We are not experienced with hydrodynamics, we just read some theory and designed the prop and duct based on this (we are still using almost the same setup as our first design, will start to improve this when everything else is up and running well). The propeller profile on our prop is a B-troost .
The idea is to rotate the water prior to the propeller (that rotates the water the opposite direction). According to theory this will improve efficiency as less rotational energy is lost downstream (the downstream should now be more “linear”). We have not done any calculations, we only based the design on theory we read.
CFD analysis of this particular setup is really a pain, as there are so many variables. But we do believe the rice nozzle we use improves thrust and efficiency at low speeds (vs open prop), and cause drag in high speeds (10knots+).
In our later design, we have moved the twisted stators to the rear part of the Hiorth duct. Mostly because we wanted a guard against fingers/toes in the propeller on the rear part as well, but we also read that twisted stators at the rear of the propeller actually can give net positive thrust (which is nice). If we actually get net-positiv thrust is another story…
We are currently designing a high speed duct (and propulsion unit), with a profile similar to NACA 0012, will see how that performs.