jeepindog, you are telling us that all of the aeronautical engineering development on propellers since the Wright Bros. has been wrong? Both props had the same static RPM?

The 13-8 three blade has a greater power load than the 14-6 two-blade. If the engine was not constrained by valve train limits, breathing restrictions, or something else, it would have to turn faster with the 14-6. Maybe your engine is at its rev limits -- if that is the case, a steeper pitch 2-b will give more thrust.

For a given diameter & pitch, a three blade has 50% more blade area and drag than a two-blade & can transmit proportionately more power, but conversely, requires similarly more power to turn at the same rpm. However, since prop blades run in the wake turbulence of the preceding blade & since the blades of the 3-b prop are closer together, there is more loss of efficiency from wake turbulence. Additionally, there is more "mess" in the root transition zone, resulting in a shorter effective blade length -- again lower efficiency. If the 14-6 & 13-8 are turning at the same rpm, the discharge velocity of the 13-8 is 1/3 greater than for the 14-6 and has significantly greater slipstream shear losses -- another loss of efficiency, whereas the lager diameter prop is exerting the same work over a larger area at a lower discharge velocity -- more efficient (which is why helicopters have large disc areas). Finally, in the case of jeepindog's pitch & diameter change, the pitch increase more than offsets the reduction in diameter, requiring even more power --- so either there is a lot of stuff that isn't being revealed (like the engine at it's rpm limit), or the post is a troll.

BTW I've been in the modelling business for well over 50 yrs, flown thousands of hrs in FS aircraft & managed a fluid dynamics engineering research lab for 20 years. The physics hasn't changed.
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I guess I forgot to mention that I am a liar. Merry Christmas, britbrat. I hope you feel superior with all of your guessing about the reality of what I wrote. Both scenarios that I outlined were run with the same fuel on the same airplane with the same engine on the same days. It was tried and tried again. The only thing that I did not reveal was the actual reading that I got with my tachometer. Both runs yielded rpm figures of 9,600. I do not run my fourstroke engines at redline, as I do not need to. Although it was impossible to measure unloaded rpm during flight, the exhaust sound was similar, however, this means little. The three-blade prop indeed outperformed the two-blade prop. If someone made a 13x6 that I could have tried, I would have, but I could find none at any local shop. 13x8 only.

Lachlan