Sistering Keel Bolts

[Al Dente]

Sistering Keel Bolts

I am decided to sister in 6 additional keel bolts into my '92 built 39' sloop. GRP with a fin keel on a good size stub, displacement of around 12t, keel weight around 4.4t I think. The keel bolts are mild steel J bolts cast into the lead keel. I am looking for any operational tips from those out there who have knowledge of the process. Don Casey's summary of the procedure looks pretty straightforward. The current bolts are so rusted that any thought of re-tensioning is not going to happen. I had these bolts X-rayed in Trinidad some years back and then cleaned them up and re coated. The X-rays indicated minimal thinning of some of the bolts just below the backing plate. After hauling the boat after the puddlejump last year some degradation of the keel/hull joint was apparent with one place weeping rusty water for a while. It is this degradation of the joint material that is prompting the action, this degradation could signify an increased level of flexing of the joint. My reading suggests that mild steel keel bolts were not uncommon years back.
A knowledgeable boatbuilder in NZ is recommending 25mm bolts in 30mm holes. In NZ I can source a Duplex (2205) or Super Duplex (2507) SS, comments appreciated. With regard to the process all seems clear...soaking a rope of cotton caulking with polyurethane for the bottom seal.... anyone got any comments on that one. Staggering the bolt depth sounds good too.
After finishing the job, cleaning and resealing the joint there is also a suggestion to laminate 2 layers of 620g biax around the joint, constructive comments please.
I attach some pics of the bolts and the joint.






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[chris95040]

Might consider using bronze keel bolts for your sisters. Silicon bronze. Very compatible to lead galvanic-wise. Also don't have the crevice + stress corrosion problems that stainless steels exhibit.

My 1964 bronze keel bolts (in lead ballast) looked like new when I popped a few out for inspection/replacement a few years ago.

Mine seemed to be sealed with oakum or something-or-other. I used 5200 for the replacements.

Can't imagine why you'd oversize the holes that much.

[Viajero]

Your fin keel is almost certainly of cast iron. The best sister bolts would be of Monel, expensive and hard to find. I suggest Aquamet propeller shafting. Have a machine shop make up the studs, threaded at both ends. Drill a clearance hole the size of the shafting to your desired depth, then a short hole sized to the tap. Cast iron is difficult to drill so plan to do a lot of drill sharpening. Be careful not to break the tap. Plan to leave enough thread at the top to place a lock nut as you'll want to draw the studs for inspection.

[Viajero]

If the keel is actually of lead, silicon bronze, as suggested by Chris, would be best. If this is the case I'm very surprised the mild steel bolts lasted this long.

[Jim Cate]

Originally Posted by Viajero

If the keel is actually of lead, silicon bronze, as suggested by Chris, would be best. If this is the case I'm very surprised the mild steel bolts lasted this long.

Why would not the 2205/2507 duplex stainless bolts suggested by the OP not be better than silicon bronze in this application? They are both stronger and quite resistant to crevice and stress corrosion.

Jim

[chris95040]

Jim,

I'm curious about that too. I've been really blown away with the performance of bronze (likely a phosphor bronze) on my old boat in a number of different applications, including the keel.

The way I understand it (and I'm far from qualified to make a compelling argument, but hell, this is the internet!!) those duplex alloys have some neat workarounds to add resistance to crevice and stress corrosion, but still are vulnerable to some degree. The exact mechanism by which it occurs is not present in bronze. Hopefully someone more knowledgable can confirm/deny/clarify this.

[Al Dente]

The keel is lead. I am inclined to go for the super duplex SS, I am impressed with what I read about it. As far as I can make out Aquamet 22 is a U.S. equivalent to 2507 Super Duplex, any material experts out there?


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[Schooner Chandlery]

Hi Al,

Edited to say --ah--J-bolts! that's why you can't remove. Bummer.

Leaving all the info below--though it doesn't apply to you

Info might be useful to you: Our boat is a different situation than yours, even so, we had keelbolts through wood floors, keelson, and lead keel, as well as holding together parts of the deadwood near the prop aperture. Ours were very old--76 years old at the time of removal. They were made of Tobin Bronze (higher zinc content than Silicon Bronze, really closer to a brass) and some of of mild steel. The bronze ones were true bolts whereas the steel ones were drifts (friction fit). They were all in rough shape and took quite a bit of effort to remove. Many of the mild steel ones were heavily corroded at the wood joints but full diameter along most of the bolt or drift. Same with the bronze ones.

We replaced all of them with Silicon Bronze but really we could have used Tobin Bronze, Manganese Bronze (what your prop may be) as well. Our bolts ranged in original diameter from 1/2" to 1" and length from 16" to over 4 feet. The hull is 54' long, wood forekeel, keelson, lead keel, wood countertimber all were bolted/drifted as needed so there were about 100 total and removing them was a pain.

Hole size should be close fit to bolt size. It is common to wrap a thread of oakum (or even more common COTTON) caulking around the bolt at the nut to act as a seal. This is successful if the hole size is close fit to bolt size. Otherwise, all sorts of tricks are used to seal up around the bolt--anything from hightech grouts to just heating up the bolt and pouring a melted wax down to fill the gap. Clearly the grouts, epoxies, and modern goos have the common problem of making things harder to remove "some day" when you have to do it again.

I hope your "someday" will be far into the future. Many people do use various SS with some success. There are some corrosion issues for use of SS in salt water and you'll just have to figure out if you want to deal with that. The only reason to do so would be cost savings as it is typically cheaper to use SS than bronze.

You're not in the USA, but if for CF members who are, you can get silicon bronze keelbolts made-to-length and threaded to your spec and very affordable (for silicon bronze) by TN Fasteners, one of our sellers, in the Schooner Chandlery marketplace. Link to the TN Fasteners here. We made and threaded our own keelbolts in 2007 and would have loved it if TN had had custom keelbolts at the time!

Best of luck in your project,
Brenda

[Viajero]

Originally Posted by Jim Cate

Why would not the 2205/2507 duplex stainless bolts suggested by the OP not be better than silicon bronze in this application? They are both stronger and quite resistant to crevice and stress corrosion.

Jim

Hi Jim, I'm not familiar with the alloy you mention but all stainless steel, as far as I know, requires oxygen to retain its corrosion and galvanic properties. In an anoxic situation in the presence of salt water such as in keel bolts, it is possible for the galvanic action from without or within the alloy to destroy the steel leaving a honeycomb of chromium and nickel which appears whole but has little strength. That said, in more than 30 years of surveying I have never seen a catastrophic failure of stainless keel bolts, though I have seen many failures of stainless propeller shafts, rudder shafts and other stainless fittings above and below waterline. Best of luck, Sam Slaymaker

[Jim Cate]

Originally Posted by Viajero

Hi Jim, I'm not familiar with the alloy you mention but all stainless steel, as far as I know, requires oxygen to retain its corrosion and galvanic properties. In an anoxic situation in the presence of salt water such as in keel bolts, it is possible for the galvanic action from without or within the alloy to destroy the steel leaving a honeycomb of chromium and nickel which appears whole but has little strength. That said, in more than 30 years of surveying I have never seen a catastrophic failure of stainless keel bolts, though I have seen many failures of stainless propeller shafts, rudder shafts and other stainless fittings above and below waterline. Best of luck, Sam Slaymaker

G'day Sam,
The duplex and super duplex stainless alloys, while not new, have recently become more mainstream in usage. Their properties in general include higher strength and much better resistance to various types of corrosion and chemical attacks than the austenitic alloys like 304 and 316. Wonderful stuff for yacht hardware of all types IMO, especially chainplates and keel bolts.

As a surveyor, you might be interested to learn a bit about these materials. This article covers the basics Article: Duplex Stainless Steels - A Simplified Guide but there is a great deal of info available on the net.

Cheers,

Jim

[Al Dente]

Thanks Jim, good info. One article I read referred to the use of these Duplex and Super Duplex Steels in the offshore oil business, their increasing use has followed the growth of the offshore oil business. Particularly the piping and valve installations where the corrosion issue is critical.
Makes sense to me.


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