2015 115 Merc on 20' pontoon

[trstanley54]

I just bought a 2015 20' Starcraft pontoon with a 2015 115 4 stroke Mercury and I'm not getting the speed I assumed I would get. It currently has a 13 3/4" x 15" prop with 2.07 gear ratio. I've only had it out twice, but I was only getting 22 mph at 5700 RPM with the motor trimmed up and 3 people onboard. It also pulled to the right pretty bad. I've never had anything with a big outboard before, so I'm a little clueless on what I need to do. Any help would be awesome.

 

[velociraptorfarmer]

It's a pontoon, and not even a tritoon. That's probably about all the more you're going to get out of it, even if you threw a 150 on there. As long as that RPM is in the 115's specified WOT rpm range, you're good.

As for pulling, I'm guessing either weight wasn't distributed properly, one of the toons has water in it, or you don't have power steering, could be a combination of those too.

 

[CaptnKingfisher]

It's a pontoon... That's probably about all the more you're going to get out of it...

I agree with this poster on the speed issue. As far as the boat pulling to the side, you can adjust the trim tab to correct this. See pic. Little adjustment goes a long way

 

[QBhoy]

I’d suggest that you probably should have been sold the CT gearbox version of the engine.

 

[trstanley54]

I’ve seen a lot of people with 90s and 115s that are getting mid to upper 20s mph. I was hoping for something similar out of this one but it’ll do everything I want it to as is. Kinda sucks though. Every guy needs a little more speed.

 

[Texasmark]

If the top of your RPM band is 6000 which I think it is on 4 strokers, you are getting about all you can get out of the engine. On turning to one side or the other, get behind the engine and look down at the lower section just above the prop.

I never saw a 4 stroke lower unit but I suppose what you want to remove and look beneath is the rectangular rubber (looking) plug on the top right of your last picture.

I see that at one time the engine had a "Whale's Tale" bolted to the lower unit. ......bolt holes in the Antiventilation plate.....horizontal plate in your last picture. With that said, your trim tab is probably missing and that is your problem. How about a picture from behind the engine looking straight at the rear of the prop taken about 3' from the rear of the engine. Post that and we will know for sure.

Remove it and peer down in the hole. If you had one, there is probably a Hex Socket screw in there. The screw is to secure the trim tab...just under the Anti-ventilation Plate). Functionally, you loosen the screw and turn the rear of the tab in the direction your boat wants to go. The more degrees off center you turn it the more effect it has on correcting the problem. If you go too far the wheel will star turning the other direction. Iboats, Ebay or your marine dealer should have one for sale.

Well I shopped iboats first and that was a joke!!!!!! So I went to ebay and here is something that should work:

Mercury 105-115-120-125-135-150-175-200 Anode Trim Tab Bolt ALUMINUM 31640T07 | eBay

Replaces: 31640T07, 31640T5, 316405, 12670. - Honda BF 75-90 (for Honda the bolt may not work properly, but thhe trim tab will). (4) Liquid like maintenance products. (3) Paint and chemicals. (1) Open software.

www.ebay.com

The device mounts up under the Antiventilation plate and sticks down.....there is a round detent in the plate to accept it. For right hand turning props (like yours), mine are set about 15 to 20 degrees with the tail to the Starboard side (right side) looking from the rear.

Hull type, loading, and hull condition play a big role in how a boat handles and performs. Your hull looks like it could use a Muratic Acid bath or a good scrubbing. When I used to go to the inlaws place on vacation, with the boat sitting in the water just a week, I could tell the difference in performance and it was a slick (surfaced) fiberglass runabout.

 

[trstanley54]

I didn't know the CT would make quite that big of difference in performance. I got a good deal on the boat so I'm not real disappointed on the speed side of things. Most of the time it won't be going fast anyways. I just wanted to have something I could wakeboard behind once in a while since we sold our ski boat last year. It has plenty of low-end so shouldn't be a problem. It will be on a covered lift all summer, so cleaning up those pontoons is definitely on the list.

 

[QBhoy]

I didn't know the CT would make quite that big of difference in performance. I got a good deal on the boat so I'm not real disappointed on the speed side of things. Most of the time it won't be going fast anyways. I just wanted to have something I could wakeboard behind once in a while since we sold our ski boat last year. It has plenty of low-end so shouldn't be a problem. It will be on a covered lift all summer, so cleaning up those pontoons is definitely on the list.

Ct makes a huge difference. Just about every test they’ve done, if not all…has the ct outperform the normal version. It also runs the larger performance props available too. It even outperformed the 125 optimax in tests too.

 

[jlh3rd]

You're in the right rpm range, so no help there. Here are my results: Just me on board....'06 Lowe 24' twin 25" tubes pontoon, double bimini.
'05 Mercury 60 hp, 4cyl., 4 stroke, efi, big foot (ct)....20 mph ( cool day, cold water)
My new mercury 115hp, CT.......26 mph, but, that's at 55-5600 rpm.....I'm trying a different prop.....

I'd see about that trim tab....the fact my motors are CT's may be the difference

 

[QBhoy]

You're in the right rpm range, so no help there. Here are my results: Just me on board....'06 Lowe 24' twin 25" tubes pontoon, double bimini.
'05 Mercury 60 hp, 4cyl., 4 stroke, efi, big foot (ct)....20 mph ( cool day, cold water)
My new mercury 115hp, CT.......26 mph, but, that's at 55-5600 rpm.....I'm trying a different prop.....

I'd see about that trim tab....the fact my motors are CT's may be the difference

Definitely think the CT makes a huge difference. I’d also say that having tried lots of props on my CT…she definitely is at her best when propped right at the very top of the rpm range. I run a tempest plus to just about or a little over 6000 rpm. Propped like this, the cruising speed is much better too.

 

[ahicks]

2 things. First, IMHO, you don't need to turn a 4 stroke at max rpms for max speed. The prop that allows for max rpm might have a better hole shot/get out of the water quicker, but there's no guarantee it going to go faster than the next size bigger prop.

I don't care for how busy a 4 stroke sounds at max rpm. I set them up with a target of about 5500, and get a much better sounding motor, and very often an extra couple mph over the prop running at max rpm. There is no doubt about the hole shot though.

2nd, it might be worth raising the motor up for a speed increase. As long as it will still turn at speed without blowing out, there is NO downside to raising. The higher you can run it, the faster it will go.....

Just a thought, but for wake boarding/towing, you may want to go 4 blade....

And last, only for reference, our 90hp Honda on our fairly light 24', runs a 15p at 5500, and runs 25-26mph. Still does a nice job pulling the kids tubing.....

 

[QBhoy]

2 things. First, IMHO, you don't need to turn a 4 stroke at max rpms for max speed. The prop that allows for max rpm might have a better hole shot/get out of the water quicker, but there's no guarantee it going to go faster than the next size bigger prop.

I don't care for how busy a 4 stroke sounds at max rpm. I set them up with a target of about 5500, and get a much better sounding motor, and very often an extra couple mph over the prop running at max rpm. There is no doubt about the hole shot though.

2nd, it might be worth raising the motor up for a speed increase. As long as it will still turn at speed without blowing out, there is NO downside to raising. The higher you can run it, the faster it will go.....

Just a thought, but for wake boarding/towing, you may want to go 4 blade....

And last, only for reference, our 90hp Honda on our fairly light 24', runs a 15p at 5500, and runs 25-26mph. Still does a nice job pulling the kids tubing.....

This particular 4 stroke definitely runs at its best up near 6000 rpm. No doubt about it. Just from experience and having a bit of a prop perfection ocd thing. Fuel economy at cruising speed is much better too, when set up near the top. Just what I’ve found with mine. After 1000 hours clocked on her.

 

[ahicks]

This particular 4 stroke definitely runs at its best up near 6000 rpm. No doubt about it. Just from experience and having a bit of a prop perfection ocd thing. Fuel economy at cruising speed is much better too, when set up near the top. Just what I’ve found with mine. After 1000 hours clocked on her.

I'll defer to your experience if you have that kind of time on this motor. My experience much more generic.

 

[Scott Danforth]

I would take care of the corrosion on the foot caused by the 4 holes from where someone bolted on a whale tail. that bubbling paint is corrosion.

 

[QBhoy]

I'll defer to your experience if you have that kind of time on this motor. My experience much more generic.

You’ll be right I’m sure Ahicks. No doubt. Suppose all I can offer is around this particular engine. I have no experience with other 4 strikes this size at all. Only W we had this and a small merc 3.5

 

[trstanley54]

I still don't understand why the CT and every other brand of 115 motors gets quite a bit more speed than the standard Mercury. I had planned on getting a 4 blade at some point, but I didn't want to end up getting the wrong pitch and making it slower. It's already borderline on speed for wakeboarding so I do have some concern with that. I bought it while everything was still frozen so I took a risk buying without driving first. I figured it would need some tweaking and maintenance. Any recommendation on which 4 blade to get?

 

[Sea Rider]

Why do you need to run that motor with trim up, under that scenario will only achieve a slight top end encrease on flat calm water condition measured with a GPS, but will surely experience the boat veering sideways and lose control at speed if with too much trim up which motor manufacturers don't recommend going for.

That toon needs to be run with deck load evenly distributed, motor set to neutral trim. 90 deg angle. Iideal once toon is on plane on flat calm water have someone pull his head side of the motor, check and report at which lower leg height is the hardened water flow passing by. Ideal is to skim right under the upper small water deflector plate or as close to it as possible, to accomplish that, play rising or lowering the motor one notch at a time till that precise scenario is achieved, Nothing to lose, much to win!!

That new motor setting will surely increase the top end speed a bit provided that the motor is revving at least middle to its max wot rpm range as loaded. Try both deck load distribution and new motor trim parameters as indicated, will see how the veer issue is instant canceled. You can always go for a prop maximization to other props if the current prop is not a nice water performer..

Happy Boating


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[jlh3rd]

You're in the right rpm range, so no help there. Here are my results: Just me on board....'06 Lowe 24' twin 25" tubes pontoon, double bimini.
'05 Mercury 60 hp, 4cyl., 4 stroke, efi, big foot (ct)....20 mph ( cool day, cold water)
My new mercury 115hp, CT.......26 mph, but, that's at 55-5600 rpm.....I'm trying a different prop.....

I'd see about that trim tab....the fact my motors are CT's may be the difference

got out on the river today with my new prop. The prop that gave me those previous results on the new Mercury 115 hp was a mercury 4 blade, 14x14 .
The new prop is a mercury, black max, 3 blade , 16x13. It was windy and a little rough today, but I saw 27 mph as a peak and 6000 rpm .....so I'm where I want to be......