Quote:
Originally Posted by beau  ... The motors should rev out at 6,000 rpm but stop at 4.500rpm with standard props. Everyone wants to sell me a smaller pitch prop which would get my rev's up. but I do not see that increasing the speed very much... |
I presume the 6000RPM figure is the maximum at Wide Open Throttle, from your engine manual. I also presume the two engines are Counter-Rotating.
Overloading the propeller (high pitch and/or diameter) results in lower engine RPM throughout its entire RPM range for a given throttle setting.
Assuming the Prop’ Diameter is correct, the standard solution to low RPMs is to decrease Pitch.
Why don’t you believe it? SPEED = (RPM ÷ Ratio) x (Pitch ÷ C) x [ 1 - (SLIP/100) ] Where:
SPEED = the boat speed.
RPM = the crankcase speed in revolutions-per-minute.
RATIO = lower unit gear reduction ratio; the number of revolutions of the crankshaft to produce one revolution of the prop shaft.
PITCH = blade pitch of prop in inches.
SLIP = 0-100 index of propeller performance (a percentage) .
C = a constant to convert inches-per-minute of revolution to boat speed*:
* 1215.2 for knots (nautical-miles-per-hour)