Hey there! Try one of the hovering props listed above first, they're likely to work best for you in this application. When you choose your props, try to keep the load on the engine such that static rpm maxes below about 13500 or so, or the engine life will suffer.

I thought you might be interested in some insights on your airboat speed experiments. There's actually a simple explanation of what you observed. Here's the results you obtained, and I have no doubt they're accurate.

44-48 mph on 11x3
36mph on 11x6
33mph on 10x6

Remember that a prop is basically a wing, same as on any plane. More speed, or more angle of attack gives more lift, until the angle of attack exceeds critical and the wing stalls and produces virtually no lift. It's speed and aoa that are playing with your airboat results, for the most part. Also, the area of the prop disc has a very large effect on static thrust.

I'll start by looking at your 11x6. That's really the best matched prop to your engine that you used (in my experience). With it, a healthy .46 will tach about 13k rpm, and produce a pitch speed of 74mph and about 7 pounds of static thrust (using ThrustHP to calculate). Since the airboat is a draggy vehicle, it's not likely to achieve a speed anywhere near 74mph, so the effective aoa of the prop will be fairly high. A portion of the prop blade will be stalled because of this.

The 11x3 prop is too small for longterm service on a .46. It must have tached in the 15-16k range, maybe even higher. Musta been a beautiful noise! It's high rpm results in 10+ lbs of thrust, and a pitch speed of 45mph at 16krpm (so you were likely turning even quicker to achieve the speed you did!). Since the boat is likely to be able to approach the pitch speed (and in fact did!), the effective aoa is very low and the prop is not stalled at all. A good pitch choice for the boat, but it would have likely been better with a 12x4 or so, to bring the rpm's down without sacrificing pitch speed.

The 10x6 had the same trouble as the 11x6, but at higher rpm, and with a smaller prop disc, so less thrust. Probably tached about 14000 static, pitch speed near 80mph, and only 5.5 lbs static thrust. Again, the vehicle can't achieve that speed with the drag it has, and so the prop is out of it's element.

Your hovering machine (cool idea, btw, what's it for? Camera ship?) will have a very low flight speed, essentially 0 mph most of the time. So your goal should be to generate as much thrust as possible (big prop disc, ie big diameter) with a low pitch to avoid stalling any part of the blades. Look to those 3d props and you'll do well. APC props are about the stiffest sport props available (without visiting the exotic cf props, etc.) and are very efficient. A favorite among sport and 3d fliers in this size class. Definately more efficient than the same size zinger wood or master airscrew glass filled nylon which is much flexier. The master airscrews, however, are more likely to survive the enevitable nose over.

Hope this helped.

J