3:1 - that ratio would allow you to run a larger more efficient prop.

True Tim, could prolly run a 66"or 68" The only issue with absolute 3-1 or 2-1 ratio is that the position of the prop becomes important, because the blades stay the same in relation to the crank, whereas if you have 2.99 -1 or 3.01-1 the prop is always moving in relation to the prop (ie: never in the same place twice) I don't know if I explained that very well... but sometimes you put the prop on a gearbox/drive and it runs rough as guts.... taking the prop off and moving it around one set of bolt holes can put the prop blades at a different relationship to the crank and make it run smoother

Hey Muz, every aircraft out there with direct drive have no problems with the prop staying in phase with the the crank.
99% of aircraft have in phase drives.
Now, when you have an odd number of blades on an even number of cylynders inertial balancing is a whole different topic.

direct drive you can have problems of how the prop is mounted. On my EA81 hand cranker, to put the prop on the best way to hand start it made it run very rough.... putting the prop where it ran smoothest made it terrible to start.... what eventually fixed it was putting it in the best starting orientation, but turned 180 degrees then bolted on? Finiky or what? Yes. I think the more blades, the better (less of a problem) it is....

All I am saying is if it is exactly 3-1 (or 2-1 or 4-1) the prop may need to be removed and re orientated to get the best result

Maybe have a 0ne blade prop? Like this one blade helicopter...

on certified stuff they have a master lug that's a little bigger then the other five the prop only goes on one way (unless you get a stubborn SOB who pounds the prop on with a sledge hammer)