Kort nozzels are great for increasing the efficiency of a large prop at low speed, but as speed builds the drag introduced by the wetted surface of the Kort overcomes the benefits. As the flow of water is unimpeded, the flow of whatever is in the water is also unimpeded, until it finds the prop.
A counter rotating arrangement with the blades very close together might mince the weed. Achieving this is way beyond me, but if patented and a commercial success, I claim a divi, otherwise somebody else said it.
At relatively low speeds, I had some modest success with a simple bar pointing down directly before the prop (OK, it was a nail) that acted to deflect the offending weed out of the water flow. Mostly.
On a long ago model I tried a mesh cage, but found that as "my" pond was beloved of dogs being taken for walkies, dog hairs would go straight through and turn the prop into something resembling a furball. Speed was severely reduced due to the resistance of the mesh cage, and probably also to the reduced area available for water flow.
As far as rudders go, have the front face swept back, and avoid having the hook effect at the top by using a deflector wire.
