Anode mounting

[John Harry]

Hi!

I have a 37" wooden gaff-rigged cutter planked on an ancient Australian timber called Huon Pine. I have an anode mounted on SS through bolts aft on the hull exterior to protect prop/shaft.I removed this to replace it recently and noticed some deterioration in the planking surrounding the bolts. I have now cleaned this up but before doing anything else I thought someone might know whether this was likely to be due to electrical activity where the bolts penetrate the hull and if so how the mounting should be arranged to prevent this for the future. Eg should I be using brass or some other bolts to subdue the cathodic nature of the SS bolts?

Any help would be appreciated.

John

[paladin]

Most likely the problem was improper sealing around the bolts....but clean it, treat it and Bronze bolts might be in order.

[RGM]

Don't use stainless, don't use brass. Do use silicon bronze (a stud). Additionally, in order to "isolate" the bolts from the wood you can do the following: fabricate some tubes out of UHMW (ultra high molecular weight plastic) or perhaps better yet out of nylon, I believe that caulking compounds and sealants stick to the nylon better. The inside bore of the tube should fit snug/tight around your stud, the wall thickness of your tube should be at least 3/16". Drill a snug hole (outside diameter of your tube)thru your planking and backing block if you're using one (you should). Choose your drilling equipment carefully, this has to be good. Drive the tube into place, it has to fit tight, use some caulking compound here if it will make you feel better. The tube needs to fit flush or a hair less on both ends. Make your silicon bronze stud with ample thread length on both ends to allow the installation of washers (use washers with O.D. that's larger than the O.D. of the plastic tube, you need the overlap for sealing), nuts on the inbd and otbd ends, the zinc, and whatever electrical/bonding connection you have on the inbd side. Do NOT use "all-thread". During the installation of this whole mess you can apply some of your favorite marine grade sealant under each of the washers in order to make things watertight. Bed your backing block in an acceptable marine grade bedding compound, not a sealant or adhesive. You may wish to use additional nuts to serve only for squeezing up and tightening the assembly thru the hull and use another set of nuts/half nuts (on top of the previous ones) to secure your zinc and inbd electrical/bonding connection. The advantage of doing it that way is that you can service the zinc and inbd connections without potentially disrupting your watertight hull penetration. Use all silicon bronze for your fastening material, don't mix metals. I hope that my description was adequate, without being there with you and seeing what you've got or sending some pictures, it may be a little confusing. Good luck.

[Mirelle]

If you want to avoid destruction of timber due to alkali build up at the cathode, don't use an anode. Assuming that you have, and use, an isolation switch, and that you don't use an electric bilge pump, are your engine and electrics set up, as normally they are, for "earth return" or for "two pole"? I heartily recommend the latter.

[John Harry]

Thanks for the posts, I appreciate the learned assistance very much!

I'll let the respondents have some shots of the boat.

John