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 |
