1976 C&C 24 CHAINPLATE KNEES

[BC&C]

Hey.

I am in the process of replacement the Chainplate knees on my boat. I am getting mixed recommendations on how to install against the hull. the recommendations have gotten so are is to install a strip of foam between the hull and the new knee to avoid hard spots but some say to just epoxy it to the hull with a epoxy peanut butter mixture. I still have to grind the old fiberglass of the hull before setting it in place. Any thoughts on which would be the best way to proceed.

 

[rgranger]

I am not a naval architect, (but I pretend to be one on this forum). Joking aside, I have read extensively on the subject. Traditional carvel-framed boats rely on the rigidity of the hull structure to distribute loads and stresses throughout the vessel. Any movement between structural members can concentrate stress at joints and fastenings, potentially leading to damage over time. For that reason, I would favor using epoxy, as it can help create a stronger, more rigid bond and reduce localized movement. The stress from the shrouds will be tranfered to the knee and then to the hull and stringers and .... and... and...

 

[Ralph Johnstone]

The stress from the shrouds will be tranfered to the knee and then to the hull and stringers and .... and... and...

and ... the leg bone's connected to the knee bone, and the knee bone's connected to the thigh bone ....... now shake dem bones aroun'.

the recommendations have gotten so are is to install a strip of foam between the hull and the new knee to avoid hard spots

Let me assure you, you DO NOT want to be around people like this when it comes to structural matters related to your boat. Or anything else related to your boat for that matter.

It's mandatory that you have something rock solid transmitting the forces from the plate to your hull along its full length. Foam don't transmit nuttin'.

Epoxy is your ONLY acceptable method of transmitting all of those forces.

In order to avoid any posible separation at the contact, you may also want to consider a filet joint on both sides made of heavy glassed cloth (or several layers of lighter cloth) rather than just filler material :

.............. that runs from right from the top to the bottom. The cloth reinforcement should spread several inches from the intersection of both the knee and the hull. You don't want to do this job again in a few years. A solidly reinforced plate will also avoid telegraphing the attachment position of the knee to the shiny hull exterior.

That joint is going to see a lot of humping and bumping once it gets heavy air.

 

[dlochner]

and ... the leg bone's connected to the knee bone, and the knee bone's connected to the thigh bone ....... now shake dem bones aroun'.



Let me assure you, you DO NOT want to be around people like this when it comes to structural matters related to your boat. Or anything else related to your boat for that matter.

It's mandatory that you have something rock solid transmitting the forces from the plate to your hull along its full length. Foam don't transmit nuttin'.

Epoxy is your ONLY acceptable method of transmitting all of those forces.

In order to avoid any posible separation at the contact, you may also want to consider a filet joint on both sides made of heavy glassed cloth (or several layers of lighter cloth) rather than just filler material :

View attachment 238554​

.............. that runs from right from the top to the bottom. The cloth reinforcement should spread several inches from the intersection of both the knee and the hull. You don't want to do this job again in a few years. A solidly reinforced plate will also avoid telegraphing the attachment position of the knee to the shiny hull exterior.

That joint is going to see a lot of humping and bumping once it gets heavy air.

Yep what @Ralph Johnstone said.

Andy Miller just did a video on this very issue.

 

[shemandr]

I was going to post that I don't think simply epoxying the plywood edge to the hull with a bead of epoxy is adequate. The post above beat me to it.
I can add that built a stitch and glue pram that used filleted joins of plywood. Once leaving a slip I got the dingy caught between my boat and a dock pole. The dink bent almost in half. My wife yelled "You killed the dink!!" But it sprung back without damage to any of the seams. Fillets are strong.

 

[Stu Jackson]

Ralph, Dave and shemandr explained it. You HAVE TO understand how the loads on the chainplates work. They will, under load, be pulling UP. The boat was most likely designed so that the bulkhead was supported by both the deck above it as well as the connection of the bulkhead to the hull. You need to make all those connections as strong as you can and have been given enough tools to do so. Good luck.

Other boats are designed differently. For example, the Catalina 36 has some of its chainplates connected to bulkheads, where on my Catalina 34 all six go to special thru deck fitting with tie rods all the way down to the aluminum stiffeners below the cabin sole as part of the hull above the keel. Same builder, almost identical boats, different methods.

 

[dlochner]

install a strip of foam between the hull and the new knee to avoid hard spots but some say to just epoxy it to the hull with a epoxy peanut butter mixture

It is appropriate to be concerned about a the knee (or bulkhead) forming a hard spot where it meets the hull. If the knee is just glued to the hull with thickened epoxy there will be a hard spot. The fillet and the tabbing distribute the loads from the knee across a larger area thus eliminating the hard spot. Fiberglass does not like sharp corners, the glass will tend to pull away from the corner A fillet provides a radiuses corner which allows the glass to maintain contact with joint.