I currently have a 135hp merc on a 17' cajun. I have a 19p prop. Running at about 5600 rpm and 54 to 55 mph. How much increase in speed would i gain by going to a 23p. Or would that drop my rpm down to much. Should i go with a cuped 21. And again about how much speed would i gain by it. IF anyone could help me on this i would greatly appriciate it.