Re: Will this work?
Just because you change to a lower pitch prop to gain rpm does not mean the boat gains speed. Remember, pitch is the factor that moves the boat forward. A 23 pitch propeller theoretically moves the boat forward 23 inches with each revolution. A 21 pitch would move the boat forward only 21 inches. So the added rpm comes from the decrease in pitch and speed will stay the same. Generally, IF an engine is being severely lugged (too much prop) a drop in pitch may put the engine back into the power band so the usual 150 -200 rpm gain per inch of pitch becomes much more than that and you could see a fair speed increase. However, if you are close to the power band, then dropping pitch raises rpm and increases hole shot but top speed may or may not be affected, and certainly not by 4 - 5 mph. In fact, rpm gain may be enough that it puts the engine in "over rev" territory. You need to learn to use and understand a prop calculator like this one:
http://www.rbbi.com/folders/prop/propcalc.htm
The optimum prop is one that puts the engine at or near the upper end of the engine manufacturers recommended WOT rpm band with what you consider an average load. Top speed will be whatever that provides. You don't shoot for a given top speed and try to find a prop that does that. You shoot for the upper end of the rpm band and speed will be the net result.
