Bronze or stainless bolts?

[Tranona]

What's the aim of these steps? It's not going to keep water away from the wood, as the rest of the rudder is not sealed or specially protected, just painted. And I don't think the bolts need to be protected from the wood - I'm satisfied that the corrosion is due to using the wrong stainless for the job, not tannin attack. In any case, the holes are pretty tight-fitting, so while I'd be able to soak a bit of plain epoxy into the edges of the wood, any thickened paste I applied would be removed again as soon as I drilled a hole big enough for the bolts.

Cheers,

Pete

Belt and braces! The holes will have unprotected end grain and you are reliant totally on the sealer. Have just had to repair some rot in my hull when I removed the anode fittings, because the person (not me) who fitted it did not epoxy the holes but relied solely on the sealer. One was OK but the other had let water in. Maybe I get a bit paranoid about such things, but when you have the same boat for 30 years you learn about long term consequences of your actions!

 

[Bosun Higgs]

I'm satisfied that the corrosion is due to using the wrong stainless for the job, not tannin attack.

You're mistaken Pete. The corrosion is nothing to do with using 304 rather than 316 and everything to do with having stainless steel in an oxygen deprived but wet location. The idea suggested is that you still use staiunless but effectively build an epoxy tube through the rudder so that the stainless can only get wet from the ends - and these you can effectively seal. Its an approach that would work if you want to go that way.

Polysulphide sealants are readily available at decent chandlers - I belive that Sika make one as well as the usual polyurethane sealants.

I wouldnt worry about stainless straps and bronze bolts - you dont get wholesale corrosion between a stainless shaft and a bronze prop do you? In any case corrosion that you can monitor is far better than corrosion you cannot see in the middle of the rudder

 

[TradewindSailor]

When I was at college the metalurgy lecturer showed us all the effects of placing an elastic band tight around a stainless steel 316 bolt. The resultant necking was due to oxygen deprevation in way of the rubber band. It had nothing to do with whether the water could get to the centre of the bolt or not.

If the ends of the stainless steel bolt are oxygenated and the middle is not there will be a difference in potential ..... i.e. a galvanic cell is set up within the metal itself. The corrosion will act at the boundary of the oxygenated environment. There is a 0.4V potential between the oxgenated (passivated) stainless steel (either 304 or 316 )and the de-oxygenated (active) part of the same material.

If you protect the entire bolt from oxygen, it will be at the same potential; and this is why you have to coat the ends too.