Three good non-wood options...
- Coosa Bluewater
- 1-1/2" schedule 40 square aluminum tubing stacked
- Pre-made aluminum transom from TBN - TBnation Aluminum Transoms
- SeaCast molded and installed as one piece. www.transomrepair.net
1. Coosa cuts and fits pretty much like wood, so it's the easy choice, and it will never rot, obviously. Costs a couple hundred bucks, last time I bought some.
2. The stacked aluminum USED to be a cheap, better way to do it, but now aluminum is $$$, so not sure the current cost. Depends on where you buy. Pretty easy to do, though. Cut them to size, lay on a flat surface and weld them together and sand flat. Requires a MIG or TIG welder with Argon gas shielding, or just take it to a shop, which should be pretty cheap. My last boat had this, and it was absolutely flawless. I miss that boat! Mine was raised to a 20" transom. Ground the top weld flat, then gave up and left the ugly beads. Added a solid transom plate, and it was awesome. Super strong, and lighter than the wood.
3. You can also buy a pre-made one from TinyBoatNation. It looks like they run $500-$700
4. SeaCast is a 3rd option. I have used a lot of SeaCast over the years, and it's an excellent product. It is about the same density as wood, (floats in water) but is super-duper-tough, extremely strong. It is about $250/ 5 gal. bucket, and it looks like my old boat would have used 2 buckets, so about the same cost as aluminum:
With the Seacast, you need to make a mold of your transom shape, mix and pour the transom, strike it flat, and then install as usual. It's really awesome stuff.
I had all of these options available when I did my last rebuild last year. I ended up using doubled plywood, sealed well. I probably won't have this boat more than 20 years, so I am not worried about ever having to replace it. I surely sealed mine better than the factory, so it will probably last even longer.
If I had a lifetime boat, I would probably go with the aluminum tube transom. I love that it's so light, keeping my transom as light as possible.