Re: do i prop down?
Ok Brent, but I need to know the outboard's gear ratio. I think it is 1.92:1, but I'm not sure? I'll use that for the example.<br /><br />Remember that pitch is the number of inches the prop will move forward in the water under perfect conditions (no slip). And by reducing pitch, the rpm will increase. Holeshot will almost always be better. Speed is a factor of many things and sometimes a lower pitch prop will be faster, and some times a higher pitch prop will be faster...it all depends on the boat/motor/prop combination. Test, test, test.<br /><br />Ok. We need to find the rpm at the prop shaft. This is the rpm of the engine divided by the gear ratio. I'll assume the engine runs 5650 rpm with your 9 pitch.<br /><br />5650 engine rpm/1.92 gear ratio =
2943 rpm at prop<br /><br />So now we know how many revolution the prop makes in one minute. And we know how far the prop advances through the water each revolution(9"). From here it's just a matter of converting to mph.<br /><br />[(2943 rpm) X (9 in/rev) X (1 ft/12 in) X (60 min/hr)] / (5280 ft/mile) =
25 mph<br /><br />You said you're getting 21 mph on the GPS. So about 4 mph is attributed to other factors such as prop slip, etc.<br /><br />Now lets try the 8 pitch you want. We know that, on average, each pitch of prop equates to about 200 rpm. Of course prop diameter also affects this, so again it is a matter of testing.<br /><br />Adding 200 rpm to 5650 rpm we get 5850 rpm.<br /><br />5850 engine rpm/1.92 gear ratio =
3047 rpm at prop<br /><br />[(3047 rpm) X (8 in/rev) X (1 ft/12 in) X (60 min/hr)] / (5280 ft/mile) =
23 mph<br /><br />So theoretically, even though the prop speed increased, the pitch of the prop was reduced enough to kill the speed somewhat. But....remember that thing about prop slip and testing. A smaller pitch prop under heavier loads will have less prop slip....and may be actually faster.<br /><br />IMO (and as the others have said) you're propped about right. Although you might find the 8 pitch will be just as fast, plus give better overall performance with holeshot and loaded. I suggest a heavily cupped and raked 8 pitch to try. Let us know.
