Bolts on rudder under waterline

[sgumedic]

I've read a lot about keel bolts, but I have bolts on my rudder below the waterline that are corroded and need to be replaced. Are galvanized bolts the best to use? The ones that came out looked like they were stainless steel.

Thanks for the help!

[Maren]

Originally Posted by sgumedic

I've read a lot about keel bolts, but I have bolts on my rudder below the waterline that are corroded and need to be replaced. Are galvanized bolts the best to use? The ones that came out looked like they were stainless steel.

Thanks for the help!

Well maintained bolts are the best. That said, you see more stainless as it's the industry standard. If the stainless is good quality they resist rusting decently well. On the other hand, you can end up with what is known as crevice corrosion (which, strangely, is anaerobic). Galvanized bolts sound like a good idea but it isn't even and once you get through the zinc, it's game on.

But...

You really shouldn't take my word for it; you should test things yourself. Do this by getting a stainless bolt/nut or two of differing qualities (make sure you get a 316 and a 304 if you are only doing two) and a galvanized bolt. Dangle them off a pier and check it once in a while.

[JPA Cate]

Maren,

The results of the experiment you describe will not have the oxygen deprivation environment that the OP will be installing his bolts in. It will be interesting, but of little use to him. I suspect duplex s/s would be good there, because it is less susceptible to oxygen deprivation corrosion.

Ann

[delmarrey]

Stainless bolts are not recommended for anything under water, unless they are attached to large SS plating in which anodes (zincs) can be attached.

So it depends on what they are attached to. Bronze bolts, like from a marine hardware, is the best material. And even then they'll need replacing over time in relation to the environment.

Iron/steel will turn to iron oxide in no time underwater, even zinc plated. The plating will come off faster then your zincs will need changing and then you'll have a bleeding bolt.

I have a few SS screws right at the waterline back by my rudder. I change those every time the boat is out of the water. If I don't, the heads will just brake off with the slightest twist. I can't find bronze screws that size or shape.

[Maren]

Originally Posted by delmarrey

Stainless bolts are not recommended for anything under water, unless they are attached to large SS plating in which anodes (zincs) can be attached.

So it depends on what they are attached to. Bronze bolts, like from a marine hardware, is the best material. And even then they'll need replacing over time in relation to the environment.

Iron/steel will turn to iron oxide in no time underwater, even zinc plated. The plating will come off faster then your zincs will need changing and then you'll have a bleeding bolt.

I have a few SS screws right at the waterline back by my rudder. I change those every time the boat is out of the water. If I don't, the heads will just brake off with the slightest twist. I can't find bronze screws that size or shape.

Delmarry: Respectfully, I don't think it's quite as simple as that. The gudgeons on my boats are SS316, as are the bolts for galvanic reasons. In my case I mixed up some epoxy and lightly coated the heads. So far they've held up well. I'm not against silicone bronze at all as it's almost at strong as stainless. However, I'm going to go out on a limb here and suppose the bolts attach at one point to a fiberglass rudder but there is contact with another metal. So we're back to the galvanic corrosion problem.

Ann: I have a fair knowledge of steel from making knives but the idea of full duplex for corrosion resistance struck me as odd since the corrosion resistance of steel comes not from the crystalline structure but from the impurities, specifically chromium and to a lesser extent molybdenum, which creates a pacifying layer. But we don't tend to keep knives underwater for months at a time and at 35ºC - 70ºC, so I looked it up. No difference in actual long term marine use but a bit better in short term (as in 2 month) tests under normal condions. However, my point was that sgumedic should test things himself rather that go off hearsay on the internet.

[delmarrey]

Ask any pro wooden boat builder why they don't use SS for bottom planks.

[sctpc]

All my rudder bolts are Bronze so is the rudder bracket ect dot know if it the best but 20 years still looks like new when i scrape off the antifoul the bolts going through I have never removed.

[roverhi]

Stainless fasteners for planking don't work long term because of crevice corrosion caused by burying the screws in wood. Granted, I'd rather use silicon bronze fasteners below the water line but stainless for OP's purposes would probably work fine. The original 304 welded SS guedgeons and bolts are doing fine after 40 years, mostly in the tropics, on my old W32. SS fasteners do have their issues but pulling and inspecting them periodically will pretty much stay ahead of them.

[Stumble]

I made the switch to titanium everywhere I could. It costs a little bit more than 316, but actually less than super duplex stainless, and will never corrode.