So, I made 5 fins for a reason. I wanted to make absolutely sure my balsa core strategy would be fine on one of them before cutting a whole set. If it wasn't, then I had a matching spare to step in. From my larger test fin, back on page 2 of this thread, I got the impression that I left more ply than I had too. i.e. I don't need a 1" frame all the way around.
The first step was to start out with a fresh 3/8" fin, and skeletonize it. Areas that are part of the tab have a 1/2" frame, while those out in the air have at least 3/4". This was done by drawing out the area, drilling holes at each turn to help out the scroll saw, and cutting freehand. The cuts are by no means perfect, but the area will be filled in with balsa anyway, so minor deviations are ok.
Starting Weight: 377g
Plywood Skeleton: 154g
The core was then marked on end grain balsa using the fin, and cut out with a cheap hook nosed kitchen knife. A little trimming and everything fit fine.
A problem I had with my test fin was that the balsa is slightly thicker than the ply, so I had a slight raise the entire way around the core. Not terrible, but more trouble than it is worth, so I decided to epoxy in the balsa this time, so I could sand it flush before laminating. I did this with laminating epoxy, taking care to wet between the individual blocks that make up the balsa sheet, so I would be left with a fully stable balsa sheet. You can see I chose to make the side with the scrim stand out, so that could be sanded off. The entire assembly dried overnight between two sheets of parchment with about 5 lb weight to ensure it remained flat. Later, the balsa was made flush with the ply in about 5 minutes with a palm sander (not pictured).
Fin with balsa core: 206g
Obviously, these are flimsy now, but two layers of CF cloth (3K twill, 196 GSM, layers 45 degrees rotated) on each side make these exceptionally strong. The CF was laid out and taped off for cutting with an Olaf Roller Cutter. A total of 16 piece of CF (8 in each orientation) were cut. Along with 8 layers of release film and 8 layers of breather material. I have been getting repeated uses out of the same vacuum bag. I simply cut off the tape each time, so it shrinks about 3/4" with each use, but otherwise going strong. I'm using US Composites 635 with medium hardener for the laminating, and I've found 225 mbar to be a good pressure to pull the bag down to, which gives a nicely laminated part that does not appear epoxy starved or rich.
Prep is everything with making composites. Once the clock starts it better go smoothly, so everything gets laid out as it will go on, and I do a dry run to make sure I have everything I need and my timing is good. I have found that supporting the parts on a piece of hard board (and having a second piece for flips) covered in parchment paper is the best way to flip and move parts once the epoxy is applied, and the layers go on.
Once all the layers are on it is into the bag. You can see in the pic below I do one side entirely, including the release film and breather, then flip the entire stack and do the other. As you can see by the timer I got it done with 5 min to spare (started on 20).
After curing in the bag 24 hours (I decide how long by smearing a thin layer of epoxy on the parchment paper and letting that cure to the desired point) I removed the part from the bag, and immediately trimmed most of the excess fabric off with several passes from a utility knife. Do the trimming at this point, as it gets much harder after 48 hrs.
The rough fin has a few small wrinkles (probably because I didn't make relief cuts at the sharp edges in the CF), but is perfectly useable, and exceptionally strong. I'm not going to, but I'm sure I could stand on this one like I did the test fin with no issues.
Rough final fin: 299g
So the total weight savings from wood fin to final composite fin is 78 g. That is about 0.7 lbs across the fin set, but I believe these are stronger, and I would have laminated the ply anyway, so I think the real weight savings is around 171 g each (Fin with balsa core versus ply fin) as the lamination would have added about the same amount to both. That makes the savings 1.5 lbs across the set. And since the V2 requires an almost corresponding addition of nose weight that is more like 2.5-3 lbs savings overall. Much worth the effort I think.