Any time you have a wing that generates lift you have higher pressure on one surface than on the other. At the tips, this pressure differential is trying to equalize by way of the air under the "wing" trying to rush around the tip to the top. This is largly responsible for what are called "wing tip vortices" (sp?). A wing tip vortex is a cylinder of rotating air at the tip of each wing (you can see this when a plane flies low over the water or dusty ground). The energy it takes to disturb this air is lost in the form of additional drag. I don't know how significant this is for turbine blades.
The same thing happens with sailboat keels. The keel has to be extra deep because the last foot or two is just there to keep the water from running around the end and provides little side force.
Some boats, intended for trailiering or "gunkholeing" (sailing into shallow water), have a much shorter keel, what is called a "wing keel" (as opposed to the normal "fin keel" or a "bulb keel). This ends flat and has a small section of airfoil (hydrofoil, actually) mounted crosswise, oriented with the more covex lifting surface toward the rest of the keel. The crosswise section keeps the water from running around, so you don't need those extra couple feet, and the airfoil helps to keep the water flow on the blade side rather than running under, too. (The body of the keel has a symmetrical airfoil shape, too, and the lift from that, when there's current from the side, improves the boat's resistance to side-slip.) A downside is that if the boat is listing, more of the keel is shadowed by the the hull and foreshortened, so you don't want to let it go over too far or you sideslip and lose way.
The similarity to a mill's prop should be obvious. The body of the prop is an airfoil, but the end is largely along for the ride due to the air running around it rather than being deflected. You should be able to pull the same hack and get more power from a given diameter.
It's been a while since I saw the Catalina's keel. But if I recall correctly the wing was a bit narrower than the keel was long. (I HAVE seen some commercial mills with a similar wing on the end. Rather small, and I think they also had it overhanging the trailing(?) edge by about the width of the blade.)
I'd probably let 'em overhang a bit in both directions to make forming the leading and trailing edges easier, and make it airfoil shaped but thin, to keep the drag down. It doesn't contribute any forward force directly, though directing more of the wind into the blade makes the blade ends work better.
Anyhoo, if I make a prop mill I may try this for the end rather than worrying about rounding, or what-have-you. If you try it let me know how it works. (I'd also screw 'em on and try it with 'em on and off to check out the power difference in a given wind. I'd replace 'em with a small hunk of curved wood - like half a really fat dowel with the ends rounded to match the blade - just to protect the end of the blade while the wings were off.)
