"Cameron, I'll take a stab
"Cameron, I'll take a stab at this one for you as well (mine is but one opinion).
Those gear ratio's are for "high thrust" motors.
To start with, you should always match the motor and the prop to the rig you are putting it on.
A standard gear ratio on say a 50 horse may be in the 1.8:1 range (one prop revolution for 1.8 turns at the flywheel). That with a 17 pitch prop may push a 16 foot alumimum fishing boat along at 30 mph just fine.
You stick that same 50 horse of a 24 foot pontoon boat, that weighs more and has hydrodynamic characteristics more in line with a rock than a boat, and that 50 is going to gasp, gag and bog down trying to get it moving.
By moving to a lower gear ratio and lower pitch prop, you can push it along at a reasonable speed, while you and your dozen guests sip your drinks. The motor isn't working as hard and it's pushing "less" water initially to get forward motion. Totally different application though....
You could use the high thrust on the fishing boat, with a rediculously high pitch (if you could find one) and achieve the same results as a "standard" set-up, but my question would be "why"? (unless you got the motor for nothing).
At 5000 rpm (at the crank), 1.8:1, would equal about 2778 rpm at the prop shaft. A 17 pitch prop should travel (shaft rpm x inches of pitch - slippage) about 3935 feet (minus slippage) per minute.
At 5000 rpm, 2.59:1, would equal 1930 rpm at the prop shaft. So to cover the same distance with the high thrust you would have to up your pitch from 17 to about 24.5. (1930 x 24.5 = 3940 feet), just to attain the same speed as the convential set-up.
The value of the low gear, is for moving more weight with a lower pitch - where you simply want to "putter" along....
Less exhaust fumes on a 4 than a 2 - not sure if the fish mind either way
"