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Old 18-03-2017, 19:31   #1
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Chainplate Replacement

Greetings,
Shane from Brisbane Australia.
I have a recently purchased Slocum 43 cutter rigged sloop. It was built by US boat builders Cruising Yachts International in Taiwan in 1984.
Hull construction is handmaid fibreglass on Airex core.
Last weekend a starboard double shroud chainplate sheered off just below deck level. The fracture is brown all through, indicating that it's been ready to give way for some time.
These chainplates are 5mm thick electroplated stainless steel, and are screwed and bolted to the hull behind the interior teak panelling and disappear into the fibreglass further down the hull. In other words, they were never designed to be simply pulled out and replaced.
The way I see it I have a couple of options. One is to cut the plate off inside the boat and lap a new chainplate over it, with boats and screws fixing the two together. This would involve on site welding because the new part would need to be bent to align with the angle of the shrouds. The advantage of this is good aesthetics. Disadvantage is that exactly the same is likely to happen again in 15-20 years, probably while cruising when I least want it to happen.
Another option is to leave the old plate intact, fill in the old deck aperture, and fix a new plate to it on the exterior of the hull. Advantages are that it allows regular exposure from the rain to wash away salts and easy access for inspection and replacement while at sea. Disadvantage is the aesthetics of visible chainplates bolted to the hull and more through fixings penetrating the hull.
My inclination is to go with the second option, exterior fixed chainplates. These parts are too important to fail at sea at some random time.
I welcome any advice on this as I'm sure it must be a common dilemma!
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Old 18-03-2017, 20:41   #2
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Re: Chainplate Replacement

Your first idea is all sorts of crazy. The second is a reasonable option though I personally hate exterior chainplates. They provide a huge area for poltice corrosion, and unless you are really going to pull them off annually for dye penetration testing they really aren't any safer.

Personally I would just have new plates made from G5 titanium. But I used to sell the stuff so I am probably biased (I don't anymore btw). It eliminates corrosion as an issue, and will Ben substantially stronger than 316 stainless.
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Old 18-03-2017, 21:51   #3
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Re: Chainplate Replacement

Are your plates through bolted or embedded? If embedded only one real way to solve your problem and that's to dig the old bolts out of the knees and rebuild the whole assembly. If left accessible and open, regular inspections will be easy, so it'll be up to you whether to put inside or outside, both have pros and cons.
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Old 18-03-2017, 23:51   #4
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Re: Chainplate Replacement

Thanks for your replies.
The bolts are embedded, and further down the hull the plate is very deeply cast into the fibreglass. I'm loath to dig the old plates out because I see them as additional strengthening of the hull, along with the thickened fibreglass. Not to mention all the teak joinery I'd have to destroy to get to them!
Titanium exterior chainplates fixed through to the old plates with bolts does sound like a good solution, although it doesn't sound like the sort of part I could get by popping down to my local fabricator. Maybe I could find something ready made Stumble?
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Old 18-03-2017, 23:53   #5
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Re: Chainplate Replacement

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Old 19-03-2017, 00:12   #6
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Re: Chainplate Replacement

For those, the ears look decorative not structural. In the US I could recommend somewhere, not sure about your side of the world. But a specialized machine shop should be able to knock some out for you pretty easily.

The cheap option is just G5 plate cut to size, with the corners rounded a bit. Removing whatyouhave is the right option, then slotting the Ti in place. If you don't want to go that route then putting the Ti external is an option, but I would really recommend Ti fasteners too. Then epoxy seal the old chainplate slots.

For the finish, mill stock is fine, but sandblasted looks a lot better and isn't all that much more expensive, it will be a dull uniform grey. You could get them polished, but the polishing will cost more than the plates and unlike stainless, polishing titanium is purely cosmetic.
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Old 19-03-2017, 01:46   #7
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Re: Chainplate Replacement

Why would you not consider exterior bronze chain plates? If you were in the US, I would suggest Port Townsend Foundry in Washington state.
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Old 19-03-2017, 03:03   #8
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Re: Chainplate Replacement

Let's say you intend to refit with exterior chainplates.

Exterior chainplates usually are through-bolted through the hull, with a backing plate on the interior of the hull, so the bolt tension clamps the chainplates to the hull, right?

If you wanted to maintain your current rig geometry, you could use your current embedded in GRP plates as the backing plates.

But you would need to drill through both hull + existing chainplates to do so.

Drilling through the hull + existing chainplates may take almost as much effort (in terms of dismantling interior furniture etc) as removing those embedded chainplates.

And you would also need:

(1) to check that the existing chainplates are solid enough to act as backing plates (that is probably the case: the corrosion is likely restricted to being around the exit of the plates through deck); and

(2) make up a template of the hull curve so you can have the plates bent to fit the hull curve (otherwise when you clamp an exterior plate to the backing plate, you will bend the hull to suit the exterior plate).

You would need both a template of the hull curve (at each of your chainplate stations, starboard and port), plus you would need to work out the dimensions of your plates. You have the thickness and probably the width of each plate, because you've plate exposed above deck. With careful use of a quality metal detector, you might be able to work out the length of each plate.

As for making new chainplates, in SE Qld you need jolly good luck to find either forged bronze plate or G5 titanium and someone to work it. I'd suggest staying away from cast bronze plate unless you know bronze and the caster well.

316 stainless is cheap. It's a compromise: subject to crevice corrosion and stress corrosion.

A little more expensive is 2205 duplex stainless. Almost immune to crevice corrosion and if in the same scantling as your 316 stainless original plates, relatively immune to stress crack corrosion (scc) too.

For bolts, you could buy future insurance and get the bolts and nuts carved from 2507 super duplex. It's even stronger than 2205 duplex and even more immune to crevice corrosion and scc.

The downturn in the mining and gas pipe industry in Qld means you can find suppliers of 2205 duplex and people to machine it for you. As Stumble noted, polishing 2205 duplex to No. 8 mirror would double the cost of the plates. Electropolishing to slightly less than No. 8 mirror is more economical.

If you wanted to go down either the El Cheapo more 316 ss route, or the long life 2205 duplex route, I'd suggest you go talk to Mykel at Stirling Stainless Steel at 51 Musgrave Road, Coopers Plains. Or look for other contact details at stirlingsaustralia.com.au You'd need to make drawings of exactly what you want, including the bolt holes. Stirlings would then laser cut the steel plate (so if you wanted to use carriage bolts, you could get the square holes cut to the right size and without stress to the metal).

Mykel could also point you to expert fabricators and electropolishers.

If you want 2205 bent to a precise shape based on a template, I'd point you to Karl at NRG Piping at 55 Christensen Rd, Stapylton (it's a small industrial park. NRG Piping is at Lot 6).
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Old 19-03-2017, 07:20   #9
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Re: Chainplate Replacement

Fantastic advice, thank you!
Alan Mighty, so great to get some local knowledge for the materials and fabrication.
I'm ok with the technical drawing as it's very much part of my profession. I do find it staggering that such a design flaw can be built into a yacht, basically a hidden time bomb. I worked in NZ during the development of solutions for the 'Leaky building syndrome', an affliction caused by water getting into otherwise sealed spaces and rotting untreated timber and ferro metals. One of the first design flaws to be eliminated by the building code was top mounted balustrade fixings into balconies and decks. And here we have them on a yacht.
I'm convinced that I don't want deck penetrations that are totally reliant on sealants for their effectiveness. Externally mounted Ti or 2205 duplex chainplates and fixings sound the right way to go.
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Old 19-03-2017, 08:21   #10
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Re: Chainplate Replacement

Quote:
Originally Posted by vfsurgpa View Post
Why would you not consider exterior bronze chain plates? If you were in the US, I would suggest Port Townsend Foundry in Washington state.
Bronze is fine but it's in the same price catagory as titanium. The limit is that you have to use forged bronze, not cast, and a huge percentage of all bronze on the market is cast. Forged stuff is out there, but it's hard to source, hard to track down, and expensive.

Titanium is obviously still a pretty specialized metal as well, but G5 is off the shelf stuff for people who do stock it. Anyone who does use titanium will almost assuredly have G5 in stock. A quick google search for 'titanium New Zeland' pope does up a good number of people with it on the shelves right now.
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Old 19-03-2017, 08:37   #11
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Re: Chainplate Replacement

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Originally Posted by shanegeard View Post
Thanks for your replies.
The bolts are embedded, and further down the hull the plate is very deeply cast into the fibreglass. I'm loath to dig the old plates out because I see them as additional strengthening of the hull, along with the thickened fibreglass. Not to mention all the teak joinery I'd have to destroy to get to them!
Titanium exterior chainplates fixed through to the old plates with bolts does sound like a good solution, although it doesn't sound like the sort of part I could get by popping down to my local fabricator. Maybe I could find something ready made Stumble?
If you're talking about getting another 20+ years out of them, I would be just as concerned about the condition of the embedded bolts as the chain plates. Drilling them out may not be as easy as it sounds and if backed with plywood or SS chances are it's wet or been wet and rot or corrosion is likely.
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