RPM's

[dabalone]

In regards to propping your boat correctly, it seems the answer is always prop it so you run in the upper end of your motors recommended rpm range. This apparently will get the most life out of your engine. So, does it hurt to run your engine below the recommeded rpm range? After all most of the time we cannot run wot and I simply don't care that much about speed anyway or listening to my motor scream at the high rpm's. Cruising around most of the time under the max rpms gonna have any major adverse affect on my motor? My motor is rated at 5750 rpms, but it purrs just fine at 4800 with good enough speed and low noise levels, this is gonna screw up my motor? O6 115 Opti.

 

[Silvertip]

Re: RPM's

Yes it is important to prop the motor for wot rpm in the band specified by the manufacturer. The prop is to a boat, like the transmission is for your car -- and it has only high gear. The prop must get you out of the hole, provide a comfortable cruise, and not lug nor over rev the engine at wide open throttle. You can install virtually any prop on your engine and it will push the boat. But if you install a prop that lugs the motor at wide open throttle, it is lugging the motor at lower rpm as well. If you install a prop that over revs the motor at wot it is also running faster than it needs to at whatever speed you are cruising. Certainly you can run the motor at 4800, or 3000, or 1000 -- after all that's why they install a throttle. But the engine needs the proper propeller and the only way to determine what's correct is to test at wide open throttle. If it's set up properly at wot it's set up for any rpm lower than that.

 

[Texasmark]

Re: RPM's

The only thing I might add to Silvertip's response is you don't want to hang around too long on the down side of planing out (just below planing speed). If you want to cruise, firewall the throttle, get up on plane (rapidly gets you through the stress fastest) then throttle back to maintain that "on top of the water" attitude and have your comfortable, quiet, cruise.

Mark