Over-Revving Engine - Tach or Prop??

[Mercury60]

I've got a 1986 Mercury 60hp 2-stroke. At WOT the tach indicates its running at 6000+rpm (yikes!). The max reccomended is 5500rpm.

The tach is pretty old, is there anyway to verify its accurate before I buy a new prop?

 

[CharlieB]

Re: Over-Revving Engine - Tach or Prop??

Before you buy another prop, HOW wa the boat LOADED? Light, medium, heavy, average for your usage?

I would much rather see a slightly undersized prop and pull back slightly on the throttle, than to find a boat overprop and unable to turn anywhere near peak RPM. The overpropped motor would certainly not last near as long.

Retest again, vary the load in your boat, get a better idea of how max RPM varies with your load, THEN consider what prop may be required for your usaul needs.

 

[Mercury60]

Re: Over-Revving Engine - Tach or Prop??

Thanks for the replies Charlie! Just for some backround, its a 1986 17ft Smokercraft Bowrider with an aluminum hull. Not sure of the manufacturers dry weight.

Completely empty with just a few gallons of fuel and myself the motor would nudge 6200 rpm. This is on perfectly calm water, no wind and fully trimmed out. 35mph if it matters. I don't do that anymore because I'm now aware that is alot higher than the max reccomended WOT RPM. When I first got the boat I didn't know better.

Now fully loaded with gear and fuel and an extra passenger it maxes out at maybe 5900RPM. I'm guessing maybe an extra 300-350lb?

The tach is probably original for the boat so I'm not sure of its accuracy. If it matters when I get home I'll post the prop specs.

 

[jerryjerry05]

Re: Over-Revving Engine - Tach or Prop??

They sell a Tiny Tach .It's a great thing to have in the tool box.You just ground it and hook up the lead to the plug and you have the reading.J

 

[sschefer]

Re: Over-Revving Engine - Tach or Prop??

Absolutely agree with CharlieB. 6000 RPM is not over revving that engine to the point of premature failure. Lugging it is far worse. 200 rpm per inch of pitch is a rough estimate of what a prop change will do for you. Like Charlie said, you should be able to easily reach your max RPM and a comfortable top speed. If you're trying to run at max RPM and calling that cruising speed then you're simply underpowered to began with. That sounds odd but if you think about it, it makes sense.