Thanks. I'm running prop programs for the Yanmar 2GM20F I plan to install and everything says that the aperture is too small for the engine to have good efficiency. The 18hp is plenty to push the boat but the prop should be a 16 x 11. three blade. By my calculations I would have to increase the pitch to at least 15 - 16 to get the prop down to the required 13" for the existing aperture. What are people running?