Transom Replacement with a plywood core
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Installing the new core: your new plywood core should be the same total thickness as the original plywood. If you are changing anything about the design of the transom (higher engine mounts, enclosed transom, bracket, etc...) you may need to make adjustments to the thickness of the core. Post details on the forum and we can help with this. Most powerboat transoms are made by laminating multiple layers of 1/2" or 3/4" plywood. It is not a problem to make a very thick transom by using epoxy to laminate many layers together, in fact it is quite the opposite.
Plywood selection is another topic covered in other tutorials, please read this for more information
The new core can be either made in one piece, then bonded into the hull, or you may laminate one layer at a time into the hull. It is generally easier to laminate the core outside the boat and bond it into the hull once, but on very large transoms this may not be possible due to weight and difficultly in handling.
To laminate two pieces of plywood, first coat the faces of the plywood that will be glued together with un-thickened (neat) epoxy resin. This is a thin coat of epoxy, just enough to let the face of the wood soak up a little epoxy. While this epoxy is wet, mix a batch of epoxy glue. Epoxy glue is made from same epoxy with wood flour mixed in to reach a "ketchup" consistency . Using a notched spreader, apply even amounts of epoxy glue to each plywood face. Now clamp the two layers together using weights, clamps, or dry wall screws (temporary, removed after glue cures). Do not apply too much pressure! Epoxy is gap filling and extremely strong, you want the epoxy to remain between the plywood layers. Once the epoxy begins to squeeze out the sides evenly it is time to stop the clamping pressure
We have videos and tutorials on mixing epoxy and making epoxy glue here.
In the example boat the customer decided to laminate each layer one by one into the hull. The method is the same as if the laminations were made outside the boat: apply epoxy, then glue, then clamp.
In the picture below you can see the first layer clamped into place (plywood is Meranti BS1088).
After 3 layers total, we have the total core thickness epoxied into place.
The gaps between your new core and the sides/bottom of the hull will need to be filled with epoxy putty. The epoxy putty is made from the very same epoxy and the wood flour glue, but to make the filling and fillet putty you will want to add a little silica to make the putty easier to spread. After filling the gap, using the same putty, you will make a fillet all around the joint between the core and the hull. A fillet is a radius of putty that allows fiberglass to make a turn around an angle.
Below is the core with a fillet of epoxy putty all around (Note: the stringer should have been cut off at least 10" forward of the transom).
The next step is the tabbing of the transom core to the sides and bottom of the hull. The tabbing comes before the wide cloth of the new inside skin. For tabbing we use 12 oz Biaxial fiberglass tape. This tape is stronger than traditional woven tape due in part to its fiber orientation (45/45 degrees). It is also very easy to work with, wets out easily, and is epoxy compatible. We use multiple layers of the 12 oz tape, the amount will depend on the particular boat. We highly recommend doing the fillet and glass tabbing all in one work session, this is called working "wet on wet" and results in a much better lamintion quality with less clean up and no grinding/sanding between layers!
Here is the core tabbed into the hull (again, the stringer should not have been tabbed back in yet as in this picture).
