Starcraft SF14LW with perforated transom.

[boatboss]

I bought a clean looking Starcraft aluminum boat a few months ago with a four stroke Mercury motor on the back, after a few times out I noticed water leaking in from behind the transom panel when I slowed the boat.
Further investigation after removing the motor, corner caps, inner aluminum and wood shows the inside of the transom is severely corroded, only where the wood touched the aluminum.
The wood is like new, no rot, no damage at all. I can put my finger through the aluminum all along the bottom edge where the wood sat. Right now, after pulling the wood away, I've got holes that are 6 inches long and an inch high, and I suppose I could poke my finger through all the way across that line side to side. The rest of the hull is shiny, clean and like new. Its never seen salt as far as I know, its powered by a manual start motor with no charging system so I don't think its an electrolosys issue, and the wood doesn't appear to be pressure treated.

I talked to a local welder and they won't touch it, he said none of the transom is solid enough to weld on.

I'm thinking of taking a sheet of .040 aluminum about 4" larger than the wood panel, and riveting it all the way around with some metal adhesive in between.
The top rail of the transom is fine, as is the top 3" or so of the metal, the main damage is along the bottom edge of the wood, roughly the lower 2" of where the wood sat against the outer panel. The inner panel is rotted too but that's easy to just replace. A buddy said I should just grind and sand the holes and epoxy some back plates in place and then use marine filler to fill the exterior damage for appearance.
I'm leaning towards all new metal but I can't weld aluminum, or at least I'm not that good at the tig welder to do something that counts like the back of my boat.
I can bang rivets though.
I need to figure out how to fix this and then how to prevent it in the future.
Since the wood is fine, I'd be putting it back in, but I'd lean toward sealing it or putting some sort of plastic barrier between it and the aluminum.

 

[sphelps]

I'm not a tin boat guy but it sure sounds like it had p/t ply in there .. I would make double sure before ya put the old stuff back in.. I'm sure the Starmada crew will chime in shortly with some good advice ..

 

[Tnstratofam]

Sorry to hear you're transom is compromised. My first guess would be that if the wood in there now isn't pressure treated, it had pressure treated wood in there at some poit in its past. The piece of metal the transom sits on at the bottom is called the z brace. If it is damaged beyond repair you should be able to get a good machine shop or sheet metal shop to fab you a new one. As for repairing the outer transom skin pictures of the damaged area would help in determining how best to repair it. I believe that putting a patch over the outside buttered with 3m 5200 and rivited in place would be the right approach, but if the bad place is parallel to the knee brace and z brace the patch may have to be oversized to compensate for the weak transom skin.

Even if your transom wood looks good if there is any rot in it at all you should go ahead and replace it while you have it out. You can use a good quality exterior grade plywood, and seal it with one of several good methods to preserve the wood for years and years. First let's see some pictures of the transom skin so we can best decide on how to proceed.

 

[jbcurt00]

Post some pix of the transom, the SC crew will help you decide what to do about fixing/repairing/replacing the transom skin.

 

[boatboss]

The 'Z' brace doesn't appear damaged at all, only the transom skin along that brace. There is no vertical line of corrosion, only lateral mainly along what was the bottom 4" of the transom wood. There's some pitting everywhere the wood touched the aluminum but the area of concern is down low. There are fist size areas that are paper thin or falling apart. The wood looked original, I had to drill out a hand full of bucked rivets to remove the inner panel and all the 'nails' that held the transom cap in place looked original. The transom panel is un painted, un coated in anyway, it looks like fir plywood but it doesn't look like two sheets of 3/4" ply laminated together. The wood is tan or brown, not green or dark brown as most P/T ply would be. The boat came from western PA, so its unlikely its been in saltwater and there's no sign of it ever having any sort of electrical system that would contribute to corrosion.

I've ruled out bolting on a patch, there's no doubt that there's little left to bolt too and a patch large enough to cover the affected areas wouldn't have enough compression force in the middle to be effective strength wise.
If I could buy a new transom panel, I'd gladly do so and then attack the job of replacing the whole rear panel but the task of reforming an entire transom by hand is nearly impossible since I would assume the original was a stamped form. I also thought about having a whole transom panel welded in place but I've spoken to four welders, two who specialize in boats, and all have said that they feel the metal wouldn't weld well and would likely warp severely. There's also the issue of melting the sealer in the double rivet seam around the edge if welding in a full panel.

The transom holds the motors well, there's no sign of structural movement yet.
I'm still leaning towards bonding a new aluminum skin on the inside which would be the entire size of the wood panel. I'd then fill the damaged outer skin with JB weld and marine filler and repaint. The wood would be encapsulated in epoxy with a barrier layer of plastic sheeting between it and all metal.

Here's a pic before I took it apart. The holes you see here got larger once the wood was out, the damage is larger inside and along the entire line along the rivets that hold the Z brace. The damage does not extend to the Z brace rivets at all, it stops where the wood contact ends. The pain on the boat appears original, the paint pattern is identical to one a friend of mine has, even down to the masking lines along the transom cap, he bought his new.
The rest of this boat looks new, no signs of corrosion anywhere else.
Without any doubt, I could poke my finger through any one of the holes and rip out all the metal along the lower rivet line without any resistance on each side.
The corrosion line stops at the knee brace, there's no corrosion on the brace, and none along the vertical line where the brace sits. Both sides of the transom are identical, the corrosion is in the same place and roughly the same size areas are affected.

 

[boatboss]

I found a cure for the rotted transom today. A buddy had a bucket of pourable transom filler and a bucket of chopped glass strands left over from a boat he did last fall. I soda blasted the inside of both aluminum panels, then reassembled the transom parts on this without the wood in place, I made up a few spacers out of junk fiberglass bits from an old boat, taped up all the perforations and open corners, then mixed and poured in the filler. Once it hardened, I used some marine feather filler on the outside panel to fill any remaining voids showing, primed and repainted the transom. The way I see it, is that the filler is bonded to the transom panels, thus reinforcing them, and the filler will never rot and shouldn't attack the aluminum. The worst part was getting the cavity sealed up before pouring the filler. It took lots of speed tape and duct tape, plus some pre sealing of the lower seams with some strait resin and a paint brush.
The best part is that the repair didn't cost anything and the boat is ready for the water now and I feel confident that its safe and that I shouldn't have to worry about the transom falling apart down the road.