The knee brace is the first place I would look for leaks. The part that attaches to the hull has 6 or 8 rivets. Those are the ones that take all the stress. Over time they can work themselves lose and cause a leak. To tighten you need to have someone hold a heavy metal object on the outside of the boat on the rivet you are fixing. A sledgehammer works. You then hit the rivet from the inside to reseat the rivet.
Another place to check for leaks is the keel at the front of the boat. Beaching the boat over time has a way of wearing the keel down and putting a hole in it. I found a hole that the PO (previous owner) tried to fix with silicone. He of course did not know how to fix the problem and it continued to leak until I used a piece of aluminum and riveted it to the keel. The patch I put on was backed by 3m5200 and has not leaked since the repair. This repair has worked for me but others may not like the way it looks on the outside of the hull. To each his own. Mine is a fishing boat and have not heard the fish complain yet. ther place that I found a leak on my boat was on the hull where it sat on the bunks of my trailer. Corrosion from the saltwater usage had made several pinholes. My repair was to take another piece of aluminum backed with 3m5200 and pop rivet it to the hull. I would prefer to put the patch on the inside but again not going to happen until I replace the deck.
Best to check to see if your transom is bad, if so the flexing will cause the rivets to loosen back up. If you find the transom is bad, then when you replace the transom you will want to replace all the rivets that hold the knee brace at that time. Your knee brace may also be in bad shape with corrosion or it may have cracks. This would be the time to repair and strength up the knee brace as well. I did see a post on this website of a repair to a knee brace and the guy used stainless steel machine screws with 3m5200 to replace the rivets. It looked like a great solution to his problem.
What do I do on my boat? I check for leaky rivets once or twice a season. I test by putting a foot of water in the boat and crawl under it with a felt pen and mark all the leaks. I then remove the water and drill out all the ones that I have marked. I then take pop rivets that are made to seal and goop them with 3m5200 and install them. This is a temporary fix as the correct way to fix them is to remove the deck (floor) and replace them with solid rivets and gluvit. If and when I do replace the floor I will be fixing it the that way. I will use an air tool and a bucking bar to make the repairs.
How do I figure out what to do to fix a problem on my boat? I go to this site and use the search function and plug in my search terms and read the results. Most of the questions have been asked before and a quick search sometimes answers your question faster and the explanation is more detailed than what I gave you on my pervious post.
One last thought as a reread your post. You mention that you thought that the leak could be from the seam that is between the transom and the sidewall. That seam has a double row of rivets and that one would be the last place that I would suspect a leak. I could be wrong but water in the hull test would remove all doubt.
Hope this all helps you with you boat and yes wait for warmer weather.
GLG
.
