115 Pro XS Command Thrust on a pontoon - prop?

[emilime75]

Will try and make this as TLDR friendly as possible. Both props tested same day, same conditions, same load. RPM confirmed via VesselView and speed via GPS. Would like to be able to hit max RPM(6300) or just above with this load as it is about as light as we're ever going to be. Also confused by top speed, research tells me this boat with this engine should be in the 30s.

2018 22' pontoon(dual) with lifting strakes and under skinning.
2018 Mercury 115 Pro XS 4 Stroke Command Thrust(2.38 gear ratio).

Original prop - 14.5 x 17P Solas Rubex aluminum 4 blade = 4400 RPM at 19 MPH.

New prop - 16 x 12P Mercury Black Max XC(Extra Cup for pontoons) aluminum 3 blade = 5500 RPM at 21 MPH.

What would be recommended as the next prop I try? Budget does not allow for stainless. Merc prop calculator suggests their 14 x 12P SpitFire aluminum 4 blade.

 

[dcwilson]

Sub'd for info on this as well. I have a 2014 115 4 stroke Command Thrust on a 22ft pontoon. I am also only getting 4500RPM at 21mph with a 15.5 x 15p aluminum 3 blade prop. Should be up between 5000-6000RPM with this engine, but not getting near there.

 

[Texasmark]

Well the Go Fast prop slip calculator says that your 4 blade is yielding a 37% slip and your 3 blade a 19% slip. Large diameter and shallow pitch are the traits of a pusher prop and being shallow in pitch means that RPMs will usually be up which yours are.

Per DC Wilsons comment it seems you are in the middle of your RPM range with the 3 blade.....not perfect but fine. I'm not a toon guy but I would guess that tubes don't give you a lot of lift and planing surface so a 19% slip isn't a bad number. Your RPMs are ok, speed is improved, I'd leave well enough alone unless you just want to puke money which you indicated wasn't on your agenda!

 

[dcwilson]

Well the Go Fast prop slip calculator says that your 4 blade is yielding a 37% slip and your 3 blade a 19% slip. Large diameter and shallow pitch are the traits of a pusher prop and being shallow in pitch means that RPMs will usually be up which yours are.

Per DC Wilsons comment it seems you are in the middle of your RPM range with the 3 blade.....not perfect but fine. I'm not a toon guy but I would guess that tubes don't give you a lot of lift and planing surface so a 19% slip isn't a bad number. Your RPMs are ok, speed is improved, I'd leave well enough alone unless you just want to puke money which you indicated wasn't on your agenda!

Just to specify, my engine is a regular 115 4 stroke with the Command Thrust gear ratio, 5000-6000 max rpm. The OP is a Pro XS engine which is rated for 6000-6300 max rpm, so he is a bit low with the 3 blade prop.

 

[dcwilson]

Bump on this thread. Trying to figure out what prop to go with for the summer. 4500 rpm at 21mph just isn't cutting it. Using a prop slip calculator I'm at 22% slip which is high from what I understand. I need to get at least another 1000 rpm and would like to be high 20's for speed. Do I have other issues with only getting 4500 rpm with a 15p prop and a 2.38 gear ratio? Something doesn't seem right. Engine has no codes or warnings. Prop calculators are saying I should go with a Spitfire 4 blade 14X12p or a 3 blade 16X12p or 13p for my application which should do 27mph at 5500-5750rpm... provided it gets there... The more reading I do the more a 12p or 13p prop sounds underpropped with a 2.38 gear case? Sure wish I could test a couple different props...

 

[QBhoy]

Bump on this thread. Trying to figure out what prop to go with for the summer. 4500 rpm at 21mph just isn't cutting it. Using a prop slip calculator I'm at 22% slip which is high from what I understand. I need to get at least another 1000 rpm and would like to be high 20's for speed. Do I have other issues with only getting 4500 rpm with a 15p prop and a 2.38 gear ratio? Something doesn't seem right. Engine has no codes or warnings. Prop calculators are saying I should go with a Spitfire 4 blade 14X12p or a 3 blade 16X12p or 13p for my application which should do 27mph at 5500-5750rpm... provided it gets there... The more reading I do the more a 12p or 13p prop sounds underpropped with a 2.38 gear case? Sure wish I could test a couple different props...

Is that numbers from the vessel view data ?
Totally different boat I have…but same engine. I run a 19 and a 20” prop on the CT box. But different boat all together.
What type of 15” is on it now again ? Is the trim used and is the height of the engine correct ? Any pics of the set up ? Sounds like you need a good prop and the revs to lift the hull a little. All the power in these engines is up the top end too.

 

[dcwilson]

Is that numbers from the vessel view data ?
Totally different boat I have…but same engine. I run a 19 and a 20” prop on the CT box. But different boat all together.
What type of 15” is on it now again ? Is the trim used and is the height of the engine correct ? Any pics of the set up ? Sounds like you need a good prop and the revs to lift the hull a little. All the power in these engines is up the top end too.

Thanks for the response. Yeah that's from vessel view data.
Yeah, non pontoon 115 CT's seem to be in that 19-20 pitch from what I can find.
Its a BlackMax XC Pontoon 15.25X15 prop (48-78116CP1). Last fall I checked the height of the engine and it seemed to be in a good position. I don't have a photo of the setup on hand, but can get one.

 

[QBhoy]

Thanks for the response. Yeah that's from vessel view data.
Yeah, non pontoon 115 CT's seem to be in that 19-20 pitch from what I can find.
Its a BlackMax XC Pontoon 15.25X15 prop (48-78116CP1). Last fall I checked the height of the engine and it seemed to be in a good position. I don't have a photo of the setup on hand, but can get one.

Do the revs increase when you trim up ? No doubt a better prop will help. One that can run close to the surface and not lose grip like the one you have will easily do. Seems a big diameter too. Maybe that’s what pontoons need. Not sure. Know nothing about them to be honest. All I know is that unless you can get that motor up near the 6000 rpm…you’re losing out. That’s where the real power is and it comes alive when over 5000 for sure. A 15x15 ally will be hard pushed to do much for you I think. But honestly don’t know how things go with these boats. I know my CT runs incredibly well with a big tempest plus on her. But different boats as we mentioned. When I was messing with props on mine…over propping her just ruined it. Despite the specs saying the range starts in the low 5’s…it was useless down there. So much better when propped up near or just over 6000 rpm flat out. I’d try running a smaller diameter prop if you can perhaps. Stainless one too. See if you can lift the hull any more. That will give you rpms. Then the more rpms you get…the more lift again.

 

[dcwilson]

Do the revs increase when you trim up ? No doubt a better prop will help. One that can run close to the surface and not lose grip like the one you have will easily do. Seems a big diameter too. Maybe that’s what pontoons need. Not sure. Know nothing about them to be honest. All I know is that unless you can get that motor up near the 6000 rpm…you’re losing out. That’s where the real power is and it comes alive when over 5000 for sure. A 15x15 ally will be hard pushed to do much for you I think. But honestly don’t know how things go with these boats. I know my CT runs incredibly well with a big tempest plus on her. But different boats as we mentioned. When I was messing with props on mine…over propping her just ruined it. Despite the specs saying the range starts in the low 5’s…it was useless down there. So much better when propped up near or just over 6000 rpm flat out. I’d try running a smaller diameter prop if you can perhaps. Stainless one too. See if you can lift the hull any more. That will give you rpms. Then the more rpms you get…the more lift again.

It's been a long winter, I don't recall if the rpm's increased when trimming up, but I want to say yes? I did try a different prop that was the wrong hub size which caused extremely bad ventilation (only went 5mph or so), however the engine did rev right up to the rev limiter so the rpms did get up there, albeit under basically no load.
Here's some photos of the engine mounted. It's mounted pretty low but it seems to be in a decent position from what I understand.

 

[QBhoy]

Looks very low.

 

[QBhoy]

That’s not a pro xs. Max rpm is 6000 on that. Looks like an xl shaft too.

 

[dcwilson]

Looks very low.

Yeah it seems a little low to me as the CT leg is longer than the regular 115. I am having a tough time figuring out what the optimal engine position is for a pontoon though. The engine "pod" varies model to model so you can't really measure from that. Any insight on where to measure from?

That’s not a pro xs. Max rpm is 6000 on that. Looks like an xl shaft too.

Correct, I'm not the OP of this thread (second post), I've just hijacked it because I have the exact same questions.

 

[QBhoy]

Yeah it seems a little low to me as the CT leg is longer than the regular 115. I am having a tough time figuring out what the optimal engine position is for a pontoon though. The engine "pod" varies model to model so you can't really measure from that. Any insight on where to measure from?


Correct, I'm not the OP of this thread (second post), I've just hijacked it because I have the exact same questions.

I’ve got a ct. it’s not like that though. That’s an xl shaft I’m sure.

 

[dcwilson]

I’ve got a ct. it’s not like that though. That’s an xl shaft I’m sure.

Yeah I thought all CT's may have had that XL shaft but this is my first outboard, don't know a whole lot about them. Learning though. Going to do some measuring but I think you're right it does need to come up a bit I think. I'm also going to get a 14X14 4 blade prop (Spitfire or Nemesis). I PM'd another member with a similar pontoon and same engine. He was able to achieve 27mph at 5750rpm with that 14X14 prop. Using prop calculators that seems to be the sweet spot to achieve around 28mph with a 15% slip or so. The next option would be the 14X12 but it's top end would be around 27 at 6000rpm so there wouldn't be much wiggle at the top.

 

[QBhoy]

Yeah I thought all CT's may have had that XL shaft but this is my first outboard, don't know a whole lot about them. Learning though. Going to do some measuring but I think you're right it does need to come up a bit I think. I'm also going to get a 14X14 4 blade prop (Spitfire or Nemesis). I PM'd another member with a similar pontoon and same engine. He was able to achieve 27mph at 5750rpm with that 14X14 prop. Using prop calculators that seems to be the sweet spot to achieve around 28mph with a 15% slip or so. The next option would be the 14X12 but it's top end would be around 27 at 6000rpm so there wouldn't be much wiggle at the top.

Sounds like you aren’t too bad as you are for props.
Standard CT 115 would be a long shaft. I know that there was and is a serious shortage of normal LS engines here. A few xl around though.

 

[jlh3rd]

Fyi and my results. I have a 2007 24' lowes ,twin tube pontoon boat. 25" tubes, no lifting strakes. Double bimini top, always up.
I replaced the merc stock 60 hp last summer with a new 2021 115 CT.
They put a spitfire , 4 blade, 14x14 on it. Also, It had one lower mount position available if needed.
With the 60, early spring, just me, cold air and water, I could get 21 mph, smartcraft tach @ 58-5900...gps speed.
Last fall with the new motor and last run of the year, warmer conditions though, I'd get 26-27 mph, but only about 5500-5700 rpm. Pulling tubes with kids and adults on board during the summer, I was in the 5200-5300, rpm range...too low....
I did have a blowout issue when uptrimming( another story) but now seems rectified with lowering the motor.
So, I put a Hustler 4 blade 14x13 on it. hope it gets me where I want...

 

[dcwilson]

Fyi and my results. I have a 2007 24' lowes ,twin tube pontoon boat. 25" tubes, no lifting strakes. Double bimini top, always up.
I replaced the merc stock 60 hp last summer with a new 2021 115 CT.
They put a spitfire , 4 blade, 14x14 on it. Also, It had one lower mount position available if needed.
With the 60, early spring, just me, cold air and water, I could get 21 mph, smartcraft tach @ 58-5900...gps speed.
Last fall with the new motor and last run of the year, warmer conditions though, I'd get 26-27 mph, but only about 5500-5700 rpm. Pulling tubes with kids and adults on board during the summer, I was in the 5200-5300, rpm range...too low....
I did have a blowout issue when uptrimming( another story) but now seems rectified with lowering the motor.
So, I put a Hustler 4 blade 14x13 on it. hope it gets me where I want...

Thanks so much for the input, this is great information to have. Yeah that 14X14 speed and rpm sounds just like the other member I talked to. I do agree a 14X13 would potentially be a better fit, however, I can't seem to find a 14X13 4 blade with a 4.75 hub for the CT gearcase. Were you able to find that Hustler 4 Blade in a 4.75 vs 4.25? I can only seem to find 4.25.

 

[jlh3rd]

well...I have a merc 2021, 115hp, CT. just the regular 115. So whatever gear case comes with that motor is what i have. It had the merc spitfire,4 blade, 14x14. The Turning Point "Hustler" 4 blade, 14x13 actually fit right on the spitfire's hub, and matches the diameter of my gearcase. So, I'm not quite sure what motor has what gearcase, or what you need.
Does the PRO series, or commercial series merc motors have a different gearcase?
I just know this prop fits my motor.

 

[dcwilson]

well...I have a merc 2021, 115hp, CT. just the regular 115. So whatever gear case comes with that motor is what i have. It had the merc spitfire,4 blade, 14x14. The Turning Point "Hustler" 4 blade, 14x13 actually fit right on the spitfire's hub, and matches the diameter of my gearcase. So, I'm not quite sure what motor has what gearcase, or what you need.
Does the PRO series, or commercial series merc motors have a different gearcase?
I just know this prop fits my motor.

That’s interesting. From everything I read, all CT gearcases have a larger torpedo and take larger diameter propellers (4.75”), the same as the higher HP outboards. This allows for more prop selection as well. You can’t run a 4.25” propeller on it because the exhaust will just blow over the prop causing extreme ventilation (ask me how I know). It will fit on the gear case fine, but there will be a gap for the exhaust to blow out. Do you have a part number for that Hustler prop?

 

[QBhoy]

Ct does have the large gearcase. So needs to run the props that suit it. It’s the same size props that usually fit the Mercury engines of 150hp and above. Also the same as the mercruiser alpha and bravo 1 outdrives.