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Posts: 197
Location: N Illinois | Running a 2002 ranger 619. Had to repower do to motor issues.
Old motor, 2002 200 hp mercury optimax. When new could reach gps speeds of 52 mph at 5500 rpm. Last couple of years getting 48-50 mph at 5500 rpms. Running a tempest prop, 21 pitch.
New motor. 200 hp Yamaha vmax SHO four stroke. After break in period opened her up. Maxing out at 5200 to 5300 rpms. Hole shot excellent. Top speed gps is 55 mph. Currently running a Yamaha vmax prop with pitch of 22.
Speed is up, but rpms down. Mid range seems good. Question is do I have the right prop with these numbers?
Thanks in advance. |
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Posts: 5874
| Max RPM on the new Yamaha? |
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Posts: 3508
Location: Elk River, Minnesota | Hiya,
From what I could find, the max rpm on the motor is 6000, which means you are well under the light load rpm for your rig. This would be a sign of being over pitched, as I would guess the hole-shot may not be so quick...not bad, but just not a quick pop up out of the hole.
If your numbers are spot on, you have a slip factor of 13%, which is pretty good (this is calculating at the 22 pitch w/5300 rpm).... If you calculate at 23 pitch, which is may more closely match how the prop might perform being highly raked and cupped, that slip value goes to 17%...a little on the high side...
The prop you have on it now is a good choice, but I think might be over-pitched by an inch to possibly 2. Assuming you were lightly loaded when you made your runs, if you down-pitched by an inch, then raised your motor up as high as you could go that would give you another 400 or so rpm, getting you to 5700. If you went down one more inch you would be roughly 5900...right in that sweet spot where you are almost maxed out, but not quite... This is assuming the motor is mounted in the lowest possible position (highest bolt hole is used)...
In dropping the 2 inches and raising the motor (given the prop still performs well at that motor height...trim, cornering, etc.), you should roughly be seeing 58 - 60 mph given a slip value of 10%. This is being calculated as a 21 pitch prop as the cupping on the trailing edge of a 20 pitch acts more like a 21 due to how well the prop will grab and hold the water. If you figure 12% slip which is still within an acceptable range due to the higher motor height, you are still looking right in the 58 to 59 mph range... at 17% you are still at the 55 - 56 mph level. All excellent situations, with the kicker of having better hole shot...
These numbers are a bit tongue in cheek as every boat performs differently... all values are general here so variations from the numbers listed above will exist.
If you can, see if the dealer you purchased the motor from has a demo prop program where you can take a 20 and 21 pitch prop out and try, and just see what you find.... Cost should be the same, so they might even swap out your current prop if it is in top condition for the one that best suits your rig.
Steve |
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Posts: 197
Location: N Illinois | Thanks. That's the kind of info I was looking for. |
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Posts: 197
Location: N Illinois | Another question. The diameter of this prop is 15 1/8. Just curious how diameter affects performance. |
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Posts: 3508
Location: Elk River, Minnesota | Hiya,
Diameter will affect handling for the most part. The smaller diameter, the less surface area you have on each blade...thus, less ability for the prop to grab the water. What it does, do, though is increase rpms when you go smaller. It is roughly about 100 rpms for every 1/4" of change. Down 1/4", you gain 100 RPM...go up 1/4", you lose rpm.
Steve |
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