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ContinuousWave: Whaler Performance
New Yamaha 90 topping out at 5000 RPM, 43 MPH
|Author||Topic: New Yamaha 90 topping out at 5000 RPM, 43 MPH|
posted 08-19-2001 11:30 AM ET (US)
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?
posted 08-19-2001 12:20 PM ET (US)
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.
posted 08-19-2001 12:20 PM ET (US)
What was the load on the boat at the time?
How much fuel aboard?
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.
posted 08-21-2001 11:06 AM ET (US)
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.
posted 08-21-2001 11:49 AM ET (US)
Try it with a pretty much empty gas tank.
A full tank knocks about a knot off my
posted 08-24-2001 02:12 PM ET (US)
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
posted 08-27-2001 10:49 AM ET (US)
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!
posted 08-29-2001 11:46 AM ET (US)
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?
posted 08-29-2001 01:40 PM ET (US)
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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