My J-24-sized 2 x 10 emergency rudder is intended to steer a Santa Cruz 27. It actually managed to steer my Santana3030 OK in 15 knots of breeze, if I tucked in a reef. I have no doubt that it will handle the Santa Cruz just fine, since it's easily 3/4 the chord and depth (though it doesn't go as far down in the water) as the stock rudder.
I must say that I'd have significant reservations about using it on a Valiant 40!
I strongly, *strongly* recommend going to the emergency steering seminar that the SSS will put on, and seeing what some of the larger boats have for their systems.
Another thing to keep in mind is that one of the likely scenarios that would cause an emergency rudder to be needed, is smacking into something and bending the rudder/rudder post and jamming the whole thing off to one side. This, of course, will steer the boat in circles and you will go nowhere. So I always spend some time thinking about how I might get rid of the rudder if I had to. I carry a piece of 2 x 4 long enough, and cut to the right dimensions so that I can ram it down the rudder tube and shove the rudder and rudder post out..
Also, it's important to think on how you'll set up a self-steering system with your emergency rudder. For me, I will lash a 2 x 4 to the stern pulpit, with the piece of wood lying on deck, just forward of the lazarette hatch. The wood is drilled in the correct location so that an autohelm tiller pilot goes in the right place, and I'm on my way....
1968 Selmer Series 9 B-flat and A clarinets
1962 Buesher "Aristocrat" tenor saxophone
Piper One Design 24, Hull #35; "Alpha"