Lightening/Earthing

[Guest]

Last year I inserted a copper braid earth strap right through the boat. It starts at the rudder stock and goes through to a chain plate, on it's way I have connected the engine, anodes and keel. My question is "because I have connected it to a chain plate, effectively earthing the mast and rigging as well, have I now turned the whole mast into a lighting conductor rod" Would I be advised to dis-connect the strap from the chain plate? Any help / advice would be welcome.

 

[Roberto]

Tough subject, almost black magic, many different conclusions to be expected...

Here is what I have done, I have been struck by lightning once and am still here to write, at least I can say it worked for me!

From your post, I understand that your mast step is not copper strapped to a keel bolt. That would be the first thing I would do, but only in the case your keel is NOT encapsulated in grp. A big strap, the biggest/shortest the better. See those "ground copper strap" available at stores? I d put three or four of them. As connecting copper to aluminum, books say, is often not a wise thing to do, put an enormous steel bar in between, check connections regularly, and above all bolt everything.

ISO rules state that all chainplates should be connected to ground as well. I am not sure this is feasible on a yacht.
When the lightning strikes the top of the mast, current finds two ways of dissipating: one through the aluminum extrusion, the other through the stays. Electric conductivity of the Al mast section is many many times bigger than that of the steel stays, so current should be invited to pass along the spar. By putting a strap at the base of the mast you facilitate 1. the dissipation of a direct strike into water/ground, 2.the dissipation of static charge before a lightning actually strikes your boat, thus reducing its likelihood. Most of the current will flow through a big conductor with low resistance, and damages should be reduced.
Grounding the chainplates should theoretically provide a Faraday cage protection, but in the event of a direct strike bigger currents will pass through the stays, once they get to the chainplate they should flow to ground as well. There is a technical difficulty though: in order to let current flow from the chainplate to a keel bolt, you should lay your copper strap diagonally straight through the middle of the cabin, something usually not easy to achieve. So one says I ll run it on the inside of the planking... in that case it is very likely that once the current gets to the "elbow" of the hull it will go straight through the side of the hull into water, causing impredictable damages... It has happened before.
So I would avoid chainplate connections. What about Faraday cage then? Well the masthead should protect you and other people on a cone whose base has a diameter equal to mast height, and it usually does.

Most dangerous, an ungrounded mast base: current would flow through the mast, but not finding an easy path to earth it would "look around" to find an easier one: part of the current would surely push through the mast base into water, burning/damaging most of the non conducive material it finds in between. Part of the current would find its way to ground "jumping" to nearby conductors connected to ground. It might be a crew handling a salt water wet rope or the anchor chain, the helmsman holding a metal tiller, or a near seacock. All unpleasant events. That is also why if the keel is encapsulated in grp, the grp would be severely damaged. In that case a ground plate should be fitted. They say one square ft of copper plated on the outside, I would easily exceed that requirement.
Beware also if you sail in fresh water, the consequences of a strike are many times more severe than at sea, and ground systems must be very overdimensioned. To maximize your chances, you should basically cover half of your underbody in copper!

The mast is the biggest conductor you have on the boat, sea is a perfect ground, just put the strongest electrical connection between them. Trying to keep most strike current through this path will help avoiding side flashes.

Sorry if I put too many do s and donts, I have by no means the final answer, and hopefully wait for others to show me my mistakes to be even more protected in the future!

Roberto

 

[vyv_cox]

Lightning

strikes are of the order of several kilometres in length. Your boat is a few metres high. The voltage involved in lightning is somewhere between thousands and millions, i.e. a vast potential energy. Nothing you can do will have the slightest effect on a lightning strike. Throughout history wooden ships with wooden masts, trees, stone buildings, mountains, all largely non-conducting, have been struck by lightning. You may be able to prevent the cooking of electronic devices by disconnecting them, but even this is dubious.

Best advice is don't worry. It might hit you, it might not. Plenty of boats get hit, with varying degrees of damage. I don't remember many yachtsmen being killed, but I do remember many golfers and joggers who were.