Bulkhead repair

I am working on replacing a section of my 40 Tollycraft aft bulkhead. The beam is 14’8” and only a section has some rot I would like to replace. I do not believe I would be able to replace the entire bulkhead in one section, so a seam is inevitable. What are options for the joint?

I made a trial of joining two ¾” pieces of Marine plywood together using the following process

1) Make a Rabbet cut ¾ deep on each piece leaving half the thickness of the ply.
2) Use polyester fiberglass structural putty to mate the two sections together.
3) Use a 3 inch planner to make a 1/16 pass above the joint.
4) Lay 3 inches of CSM into the groove, and use polyester resin to lay it in place to improve adhesion.
5) Use a 9 inch strip of biaxial 1708 over the CSM and joint.
6) Repeat on the opposite side.

Your thoughts about this joint or others would be greatly appreciated. Thank you.

 

Welcome to the Forum Tolly.
We shall look forward to seeing your photos - as the saying goes, 'a picture is worth a thousand words'.
I think you have to post a certain number of posts before you can then post photos (?)

In the meantime, how large is the area / section that you plan to replace?
For example 12'' square, or bigger, or......

Re how 'a seam is inevitable', what sort of finish do you intend to have on the bulkhead?

Re your laminate process described above, I think it would be much easier / stronger / simpler if you used epoxy rather than polyester resin to glue in the new section, and then apply a layer of cloth with epoxy over it.

 

Welcome to the forum.

Unfortunately that is a recipe for a weak joint.

Better options
-Increase width of the 1/2 lap to 8 times the plywood thicknesses.
-Use epoxy.
-Forget about the CSM

Good luck

 

Try a plain scarf, if you work tidy the glue joint will be almost invisible, no glass is needed. It is difficult to cut the scarf in place you need to practice on a vertical piece of ply. For a stepped scarf make more larger steps to increase glue area.
Whatever method you use, only use epoxy, saturate both sides with neat, then clamp with thickened.

 

3x - don't bother with poly

you can use an oscillating tool with a better quality carbide and follow Rumars advise and make a nice scarf joint on some scraps; a dull cutter or too fast a cut will burn and blacken the wood forever

thicken the epoxy with fumed silica; this is a simple repair done right

if you don't have cutter tool distance for all the scarfs; do your best or combine the half lap and scarf using the oscillating tool and a chisel. An open joint is best because the thickened epoxy will fill any errors with too big a cut.

 

This is a scarf done in-place.
Lapstrake repair | Boat Design Net https://www.boatdesign.net/gallery/lapstrake-repair.27623/

 

Thank you for the feedback. We just purchased the boat, and it is in Ohio, and I am in Connecticut. The Survey revealed the bulkhead issue, but bit the extent, so there are some unknowns... I am going out next week to make the repair, and will be able to send some more photos of what it looks like once I get out there and teardown the insulation. The scarf joint looks like the best option so far...

 

The scarf joint looks neater. However, you can butt block the repair if one side is hidden.