1991 StarCraft Mr 190 high rpm

[Bv24]

Hello I'm new to this fourm and boat ownership in general so any advice will be helpful. I just recently purchased a 1991 StarCraft aluminum center console with a 115 mercury on it. The motor runs excellent but it is running at about 5500-5750 rpms wot and only getting speed right around 30. When I bought the boat it had a prop with a 10 pitch. Something I asumme the guy had laying around and it would run at about 6500 rpms and go about 15mph. I purchased a new 21 pitch prop and now I am getting on a plane quickly but running the high rpms and only getting a top speed of about 30mph. I know the boat is only recommend to run a max of 5250 but as far as I can tell I cannot go any higher in pitch to drop the RPM. Is there any other way to lower the RPM and any other way to get too speed. The boat also has a black rubberized bedliner like coating on the bottom of it. I know it will loose a little bit of speed because of it but I didn't think it would be that much. I'm going to peel it off just thought it might be helpful. Also i have already checked the setting on the tach and it is set on "2". The tach does sit at 1000 without the engine on. Any and all advice or ideas would be helpful. Thank you in advance Brett

 

[jbcurt00]

Moved to the Mercury outboard forum

:welcome: to iboats

 

[Chris1956]

What year Mercury? Does it have power trim? Is the boat aluminum and what length? That rubberized coating will rob some speed.

If your tachometer is accurate (doubtful), the motor is turning plenty fast, however, 23-26" pitch props are usually available if necessary to reduce RPM. However, I would wait until you know your tach is accurate before repropping.

 

[Bv24]

It is a 1991 and yes power trim.it is a 19 foot aluminum hull. How could I tell if the tachometer is accurate? I know there is the setting knob on the back but I'm not sure what to set it at.
Thank you
Brett

 

[Chris1956]

Tach should be set to 6 pulse or 12 pole, whichever it has. I would think that boat at the RPM you said would be hitting 40+MPH.

 

[Bv24]

The tach has the setting of 1,2,3,4. Any idea which to use? And that is what I had thought right around 45 or so. I have herd to switch to a 17 pitch? What are your thoughts?

 

[Chris1956]

If she gets on plane quickly with the 21P prop, I do not see the need to switch to a lower pitch prop. You might look up your tach on the internet and see if it compatible to your motor. OB tachs count stator pulses to determine RPM.

 

[CharlieB]

Look into a cheap inductive pick-up tach. I have one that is just a little black box slightly bigger than a small box for wooden matches. It only has one wire that you simply wrap around any spark plug wire. Very handy to test any gas engine, lawn mower, weed-wacker, etc..

Once you know your motor RPM then you can judge whether your prop is close enough. Sometimes a prop can be close, but the next pitch may be too much. When that happens take your prop to your local prop shop and ask if cupping it a bit will drop your RPM the amount you want. Until you know RPM I wouldn't change anything yet.

 

[Bv24]

I will look into that. I am thinking that a large part of it is the black stuff on the bottom a out like bedliner. Any thought on taking it off? Good idea or bad?I don't want it to leak a ton of water.

 

[Watermann]

If that bed liner garbage is allowing water under it then you'll have serious corrosion issues to deal with. I would remove it but I don't envy you trying to get that stuff off the bottom of the hull, it would be bad enough working on it where you can get to it. I have no idea what would remove that junk, maybe some chemical stripper? If the boat was gutted and you could flip it over to leave the stripper on a flatter surface then it might work pretty good.