New Suzuki 115hp outboard top end speed and rpm issue

[Fishiboaters]

Hello, I am having an issue with my new Suzuki 115hp outboard. Stingray 191DC. The motor is topping out at 36mph at 5100rpm. Suzuki has a performance bulletin with my same motor and looks like same prop and they showed 46mph at 6000rpm. I have the 23x14 aluminum prop that came with the boat. I posted the question on Suzuki's facebook messenger and a rep told me it could be weight (I tested while alone on the boat, no other significant weight), trim (I cycled it through the entire range), motor too low (I will need to check this but I think it is the same as what was on the test boat), or prop pitch too high (this is the standard prop that comes with this boat and the same as was used in their test). Otherwise it seems to perform fine, some vibration at certain rpms. Any advice? I am having trouble connecting that this could be an over-prop issue, wouldn't propping down lower top speed which is also already low?

 

[jimmbo]

If the engine rpm is in the lower half or below the recommended RPM range, a lower pitch prop often raises the top speed of the boat. There is an RPM that produces the most power. If you can pick a prop that hits that, you will get your best speed for that prop line and setup. Propping to run at RPMs on either side of that ‘peak’ will result in a lower top speed. There can be several props that will hit the optimum RPM, but will yield different top speeds. Picking a prop is part Physics, part VooDoo

 

[NYBo]

What was the boat in the Suzuki test? A 23" pitch seems like a lot of prop for a 115.

 

[Fishiboaters]

Thanks for the reply. So, sounds you think my issue is in the prop selection. Any thoughts on aluminum vs stainless and how I find the optimum prop without complete trial and error (and expense)? This is a new motor, should the dealer be providing me with alternatives since they are responsible for propping the boat correctly?

 

[NYBo]

First, verify your tach and your speed reading. Are you using GPS for speed?

Talk to your dealer. They may let you borrow some different props to dial this in.

 

[Fishiboaters]

The boat in the test was a Stingray 182sc, the exact boat without a windshield. The 23 pitch seems to be standard for the Suzuki 115hp. For speed, I was using the speedo and GPS, both were consistent with one another. I am not sure how to check the tach. Good suggestion on borrowing props from the dealer, thanks.

 

[NYBo]

Hm, that sounds like a smaller boat with a single console vs your DC (double console?). Do you have the specs available for both?

EDIT: Okay, I checked the boat spec page here at iBoats and they are the listed as having the same length and weight.

 

[jimmbo]

There is no such thing as a 'Standard Pitch', while each application is really a custom fit, there are some patterns that can be used as a starting point. While a 23 does sound high, with a numerically high enough gear ratio, it would be fine. What is the WOT rpm range for your engine?

 

[Fishiboaters]

5000 to 6000rpm is the WOT range