The white material you see on these parts here is not soft silicone. There’s no reason to use anything like that on a build like this. these cedar strips are glued together with regular wood glue, then the hull and the deck get glassed inside and out. so what you see here is the inside or under side of the deck that’s been glassed. The cross pieces are made out of ash, glued in place with thickened epoxy over the glass and then I put the fillet all around it. I use the west systems 403 filler which is milled cotton and the 406 colloidal silica. I use a little 403 and mostly 406.
If you did a test, if you made a solid piece of just epoxy and did a hammer test, it would break really easy I think. When you mix anything with it as a filler, it makes it really strong. I don’t know if just colloidal silica is stronger by itself, maybe.
If you called west systems they would be glad to tell you about this in detail because I’m sure they have done tests with the different fillers. Also there is this forum of builders that know a lot about using epoxy.