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Author Topic:   Calculating Engine Crankshaft Speed
frauke75 posted 08-14-2014 02:40 PM ET (US)   Profile for frauke75   Send Email to frauke75  
Is it possible to calculate engine RPM when at full speed without a tachometer? I have a 1968 Nauset with a 1973 Johnson 65-HP three-cylinder loop charged outboard engine spinning a aluminum OMC 13-3/4 x 15 propeller. This propeller my dad switched to ski with. Another prop was faster but now in bad shape. This is fine as I am now in the same boat as he was (literally)---pulling my kids around with it. I am happy with the performance as is; [the boat] jumps on plane and maxes out at 30-MPH. It will do this regardless of number of people and gear, wind direction or chop; always 30 MPH. Pulling a tube might be a different story. I have not looked at the GPS when tubing.

Am I achieving max RPM? (5200) The motor is strong, just over 100 hours on it and [peak pressure is] at 120-PSI in all three cylinders. I just do not want to overload or over rev it. I am confident that the propeller is correct but would feel better knowing and also just curious how well the old girl is still running. Is there anyway to calculate the engine crankshaft speed?

dg22 posted 08-14-2014 03:09 PM ET (US)     Profile for dg22  Send Email to dg22     
Jimh has a prop calculator which you can enter everything but engine speed and it will calculate that for you.

Note: I usually put 5 for slip. I think the gear ratio for your motor is 2.41 to 1

frauke75 posted 08-14-2014 04:01 PM ET (US)     Profile for frauke75  Send Email to frauke75     
Thanks for the link and help with the slip input.

That makes it about 5400... which I think is good.

I do not know what the correct max RPM for this motor is, but information I have found on similar motors lead me to believe it should be 5000 to 5500 rpm.
Anyone know were I might find out the max rpm for this motor?

I do not know how to truly determine the slip but will need to be a bit carful as if the slip goes over 7 I am over 5500 rpms. My current prop has some dings and might be slipping more because of it.

I rarely am at full throttle but good to know I might be better to back it down a notch just to be safe.

tedious posted 08-14-2014 09:36 PM ET (US)     Profile for tedious  Send Email to tedious     
Redline is 5500 but those OMC triples are tough - overrevving a bit is not going to hurt anything. The newer versions are rated for 6000.


Teak Oil posted 08-15-2014 10:15 PM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
30mph is pretty weak for top speed, I would expect 35 mph at least, though I am guessing you do not have power trim which will hurt top speed considerably.

Those three cylinder loopers like rpms, you need a tach and a prop to get it over 5500 rpm

jimh posted 08-15-2014 10:24 PM ET (US)     Profile for jimh  Send Email to jimh     
Buy a tachometer.
seahorse posted 08-16-2014 01:06 AM ET (US)     Profile for seahorse    

You can get a Tiny Tach and Hour Meter model TT2A for about $55 from

jimh posted 08-18-2014 12:20 PM ET (US)     Profile for jimh  Send Email to jimh     
Generally no one ever has tried to deduce the engine crankshaft speed by measuring the boat speed and working backwards, making guesses at the propeller slip and gear ratio. The direction arrow is backwards in that sort of calculation.

Engine crankshaft speed is usually measured by a tachometer. If installing a tachometer is completely out of the question, perhaps you could borrow one for a day of testing.

dg22 posted 08-18-2014 01:45 PM ET (US)     Profile for dg22  Send Email to dg22     
I agree with jimh, it's best to buy or borrow a tach to get an accurate reading -- ball park figures are not a good idea when it comes to engine RPM.

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