If you have a target rpm, and speed, you can choose a prop. If you think the airboat would run say, 30-35 mph on the power available, then choose a prop with a diameter and pitch that will turn the rpms the engine is capable of while providing the target speed.

Example: A 6" pitch prop turning at 6000 rpms gives a theroetical speed of 35 mph. Now just choose a prop diameter that will allow the engine to turn 6000 rpms.

Note that choosing a prop with too much pitch will not necessarily make the boat go any faster. If there is not enough power available for engine to move the boat through the water faster, added pitch will just reduce overall performance. One has to be realistic about the target speed, whether it is an airplane or a boat.

In this case, I have seen videos of a three point hydro airboat running around 45 mph with a G-26, hence my guess at 30 to 35 mph for the airboat in question here.

AV8TOR