The blades of fixed rudders appear to be thicker in section than the lifting types, possibly simply because available rudder stocks limit thickness.
A thin section creates less drag when travelling parallel to the water but when the rudder is angled to steer I am told the thicker sections are less prone to stall.  (www.philsfoils.com is interesting).
But what happens in practice?   Is there a speed advantage in using a thick (12% or more?) fixed rudder, assuming passage between launching and getting the rudder on goes without calamity?   Does a thin section stall, or start to so that excess helm/handbrake is being applied around the course?

Nationa 12's experimented with rudders that give positive bouyancy, these were quite big in section. I think they did increase drag a little. It should not be necessary to use too much rudder so stalling shouldn't be a problem, if you are stalling the rudder chances are you are using too much. I know I often use more rudder than I should when frstration at the slowness to tack a modern Merlin but even then the rudder doesn't seem to stall. I would go for the thiner section. There has been some research on this subject, it might be worth trying to find a white paper. Dave Greening (designer, X-Merlin man and N12's) has a lot of knowledge on this. Maybe a post to him via the 12 site would get some useful info?

Try C A "Tony" Marchaj's book "Aero and Hydrodynamics of Sailing", which has a lot on effectiveness of thick v. thin foils. In essence, I think he says you can use thicker foils up to 18-20% t/c ratio, BUT they have to have squared off trailing edges, rather than ones that taper to a knife edge. NB, this is the second of Marchaj's books, published about 1976/7 I think, not the first-titles are similair, I may have quoted the title of the first one.

One thing i do know for absolute certain no matter what the scientists say a razor sharp trailing edge is far more efective than a squared off one no matter how sharp the edges, hard to maintain but it just works better in any class in any weather. As to thickness the plane designers who spend far more on research than the boat an ship chaps say 12% of the cord + a sharp trailin edge and I've just been looking at an MD11 wing admitedly from inside the plane!

True for a plane, but it's hardly likely a sailing dinghy rudder is operating at the same Reynolds number as a jumbo's wings!? Also, when plane wings are called on to operate at high angles of attack, they suddenly grow flaps, fences, slats etc, at which time they most certainly don't have a t/c ratio of 12%. However, how much time does a rudder really spend at a high AoA? as a % of yer typical race, < 1%? In which case you get more benefit from low drag, rather than high stall resistance. In which case thin is best, Go thick to win the start in a match race?

Go thick if you need think you may need to be able to steer when whilst planing down a windy run your crew who was sitting opposite you suddenly appears still attached to boat vertically above you !!