Re: Transom Repair
Well, there a couple ways of looking at this. Given that the rot is in the middle of the transom I think I would look at approaching this as a partial replacement. Going this route eliminates alot of fiberglass and gelcoat work. That being said, the area that you will have to cut open will be larger than the rotted area alone. If the damaged area is 3x2, I would expect to cut open an area 4x3. With all of the mechanical out of the way I would do this from the exterior side of the transom.
Cut off the outer skin of glass with a thin kerf blade. Fein makes a great tool for this (multimaster) but is a little pricey for a one time use ($300?). A skill saw with the depth set to the thickness of the exterior skin of fiberglass would also work but will make alot more dust (make sure you're wearing a good respirator, tyvek suit and eye protection!)
An important part to keep in mind with the planning is that you need to stagger the joints between the inner and outer layers of plywood making sure that one overlaps the other.. You don't want to have the joints from these two pieces lining up otherwise it will be a weak spot. For example: If the actual rot area is 24"x24", I would cut the outside skin 36"x36" (only the skin, not the plywood), then the outer layer of plywood roughly 3" smaller all the way around, and the inner layer of plywood 3" smaller than that all the way around leaving you a square that is to the actual size of the rot; 24"x24"..
Best ply to use is a marine grade (okoume or merriante) and laminate in place with epoxy. Use screws to temporarily suck everything together so that there is a good bond between the two pieces of ply as well as the inner and outer layer of fiberglass (#10 screws 1 1/4" with fender washers every 12" or so) Actually screw the inner layer of ply to the inner layer of glass. Let the epoxy set, remove screws and repeat the process screwing the outer layer of ply to the inner layer of ply. Continue the stacking process until everything is in place. I wouldn't try to do both layers at the same time. The goal is that during the re-building process, each layer is larger than the previous so that all joints are overlapped. If there are any gaps along any of the cutout seams, pack them with thickened epoxy so there are no voids..
After everything has cured, remove the temp screws and epoxy the outer layer of glass back in place (again, using screws to pull everything together) making sure to get it in the original location. grind the edges of the fiberglass skin back to a taper roughly 4" wide; 2" on each side of the seam) and layup glass matting (1.5oz) over the entire seam (probably 4-5 layers). Sand down with 80 grit so the glass layup is flush (or even a little low) with the rest of the transom. From there, get some gelcoat colormatched (
www.spectrumcolor.com), lay it up and wetsand it smooth and flush. Make the cutout through the new plywood and re-mount the mechanical with a marine grade bedding compound (NOT SILICONE!! I recommend 3M 4200). This is a pretty concise explanation. I've done this many times and it isn't all that difficult, but it does require a bit of fore-thought and planning. Hope this helps, pm me if you have any further Q's.
Good luck!
