Chainplate Knee Rebuild

[actual.lee]

It appears as though I need to rebuild the port chainplate knee on our Tartan 27-2. It looks like it has been repaired (misrepaired) at least once in the past and unfortunately water has gotten into the wood, which is encapsulated in fiberglass. The chainplates bolt directly to the knee, they are not encapsulated.

I'm planning to laminate new marine ply to 1" thickness, epoxy it into the hull, tab it with several layers of 10 oz cloth and epoxy resin, and then encapsulate the wood in fiberglass, before drilling (and filling with epoxy then redrilling) new holes for the chainplates.

You'll note the knee is tabbed on the bottom half. It's free floating on the top half (original design). I'm wondering how far in either direction along the hull I should tab in the new knee. 4"? 12"? What's reasonable?

This is also your chance to correct my planned approach. I've worked with fiberglass, but never on something so structurally integral to the rig. I've also read where others attempt this approach by removing only one fiberglass skin, dig out the old wood, then laminate new wood. The theory is this maintains one of the original fiberglass bonds to the hull. I'm considering this method.

I'll try to attach a photo of the existing knee.

[jimbunyard]

I'd just ditch the whole wood concept, buy an appropriately-sized piece of 3/4" G10 from Mcmaster Carr, and glass that in instead.

Or more likely, given my cheapo nature, just lay up a polyester resin/roving 'board' to the proper dimension.

Does away with future wood problems, as well as filling bolt holes through wood core.

[alaskanviking]

My chain plates are mounted in the other direction, so they are just little angle blocks essentially. When I pulled my plates the knees were so waterlogged they felt almost like a sponge. Full of water. So soft I could poke a finger into it.

I had the yard replace these with G10, and glass them in. Solid and It won't absorb water or degrade.

In hindsight these were pretty easy to make but labor intensive. If I did it again I would have made these myself using G10 the correct thickness.

Get G10 the thickness of the core. Sand it good so the resin adheres. Use the same amount of fiberglass you would normally. Will be strong and water proof, never have to fix it again. Make sure you tab it to the hull well

[actual.lee]

Originally Posted by alaskanviking

Make sure you tab it to the hull well

This is one of my key areas of uncertainty. How far along the hull should it be tabbed? When I read about bulkheads, it seems a lot of folks only tab 4" from the edge of the bulkhead along the hull. My gut tells me this is insufficient for chainplate knees, but maybe I'm wrong.

[alaskanviking]

Originally Posted by actual.lee

This is one of my key areas of uncertainty. How far along the hull should it be tabbed? When I read about bulkheads, it seems a lot of folks only tab 4" from the edge of the bulkhead along the hull. My gut tells me this is insufficient for chainplate knees, but maybe I'm wrong.

Originally Posted by actual.lee

This is one of my key areas of uncertainty. How far along the hull should it be tabbed? When I read about bulkheads, it seems a lot of folks only tab 4" from the edge of the bulkhead along the hull. My gut tells me this is insufficient for chainplate knees, but maybe I'm wrong.

https://youtu.be/c9hFJ0tLON0

He goes into it here. There's a lot of overlapping layers. after the first layer of glass the subsequent layers extend onto the hull ~3". Then he overlaps that with the tabs, It looks like ~6". Several layers though, some smaller 4"ish followed by the wider pieces.

My understanding is that you continually overlap with wider pieces. I'd start with 4", and move to 6" at a minimum. If it's feasible to do so, or structural I'd go with 8" or 12". Structural I'd get 17oz biaxial cloth cut into strips, similar to what he is using looks like.

I'm fairly good with glass but not an expert. I generally overbuild unfortunately. So don't take my advice for gospel. Structural stuff like knees kinda freaks me out. The video is pretty self explanatory, and not really difficult. I'd be constantly thinking and worrying about it, with every creak of rigging. Ironically I am glassing and tabbing a bulkhead myself. Full bulkhead, never had any glass and only marginal tabbing. Will be fully sheathed in glass and heavy tabbing when I'm done. Overkill definitely, can't help it. Structural but not structural so it doesn't worry me, I'm only adding too the bulkhead, so what I do will be better/stronger/whatever than it was before. This is definitely a "me" thing.

[jimbunyard]

This is especially a case where preparation is very important.

The area where the new fixture is to be bonded needs first to be ground down to solid hull fiberglass; all the existing tabbing needs to be completely removed.

Since the width of the liner cutout on the lower portion of the hull around the knee limits the width of the tabbing possible (unless you [for no good reason] choose to widen it) the tabbing of that part will be limited by the area that to which you have access; what appears to be 3 inches on each side, which is completely adequate.

For a truly bulletproof, worry-free installation, my preference would be to make a cardboard template that closely matches the hull curve from the bottom end of the knee to the hull/deck joint, and then bond the 'glass board' (be it G10 or of homemade variety) made from that template in place with a paste made from a structural filler (like milled fibers) and epoxy, followed by three or four layers (depending on which glass you use) of tabbing along the entire length. Using staggered widths of tabbing makes for a much neater and professional-looking job, but I'm not sure that it's strictly necessary. Though that's what I always do. Also, I've always heard that the 'correct' way to do it is start with the wide pieces, but I usually do it the other way 'cause it looks better.