2005 Yamaha F115 Low Rpms

[pilot4net]

I have a Yamaha four stroke on the back of a 18'6" Xpress Bass boat (aluminum boat) and I cannot get the rpms above 5250 rpms and Yamaha's rpm range is 5000-6000 rpms according to their website. Presently the prop is a 13-1/4 x 19 inch three-blade prop which is what Xpress says Yamaha recommends for this application. I have read that for optimum performance as for as power and economy, the engine should be running near the top end of the recommended rpm range at WOT, so I would say that my rpms are a little lacking in that aspect. The manufacturer is being real good with this and is sending me another prop to try out which will be a 13-1/4 x 17 inch four blade from PowerTech. He wants to see if the lowered pitch will increase my rpms and he chose a four blade to try to remedy a porpoising problem I was having. If my engine isn't coming up to the top rpms, then the porpoising may have been because the motor wasn't putting out it's full power decreasing the bow lift and causing it to fall. Here are my questions: What could be causing the engine not to come up to the recommended rpms? It's still a new engine and I haven't had the 10 hr. tune up done yet. Could this be the problem? I could get the rpms to come up some more, but the porpoising becomes unbearable at about 5400-5500 rpms so it is not practical. Do you think the new prop will help my problems or make them worse? Any info you guys can give me will be greatly appreciated. Thanks.

 

[imported_JD__]

Re: 2005 Yamaha F115 Low Rpms

Are you over trimming the engine in trying to get your WOT? That will cause porpoising as well as improper weight distribution (too heavy an engine, all batteries & fuel tank in rear).<br />Could be that the transom has very little slant or angle. So much so that the engine can't be trimmed down/tucked enough to prevent porpoising. Transom wedges will correct this but if the boat is new you shouldn't have to be "doctoring" on it just to get it to perform normally.<br />Nows the time. I would make them get it right or take it back.

 

[Ray Neudecker]

Re: 2005 Yamaha F115 Low Rpms

Sounds like a good time to put a straight edge on the bottom to see if you have a rocker in it.

 

[rodbolt]

Re: 2005 Yamaha F115 Low Rpms

I am with JD<br /> and the only prop reccomendation that yamaha will make is the one that allows the engine to turn near the top of the recommended RPM range. a quick run with test wheel part#YB1624 will tell ya if its prop or engine. the porposing is a mounting/hull issue.

 

[pilot4net]

Re: 2005 Yamaha F115 Low Rpms

"Are you over trimming the engine in trying to get your WOT? That will cause porpoising as well as improper weight distribution (too heavy an engine, all batteries & fuel tank in rear)."<br /><br />I can run the boat all day long without porpoising with the boat running 48-50 mph with an rpm of about 5100-5200. When I try to trim the motor out some more in order to get more of the boat out of the water and increase the rpms as well as speed, the boat will start picking up speed and rpms start to increase but the porpoising starts and gets worse the more I trim the motor out. To stop the porpoising, I just have to trim it back in and the speed goes back to what I showed earlier as well as the rpms; all of this at WOT. The boat performs great as long as I leave it at the 5100 to 5200 rpm range, but when I try to get the rpms to increase in order to achieve better economy/performance, my only way is to trim the motor out since I'm already at WOT and the porpoising soon follows.