Originally Posted by Martyn McKinney
The 12X6 prop would be a few percent less efficient than the 12X8 prop.
Prop efficiency increases with increasing P/D ratio.
Yep that's one thing I expected to hear.
Bruce seemed to be indicating that the 12x8 would have a higher zero thrust speed, does that sound right to you? They should both have more or less the same geometric pitch speed, I'm interested to hear more about differences in zero thrust speed if you're aware of them.