I have been calling around the tri-county area getting costs on marine ply to repair my rotted floor on my 17'. I have checked out alternatives to the marine ply with the hopes of saving a a few dollars. There are an amazing amount of different forums accessible via the Internet where the say one thing and the next forum says the complete opposite. I have come to the conclusion that to be "penny wise, is to be pound foolish" as the saying goes. I found a place called NAUTICAL LUMBER in Warren as a source. He is $3.00 higher per sheet than John's or Consumer's, but he is local and does nothing but sell marine stuff. and I don't have to drive to BFE to get it. As for the additonal $3.00, I figure if I pick his brain for knowledge of what I need to do and how to to it properly, it will be worth the extra few dollars. I can probably buy all the epoxy and glass cloth there as well. I did not include prices or more detail because I didn't want this post flagged by the SPAM POLICE. 

LOL aty the spam police. I think they are taking a break from this site because they are upset with the shoutbox...

All I can tell you is to be carefull taking the old floor out so you can use it as a template. I got into replacing the front deck of an old wood Thompson thanks to my buddies I fished at the time, luckily they didn't ruin the old deck the worst part was making a new rub rail out of full 1" x 3" white oak. I had to use the old deck and make a steamer to bend and hold the oak until it dried but the worst part was cutting the angle of the hull o it would fit properly but thanks to my old bench saw and a good carbide blade and some major thinking it worked out great. What you are doing should be fairly easy good luck.

The floor was so wet that 95% of it could be pulled out by hand as it was deteriorated to strands and clumps the at one time resembled plywood. Nothing came out that could conceivebly be measured except fo a small piece of plywood that let the main bilge empty into the well with the bilge pump. The only thing that stopped someone from falling through the floor was the fiberglassing some one did. Now see that a major portion of the portside interior transom is rotted also. It looks like this was done once by a previous owner who did not plan on a long term proper fix. I purchased the boat last fall from my Uncle who bought it many years ago from a friend of his. The rot is worse than I thought it was going to be, but then I never really inspected it previous to buying it. He said whatever it cost to fix deduct it from what I am paying him. I am certain the rot is more than he expected so I won't hit him with the full cost. He did give me an good price to start with. I have two other boats so I won't miss fishing this spring, if it takes me a while to do it. I want it done right. Thanks for your input.

Use a big piece of say a refrigerator box cardboard to make a templete to trace on the wood.

h2o

I tried the big piece of cardboard Greg, and it worked like a charm. First, I traced the transom on it from the outside, then I cut it slightly smaller. After  several minutes of trim and fit from the inside, I had a great template. Best of all it was foldable. My son and I traced the template on a 7/8" 4' x8' piece of marine ply and started cutting. We managed to get two transom layers out of one 4' x 8', The new transom will consist of several fitted pieces but as long as I epoxy resin them and keep the seams at least 6" apart on the two layers, I should have no problem. That and making certain there is not a gap between the innermost layer and the actual fiberglass part of the transom should be plenty. After that it is on to stringer repair  / replacement and then the new deck. I have cut the old stringers back about 3-4' from where they originally ended and still have signs of wood rot. Close to the transom all I had was the roving. The wood had disintegrated. It is a bigger project than originally planned for but it is going nicely. If it wasn't for the rain and my gimpy knees, I would be closer to completion. It really brings meaning to the phrase" If you don't have time to do it right the first time, how are you going to find the time to do it again" ? Any volunteers to help on the stringer replacement? It is very critical to use the same type of wood as the original and keep them level. A cardboard template 12' long will help with the correct contours. A 4' level, some epoxy, stringer to hull adhesive and glass cloth should do the trick. Epoxy thickened to the consistency of peanut butter will make a good stringer seat. Some compartments between the stringers and two part foam will give the boat some needed buoyancy.

Sir, you know I'd be the first one to volunteer to tinker with the boat, but my back is in just about of a bad shape as your knees. After all that physical therapy didn't do squat, i can hardly lift my youngest son, so please, don't hold the lack of volunteering against me here