Batteries - Current & Lightning - Oh My

[Theis]

The Kittiwake sotry is interesting, and I believe he is correct about the cause of the problem. I have no confidence that he has corrected the problem now however.

I am assuming his mast is aluminum. The stainless steel bolt he is using will galvonically corrode when connected to aluminum. That WILL happen - and again there will be no electical connection. That is why when you put stainless steel screws into your aluminum mast, alwsys coat the screws with anti-corrosion cream (which is not a conductor) but prevents the threads from corroding and the screws from pulling out.

Keep in mind that the lightning does not "strike" the mast in the colloguial sense. What happens is that the mast head, if not adequately grounded, builds up a positive charge - and that positive charge can be quite high in electrical potential (that is why people's hair stands on end before they are struck) The way to dissipate and prevent that charge build-up is through the very low resistance path to water ground. If there is even a small resistance path, then the charge will build up, the point of resistance will heat up further increasing the resistance and the boat will attract a lightning strike. If there is a low resistance path, a positive charge can not build up on the masthead.

This positive charge build up on the mast head attracts the negative charge on the bottom of the cloud. The positive "streamer" actually goes up towards the cloud to meed the negatively charged streamer coming down from the bottom of the cloud. When they meet, BANG!

The fact that the bolt blew out is indicative that there was a high resistance path at the connection with the mast. Otherwise there would have not been heat generated at that point and the bolt would not have blown out. Once the main path was broken. The lightning went whereever it decided to go and blew out everything.

In my Ariel originally, all the shrouds had a 12 ga. wire connecting them together (on both sides, but not connecting the port to the starboard side). I have now increased this to a 6 ga. Because the stainless shrouds are individually lousy conductors compared to the aluminum mast, it is worthwhile having them all shrouds connected together so there are six parallel paths (grounding the mast base with an aluminum bolt is evern better. Originally here was then a 12 ga. cable going from one of the starboard shrouds to the water intake. I connected a 6 ga. wire from the port side shroud to the bow pulpit, and a similar 6 ga. from the bow pulpit to the starboard side, so the base of both sets of stays are connected both at the masthead (by a stainless steel bolt and plate - so galvonic corrosion is not an issue at the masthead. From the base of the shroud connection, the #6 cable goes to a 3/4" copper bolt connected to the copper plate. The bolt has to be large - much bigger than the wire size. Current only flows on the outside of a conductor. The wire is multi-stranded so it can be smaller in diameter than the bolt if the wire has multiple strands. The bolt, on the other hand, is like a single strand. And remember, a stainless steel bolt is a lousy conductor. I would suggest you only use copper or bronze (or aluminum in the worst case) for your through hull bolt, and make it as large as is reasonably possible.

The entire plate/bolt/whatever combination I bought at West last year (about $38 as I recall) and it came with complete instructions. Look in their catalog under lightning or contact West Marine (I don't have a catalog here or I would do it for you).

Many people just use battery cables laying over the side but connected to a shroud (assuming all shrouds are connected to one another internally as I commented), and I won't fault it in addition to the grounding plate as a good idea just for piece of mind. That is better than having lightning going through a through hull fitting if the main system fails. The problem with the cables, is that the area of surface contact with the water is is not considered sufficiently large.

The copper plate, or lightning shoe, should not be faired. It is the edges (and the corners) that prinicipally dissipate the electric charge, not the smooth surface. If you fair the plate, you will lose the edges. And NEVER paint the copper plate or your shoe. All metal through hulls should be tied (bonded) to that shoe so that there can not be any voltage buildup between various through hulls (i.e. lightning can not jump from one to another as happened in the Kittiwake).

Another safety precaution is to annually take a volt meter an make sure there is no resistance between the grounding plate and the mast. If Kittiwake had done that beforehand, my bet is that he would have realized his mast was not grounded.