BOAT REPAIR

if you have the tools, time and know how, go ahead and do it. it's labor intensive tho.  buddy and i replaced the transom on my 18 ft. aluminum (grumman)  3 years ago.  drilled out all the rivets and had to remove more of the back of the boat than we originally thought in order to pull the old, decayed transom out.  seemed we were constantly moving 3 steps forward and 1 back as a new problem popped up daily.  took us about a week, but we got it done.  if theres anything left of the old transom when u pull it out lay it on a large piece of cardboard and make a template and cut your new one from that.  mine was so rotted we had to drop the cardboard into the boat and shape the template from there. we constructed the new one out of marine grade ply wood (3 pieces) and glued them together with if i remember correctly a product called west coat system. also coated the entire transom with this as it dried to a thick rubber coating around the entire transom. sealed the boat back up with stainless nuts, bolts and lock washers and plenty of silicon caulking. i was told afterwards on this site that i shouldn't have use the stainless that it would react with the aluminum of the boat and cause little rust spots all over.  still not seeing this and its been 3 years.  i posted alot of questions on here when i was doing this and got a ton of replies along with names of people that will do it. theres a pour in transom out there too, but that was pretty expensive if i remember. good luck. if you have questions, pm me, and if i can't answer i'm sure my buddy can.

Did my dad's boat a few years ago. We used the SeaCast for the new transom. It is a little pricey but does a really nice job. Getting the old wood out is a PIA but after researching quite a bit I used an electric chain saw to chew most of it out. What ever you use to replace it I would recommend 3M 5200 rather than any other type of calking. Definitely DO NOT use regular treated plywood, it is treated with a copper solution and you will have a dissimilar metal reaction that will eat little holes in your aluminum. Check out www.jamestowndistributors.com and www.transomrepair.net  

My dad's boat is a Sea Nymph GLS195 and it has an inner and outer skin on the transom. I think most of the larger aluminums have both. You can also build the inner skin if necessary. I was happy with the end result...

I know that my Starcraft 22' CC doesn't have an inner skin.
 
Do yourself a favor, use the Starboard(or similar UHMW).  I replaced the transom on my old 16' fiberglass in 2000 and thought I did a bang up job.  When I cut it up for scrap two years ago the wood was already starting to get soft.  Not anywhere near rotten, but maybe another 5-10 years and it would have been gone again.  Won't ever replace a floor or transom with wood again.

bulldog1 
Either way replacing a transom is a big job with lots of pain involved. Good luck to the OP...

 
Very true. If you can get a decent turn around on the job just take it somewhere.  We re did the transom on a sea nymph 195 gls a few years back only because the shop was backed up 3 months and didnt want to miss most of the summer. I think we were quoted around 1500 for the job.  We asked the shop what we needed to do to replace it and they helped us out. We ended up using marine grade plywood and coating it with fiberglass, then when we put it in we added sealant to every screw/bolt and around the top.

CRAPPIE_SLAYER

+1 on using 3m 5200 for sealing everything.  That is some great stuff.  Like eye crosser said, check out tinboats.  Lots of info and very helpful people.

 
For sealing, I prefer 4200. 4200 and 5200 have the exact same sealing properties, 5200 is a much more powerful adhesive however. If you ever want to get something apart again, use 4200 rather than 5200. If you need a powerful adhesive and sealant, 5200 is the way to go. If you just need a sealant, then opt for 4200.