You know, I think ThrustHP is a good program. Remember, static thrust is different from thrust while the prop is "unloaded" in the air. The thing to note is how much horse power is required to turn a given prop at a selected RPM. I have a Magnum .91 4-stroke in a Super Kraft Cap 232 Sport, and I've tried a variety of props. A 12x8 Scimitar will turn at 10,000 RPM and require 1.2 horse, and give me 6.2 pounds of thrust. The model weighs in at 6.5 pounds It won't pull it strait up but close. A 13x8 shows 8.5 pounds at the same RPM. It will pull it strait up, but not too fast. My point is, it's a good place to start when trying to find a prop for you engine.

Remember to see what your engine is rated at power wise, and note the power requirements to turn a given prop to a selected RPM. Then ask yourself if the engine can pull that many RPM.