Re: Prop change
So what effect will going up 1.5 inches have?
Obviously you are pushing more water. On a flat ratio basis you might do a ratio 14.25/15.5 = about 9% speed reduction possibility.
On pitch, a 1" change is said to be 200 rpm change which as Bondo said has more effect than diameter for a comparison. So if you have a 17P and went up to a 19P for example, you would theoretically decrease your rpms by 17/19 (for a number) or 9% and speed might or might not be effected....depends on the engine torque curve.
So, just to throw out some numbers, if you are running 35 mph with your current prop, expect to run not more than 9% less mph.
But the load on your engine will also be affected and that depends on the torque curve of the engine so that could change more than 9%.
Case in point, I put a 14 1/4, 23P prop on an engine that was running 56-5800 with a 13 1/4, 24P prop and had a couple of other things different that could have assisted in dropping the WOT rpm (different brand prop and different material). As you would expect my "just on plane" performance was really nice due to the larger diameter and less pitch, as the boat was easy to control at that point....stayed on plane easily, but the top end fell off to 5000 rpm and speed dropped off drastically...due at least to one less inch of pitch and 6-800 less rpm.
So, the only way you will know for sure is to run it!
Mark
