Chainplates - Redesign?

[andreas.mehlin]

I have been thinking about how to improve the chainplates on my boat. Today they have been fitted thru deck and bolted to bulkheads....and yes it's a prone area for water damage.

How about if I use a L-profile between deck and bulkhead. On deck a straight plate with welded (or bolted) U- fitting...and holes to bolt thru to the L-profile?

Good or bad idea?

edit:// btw....Going with dyneema rigging

[psk125]

You would probably need to discuss this with a structural engineer. Our chainplates go through the deck and are through-bolted to the main bulkhead. They are angled to match the angle of the shrouds. Your L-profile setup does not do that, so might set up forces on the bulkhead that could pull it out, or not support the mast sufficiently and lead it to break. An engineer would be able to tell you more. It would appear that the setup you envisage would be easier and less expensive to build than the bolted-to-the bulkhead original design. There must thererefore be a reason for the builder not doing it that way.

[andreas.mehlin]

Originally Posted by psk125

You would probably need to discuss this with a structural engineer. Our chainplates go through the deck and are through-bolted to the main bulkhead. They are angled to match the angle of the shrouds. Your L-profile setup does not do that, so might set up forces on the bulkhead that could pull it out, or not support the mast sufficiently and lead it to break. An engineer would be able to tell you more. It would appear that the setup you envisage would be easier and less expensive to build than the bolted-to-the bulkhead original design. There must thererefore be a reason for the builder not doing it that way.

My setup today is a straight 1/4" stainless chainplates that passes thru the deck and bolted to the bulkhead with 5 bolts. So very simple....

My new idea is far more advanced or probably cost much more do welding etc.
It will, just like the current ones, be bolted with 5 bolts to the bulkhead.

[RaymondR]

The tangs or straps are simple and effective and help carry the loads down to a large area on the bulkhead away from the edges.

[Wotname]

I understand (but don't know) that much of the load on a chainplate is taken by the friction between the chainplate and the substrate and not by the shear strength of the bolts. The bolts primary purpose is to provide enough pressure to ensure the friction is there.

If this is true, then the surface area of the chainplate (against the bulkhead etc) should be maintained.

[RaymondR]

Originally Posted by Wotname

I understand (but don't know) that much of the load on a chainplate is taken by the friction between the chainplate and the substrate and not by the shear strength of the bolts. The bolts primary purpose is to provide enough pressure to ensure the friction is there.

If this is true, then the surface area of the chainplate (against the bulkhead etc) should be maintained.

Or increased by extending them and bonding them to the bulkhead as well as bolting or, increase the surface area in contact with the bulkhead by underlying them with say largish 2mm SS or glassed in carbon steel plates to dissipate the loading over a greater area of bulkhead.

[Wotname]

Originally Posted by andreas.mehlin

I have been thinking about how to improve the chainplates on my boat. Today they have been fitted thru deck and bolted to bulkheads....and yes it's a prone area for water damage.

............

One way to improve the sealing of the chainplate penetration is to weld a collar onto the chainplate so there is a much greater area for the sealant to work.

[Eigenvector]

Originally Posted by Wotname

I understand (but don't know) that much of the load on a chainplate is taken by the friction between the chainplate and the substrate and not by the shear strength of the bolts. The bolts primary purpose is to provide enough pressure to ensure the friction is there.

If this is true, then the surface area of the chainplate (against the bulkhead etc) should be maintained.

This is a true statement and the proper way to design this connection.


There is nothing wrong with the OP's design. Just be sure that when bolting the deck plate on that you don't over stress the bulkheat-to-deck joint due to uneven or mis aligned surfaces.



The bolts thru the deck are only to transfer the load to the bulkhead since the deck has minimal load carrying ability in the transverse direction. It will make it easier to seal.


My advice would be to dry fit with a bedding compound (epoxy with filler) hand tight. Using a release agent wouldn't be a bad idea either. Once it sets up hard you've got your good mating parallel surfaces. Then torque up and you're set.

[Cheechako]

It's hard to beat the simplicity of a plate through deck bolted to a bulkhead. Make sealing plates that go around the chainplate if sealing is a worry for you. Like Wotname shows in post #7.
Your scheme would work if done properly though. But the reality is there is likely a large fiberglass fillet attaching the bulkhead to deck so those L plates will need to be a ways away from the bulkhead. Bolts are heavily susceptible to corrosion, so you still have a corrosion risk if the sealant fails either way. Then there is the loop or welded tang. Welds are subject to corrosion if not annealed or polished properly after welding. So you risk corrosion with the new design too.

[Smokeys Kitchen]

Originally Posted by Wotname

One way to improve the sealing of the chainplate penetration is to weld a collar onto the chainplate so there is a much greater area for the sealant to work.

This is exactly what mine look like on our Pacific Seacraft. No leaks to date.

[Jim Cate]

Originally Posted by Wotname

One way to improve the sealing of the chainplate penetration is to weld a collar onto the chainplate so there is a much greater area for the sealant to work.

would not those welds create stress risers and potential weak points in the chain plate itself?

We've had similarly shaped collars that were simply screwed to the deck, but not welded to the plate... dunno if they really help, though!

Jim

[stormalong]

Why are you trying to improve your chainplates? Is there a problem? What is it?

The best improvement that you could do is to make them thicker and use 316L stainless.

Any welding is a potential source of corrosion and cracking. Nothing would be as strong as an in-line pull on a single piece of stainless steel.

[Psy at SEA]

Originally Posted by Jim Cate

would not those welds create stress risers and potential weak points in the chain plate itself?

We've had similarly shaped collars that were simply screwed to the deck, but not welded to the plate... dunno if they really help, though!

Jim

I would agree that the welding would have to be MPI'd again once those plates are welded to the chain plate, it will also have to be fitted at the correct angle to the deck.
Good idea ?

[Wotname]

Originally Posted by Jim Cate

would not those welds create stress risers and potential weak points in the chain plate itself?

We've had similarly shaped collars that were simply screwed to the deck, but not welded to the plate... dunno if they really help, though!

Jim

Most probably Jim, it is always a trade off. In my case, all six chainplates penetrated the ply deck and all had leaked over years before I became the "new" owner.The deck was soft around all of them and they were alive with crevice corrosion. I moved the four lowers to the outside of the hull but for a variety of reasons the caps had to remain in the original position. To mollify the welding concerns, I doubled the cross section area of the chainplate and had the welding done by the best certified aviation welder I knew and he had his NDT mate to give it a once over - all paid in Kimberly currency (i.e. a carton of one's choice) .

I think no-welded collars are a slight improvement compared with no collars but it seems to me there is still plenty of movement available in the chainplate / deck interface for the sealant to fail.

However I am DIY sort of guy and if my ideas fail, I live with the knowledge it was my idea

Originally Posted by Psy at SEA

I would agree that the welding would have to be MPI'd again once those plates are welded to the chain plate, it will also have to be fitted at the correct angle to the deck.
Good idea ?

Yes, getting the correct angle did take a bit of effort but with dry fitting and spacers and using masking tape (and suitable language) it was achieved.

I don't recall but I think gap between the deck and the collar was nominally a little over 1/8" dry and around 3/16+" in service. This gave a large area and thickness of sealant for it to work properly.

It certainly worked for the 10+ years I had the boat.

[RaymondR]

Originally Posted by Wotname

One way to improve the sealing of the chainplate penetration is to weld a collar onto the chainplate so there is a much greater area for the sealant to work.

I would not do that with SS chain plates. SS is very prone to failure when welded and even 316L is not immune from the process which causes the cracking problem.

A separate plate which slips over the chain plate and is screwed down to the deck is a better system from a crack prevention viewpoint. The secret to preventing leaks is either to use a thick but softish rubber gasket or use spacers under the goo until it dries and firms up then tighten them down a little.