Different Outdrive Ratios 1:5 Or 1:36
 

which will propel a boat faster a 1:50 with a higher pitch or a 1:36 with a slightly smaller pitch

Conventional wisdom seems to be that a higher pitched prop with the lower gears will be faster until you run out of higher pitched props.

Prop efficency is supposed to drop off around 34". I'm told the 1.5 gears will accelerate faster than 1.36 gears. Also, props in the 30's are pretty hard to find.

I'm definitely no expert on this, but I've always heard you should

"spin more pitch slower"

which is the same as what laster said.

I've also heard that you plot speed (y-axis) against rpm and look to see if the curve flattens out (inceasing rpm with diminishing increase in speed). If so, lower gears (1.5, 1.65, 1.84, 1.98) may help increase top end.

Again, I'm no expert, just what I've heard.

I called Mercury racing and they said it shouldn't matter but I might gain 1 mph with 1:36 but holeshot would be slower, they also said 1:36 is easier on the lower unit due to ft. lbs. of torque on the shaft or somethin like that. Anyway I'm testing this weekend 1:50 against 1:36

More pitch, running slower, tends to...
 

throw a lighter boat around a little more. More torque reaction from a bigger prop wants to make the boat lean over when you hit the throttle. A heavy boat doesn't torque over as easy as a smaller boat so it effects the boat attitude a little less. Just put a fat mother-in-law on the side of the boat that wants to rise!
Dennis Moore

I don't have a fat mother in law, Weight is weight so I guess my sister will do .

did some testing this weekend I'm sure all boats vary though, but on mine 1:36 Bravo drive with an Imco 2" shortie unit 21p Mirage Plus @5200 = roughly 67-68 mph. Bravo1 22p @5000= 67.3 GPS. on a full tank. Sun. switched outdrives to 1:50 with stock lower unit filled tank again 23p Mirage @5200 pretty much same speed. Powertech 23p 4-blade @ 4800 probably about 60mph. This prop sucks. I'll be switching the Imco drive to the 1:50 upper and continue my tests.