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hey everyone i'm building a fiberglass over plywood on douglas fir cabin cruiser. The original boat was 19' in length, but the new one will be 21' to allow room for a small head. Because of the extra length, i'm beefing up the keel from two inch by four and a half inch to two and a half inch by 5 inch. The lower stringers (nearest the keel) are also getting beefed up as the engine is mounted onto them, and bigger transom knees. I'm using the original 4 cylinder diesel engine and sterndrive. The original transom is made from plywood, a 10mm outer skin with two layers of 3/4" laminated inside. These inner layers only extend up a couple of inches above the stern drive. From there is only a flimsy frame screwed and glued to the 10mm outer skin for the sheer clamps and upper knees. Initially I was going to laminate three layers of 3/4" plywood together the full size of the transom. However this will add a lot of weight to the stern so i'm looking for other methods of constructing a transom strong enough but lighter? any suggestion or examples are welcome. cheers

I agree that adding excess weight at the transom is usually not a good idea - particularly a boat that operates at planing speeds. If it were me I would stick with the two layers of 3/4". If I was concerned about strength I would add a "beam" at the top of the double layers. This could be something like a solid 2" x 4" set on edge running the width of the transom. It would make the transom more rigid but not add a lot of weight. The drawing below might better explain what I mean. It is a side view with the red square being the beam.



Or even a 6" wide piece of 3/4" plywood attached to the top of the double layers like the drawing below would help add stiffness.

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So Many Rivers,
So Little Time....

cheers raymacke, that sounds like a good idea. I'll need to add some sort of framing to the upper single plywood skin for the sheerclamps to grab as well i think.