Lightning on a Boat

I just finished sitting out in the cockpit under the tarp watching a great lightening storm blow right over top of the boat. Lots of great thunder and tons of bright flashes – such power and regal majesty yet ultimately one of the more dangerous things to those living on a sail boat. Basically, you’ve got lots of lightening flashing all over while you are sitting under a large metal pole that rises some 40 or 50 feet into the air. Does this really sound safe?

This brings me to the topic of lightening dissipaters and grounding plates.

A lightening dissipater is a brush like appendage that is attached to the top the mast. It actually looks like the metal brushes that are used on top of signs and under bridges to keep the pigeons off. The idea is that the many bristles of the brush allow the positively charged ions to dissipate into the air thus preventing the completion of a circuit between the clouds and earth. (Most lightening is the movement of energy from a negatively charged cloud to a positively charged earth. See more information at Wikipedia.)

A grounding plate is a large area of metal bonded to the bottom of the boat which is connected to a lightning rod which is at the top of the mast. The theory is that the lightning strike will move down the rod/mast to the grounding plate which will dissipate the charge into the surrounding water.

There are problems with both these systems. Basically – sometimes they don’t work. The lightning dissipaters often cannot discharge a large enough amount of energy or are not fast enough to prevent the flow of electricity. Grounding plates can only handle so much electricity. There are many stories of grounding plates that have been vaporized during a strike with the result being a hole in the boat below the waterline (or water in the basement so to speak.) So, what to do?

Well, you can do what I do – nothing.

Sit out in the cockpit, enjoy the show and hope that your neighbors mast (you know, the goldplater with the mast that’s 20 feet taller than yours) attracts all the lightning. Another bit of joy and wonder that you get to experience while living aboard a boat.


  • Anonymous

    Well, i guess in terms of charge, everything is relative. But the Earth is generally considered to be neutral in the big scheme and not positively charged. Of course, it is more positive than the bottom of those stormclouds so it works like you say.
    Erm, sorry to be pedantic.

  • JD Tenma

    I wonder if it would be practical to have the ground plate on a thick cable. When a storm approaches, let out the cable so the ground plate is 5 or 10 feet below the lowest point of the keel. That way, if it does vapourze, it will not blow holes in your boat.

  • Reed

    The problem is that lightning dissipators don’t work. They only dissipate funds from the buyer. An additional problem is that boats in fresh vs salt water are subject to greater damage since the fresh water cannot dissipate the charge as readily after a strike.

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