A doubler is a piece of material laminated to another piece of material to increase the strength of the second piece. Let me give you an example of how this works, using your boat hull and a 1" ball bearing:
- You glue the ball bearing on a table top.
- After the glue dries, place the rear(transom) of your boat on the ball bearing and push against the bearing with the force being applied to the boat's nose. If you look at the transom, from above, you will see that it is pushed inward with the furthest place in being where the ball is in contact with the transom and the amount of deflection decreases as you move toward the sides, bottom and deck
- Now, take the boat off of the bearing and epoxy a piece of ply onto the inside of the transom. We're not talking running the length of the boat, only along the inside of the transom. After the epoxy has had a full day or two to cure, we repeat the process from the previous step.
- When pushing down on the boat's nose, the transom will be stiffer, not allowing the inward deflection. In fact, the only way the transom will bend is if the plywood, epoxy or CF fails.
The same thing is what is happening with the engine mounted on the transom. The weight of the engine pulls at the top of the transom, trying to get it to move outward, along with pulling down on the screws holding the engine in place. As the boat is running, the thrust from the prop is trying to push the bottom of the transom in. When you are starting your boat, the starter is placed on the starter cone at the top on the engine, forcing the engine against the screw hole lower side in the transom. All of these forces will cause the CF to fail in a short period of time because all of the force is at the screw hole locations, through the fairly thin CF. The addition of a plywood re-enforcement will take a lot of the load and spread it over a much greater area, reducing the stress on the CF at the screw locations.
Hope that makes sense