Aluminum chainplates

[mrgnstrn]

Hello all.

I recently had my mast unstepped for some overdue rigging work.
As a result the chainplates became clearly visible for the first time since owning the boat (only 3 years).

The chainplates are all aluminum, 4" wide plate, 1/2" thick and run from the deck through the cabin, and are secured by a bunch of bolts into massive stringers glassed into the hull.

Since there was this aluminum-stainless connection, the expected thing happened, and the aluminum has flaked off in the areas where the turnbuckle toggles where coving.

So, something needs to be fixed.

Going to all-stainless is the obvious solution, but really expensive. These buggers are 40" long, 4" wide, and 1/2" thick. We could go to 1/4" thick plate, but still has to be 4" wide to have enough room for 3-shrouds to attach to it.


I am currently exploring the keep-it-the-same approach because it has taken 21 years to get to the point where something must be done, all on the salty Chesapeake. Plus, the 37" of material that is inside the boat, cabin, bolted to the stringer, is perfect looking. No defects at all. The problem is isolated to the 3" that stick up on deck.
So, anyone have a clue what material they might be made of?

Pictures here: http://members.cox.net/btsrefit/chainplate.html
Other pertinent details:
1987 C&C 35

Keith

[Brion Toss]

Hello,
Skene's rates 1/2" thick plates for 7/32" wire, and a 7/16" pin; what size are your wires? I'm guessing much larger, but then Skene's is very conservative on this subject. If there were a vast surplus of strength I might feel good about re-using the existing chainplates, but I really need to know the loads.
If they aren't reusable, and if the dimensions make sense, simply replacing them with aluminum ones is ideal, assuming you isolate the metals better in future.
Fair leads,
Brion Toss

[mrgnstrn]

There are three shrouds attached, all navtec rod rigging:
-10
-6
-10
in that order (upper, intermediate, lower).
So the total, maximum load with all three at near breaking strength is about 26900 lb.
With a total tensile area of about 2 square inches, that is 13450 psi.
What does Skenes say?
Is this even a design condition that makes sense?
What grade and heat treatment/temper is likely to be the one used?

Thanks,
Keith

[Brion Toss]

Hi again,
You can likely count on about 40,000psi strength for aluminum, so if the radii are right, I think you are in good shape. I will get back to you about the alloy to use; do you have a fabricator in mind?
Fair leads,
Brion Toss

[mrgnstrn]

Brion,

Thanks for your reply. As for radii...?where? Do you mean surrounding the holes for the pins for the turnbuckles.
Pictures are posted here:
http://members.cox.net/btsrefit/chainplate.html

I have two leads that I am working on for getting them fixed.

#1 is to go back to the "old" C&C factory guys up in Canada, who said they have the original drawings, etc.
#2 is to go to a local machine shop, and have them mill off the corrosion, and fill it back in with weld-metal.

Yes, better corrosion protection (e.g. lanocote) would go far to help this in the future....except that I have only had the boat for 2 seasons so far....no idea what all the previous owners did.
(funny how all previous owners are scapegoats....)


-Keith

[Joe Henderson]

Dear Keith,

You must forgive me for jumping in at this late stage in this topic.

I have always hated chainplates with welded-on keeper plates. When I see a weld across a load path my left eye starts to twitch and an uncomfortable feeling creeps up my spine.

That said, your plates have been in place with no problems for years. I think this is due to high quality materials and workmanship rather than any design virtues. They are large and look to have been well finished with a good anodising job and fitted properly. I think you will be hard pressed to duplicate the quality.

If I were you, even though the cost is high, I would consider the stainless steel option.

The only problem you will have with new assemblies is getting the keeper plate to press on the deck correctly. I bet that when they were first fitted at the factory, the whole assembly was offered up to the deck/hull, the keeper plates were dressed to fit, and then the lower attachment bolt holes were drilled through the bulkhead/glassed in web.

This would ensure the keeper plates were flat down on the deck and did not leak.

Trying to match the lower through bolt holes will be a nightmare. You may want to consider seperate, thicker, larger keeper plates that are fitted afterwards onto a good bed of mastic as per the bedding deck hardware thread. This will also allow you to have the chainplates lazer cut from plate and then shadow drilled off the originals before you put in the required crank.

You may have to remove the existing welded-on keeper plates then straighten out the cranked portions of your old chainplates so they lay flat against the new blanks for use as a pattern.

As always, it is entirely up to you, and will be influenced by the availability of the correct materials and labour.

Why not send the patterns for replication to Brion. You can then be sure of one of the most important components on any sailing vessel.

When discussing chainplate inspection and/or removal I always ask the client the following question:-

"If the need arose, would you be happy to lift your boat, properly slung, attached and balanced, using only the chainplates?" This usually results in much sucking of teeth and a grudging decision to inspect the chainplates after all.

On the subject of cost, and as another hypothetical question, how much would the average owner be happy and eager to pay to have their mast and sails magically back up and working just after the chainplate broke in mid passage?

Regards,
Joe Henderson.

[Brian Duff]

I think that overall aluminum is a better choice than stainless steel for any structure to be used in the marine enviroment. It doesn't suffer from as serious, and hidden, corrosion problems as stainless steel does. Aluminum shows its corrosion quite clearly, where stainless can dissolve deep while still appearing to only have 'surface corrosion'.

Reproduction of these chainplates should be an easy job for any machine shop.

The comments about the seperate deck plate have merit due to the serviceablility of this type of installation - a leak can be addressed without removing the entire chainplate. Thats a plus.

Using plastics to isolate the aluminum from the stainless steel where possible is always a worthwhile effort.

Good Luck.