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Ed Anderson wrote:
While increasing the blade chord is the easier option, it is less efficient because the aspect ratio of the blades is decreased resulting in some loss of aerodynamic efficiency.
One reason the thick chord is less efficient is that it creates a much larger tip vortex. It takes a lot of energy to make those tiny tornadoes at the end of a wing. Paul Lipp's propeller minimizes this by putting all the chord at the center and then thinning out to nearly nothing at the tip. Wittman did the same by putting triangular sections on the ends of his main wings.
So, I'll modify the earlier rule. The best prop would be the longest one that would leave some ground clearance, and then absorbs any remaining Hp by increasing the chord, with the chord increase starting near the hub and thinning as it moves out to the tip.
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