The dowel rods have virtually NO support in the end grain, the screws pull out very easily. There is another option for the turn fin support. When building the sponson, insert a section of 3/4" OAK block to the inside of the sponson at the turn fin area, longer than the length of the screw holes. Using the inside sponson plate as a pattern, drill the fin holes through the wood. Insert blind nuts (use 8/32) on the back side of the oak block. You now have bolts to use to secure the fin to the sponson and they are pulling into the wood and not out if it. It will hold the fin in place.
David Preusse and Mike Hughes makes 45 turn fins for the JAE.
If you want the brass tubes to stay in place, insert some oak blocks where the tubes go through the sponsons. Redrill the holes using a drill press. The tubes will stay in place when glued to the oak blocks.