washer for keel bolts
I am replacing the backing plates, washers and nuts (while I'm at it) on my keel bolts. The bolts are in great condition (from what I can see) - but for peace of mind sake I am replacing the hardware.
Having custom stainless backing plates made (getting quotes now - not sure if I should go with 304 v 316 as I read that 316 is subject to stress issues), but am looking for washers and nuts - the bolts I am replacing are 5/8" (for some strange reason my keel has 8 bolts - where 2 are 3/4" bolts, and the rest are 5/8). At any rate, for the washers and nuts do you think I can use 18-8 stainless or do I need to go with 316? What is difference in strength (if any). Of course the 18-8 are easier to obtain. How about Monel (McMaster Carr sells them for $7 a pop!)
Appreciate any thoughts here.
I am very wary of stainless where it may get de-oxygenated in the presence of chlorine.
Man, does it break down quickly if the conditions are wrong.
My question is this... what are the bolts in the keel made of??? If they're 304 stainless, you'd probably be better off getting the backing plates, washers and nuts in 304 to prevent any galvanic corrosion issues between two different grades of stainless steel.
316L is a better material, since it is more corrosion resistant and doesn't suffer from the chloride stress cracking that 304 is subject to having in a chloride ion rich environment, like salt water or chlorinated pool water. However, 304 is about 15% stronger than 316 IIRC.
Monel would be great, but if the bolts are stainless, there will be galvanic corrosion issues whenever the bilge is wet.
Don't Mix and Match
SD is right.
Find out what the keel bolts are and use that material in the washers and nuts. If you don't know, opt for the 316.
316 is best for this application, not 304.
Stainless other Materials
You don't say..
You don't say what boat, how old etc. etc.. Many times the nuts come off and the bolts look fine but it the sealant/bedding compound has been leaking galvanic corrosion may still be eating away your bolts where you can't see them inside the keel stub.
Galvanic corrosion of keel bolts from the Ericson Owners Forums:
This is what my backing plates look like. They are original and the boat is a 1979 so I guess they used a high quality stainless from the factory.
All, thanks for input.
To answer some questions.
1) Boat is a 1985 S2 27
2) keel bolt material - i don't know it they are 304 or 316 - how can I tell - since S2 no longer mfg sailboats, and most of the knowledgebase is not there anymore.
3) Good point about monel - will bag that idea...
Here is a chemical test kit...
but it is expensive.
Alloy identification, metal testing, kits
I am sure that there are lesser expensive ones or you could mix up the chemicals yourself and do the test. Basically the main difference between 304 and 316 is Molybdenum. 316 has it, 304 doesn't. You're testing for the presence or absence of Moly.
Stainless has a weakness in saltwater for pit corrosion, where the ions setup a cell and eat away if there is a lack of O2 to form the protective oxide layer.
If you are going to the trouble and suspect any possibility, seperate, inspect and rebed the hull-keel joint. I used dry film lube on the studs to prevent gauling during the torquing. Lubing the threads also changes the clamp load to torque by modifing the rotational friction.
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