Top speed and RPMs

[mzodarg]

So I've been chasing down correct prop for my 22' tracker Fishn Barge since I purchased it.

Yesterday I had a Mechanic from the local tracker dealer meet me at the lake.

At 3/4 throttle, I am turning 4900 rpms and at full throttle I'm turning 4900 rpms. When at full throttle you can still open the butter fly on the fuel injector by hand roughly .060.

No resolution on this yet, still need to hook computer up and verify TPS settings.

Below is GPS at WOT 4900 RPMS with a 15x17P prop.


My hunt continues.

Mike

[rancherlee]

Your command thrust lower unit is very close to the same ratio on my DF140. A 17p prop on mine is good for 36-38mph @ 6000 rpm. It too much prop for a 90hp, and your actually spot on RPM with that pitch with roughly 15% slip. A 15P version of the same prop would bring you up to 5500-5600 range which I think is in the proper RPM range for your engine. A 14p would bump it to 5900-6000rpm and would be good for heavier loads and give up a touch of top end speed. I don't think you will pickup any more speed though, that big 2.1l engine is probibly already making 90hp+ @ 4900 rpm, I'd actually bet on that!

[wrd1972]

Way too big a prop. IMO, RPM is king and makes speed. I routinely run 5.8 - 6K PRM with one of two props. Which one I run depends on how many are on-board. One is 13.5 x 16P and the other is 13.75 x 15P. Both of these are very optimal for my boat for nearly every condition.

I you are not using every RPM you motor is capable of, your leaving speed on the table. Again, IMHO.

[ROLAND]

Yeah, I agree with some of the others here.. think you may be over propped... don't own a mercury so I can't give you specs, but I have a yamaha 75 4 stroke and @ wot I'm bumping 6000 rpm's.
I believe the alarm sounds at 6250, but not positive about that.

[mzodarg]

Thanks for the replies!

I've been chasing this for awhile. Tracker swears that the prop they recommend (15x17P) is the correct one for this setup. I say it's not. Not looking for speed, just want the rpm range at WOT up around 5700-6000. IMHO, wide open throttle at 4900 is lugging the motor and can't be good for it.

Boats under warranty, so I'm going thru the process.

Mercury's web site recommends a 15.5 x15P for my particular setup on there prop selection web site.

Another concern is at WOT, there's no change in rpm's when throttle is at 3/4.

Mike