Hi Vance. Phil has nice thrusters, for sure. Looking at all the Kort nozzle pictures and literature available, there's a lot of differences on the actual "airfoil" shape of the nozzles. I'm sure there's a lot of highly technical engineering that goes into designing one of those.
For a home builder, ( with limited computer skills ) I think a simple airfoil shaped nozzle could be fabricated without too much trouble.
I was thinking that a large PVC coupling, or any type of tube slightly larger than the prop used could be used as the base. I've been using regular sheet rock mud over foam to make molds for the various fairings and dive planes, and it's easy to work with, cheap, and shapes up easily. It sands down very quickly, and is easy to shape. Then I apply a couple of coats of left over house paint to "toughen up " the surface.
Spray on some mold release and apply the fiberglass. Lots of sanding and more layers of fiberglass, a final color coat with UV protector, and it's done. ( sounds simple doesn't it ??? How come it takes so long ? )
Seems like a Kort nozzle could be made the same way.
I bet even just a big thick piece of foam could be carved out to approximate the whole nozzle piece, and if the fiberglass was thick enough, you wouldn't even need to remove the foam.
It is pretty simple to "melt" the foam out of the inside with a little gasoline, but kinda messy.
I see what you mean about the plastic props being light and therefore easy on the motors. That makes a lot of sense. Maybe an aluminum prop could be made. I've been playing with the idea of trying the home forge method of aluminum casting shown on the psub site. Not sure how closely I could get a metal prop to balance though. Might be fun to try it.
The prop rings I made for the two rear main thrusters were primarily to keep weeds from getting into the props. The clearance between prop and ring is about 1/2 inch, and the props are the regular Minnkota "weed wedge" ones. Probably not the most efficient for a shrouded prop, but I gotta believe Minnkota spent a fair amount of time designing them for maximum efficiency with their motors.
The plus is they are cheap ( $36) and easily replaceable, so carrying a couple of spares won't break the bank.
Frank D.