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Stainless Steel Bolts for Strut & Stern Tube

Mar 17, 2020
6
Hunter 340 Somewhere wet
I bought a 1998 Hunter 340 a year ago. I'm swapping out the engine & doing a minor refit. One of my concerns is the stainless steel bolts on the strut & stern tube. Normally I wouldn't put stainless below the water line, but some builders do to save cost. Can anyone tell me if a H340 of this vintage normally came with stainless bolts? Have these installations run into problems with crevice corrosion (or any other type of corrosion)?

The original owner of 19 years generally did the right things & did them right. The next owner (of 6 months) had little knowledge, but that didn't stop him from messing around with things he shouldn't have. So I don't know if these are original or recent.
 
Jan 22, 2008
1,497
Hunter 34 Alameda CA
My 1985 Hunter 34 has stainless flathead machine screws, washers and nuts that came with the boat. I had to replace the bronze strut last September. Those fasteners looked perfect. My strut has never been bonded. Apparently the fasteners outlast the strut. I replaced the stern tube too as it was shot. The carriage bolts were also perfect. I bought both right here at the SBO store.
 
Jun 8, 2004
7,813
-na -NA Anywhere USA
Yes Hunter used a high grade stainless steel nuts, bolts, screws, etc. unless brass was called for as stainless generally would not rust. You must be thinking of regular steel which will rust. Manufacturers had to be careful when ordering so they did not get Chinese stainless which had impurities in stainless causing it to rust
 
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Mar 17, 2020
6
Hunter 340 Somewhere wet
Thanks for the info, specifically with whether Hunter used SS hardware on the strut & stern tube, and whether it was Hunter's practice to bond their fittings.

SS (of any type) below the waterline is generally a poor practice. SS doesn't corrode (rust) because in the presence of oxygen it quickly forms an invisible layer of oxide on the surface. Under water it can't form that layer, so when that layer gets abraded, scraped off, etc. it is subject to corrosion.
 
Jun 1, 2009
1,223
Hunter 49 toronto
Thanks for the info, specifically with whether Hunter used SS hardware on the strut & stern tube, and whether it was Hunter's practice to bond their fittings.

SS (of any type) below the waterline is generally a poor practice. SS doesn't corrode (rust) because in the presence of oxygen it quickly forms an invisible layer of oxide on the surface. Under water it can't form that layer, so when that layer gets abraded, scraped off, etc. it is subject to corrosion.
I think you are thinking about aluminum re: forming an oxide layer.
 
Jun 1, 2009
1,223
Hunter 49 toronto
I think you are thinking about aluminum re: forming an oxide layer.
Or, possibly, you are referring to chromium oxide, which indeed does form on the stainless surface. Unlike aluminum oxide, the chromium reaction happens almost immediately, and is virtually invisible, being at the atomic level.
Now, if you used really kinky stainless in deep salt, this wouldn’t be optimal. But a 316 should work.
I’m in fresh water (lucky me) and my strut screws look like they came out of the box
 
Mar 17, 2020
6
Hunter 340 Somewhere wet
As you point out, stainless steel forms a very thin (atoms thick) layer, which for some types of stainless is sufficient. Using it underwater is generally not a good practice because when the oxide layer is damaged there isn't enough oxygen in the water for the oxide layer to rebuild itself. Lots of builders do it, though, for cost reasons. The other reason not to use it is that it can be susceptible to crevice corrosion, especially in way of the cutless bearing.

BTW, that's also why you should sponge the water out of your bilge to keep it dry, at least the part where the keel bolts are. They're generally stainless, and you don't want them corroding. They're especially susceptible to crevice corrosion, which can be hard to detect.

My question was whether Hunter had used good enough stainless (typically 316 or better) that other folks haven't had issues with it. I don't know if the SS bolts in my strut & shaft log are original, but since they're in good shape I'm going to leave them for now. (If they were replaced they probably came from a big box store, and are unknown composition.) I'll keep an eye on them when I dive down to replace my zincs.

Thanks for the help & info!

Why doesn't stainless steel rust?

Stainless Steel and Corrosion
 

dLj

.
Mar 23, 2017
1,019
Hunter 30 Snug Harbor, Lake Champlain
Stainless is generally said that it should not be used below the water line. However, there are caveats. The biggest problem for stainless below the water line is in stagnant water applications. They perform better where there is moving water in underwater applications. Moving water allows for better reformation of the passive oxide layer as there is more oxygen available in moving water. But to have the stainless void of stagnant locations is a difficult design problem.

There are stainless alloys that are more reliable in underwater applications than others. We talk here a lot about using 316, that's an easily found alloy that is pretty good, but not really ranked for underwater usage. Alloy 20Cb is substantially better and can be used underwater. 17-4 PH is also quite good, but it needs to be in the H1000, or higher temperature, heat treated condition. It is most frequently found in the H900 condition

dj
 
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