Transom rebuild, what is strong enough?

[gcornelisse]

I posted this earlier today in "Powerboats" which was probably the wrong forum. Not sure there was a way to move it to here, so I reposted.

I've just, hopefully, completed rebuilding the transom in my 1984 Tide Craft Spitfire bass boat. I guess I'm having second thoughts as to whether or not I built it strong enough and so I'm looking for some of your thoughts....

Let me tell you what I've done:
- cut away the inside skin and thoroughly removed every last hint of old wood for the transom and the first 48" of the stringers that supported the transom
- removed as must paint, gel coat, and loose fiberglass from the surrounding areas as reasonable that would later be covered with new fiberglass
- cut a 2 new transom layers and stringers from good quality 3/4" oak plywood. Stringers are 3 layers of 3/4" on one side and 4 layers on the other mostly to fill space.
- treated both pieces of plywood and new plywood stringers with CPES from the Rot Dr.
- sanded everything down with 60 or 80 grit sandpaper and then wiped everything with lacquer thinner
- Using non-blushing epoxy resin (instead of polyester or vinyl ester resin) reinstalled the transom and stringers making sure there were no voids or large bubbles. We did have a little trouble getting the transom to bond flat to the old outer skin. We poured a thin batch of resin between the skin and the transom in several places to make sure everything was solid. We also laminated the two pieces of transom plywood together with one layer of fiberglass as it was being installed in the boat
- For added strength we used 1/4" x 2" galvanized threaded rod to span between the new transom and the sidewalls of the transom area into the old built-up fiberglass. These were installed in holes between the 2 transom layers through to holes in the old walls of the transom area. While fiberglassing in the new transom we put these pins in with a thicker mix of resin.
- covered the outside of the transom skin with 2 layers of fiberglass that came over the top to the inside. And, overlapped the sides of the transom covering at least 6" of old fiberglass
- 2 more layers of fiberglass on the inside coming over the top to the outside alternating with the layers originating from the outside of the transom
- added 2 more layers of fiberglass at the top of the transom covering at least 4-6" inside and outside and overlapping the old fiberglass to the sides
- the stringers were bedded with thicker fiberglass mat to the bottom of the boat and to the transom. I used a slightly thicker mix of resin to fill and hold in the gaps and imperfections between the boat, transom, and layers of the stringers.
- stringers were fiberglassed with 2 layers to cover exposed wood.
- used wide strips of fiberglass covering the connection between the transom and stringers. These strips were also alternated between the layers of fiberglass used to cover the transom.

The boat was originally design to take up to a 90hp outboard. I've only got a 35hp on it now which should be adequate power. I took a lot of old fiberglass out of the boat while taking the old transom out. In some places it was 1/4" or more. I'm suspicious about using that as a gauge for how much fiberglass I need to put back in simply because I can tell that the transom has been replaced once before by someone who took much less care than I have. However, I still want to make sure I've got enough support for the weight of the engine and the wear-n-tear its likely to get through normal use. Do I need to build up more layers of fiberglass for added strength?

Swiss cheese


New transom laminated in place and clamped just enough to hold in place, but not enough to push the resin out


Outside of the transom with 2 layers of fiberglass on top of the original skin and 2 more strips at the top for extra strength that go over the top from outside-to-inside


New stringers fiberglasses in and lightly screwed into the transom with pocket screws...mostly to hold it in place while it sets up. Used wide strips of fiberglass alternating with skins on the transom to span the 90 degree joint between stringer and transom

[dereksireci]

gcornelisse

I had to reply since nobody else has.....

Very neat work.

When you say "covered with fiberglass" it is a very vague statement.
Chopped stand mat, or just call it mat.
Was it cloth, woven roving or a knitted fabric (non-woven)
All these different types of fiberglass have uses for which they are appropriate.
Look at each area and try to imagine the forces acting on it when the boat is in use, or being towed over a rail road crossing on a trailer at high speed when you're finished fishing and want to get home.

The engine will try to force the transom the flex inward and at times outward, at the same time the transom will tend to compress the tops of the stringers. The flexing of the stringers will cause a buckling moment on the sides and want to pull them away from the hull. And so forth.

Orient the fabric to resist these forces. Chopped strand mat is equally strong, (equally weak) in all directions. A woven roving fabric's strength is greatest in the 0 and 90 degree directions and weaker at 45 degrees. Woven roving must be used in conjunction with mat to achieve a proper bond.
The purpose of adding glass to the stringers is to add strength more than to cover exposed wood.

As you can see there is a lot more to it and I've forgotten what your question was in the first place.

The short answer is for a 35hp motor you should be ok. If you used all chopped strand mat fabric, make sure you have life preservers for all on board. Good luck.

If you have further questions, now is the time to register for our "Intro to Composites" class here at NC Martec. We have day and evening sessions.

[gcornelisse]

I used woven fabric for nearly everything. I'm not sure which you call it. It was not sprayed on. I wish I could tell you the weight, but I've had this huge roll for years from another boating project my father was working on. Same sort of stuff I've seen people use with cedar strip canoes. My impression is that it was about the right weight or heavier. I only used chopped strand mat between the stringers and the bottom of the boat in a double layer to even out some of the inconsistencies, but used plenty of epoxy to make sure there were no air bubbles and a little weight to keep the stringer compressed because of the sponginess of the mat.

Except for some of the areas around the bends in the stringers the weave was 0/90 degrees to the major lines of the transom and stringers. I was careful to keep the weave of the fiberglass in good shape, avoiding pulls, while positioning everything.

[gcornelisse]

...and when ever possible I tried to do my next layup within 18 hours of the last. The epoxy I was using had a 24 hour curing time.