Mercury 115 fourstroke high rev low speed

[dussbake]

Boat and Engine:

Brand new Mercury 115 Fourstroke CT for a 24' pontoon.

Problem: High RPM's but not much thrust

Prop: 3 blade 14X11 (issue started first time on the lake with new motor. Bought a new 4 Blade Spitfire 13.8x14. Problem persisted. Also need to mention new prop came with new Flo-Torq II hub so we can rule out spun prop.

Motor is mounted all the way down and when trimmed down prop sits a good 18" - 24" in the water.

Best guess: some sort of ventilation with exhaust or pitch is just to much for a heavy boat. I'm at the end of my rope and hope the experts can chime in. If it is exhaust ventilation I can't be the first to experience this, right?

Review: Not a spun prop... trimmed all the way down... motor mounted all the way down...don't mean to sound like a smart ***** just trying to be clear and specific as possible. Thank you for your help guys ( and gals)

 

[dussbake]

Please Help! RPM issues

Good day all,

I have a 24' Fisher Dlx pontoon that came with a Mercury 115 4-stoke. Long story short, I have re powered with the new Mercury 115 4-stroke Command Thrust which contains the bigger gear case (2.38).

The issue I'm having is that I cant get the prop to "bite" when given throttle. I took my old 14x11 3 blade prop that the original motor came with and put on it so I figured it was a spun prop. Bought a new Spitfire 13.8x15 4 blade with flo-torque II hub and got the same result.

I can put-put around but that's it. My next best guess was ventilation but the motor is mounted all the way down and with motor trimmed all the way down the prop sits about 18"-24" below the surface.

The only other thing I could think of would be cavitation. Could the larger gear case spin the slightly smaller diameter (13.8) so hard that it's causing the low pressure bubbles? I figured the extra surface area of the 4 blade would make up for the slightly smaller diameter. Maybe a bad move on my part, idk.

Lastly, I can't gather data because I can't get to WOT to record RPM's. I obviously think I need a larger diameter but unsure on the pitch because of this. Im within a 1/4" of what the prop selector tool called for on diameter but I feel as if I'm a long way off because of lack of data. I just need something to "bite" so I can log the results.

So I'm at the end of my knowledge and appreciate yalls input. I realize this is probably more of a prop question for the other forum but nobody is responding over there, I guess because I'm talking about a pontoon. Lol

After all of that what I've learned so far is that it's not a spun hub, motor is mounted as low as it can go, and trimmed all the way down. I do apologize for the long post; I just wanted to provide as much detail as possible.

Happy Father's Day to all the dads out there!

 

[dussbake]

Please Help! RPM issue

Good day all,

I have a 24' Fisher Dlx pontoon that came with a Mercury 115 4-stoke. Long story short, I have re powered with the new Mercury 115 4-stroke Command Thrust which contains the bigger gear case (2.38).

The issue I'm having is that I cant get the prop to "bite" when given throttle. I took my old 14x11 3 blade prop that the original motor came with and put on it so I figured it was a spun prop. Bought a new Spitfire 13.8x15 4 blade with flo-torque II hub and got the same result.

I can put-put around but that's it. My next best guess was ventilation but the motor is mounted all the way down and with motor trimmed all the way down the prop sits about 18"-24" below the surface.

The only other thing I could think of would be cavitation. Could the larger gear case spin the slightly smaller diameter (13.8) so hard that it's causing the low pressure bubbles? I figured the extra surface area of the 4 blade would make up for the slightly smaller diameter. Maybe a bad move on my part, idk.

Lastly, I can't gather data because I can't get to WOT to record RPM's. I obviously think I need a larger diameter but unsure on the pitch because of this. Im within a 1/4" of what the prop selector tool called for on diameter but I feel as if I'm a long way off because of lack of data. I just need something to "bite" so I can log the results.

So I'm at the end of my knowledge and appreciate yalls input.

After all of that what I've learned so far is that it's not a spun hub, motor is mounted as low as it can go, and trimmed all the way down. I do apologize for the long post; I just wanted to provide as much detail as possible.

Happy Father's Day to all the dads out there!

 

[alldodge]

Good day all,

I have a 24' Fisher Dlx pontoon that came with a Mercury 115 4-stoke. Long story short, I have re powered with the new Mercury 115 4-stroke Command Thrust which contains the bigger gear case (2.38).

The issue I'm having is that I cant get the prop to "bite" when given throttle. I took my old 14x11 3 blade prop that the original motor came with and put on it so I figured it was a spun prop. Bought a new Spitfire 13.8x15 4 blade with flo-torque II hub and got the same result.

I can put-put around but that's it. My next best guess was ventilation but the motor is mounted all the way down and with motor trimmed all the way down the prop sits about 18"-24" below the surface.

The only other thing I could think of would be cavitation. Could the larger gear case spin the slightly smaller diameter (13.8) so hard that it's causing the low pressure bubbles? I figured the extra surface area of the 4 blade would make up for the slightly smaller diameter. Maybe a bad move on my part, idk.

Lastly, I can't gather data because I can't get to WOT to record RPM's. I obviously think I need a larger diameter but unsure on the pitch because of this. Im within a 1/4" of what the prop selector tool called for on diameter but I feel as if I'm a long way off because of lack of data. I just need something to "bite" so I can log the results.

So I'm at the end of my knowledge and appreciate yalls input. I realize this is probably more of a prop question for the other forum but nobody is responding over there, I guess because I'm talking about a pontoon. Lol

After all of that what I've learned so far is that it's not a spun hub, motor is mounted as low as it can go, and trimmed all the way down. I do apologize for the long post; I just wanted to provide as much detail as possible.

Happy Father's Day to all the dads out there!

Sorry no one has responded to your question but posting in two other forums is not a good way. Best to post "Bump" in another post to bring it to the top

In any case Howdy

Your saying your getting high rpms, just how high is the rpm's?
Was the original 115 4-stroke moving the toon OK?

 

[HotTommy]

dussbake,
First, there is a forum here just for prop issues. It can help with problems like this too. .... Second, I have the same engine on my two log 24' pontoon boat. I'm running a 14.8" x 15 pitch four blade propeller with no issues. The only other thing I can suggest you check is how far your prop goes below the motor pod.. There are people here who know more than me who can suggest how far below. But the prop does need to be clear of turbulence from the pod.

 

[ahicks]

Is this a conventional 2 tube 'toon, or a tri-toon?

The horizontal/flat part of the lower unit that's located right above the prop is called a cavitation plate. It's supposed to prevent air from being sucked from the surface and ventilating the front of the prop.

That horizontal surface needs to be below the bottom of the transom. How far below depends on the design of the engine pod or center log depending on the design of your boat. If it's even with the transom, or slightly above, you can get into the problem you describe pretty easily.

 

[GA_Boater]

Your three threads have been merged into this one. As AllDodge said, starting more than one thread on the same subject causes too much confusion.

The Prop Questions section seems to be the proper place for your thread.

 

[dussbake]

I apologize for the duplicate post... I just figured since I wasn't getting a response here that I would post in the pontoon forum...anyways, thank you all for taking the time to help me figure out my problem. It is much appreciated.

It only has two floats and the cavitation plate sits just below the surface of the water. I am switching back to the original prop today and putting the new hub that came with the other prop in it today, torque it down, pray over it and see if there's a difference.

The motor is already mounted all the way down so I'm not sure what to do if it needs to go deeper. Trimmed all the way down, the water level sits just below the pee hole.

Not sure what to do if it is just turbulence causing the issue but surely there is something that can be done.

Thanks again guys!

 

[dussbake]

I will take pictures if I can and post them later on if I can. Weather may be an issue today so not 100% positive I can accomplish this.

 

[ahicks]

I apologize for the duplicate post... I just figured since I wasn't getting a response here that I would post in the pontoon forum...anyways, thank you all for taking the time to help me figure out my problem. It is much appreciated.

It only has two floats and the cavitation plate sits just below the surface of the water. I am switching back to the original prop today and putting the new hub that came with the other prop in it today, torque it down, pray over it and see if there's a difference.

The motor is already mounted all the way down so I'm not sure what to do if it needs to go deeper. Trimmed all the way down, the water level sits just below the pee hole.

Not sure what to do if it is just turbulence causing the issue but surely there is something that can be done.

Thanks again guys!

Not sure how both of the highlighted comments can be true? My bet is something is getting lost in translation here, but I don't know what.

An engine so low the pee hole is just above the water level, would be pretty unusual, unless we're talking about a badly overloaded boat? The cavitation plate is generally checked out with the boat at rest, so you can compare it's height with the bottom of the transom.

 

[dussbake]

I've got pictures but it says they are to big so trying to figure out how to resize them. When I say "pee is just above the water I'm talking all the way trimmed down. All looks normal. Boat is not weighted down and previous 115 4S pushed her along just fine even weighted down. Beginning to lean towards shifter cable issue rather than prop because it just doesn't make since with new prop/hub/motor. When you shift into reverse there's a definite "cluck" but not into forward even though you feel it change. May not be engaging forward all the way. No grinding just high revs and little thrust...can't go over 5 mph.

 

[ahicks]

I've got pictures but it says they are to big so trying to figure out how to resize them. When I say "pee is just above the water I'm talking all the way trimmed down. All looks normal. Boat is not weighted down and previous 115 4S pushed her along just fine even weighted down. Beginning to lean towards shifter cable issue rather than prop because it just doesn't make since with new prop/hub/motor. When you shift into reverse there's a definite "cluck" but not into forward even though you feel it change. May not be engaging forward all the way. No grinding just high revs and little thrust...can't go over 5 mph.

Yes, something not right there. I think maybe we've been on the wrong track to get to the bottom of this issue. Checking out your adjustments, maybe making sure cable ends are anchored properly, etc. a good plan. Best of luck!

 

[alldodge]

Use something like google docs, photobucket or other third party site and link back to here

 

[Sea Rider]

You seem to have a bad OB/transom height match along an incorrect trim setting, once both are dialed right can go for a prop maximization to prop OB right for toon to perform as expected as usually. loaded.

Happy Boating

 

[dussbake]

No Title

Don't think it would be a bad height transom match as the previous motor was a 115 also with same 25" shaft. if it isn't the prop and I shifter cable adjustment issue would this result in slippage of dog gear under load even though there is no grinding or vibration? I looked for a 3rd party manual but nothing that new came up. Latest I could find was 2007

 

[dussbake]

Quick update: To tired to mess with it today...I will get better pics of the boat out of water tomorrow as long as the good Lord is willing and the creek don't rise

 

[dussbake]

No Title

Well, I lied... I couldn't quit thinking about it so I took the cowling off and observed shift cable position in FWD, N, and REV. It was slightly different when shifting from Fwd back to Nuetral than it was when I shifted from REV back to Nuetral. So I made a slight adjustment ( Maybe about 1/8") and in seemed to make the positions more consistent. The shift also felt better but won't know until I put it in the water under load. Also put the new prop back on ( not seen in pic)

Hopefully the pics will give y'all a better understanding of where the cavitation plate is relative to the bottom of the transom hull.

 

[ahicks]

Well, any thoughts about the engine being mounted too high are pretty much out the window. If anything, it's too low!

Hopefully your adjustment did the trick.

 

[GA_Boater]

A motor mounted too low can cause this kind of issue. The midsection causes much drag and more than the midsection is in the water at rest.

How did she sit and perform with the old motor? Any water in the toons?

The stern is pretty low and you may not be able to raise the motor enough, but I would raise it as much as possible to see if there is any improvement.

 

[dussbake]

She sits in the water well and and I can raise the motor completely out of the water. I get the same result at all different positions so no improvement there. Any difference between old Mercury 115 and this one is slight if any. That's why this is so puzzling. No spun hub and should be deep enough to rule out turbulence. No water in toons.