My rudders float as well, and I would estimate that they provide slightly less than .5 oz of positive bouancy. While it is a good thing, it is not much of one. I keep track of every ounce that goes into the boat, and you can work up the arm for the mean of the rudder and multiply out the moment that it generates... but I don't think you will be overly impressed. Still.. it is a good thing as opposed to a bad thing, and to those scores I do pay attention and keep count.
Actually I missed the question, and the correct response would be not that they require more weight to ballast down the floating rudder they actually lift up on the stern of the boat. This changes the pitch of the boat (angle of the longitudinal axis relative to level) in a measureable amount, but overall buoyancy in a miniscule amount... similar to the fat kid and skinny kid on the teeter-totter.
"Give me a lever large enough, and I shall lift the Earth"
The arm is more crucial here than the amount of lift... but again we are dealing in tiny amounts (micro-knots). I believe in the doctrine of "no stone left unturned" but.... I would rather practice with an average boat everyday for 2 weeks before an event, than go their with 2 days practice and the fastest boat.[B)]