The Offshore Voyaging Reference Site

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.


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John Harries

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

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.

John Harries

Hi Matt and Erik,

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

Marc Dacey

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

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.

Eric Klem

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?

Eric

Matt

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

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

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:
http://www.marinelightning.com/EXCHANGEOct2007Final.pdf

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:
http://www.marinelightning.com/Siedarc.htm

Also, ABYC Technical Report TE-4 “Lightning protection” has been revised to take account of this new research.

Cheers,
JCFlander

Chris

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.

Chris

Yep,
Maybe to folks in tinfoil hats are on to something!

Hoftman

And it all comes standard in the Adventure 40!

[Right?]