varying values between the negative of the converter and the hull. Said capacitors will help to pass spikes, but not DC current that can cause corrosion.
      • It would also be worth looking into installing surge suppressors at the DC inputs of all of the electronics.

      Having said all that, do keep in mind that this problem may easily be poor gear and network design and nothing you can do will fix it for sure. Typically what happens in these cases is that the manufacturer brings out a new model or fix(s), but never admits that they had a problem in the first place—sad but true.

      This is one of the reasons we tend to stick with commercial gear from companies like Furuno and Icom that must survive on fishing and commercial boats where huge inductive voltage spikes from heavy winches etc are the norm.

      Hope that helps.

      • Petter ;-) Sep 10, 2015, 6:07 pm

        That was one long response John. Thanks a lot! In addition I hope more members join the conversation to add to the knowledge base.

        Based on your ideas, here are my renewed thinking based on your thoughts;
        1a. For the sake of the argument, let us assume that the resitance measurements are flawed. And it in a way makes sense to me as I also made a measurement directly on internal the aluminium blocks where the screws that holds the main hull zincs are fitted. (Screw is stainless, but still.) If what we have done is flawed, is there a sensible way to test resistance between hull/mast and the water?

        1b. As for electronics not being fully tested and rugged, I definitely think you have a point. The multifuntion plotter/rader screen suddenly entered a eternal boot mode, saying software upgrade failed, when no upgrade had been applied. That required a full firmware upgrade. Luckly it happened in a location where it was possible to download firmware over the internet. Just consider this during an ocean crossing or in foggy iceberg waters. Not fun! … and that is just one of the issues, I have had to deal with.

        2. However, both times the gear failed there were thunder and lightening in the near area and in my mind a much higher level of static energy than on a normal day.
        Both failure situations occured in a stable motoring situation. No gear was engaged close in time to failures.
        Early in the sason, it happpend that I got a static spark from touching the stays while walking on deck. What could cause this, if not accumulation of static?
        I assume (and will verify) there is no electrical leak present. The following leak meter is installed and measures both positive and negative leaks down to very low levels.
        3. Fitting capacitors to protect hull integrety appears to be a no-regret solution. As I am far from an electronics engineer, could you give me a hint what the specs of the capacitors should be (size and how many? I guesss they could be built into a small plastic enclosure with just two connecting wires exiting?
        4. I have also considered to add inner/outer DC blocks to the VHF antenna cable. Views? (
        5. Adding surge supressors; In the last failure the rudder angel indicator was blown. This unit is only powered via the NMEA backbone.
        As for the surge supressors, would you care to direct me to what they would look like?
        If and when you, John, or somebody else find time and opportunity, I am still eager to learn more about this issue.

        Petter 😉

        • John Sep 11, 2015, 8:52 am

          Hi Petter,

          Given your experience at the time, you may be right that atmospheric electricity was to blame. However, don’t be too quick to discount inductive spikes since the damage from them tends to be cumulative over time.

          Anyway, both risks will be reduced by the suggestions I have made, so I would still make those changes.

          Sorry, I don’t have specific surge suppressor recommendations and ideal capacitor size will depend on a whole bunch of factors that are beyond my knowledge to calculate. I have heard that a bunch of 0.15uF capacitors will pass RF from a single sideband, so that’s probably a good place to start. (Note that multiple capacitors in parallel act as one.) Your electronics people should be able to recommend a good surge suppressor. (If they can’t, you need new electronics people!)

          There is no good way to measure the resistance between the hull and the water that I know of. The best thing to do is wait until you are next hauled out of the water and then use an ohm meter to check the resistance between the rig and the hull and the hull and the zincs. If you get this down below a few ohms or so, then you are connected to the water as well or better than any boat.

          I suspect that your zincs may not be well connected to the hull. This is a common problem. I remove the zincs from our boat and clean the contact area to the hull every time we haul.

          Finally, I fear that you are simply dealing with very fragile and overly complicated “yacht” equipment and that this may be on ongoing battle. I know this is not good news, but it’s as well to be aware of it before you do strange things to your boat.

        • John Sep 11, 2015, 9:30 am

          Hi Petter,

          One more point. Don’t put any store in the fact that you got a static shock off you rig. I’m almost certain that the issue was that you were holding a charge and when you touched the rig you discharged to ground. This proves that the boat is well grounded, rather than the other way around.

          This is just the same as in your house on a low humidity day when you get a shock off say your refrigerator. It’s not the refrigerator that has a static charge, it’s you.

          • Petter ;-) Sep 11, 2015, 12:14 pm

            Again many thanks for the very helpful input John. Will be off to the wilderness for a large week (not net, radio, newspapers etc. etc.), and will revert on the issue upon return.
            – petter

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