Obviously, such a thing does not exist for the Mazda rotary. See Guiding Principle above. The recommended prop for our gear drive is a wood prop, either 2 or 3 blades. The use of a prop extension is not recommended. Boats benefit from lower weight engines, such as the Mazda rotary, as much or more than airplanes.
Lower weight helps lower the center of gravity, thereby improving stability, in airboats.
If you would like a closer look at the hardware, call to schedule a visit. It will probably answer questions that you would not have thought to ask.