I wouldnt cut my existing rudders and try to extend them, you are asking for trouble. I've done a fair amount of work with composites and unless you've got some experience, this is going to come out with a less than perfect blade. Plus using wood it asking for trouble, it will rot over time. You will probably get Red oak dowels and they wont last. I would start new, and keep your exising boards in one piece. If you can find some broken ones, or a second set, then experiement on those.

I have a set of daggerboards that I made out of foam/epoxy and carbon that I dont want. They would be perfect for your project. They need some fairing, and you will need to cut our your own profile, but they are plenty long enough. And if you dont like your new boards, you can always go back to the old ones.

Dont cut the existing ones up.

Send me an email if you are interested wjvining at yahoo

Bill