Let's see...

[dbuffington]

Let's see if I've got this right...

I have a Mercury Mark 55e outboard. The max recommended rpm is 5400.

I finally found a tach to use with the motor, and this past weekend, on a 60 degree day with a 10 inch pitch prop, full throttle produced 4900 rpm.

So...

To get closer to the 5400 max, I should use a prop with more/higher pitch, say 11 or 12 inch. Correct?

Thanks!
Dave

 

[Barnacle_Bill]

Re: Let's see...

Are you sure that prop has a 10" pitch? If it really does then you need to go down to an 8" pitch. But that don't sound right. Anyways to increase RPMs you go down in pitch. 1" change = about 200 RPM.

 

[sublauxation]

Re: Let's see...

I'd also try to confirm that the tach is reading right, the motor isn't mounted to low, and you're using tilt and trim properly because I'd agree that those numbers don't sound right.

 

[Frank Acampora]

Re: Let's see...

Unless you are using it to push the Queen Mary! What size boat is it on? But agreed--the numbers don't sound correct. Are you sure the prop isn't a 10 inch diameter with a different pitch?

 

[Silvertip]

Re: Let's see...

To illustrate the effect of pitch -- think of it this way:

More Pitch: prop takes a bigger bite on the water meaning rpm slows down
Less Pitch: prop takes a smaller bite on the water meaning rpm increases.

 

[dbuffington]

Re: Let's see...

Unless you are using it to push the Queen Mary! What size boat is it on? But agreed--the numbers don't sound correct. Are you sure the prop isn't a 10 inch diameter with a different pitch?

It is indeed 10 inch pitch, according to what's stamped on the prop and according to multiple Mercury references.

And no, the boat's not the Queen Mary. It's a 16 foot Whirlwind. Molded plywood, very lightweight. (I'd guess 1000-1100 pounds boat, motor, fuel, gear and me.)

And, yes. The motor is mounted correctly, with the anti-cavitation plate just below the waterline.

The tach? A new-old-stock Airguide. It appears to indicate idle rpm correctly, but I have nothing to check it against.

Yws, I'm puzzled too, but that's the data.

Thanks!
Dave