Suitable metal for replacement keel bolt in cast iron keel?

[OrangeCrush]

Hey all,

Having successfully removed four of the eight keel bolts on my 1978 Pearson 26, I am ready to replace them. See my other post about it if you like: http://www.cruisersforum.com/forums/...ime-96927.html One of the old bolts was being eaten away by water intrusion, but three out of the four looked very good. I'm leaving the other four in since after all this I'm confident they look good as well.

On the P26 fan site, they have a link to a black alloy steel bolt as a suitable replacement on the McMaster Carr website (I bought one, see picture). Although this bolt is the perfect shape, it is not high strength steel and has no resistance to corrosion, so I'm not sure it's the best choice. I can get a Grade-8 Steel bolt with a salt water galvanized coating on it, but only in a hex head, which would not spread the load out as smoothly as the beveled heads on the original bolts.

I have read that stainless steel bolts are out of the question with a cast iron keel, since they can suffer catastrophic pitting when deprived of oxygen and since SS can react galvanically with the cast iron. I'm assuming this is true, even though people still keep suggesting stainless to me.

Having shopped around at a number of companies, the best thing I can find with the same dimensions is a Grade-8 Zinc plated bolt. Nobody seems to make it in galvanized, which is what the old one looks to be. (See picture) I'm thinking of putting a little anti-seize under the beveled head so that it doesn't bond with the cast iron, using life-calk to waterproof from under the head up the shaft of the bolt and under the washer, then capping the exposed bolt end and nut with epoxy on top. I'll epoxy over the top of the bolt in the keel recess and fill the recess with faring compound, then epoxy over that again to keep the water out.

So, is this Zinc-plated Grade-8 Steel bolt going to be an acceptable substitute? I'm hopeful that I can keep the new ones as dry as they have been for the last 25 years, but I would hate to replace them with inferior metal and have four rusty bolts a year after I'm done with this.

Thoughts?

Thanks!

[Cheechako]

Personally I wouldnt worry about the grade in that application and focus more on the resistance to corrosion. Todays' zinc coatings dont seem to work that well to me. That bolt you have pictured looks like black oxide coated......? a hot galvanized bolt would be good maybe...? Your whole boat can be lifted by one 3/8 bolt..... in theory...

[OrangeCrush]

Yes I was hoping to find something hot dipped galvanized but it seems like a pretty rare size and shape. Any other suggestions as to where to look?

Yes, that McMaster bolt was black oxide coated 'Alloy steel'. It came with no markings on the head, and the coating scraped off just putting the nut on. It really doesn't seem like it will be up to the challenge.

Besides not being as reliable against corrosion, is there anything wrong with a layer of zinc being in there between the cast iron and the steel bolt?

[OrangeCrush]

I could get a superior bolt for corrosion if I were willing to use a hex head. I'm thinking that putting all this weight on the 6 corners of the bolt head (since the hole in the iron is conical) would not be the best for strength purposes, and that I need to get the countersunk head on it. Do you agree?

[barnakiel]

Can you use SS? We have cast iron keel, bolts are SS.

b.

[Boracay]

Why not a galvanised hex head bolt with a few oversized galvanised washers underneath to spread the load?

[OrangeCrush]

Yeah I was considering that, but the some of the other people I spoke with seemed to think the beveled mating surface was critical. It seems like it would be stronger and also a tighter fit to prevent water intrusion in case it got past the faring compound. Maybe that is the lesser of two evils, though.

As for Stainless Steel, Don Casey wrote the following:

"Because the iron is less noble than the steel alloy, galvanic action between the two tends to enlarge the hole in the keel. To mitigate this, stainless keel bolts are typically coated with some insulating material before they are installed.

Stainless keel bolts have one more drawback—the most significant one. Stainless steel loses its resistance to corrosion when immersed in stagnant seawater. Because of this, it is an inappropriate material for underwater fittings. It is used for keel bolts based on the optimism that the bolts will remain dry. If they do, the bolts are good for half a century. If they don’t, life expectancy can drop to half a decade."

[Benz]

Have you considered looking for cast iron bolts? They might just exist, and it would keep your metals similar. Or you could find a blacksmith somewhere (they still exist in various nooks and crannies) and have some iron bolts forged to order. I'd want to research possible brittleness in grade 8 bolts; sometimes extreme hardness or tensile strength come at the cost of more brittleness or more content of some corrodable alloy. I know this all sounds expensive, and probably is--boating is always a tightrope walk between affordability and quality. Good luck

[Sandero]

I don't know what my bolts are... but the are completely encased in the bilge and have been there for 28 yrs. The sketch detail is identical to what Shiva has aside from the fact that it solid GRP and no plywd.

Actually now that I look at the photo It looks as if there is an encased steel plate as well.

Why are you replacing them if I may ask?

[ErikFinn]

I received the attached document from Beneteau, for my Beneteau. I know your boat is not a Beneteau and boats are different and your boat's method of keel attachment is different from the one on the document, but I thought to post this anyway to share some information on the subject. If not appropriate for your boat but hopefully helpful for members looking for information. I have a cast iron keel and it uses galvanized bolts. Not stainless steel.
-Erik

[Anonymous7500]

Find a local supplier of Brighton Best fasteners. They are very high quality American made bolts. They have a flat head that is very similar to the ones pictured. Otherwise, you could have a machine shop make some up and have them galvanized. This option will cost about $20 each though, at least.

[manitu]

This i the advice I got from Minaret.
I went for Stainess A4-80

Looking at the scale of nobility-

Zinc
Galvanized steel
Aluminum
Mild steel
Wrought iron
Cast iron
Stainless steel
Lead
Tin
Manganese bronze
Naval brass
Inconel
Yellow brass
Aluminum oxide
Red brass
Copper
Silicon bronze
Cupro-nickel
Comp G bronze
Comp M bronze
Inconel (passive)
Monel
304 Stainless
316 stainless, 3% molybdenum
Titanium


You can see from the scale of nobility that for a cast iron keel, as this probably is (it may be pig iron or some other alloy), mild steel or stainless would be about equally good (not considering crevice corrosion) where dissimilar metal issues are concerned. Monel or Silicon Bronze would be the worst choice, because they are on the other side of the scale from cast iron.

.manitu

[OrangeCrush]

Thanks for all the input!

Defjef, I wanted to replace or at least inspect them because they were 26 years old and showing a lot of rust. It turns out they were in great shape overall and it was probably unnecessary. Almost all the rust was coming from the washers. But, since one of bolts was indeed letting water in and getting corroded, I'm happy that I now have the piece of mind. To be super sure, I ended up pulling four of them (half), but the rest I am happy leaving be.

Thanks, Erik. That reaffirms what I've heard about cast iron and stainless not being the best match.

I'm now leaning back towards using the better galvanized bolts I have, even if the hex heads don't snugly fit the beveled holes in the cast iron. Still, I'm worried about causing stress fractures in the cast iron if the load is only resting on a few spots, or on a thin circle if I used a washer there. The cast iron is only about an inch thick above the hole. Does anyone think this would be an issue? I could also pack some high density epoxy under the hex head I guess.

[Anonymous7500]

Originally Posted by OrangeCrush

I'm now leaning back towards using the better galvanized bolts I have, even if the hex heads don't snugly fit the beveled holes in the cast iron. Still, I'm worried about causing stress fractures in the cast iron if the load is only resting on a few spots, or on a thin circle if I used a washer there. The cast iron is only about an inch thick above the hole. Does anyone think this would be an issue? I could also pack some high density epoxy under the hex head I guess.

Putting a hex head into a hole designed for a flat head is a bad idea. Some forms of cast iron have a tendency to crack at stress points.

[OrangeCrush]

Ok, thanks for saying, A7500. I'm much happier hearing that now than later!

Anyone know any other companies/distributors that might make what I'm looking for:
5/8" x 3-1/2", flat head (countersunk) hot galvanized steel bolt (or machine screw)?

A company called Portland bolts can make them for me but they will be about $500 for the four bolts. Maybe cold-galvanizing spray or getting someone else to hot galvanize if I bought an uncoated bolt?