Re: How much difference does a inch of diameter make?
In light chop, I top out about 33 mph, but on smooth water its more like 30 mph with two guys and gear. I can't say for sure on the RPM, (No tach), but and educated guess would be about 3400 to 3500 RPM. I don't push it much since its so old, but it gets the job done and is great on gas. The original manual says 4400 max RPM, but I don't push it that hard. It spends most of its cruising at about 2800 RPM or so just under 30 MPH. It's not a problem since the river is full of no wake zones anyhow and it rarely ever sees wide open throttle. It don't gain much speed over 3300 RPM regardless of which prop I use. The hull design probably has a lot to do with that, as does the fact that its a bit stern heavy.
It had a 15 1/2" x 19 on it when I got it and did a little more MPH, but was weak out of the hole and I didn't like how it felt with a full load on. I actually keep several props for various areas, but I tend to stick with the 17P most of the time, I on occasion put on an 18P if I am staying in the river or running light.
It's also on an older 17 1/2' trihull so it's not the most hydrodynamic hull ever made. The hull weighs in at about 1300, plus 18 gallons of fuel, spare oil, a spare prop, assorted tackle, safety equipment, bimini top, and two guys I figure it's on the heavy side of 2000 lbs afloat.
I did try a 23 pitch prop once, with a light load, (just me), it was super sluggish out of the hole and had trouble getting on plane and staying there. It just didn't have the power to pull that pitch prop. I had tried that prop just out of curiosity and it was what I had to use while the one I wanted was on order. The best all around prop is probably the 15 1/2" x18P but I use this mostly just for fishing and on most trips, the boat is near capacity in my opinion.
The one thing I did notice is that the two props have very little affect on fuel usage, with the lower pitch prop probably being a little easier on fuel. It's way better on fuel than any of my outboard boats. I've had a tank last a full weekend of fishing and then some.
My biggest problem is that it takes more fuel to get this one to the water, I need to use a larger truck to tow and launch it, so that cost me as much as the boat burns just to get it there with the bigger truck. I can tow and launch my outboard boats with my Ford Ranger which gets almost 20 MPG towing compared to 10 MPG out of my full size Dodge van.
Just a note and back to the original topic, from dealing with outboards, it's been my experience that a one inch drop in prop diameter without changing the pitch, results in about an 800 RPM increase in engine speed. I would venture to guess this to be about the same with an stern drive too.
One step up or down in pitch makes about a 200 RPM difference given prop diameter says the same.
Other than RPM, the other factors will vary boat to boat. On mine, a smaller prop created a slower hole shot and blew out easily under way. Keep in mind that a Stern drive engine makes much more torque than do most 2 stroke outboards and it's been my experience that they do better with a larger diameter prop.
The only way I would consider a drop in diameter is if going from an aluminum to stainless prop which in turn has far less flex in the blades.
In short, a reduction in diameter will have far more effect on RPM than a step up or down in prop pitch. If your RPMs are a bit low, drop the pitch to a 17 or 18 but I'd leave the diameter alone.
