Determining RPM

[huntin dawg]

Is there a way to determine roughly what RPM my motor is running at knowing pitch and WOT speed of my boat?
My little jon boat runs just shy of 17 MPH at WOT with a 7 pitch prop.
The motor is a Mercury 75, 7.5 HP year 1973.

Thanks,
HD

 

[180shabah]

Re: Determining RPM

If we added gear ratio and the speed is gps not dash gauge, we could guess, but could still be off several hundred RPM.

You need a tach to know for sure.

 

[steelespike]

Re: Determining RPM

A tinytach is about $40 and easy to install.One for your motopr will fit any 2cylinder 2 stroke.Be sure to read all the info at the tinytach site.

 

[huntin dawg]

Re: Determining RPM

If we added gear ratio and the speed is gps not dash gauge, we could guess, but could still be off several hundred RPM.

You need a tach to know for sure.

The gear ratio is 2:1 and the speed is GPS 17 MPH.

The manual says WOT should be 4500 to 5500 RPM.

Just wondering if it could be roughly figured without a tach.

HD

 

[180shabah]

Re: Determining RPM

Assuming a slip between 10 and 25 percent, best guess on RPM is 5700-6000+

But again, this is no substitute for an actual tach reading.

 

[trendsetter240]

Re: Determining RPM

Roughly 5700 to 6400 is your current RPM. 5700 with 10% slip and 6400 RPM with 20% slip.


(SPEED X RATIO X 1056) / PITCH X ( 1 - (SLIP/100) ) = RPM