About 16 months ago, I had a new shoe fabricated from bronze to hold the bottom end of a skeg mounted rudder - the rudder itself has a stainless shaft with a delrin bearing (I think, plastic anyway) and the skeg is GRP. The old skeg shoe had lasted ~40 years but had developed some nasty cracks, I assumed due to stress. I had the new one made a bit thicker and heavier than the old one.
I fitted the new shoe with 3 silicon bronze hex bolts and nuts, thinking that they would be galvanically compatible, and none of the problems with stainless steel underwater.
When I took the boat out the water (she came out about a month ago, but I only had a good look on Monday) I discovered that both the bronze nuts and the hex heads of the bolts have been severly eaten away. The damage doesn't look to have started from the parts of the nut and bolt that are in contact with the shoe, but rather they have been eaten away from the outside, the flats of the nuts are gone and in fact both the nut and the hex head are a fraction of their former size. The threaded part of the bolt looks pretty intact. There is no pinkness or signs of dezincification in the nuts and bolts and I'm pretty confident they were bronze rather than brass - they came from the same reputable supplier who fabricated the shoe itself (and did a nice job of that).
I wish I had taken some photos, but I didn't and the boat is about 60 miles away just now, so no photos I'm afraid.
So I'm really wondering what happened, and more importantly what I can do better when I replace the bolts. The bronze skeg shoe isn't bonded to an anode (and never has been) - the metals were supposed to be galvanically OK togetehr. The boat has been in a marina for the last 9 months, so I wonder about stray current from neighbour boats - I never left mine hooked up to power except for the odd night when we stayed on board.
The bronze prop seems to have survived perfectly OK (its bonded to an anode though) as does the shoe itself that holds the bottom end of the rudder. I also can't see any obvious problems with bronze skin fittings, I'll check them all pretty carefully though. Its a bit of a mystery to me what's been going on, but a bit of an alarming one.
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19-04-12, 13:00 #1
Bronze bolts - what happened to them?
19-04-12, 13:12 #2Registered User
Location : Southampton
- Join Date
- Nov 2009
Your story sounds identical to mine. You're not at Kemp's Quay on the Itchen by any chance, are you?
The local wisdom there is that this corrosion of bronze is due to metallic runoff from the scrapyard across the river having accumulated in the local mud over many decades; our rudder shoe sits in the mud at low tide but the prop does not, hence only the shoe and particularly its bolts get corroded.
The advice we were given was to paint the whole lot (with replacement bolts) with a couple of coats of epoxy, which I have duly done. We'll see how it looks in January.
19-04-12, 13:26 #3Registered User
Location : Location: Location
- Join Date
- Jun 2006
19-04-12, 13:53 #4
Very interesting indeed! Yes, the boat spent a year at Kemps Quay after being launched in 2010 before I sailed her up to the Firth of Clyde last May. I dried her out on scrubbing piles before I left, but the bronze could easily have been gone by then without me noticing. I can see how dragging the bolts through contaminated mud twice a day might do that kind of damage, the way they are rounded.
Well, maybe I'll just go for new bronze bolts, with a few coats of epoxy for luck even though the dreaded mud is 725 miles away now as the boat floats. If nothing else, the explanation will make me feel better, and I'm going to be keeping a close eye on it all anyway. Bronze bolts in themselves don't cost too much, I'm more worried about the rudder falling off. Thanks!
19-04-12, 15:37 #5
It might be worth trying phosphor bronze instead of silicon bronze. The latter is an excellent fastening material but if the problem is some local chemical pollution a change of material might be for the better. Unfortunately there might be a cost penalty.Answers to some technical queries at http://coxengineering.sharepoint.com