Re: Outdrive gear ratios
this is a bit of an oversimplification, but helps to think about it.
generally, props with a ratio of pitch to diameter of 1.8-2.0 to 1 are most effective at converting torque from the drive into forward thrust. This is most applicable for boats that will run 50 or more, but doesn't change a lot when we move down to the 30-50 range.
that is for a 14" prop, pitches in the range of 26-28" are going to be the best for performance applications. Note that this assumes modern prop designs (prop design has come a LONG ways even in the past 10 years).
for strictly top speed, then based on the horsepower (best correlation to top speed), you should choose the gear ratio that allows selecting a prop in the right range for the best performance. Turns out that this almost always gives good bottom end performance as well with a properly designed marine engine and wide open throttle set right at the engine's peak horsepower.
If you need more bottom end, and don't want to underprop to get it, you can use a vented prop which lets the prop slip more during the holeshot and lets the engine rev a little higher to where it makes more power.
outdrive / engine combo manufacturers have to put together "one size fits many", doesn't require too many versions and options (more inventory overhead cost, etc...), and will give good reliability in heavy applications. So they are very conservative. Also, when the overall designs were set, the technology was not available to cost effectively make props in the higher pitch to diameter ranges so the availability of good props, especially inexpensive aluminum props, was mainly between 17-23" pitch... My understanding is that it was these reasons that the "standard" gear ratios were chosen by manufacturers like mercury.
props with lower pitches have to spin more times to go through the same amount of water and aren't as efficient at turning torque into thrust, BUT they deliver power more smoothly (notice that most ski boats run less mechanical reduction and lower pitch props, they sacrifice top end / all around performance for the best performance in towing between 15 and 30 mph...).
so the short answer is, that your boat, assuming your uysing the best prop for the gear ratio you are running, WILL be faster with the higher mechanical reduction because it will allow you to run a prop that's better at converting torque into thrust.
being a skeptic and an all around pita, I actually went through the exercise of changing gear ratios and props in the same boat with the same engine, etc.. and comparing. so I've run in my 17 1/2 ft, 383 engine boat with a standard alpha drive geared: 1.84, 1.65, 1.47 to 1 and have gone through the exercise of trial error prop selection for each. I got the highest top speed by a good bit with 1.84 and lost several miles per hour (3-5 depending on which props used). I'm currently running a shorter, different gear case performance version alpha ss drive with 1.32 gears which should be a good bit faster, but because of the different gear ratio, runs about the same speed or a couple mph faster than the 1.84 standard length drive.
note that if you have one drive and don't want to spend a ton of money, you can almost always find decent props to give decent performance in your application. The exception would be a larger engine in a smaller boat with a 1.98 to 1 gear ratio, that's an uphill battle. Also that is a reliability problem because the prop your using can bite a lot harder in the water and the high amount of mechanical reduction puts a lot of stress on the lower gearset and shaft, etc... Other than that example and in ythe context of merc drives especially, you should be able to prop any drive to give decent performance.
If you're starting from scratch, then you choose a gear ratio and prop pitch range that will work best for the specific application.
sorry so long winded - got carried away! Hope this is helpful.
ps. this is for i/o drives, assuming that the prop is running submerged, etc... there are some different considerations when you start breaking the surface with the prop, etc...
