can not get prop right (now what?)

OK i tried another tach and it says the same thing, i do not doubt your word on a 90 not turning that many rpm's. so i guess the next question is where does the rpm signal come from. it must be getting the wrong signal, right?
i have just read the very informative post by walleyehead in the frequently asked questions and his knowledge matches all the available text that is on the manufacturers and prop sites. and i still can't get my prop right. i have a 1998 suntracker signature pontoon with a 1999 merc 90 hp. i got the boat used but in great visual shape. it came with a 15 pitch prop just as the tracker factory book says it should. first time in the water it had terrible performance. it felt as though it was pushing water and felt and looked nose down. trimming did not help and it blew out with minor adjustments up on the tilt. also the rpm's were over spec at nearly 7000. i went to a 17 with some improvement and about 200 rpm drop. so i tried a 19, still better and the nose was coming up and i had more trim use but still to high on rpm. it came down about 100 more. next step was a 24, because it was available. this one is steel where the others were aluminum. and it came from a 90 hp merc on an aluminum express bass boat. still to high at 6500 and not much better feel on performance. my tach started fluctuating wildley during my test so i thought maybe the tach was bad so i borrowed one from a friend and got basicly the same reading on it but without the fluctuations. what do you think, bad rectifier?, need more pitch?, need more blades?. i never heard of a pontoon boat with this big of a prop. scratching my head.

Re: can not get prop right

Re: can not get prop right

I would seriously consider a new tach before I went any farther...
That 90 is not spinning a 24 at 6500RPM on a toon.
It may be as simple as the tach being set in the wrong position (setting on the back).

Re: can not get prop right

Re: can not get prop right

I don't know the length of your toon, but I punched in 26ft in the merc prop selector for the 90, and it says 13-15 pitch.

Re: can not get prop right (now what?)

It sounds to me like you may be cavitating and causing your rpms to go so high. I had the same problem with my express X54. A 17 pitch prop would run about 6400 rpms on a 90 4 stroke yamaha. I went with a 19 pitch solas and rpms were 6400 rpms. I added a 6 inch jack plate and the rpms went down and the aluminum props stop showing signs of cavitation burn on them ( paint coming off the edges both leading and trailing). There would be some pitting on them too. A polished stainles prop wont show signs of burning for a while, you may want to borrow an old prop from someone and paint the edges if they are not already painted and run it for about 3 to 4 trips and see if the paint looks like it is pitted or comes off like something that has been put in a sandblaster ( only on the edges) and see if you are cavitating. Most places recommend that you put the cavitation plate of the motor even with the bottom of the keel of a bass boat, I'm not sure on a pontoon.

Re: can not get prop right (now what?)

I agree that there is no possible way a 90 HP engine can twist a 24P prop in excess of 6000 rpm on a pontoon unless it is ventilating. Venting and cavitating are two different problems by the way. A nose down attitude would add to that problem. When you are sitting at the dock with the engine running, what does the tach read? If it is not 750 RPM or so, the tach is just one of the issues. If the idle rpm is off, the top end and everything in between is off as well. A nose down attitude on a pontoon is also an indication that there may be water in the tubes or that the boat is simply loaded heavy in the nose. Even a properly set up pontoon will perform poorly if everyone dashes to the front. You need to take a more logical approach to this problem rather than just throwing props at the boat. Is the engine mounted as low as it will go. Is something hanging down under the boat that is disrupting water flow to the prop.

Re: can not get prop right (now what?)

Yep, a 24 on a 90 on a toon at 6000+ is pretty unusual. Might be a good rig to run a custom 24 or 25 or 26 on. I just happen to know where some are.....

Seriously, I thing the guys are on the right track with the tach setting and the slipping issues.