My stringers had rot in them about 1.5 feet from the transom. From what I could tell, Starcraft hadn't sealed the wood from the inside (leaving the internal parts of the stringer exposed to moisture) and had left air gaps at the bottom of most of the boards. I don't believe the stringers were bedded in resin, just set in place and glassed down. They also drilled a hole in the side of the stringer, presumably to allow air to dry out the stringer, but since they'd not sealed the bottom or sides of the wood, when water entered it just sat in there, traveling down the length of the stringer along the bottom, rotting it from the bottom up. I'll be fixing this problem with my repair.
A couple points about stringers:
The top of each battery tray and some of the side supports were rotten as well. Those are less of a problem to fix. I'm not sure why they rotted, it could have something to do with the unsealed, through-hull holes in the transom that the manufacturer "installed"....sigh.
Luckily the rot hadn't traveled too far along the stringer, and from that 1.5 foot point to the bow they appeared to be solid. To fix them I started by cutting the boards back past the rotten sections until I was at least 3 inches into clean, dry wood. I cut them at a 45 degree angle, one at a time. I started with the port side stringer:
Once I had the wood ground to about a 35 degree angle, I fitted new boards in place of the pieces I'd cut out. My stringers were made using 2x4s and the battery tray supports were made from 2x8s, the rest was plywood. Construction and fitting went very quickly. I added an additional vertical support inside the stringer underneath the engine mount right where the old and new wood meet, just to be safe.
Here is a picture of the fully assembled stringer, before I had epoxied and glassed it all together. The engine mount (dark brown wood sitting on top of the stringer box) doesn't come up to the proper level of the old wood as I haven't included the glass mat between the layers yet.
This picture shows the stringer after I had coated the wood with epoxy, fiberglassed it to the hull internally, screwed it together with stainless steel wood screws and bedded all the wood in thickened epoxy.
Once the stringer was constructed and cured, I went to work on the foam compartments and battery trays. I cut the rotten parts of the trays off, leaving any good wood that made sense to leave in. Then I cut new boards to fit, ground them until the fit was nice and snug, then bedded everything in epoxy. I used stainless steel screws to hold it all together.
To attach it to the hull, I thickened some epoxy resin and ran 2 strips of fiberglass mat along the sides and tabbed the trays into the transom and hull sides. Easy enough, I was just copying what the factory had done. Lastly I drilled 1" holes in the battery tray tops and poured floatation foam into the compartments and let it expand. Once it was done expanding I cut the foam that'd popped up through the holes off flush at the surface of the tray, and it was done for the night. Here's a picture from when things were still curing:
Here's a shot of the starboard side. If you plan your cuts right, getting everything out only takes about 45 minutes! What a difference a little experience makes.
Willing and supportive helpers are always nice to have around.
Here's a picture of the starboard side all done. This one went really fast as I didn't patch anything together. If I were to do the port side over, I'd tear it out completely. They're both strong, but patching takes much more time.
Yep, that's me standing on the tray. It didn't even move. Yay!
One disappointing thing I found on the starboard side was a 2 foot square section of rotten floor, mostly contained under the rear bench seat. I was able to salvage enough of it to patch it into my lower battery tray/rear floor. I'll be pulling the floor up next year to address stringers and whatever else needs attention, so I chose to leave the floor as-is for now. The alternative is to rip it all out now, but I plan to use this boat this year and don't want to take another 2 months out of this season. Spring is here now, hot summer days will be here very quickly.
Another point to make: In the picture above of my "helper" you can see a big sheet of foam insulation standing against the wall. After using pour-in foam insulation on the port side I was fairly well disappointed in it's cost vs. effectiveness department. Foam's primary purpose is water displacement to slow or prevent the total sinking of your craft, and pink foam insulation is (by many people's standard on http://forums.iboats.com ) as good, and the price is easily 1/6 that of the same amount of pour-in foam. If you need the pour-in foam for structure, by all means, use that. I didn't need it for structure, so I used the pink stuff on the starboard side.
It was at this point in the project that I realized I couldn't afford to purchase West System epoxy any more. It's an excellent product, but the price is prohibitively high in my opinion. I chose to purchase 2 gallons of US Composites epoxy instead. This epoxy is harder to work with than West System. The hardener is very thick (like dry honey) and needs to be heated if its below 70 degrees or it won't come out the pump. The resin is about the same. When it dries, its snot green, but in my experiences, just as hard when cured and about the same effort to spread.
For the final fiberglass layup I pre-cut pieces of biaxial or "1708" cloth. When pre-cutting make sure you don't have big air gaps. Cut things appropriately so you maintain as few pieces as possible and are touching the structure everywhere. If you need to start a new piece, overlay it with the one next to it by at least 3 inches.
The final step in repairing the stringers was to mix up a big batch of epoxy and finish laying up all the biaxial cloth on both sides, then hit everything with a final brushed-on coat of epoxy to seal it up. It's tough to see here, but the biaxial is now stuck and coated (and fully wet out, so it becomes mostly transparent).
On to the next step: Painting the Bilge