Mercury 115 fourstroke high rev low speed

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titaniumneck

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I'm just asking, because I really don't know, but on a pontoon boat does the motor have to be the same depth like on a bass boat, with calvintation plate just above bottom of boat?
 

GA_Boater

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Your raise and my raise are different. Maybe I'm not being clear. I mean lift the motor on the transom, not by using tilt and trim.
 

ahicks

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Nope. These things are much more difficult to guess correctly as the tubes never really get on top completely, and the bottom of the transom is quite a bit higher than the bottom of the tubes.
 

ahicks

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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.

The guys are talking about the installed height of the engine - how the engine lines up on the boat vertically. When we say raise the engine, it's about using the holes that are lower in the engine mounting bracket - not tilting the engine out with the trim.
 

dussbake

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Recommended mounting height according to manual says 20" from top of outboard mounting brackets to bottom of transom. I will check where it's at this afternoon. By looking at pictures I would think its close. I can fine tune once we figure out the issue but I wouldn't think that not being in "sweet spot" would cause the symptoms I'm having. I would at least have enough thrust to move faster than 5 mph I would think. It was installed by a Mercury dealer as well.
 

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GA_Boater

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Did the dealer water test after installation? Doubtful he did if it was performing like this.
 

Sea Rider

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When combo does not performs good , the usual way to go is playing with trim positions with no positive avail, OB/transom optimizations along a prop maximization it's a day/night difference. If possible post a lower leg pic shot "sideways showing both horizontal plates" if combo is out of water, if not, fully tilt OB and take a good broad shot.

In real boating world OB heights, trim positions, should not be dialed on wild guesses, assumptions, a faster way to determine what needs to be done is looking what's going on at rear lower leg....

Happy Boating
 

dussbake

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Current mount height is 19.5" from top of mounting brackets to bottom of transom. Well within reasonable limits to perform. I agree optimization may be difference between night and day but that's for a later date. I agree focus should be lower foot and since prop/hub shouldn't be issue I'm leaning towards internal components I.E. dog gear engagement which should be resolved by shift cable adjustment to make sure they totally engage, right? Given the circumstances I'm pretty sure they didn't test it in the water.
 

ahicks

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OK, back to basics.
If you look at the picture showing the view from in back of the engine, it looks like the prop is located below the bunks that are supporting the 'toons. That means the engine is sitting plenty far down, and the potential for venting at low speed should be pretty much impossible. As well, I don't see anything in front of the prop that might screw up the water flow to the prop. So.......

Back to potential for possible slipping somewhere in the drive train. You might try standing on the prop blade with the engine in gear. Check while in forward and reverse. If it slips, try standing on the other side of the prop. One side should hold in both forward and reverse. If it won't go 5 mph, if the prop is bad, or linkage not working for whatever reason, something should show up in that test. If it checks OK, then something else is going on that's not allowing that prop a proper bite on the water.

Is there any chance the old engine uses a different prop shaft, and we have the wrong prop on this new engine? Have you discussed this problem with the selling dealer?
 

dussbake

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Will do. I have called the dealer but that was in regards to the initial spun prop thought. I will give them a call today. Quick question: If in gear there should be resistance but eventually you would turn the engine (crank turn over), right? I know I've said this before, but I appreciate all of you and your patience in helping me sort this out.
 

ahicks

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I'm a pretty good size guy, and I've always been able to tell pretty easily if there's something going on in this test. If the prop holds, you'll know it. I don't ever remember having an engine turn over on me, even down into the 40hp range.
 

dussbake

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It held in FWD and REV. I wish I would have done it before I adjusted the shift cable the other day but maybe that fixed it as well. We are having some serious rain because of tropical storm Cindy so will test in the water as soon as I can.
 

dussbake

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I thought I would give a follow up for anyone that experiences this issue with a Command Thrust engine. The solution was a bigger diameter prop. Finally got to speak with someone at Mercury that helped me. Since you don't see many CT models on pontoon boots it seems this issue was rare.

Basically, because of the bigger gear ratio it was causing cavitation with the normal size prop. Its like having a big motor on small tires. Jumped up to a 15 1/2" diameter prop and it solved the issue. Also have 15 degrees of pitch and my rpm's are right in the middle of the upper WOT rating. (5500 rpms).

Thanks guys for all your help and I hope this thread is useful to someone else in the future.
 

dcwilson

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I just wanted to bump this thread with another possible solution to anyone having this problem. I have a Mercury 115 CT on a 22ft pontoon as well and was getting bad ventilation on the prop at higher RPMs. Turns out the Prop Bore Sleeve was missing. The small white nylon plastic spacer that seals the gap between the lower unit and the front of the prop hub.IMG_0885.PNGIf it's missing (which is very common because they break) it allows exhaust gases to flow over the front of the prop at higher RPMs causing ventilation. So it's important to make sure the bore sleeve is intact and you are using the proper hub size propeller for the CT which is the same size as 150+ mercury motors. Standard 115 props have smaller diameter hubs which will cause extreme ventilation. Hope this helps someone out.
 

emilime75

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I just wanted to bump this thread with another possible solution to anyone having this problem. I have a Mercury 115 CT on a 22ft pontoon as well and was getting bad ventilation on the prop at higher RPMs. Turns out the Prop Bore Sleeve was missing. The small white nylon plastic spacer that seals the gap between the lower unit and the front of the prop hub.View attachment 349150If it's missing (which is very common because they break) it allows exhaust gases to flow over the front of the prop at higher RPMs causing ventilation. So it's important to make sure the bore sleeve is intact and you are using the proper hub size propeller for the CT which is the same size as 150+ mercury motors. Standard 115 props have smaller diameter hubs which will cause extreme ventilation. Hope this helps someone out.
Yup. I had the same issue with my 2018 115 Pro XS CT. It was missing the prop bore sleeve and would ventilate at about 4000-4200 RPM. Luckily it's only like a $5 part and takes about 5 minutes to install. I ordered extras to keep on hand as it appears very common for them to break. With that, I'm having a hard time choosing the right prop for my 21/22' pontoon. Starting another thread about that so as to not muddy up this one.
 
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