Well, I had a nice patch of time carved out on Sunday afternoon to do the fix, and then I got a call from my brother that the motor in our co-owned boat had overheated and wouldn't start. Turns out, number 4 cylinder was full of water.
So instead of fixing the tank, I spent the afternoon pulling the head off...
So instead I'll just walk through the planned repair. As a refresher, the tank is made from 3/4 hardwood ply. It is held together with polyurethane glue (i.e. Gorilla Glue), 3" plated deck screws, and on some seams, biscuits. The top is braced with a double-thick 4" wide eurobrace made from the same 3/4 hardwood ply. The crack is essentially a delamination in one of the eurobraces, i.e. the plywood split apart. The crack is in the "back" 6' side of the tank, and runs almost the entire length of the brace in question. The crack is "tight" in the sense that it more or less holds itself closed. But a putty knife can (carefully!) wedge it open maybe a mm or so.
Besides the fact that the aforementioned pinhole allowed water into the brace, I'm sure the splitting was somewhat "helped" by the screws. Even though I predrilled all holes, I'm sure that putting a screw that far into the edge of a narrow strip of plywood isn't exactly a good idea. The drawing should make clear what I'm hinting at here.
Speaking of the drawing, I'm sure you will all be yet again blown away by my MS paint skillz:
This is a cutaway view of the side of the tank and the eurobrace. Text is color coded.
My planned repair is partially under way. I sanded the face of the brace down to bare wood, i.e. removed original epoxy. This was to help the epoxy for the repair bond well. I didn't just want a mechanical lock into the old epoxy, I wanted new penetration into the wood. I also drilled out the pinhole that I suspect caused the problem to ensure there were no contaminants about to get locked away forever.
I have on hand for the repair more of the original epoxy, some powdered thickener (it's a mix from uscomposites but I suspect it's mostly glass microbubbles), and some heavy glass tape - I've had it laying around and don't remember the weight but it's fairly heavy for hobby use so I would probably guess between 5 and 10 ounces. My plan is to wet out the wood, working epoxy into the crack with a brush. Then, I will wet out the glass and apply it to the face of the eurobrace. I sold off my vacuum bagging rig long ago but I have some bagging film left that I will cover it with, holding it tight with tape and clamps, to keep the fabric pressed against the face of the brace. Once this is cured I'll touch it up with more epoxy if it needs it.
Essentially there aren't any inherent forces trying to separate the crack, so the repair probably doesn't really need to be that strong - my biggest worry is just that the section of brace below the crack would completely split away, but it's still very much intact as-is and should get totally locked in place by the proposed repair.
The second "phase" of this rehab effort is to lower the overflow box within the tank. I've already cut it away from the wall (I am EXTREMELY glad that I siliconed an acrylic box into the tank for an overflow, instead of building one permanently into the tank out of wood and epoxy. Cutting it out and getting it all cleaned up only took about an hour). So, I'll just glue it back on an inch and a half or so lower. This will keep the eurobrace completely dry, and dropping the water level that much will cut about 150 lbs of water weight out of the system! The water level will still be about half an inch above the top edge of the glass viewing panes, so I don't expect the perception from viewers to change significantly.
Also, just throwing this tidbit out there. There's a box full of brand spanking new LEDs and optics sitting on my desk at home...
) surges and lowers the overall stress in the tank further yet.
