New Suzuki 115hp outboard top end speed and rpm issue

[Stingrayzuke]

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?

[D'oh999]

I doubt a 115 HP, could push a 23" prop on a 2000 lb boat with 19 Degrees DR.

I run a 23" on a 140 hp, on a 1300 lb boat with 10 degrees DR. At 6100 RPM, 1 person, 15 gals of fuel.

[Stingrayzuke]

So are you saying I am over-propped? Top speed is about 10mph less than it should be and RPMs are too low. Wouldn't propping down also lower top end speed, where it is already too low?

[Harper]

. Wouldn't propping down also lower top end speed, where it is already too low?

Simple answer.....NO. The increased RPM more than compensates for the reduction in pitch. You're not doing your motor any good with the prop on there now. You're lugging the motor.

[Stingrayzuke]

Thanks for the response. I will definitely look at reducing the pitch. Any thoughts on what stainless would do for me?

[Moonlighter]

New motor - its your dealer’s responsibility to get it set up right, height wise, and prop wise.

You should contact your dealer and ask them to sort it out, thats their job. Avoid running it like it is, you are badly lugging it at the moment. Same advice you got over on THT forum.

A good stainless prop will always be better than a alloy one. Suzuki stainless are pretty good, I run a 14 x 20 on my DF115. 18 degree V hull, 18 ft boat, pulls 6150rpm.

[Kartergreg]

I repowered my 1903 Trophy with a DF115 last year. The dealer mounted the motor low on the transom so I had to raise it later. Also the dealer put a 14 x 22 prop on. I had similar problems. I could only turn 5400 RPMs and it was rather sluggish getting up on plane. I swiched to a 14 x 20 prop and I can hit 6000 RPM with a light load. Tops out at 40 mph. Unfortunately dealer was no help.

[Stingrayzuke]

Thanks, Kartergreg. Just tested the boat and the water line at plane is about 2" above the cavitation plate. Your information is useful.

[Moonlighter]

Sounds like you need to lift by 2 holes at least. If the solid flow of water is way up there, the motor is set far too deep. Is the engine throwing a lot of spray around too?

I will go out on a limb and say you will pick up about 300-400rpm if you lift it to the correct height.

There is a sticky thread on this forum (towards the top) about prop selection and engine height you might like to read.

Still, report your results to the dealer who installed the engine so they know what you found.

[Kartergreg]

Stingray, I forgot to add that when I was looking for a new prop I called PowerTech Propellers, www.ptprop.com, and they were VERY helpful. They just needed all the info about the boat and how it was perforning at different RPMs and at top end and while I was on the phone they did their calculations and gave me their recommendations. They do not sell direct so you have to buy through a dealer. I purchased online from www.dansdiscountprops.com. $270 for a really nice stainless prop, drop shipped to me from the factory.

When I went to talk to my dealer he was useless. I had just done the break in and for a while I had never run it much at WOT so I did not even know it was the the wrong prop. When I told the dealer I needed a prop with two inches less pitch, he suggested I go to a prop shop in town and have them shave off the two inches....at my expense LOL. That is when I noticed my dealer was buying some props from PowerTech.

Not happy with my dealer (but he did give me a great deal on the motor) but I am very happy with the prop I finally ended up with.

[Stingrayzuke]

Moonlighter, It seems to be throwing an average amount of spray. I am not an experienced boater, however, so I am not sure what is normal. I will definitely check out the sticky you are referring to.

Kertergreg, I will check out PowerTech. I am weary about how much the dealer will help. I already know they said it will be 2-3 weeks to even look at it, there goes a good chunk of the summer. It is under warranty so I probably don't have much choice. That is why I want to educate myself as much as possible first.

[Stingrayzuke]

Here is a photo of the motor while on plane. The arrow in the second photo is where the water line appears to be. There also seems to be a lot of spray, which I had not noticed before.

[Moonlighter]

Throwing lots of water around is typical of an engine set too deep, which is why I asked the question. Water flow is hitting the front of the leg where it shouldnt be, creating unnecessary drag.

If that arrow is where the solid flow of water is hitting the leg at cruise speed with the engine trimmed out to optimum, then it is set waaaaaaay to deep and IMO needs to come up at least 2 holes. When you get it up to the right level it is going to transform how the engine runs and how the boat performs. I bet you will be amazed at the difference!

I would be having a firm conversation with my dealership manager/owner about having this being addressed quickly. Poor form to send a boat out with the engine fitted like that.

Reality is that lifting an engine up is a 30 minute job for a professional workshop with the usual lifting equipment and competent staff. Sheesh, I could do it at home in an hour with a couple of friends to help!!!

[Stingrayzuke]

Took it into the dealer today. I will provide an update once I get it back and test it.

[Stingrayzuke]

Update: Everything with the motor checked out fine at the dealer mechanically. The dealer raised the motor up 2 holes. I took it to the lake by myself and wide open it hit 46mph at 5500rpm. Since I was by myself I was not able to observe the anti cavitation plate related to water level but I will do that next time out. I don’t think I would go higher though, it does well in turns but will cavitate if trimmed up much so I don’t think I want to go higher with motor position.
So the max speed seems fine, but should I be looking for additional rpm by propping down? I am just looking to put the setup in optimum position for performance and longevity of the motor, so if it should go up further in rpm, I will pursue that.