SYP works
Breadman,
go for it! I have used Southern Yellow Pine to make rudders for a couple of different boats. Like you, I intended to make them as prototypes and make the final product out of a better wood, but after I got them finished, with a coat of epoxy and then paint, I was happy with them and never did. They have held up great; one is going on five years old.
Good Luck,
Bert