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  New Yamaha 90 topping out at 5000 RPM, 43 MPH

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Author Topic:   New Yamaha 90 topping out at 5000 RPM, 43 MPH
Chesapeake posted 08-19-2001 11:30 AM ET (US)   Profile for Chesapeake   Send Email to Chesapeake  
Got the Nauset with the new motor in the water this past week. After the 10 hour break-in period, I noticed a modest uptick in RPMs to 5000 at WOT and noticeable increase in speed to a max of 42 - 43 (From a Lowrance X-85 reading). There is a 16 pitch SS prop on the motor.

The speed is fine, but it seemed that the RPMs were a little low at just 5,000. Motor has about 20 hours on it now. Dealer thought I should get to 5800 max RPM.

Seem low to you?

simonmeridew posted 08-19-2001 12:20 PM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
Your question is about rpm at WOT. But since you mentioned speed indicated on your x-85, I thought I'd mention some experience I had. Comparing speed indicated on my GPS unit vs x-85 indicated speed, I found the speed unit on the x- 85 needed considerable calibration(via a software change on-screen in the menu function). It was indicating much higher speeds than I was really going. I took numerous GPS readings to try to find a non- current part of the lake, then adjusted the speed calibration function on the x-85 , as I recall by a factor of (-.83). I can't be sure of the exact figure but I do remember that the first time I tried to calibrate the speed differential (gps vs x-85 indicated)I kept getting worse instead of better. So I figured out how to go into the negative numbers and fine tuned it from there. Since the speed indication is not linear, at least on my unit, I calibrated for cruising speed, ie 3800 RPM. It will not be accurate at trolling speeds, off by at least 40%. Not exactly the question you were asking but one more piece of the puzzle.
jimh posted 08-19-2001 12:20 PM ET (US)     Profile for jimh  Send Email to jimh     
What was the load on the boat at the time?

How much fuel aboard?
How many passengers?
Any headwinds/tailwinds?
Canvas up or down?
Wave conditions?
Trim tweaked for optimum flat water running?

Changing all of these in the same direction could add several hundred RPM to your engine speed.

I would let the motor break-in a bit more, then try the WOT run again with the expected minimum load. See if you get closer to your 5800 RPM target.

I am sure you know but I thought I would mention: the paddlewheel-derived speeds can be rather variable. They are subject to recalibration in the fishfinder setup software.

Chesapeake posted 08-21-2001 11:06 AM ET (US)     Profile for Chesapeake  Send Email to Chesapeake     
Jim: Just me in the boat at 180lbs. Very calm lake w/o much wind. Two pretty much full gas tanks. My wood console is probably 100+ lbs versus a fiberglass console. No canvas and trimmed up for speed.

So, I also have a 15 pitch aluminum prop in a box. I will run the motor with the new prop and see if that works.


triblet posted 08-21-2001 11:49 AM ET (US)     Profile for triblet  Send Email to triblet     
Try it with a pretty much empty gas tank.
A full tank knocks about a knot off my
top speed.


Bigshot posted 08-24-2001 02:12 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
reconfirm the max rpm's. My 90 Yammie redlines at 5500 and that is where it is with 1 or 2 people and a 17" SSprop. I am trying an 18 polished with vents this weekend, will post it. Nick
Bigshot posted 08-27-2001 10:49 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Tried the 18 with vent holes. TERRIBLE! 5400 max at 37mph(vs 5500 @ 43+). cruise at 3700 was 20mph down from about 28. Put my Yamaha 17" SS back on and AHHHHH!
Chesapeake posted 08-29-2001 11:46 AM ET (US)     Profile for Chesapeake  Send Email to Chesapeake     
Bigshot: Thanks for the info. I was testing with a 16 SS since break in (unfortunately we bumped some submerged wood and scratched a little - so the test is over and now I own the prop!). I have a 15 aluminum that I have not yet tried.

Should I be going to up to 17 or down to 15 to try and generate increased RPMs? I thought drop to a 15, decrease initial hole shot, but gain on the top end. Is this inappropriate marine physics?


Bigshot posted 08-29-2001 01:40 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
You should be running a 17" or maybe even a 19 depending on how it is always loaded. The 15" should be WAY too small unless they changed gear ratios recently. I would check the tach(I know it is new) but your top end is there but 5k is way low for a 16"ss. The 18 I used still hit 5400. Take the 15 back and get a 17 if still new.

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