RT188 Perfomance issue, need suggestions

[BrianinMD]

Took my RT188 out for a 3 day weekend, had some great fishing but had a performance issue that I am not sure what is going on so need ideas.

The issue is the hole shot, when I did break it the boat jumped onto plane very fast. And at WOT ran 47 mph at 5800 rpm.

Now the hole shot takes a few seconds where the motor seems to be bogged down, then it hits a point the motor goes from being bogged to winding up RPM's very quickly and the boat jumps onto plane. A couple times it took multiple attempts to get on plane. At WOT speed was 41mph at 5100rpm.

The things that changed were changing batters which added 60lbs, changed props from the stock aluminum 4 blade 19pitch to a Spitfire X7 19pitch solid plugs in, and instead being by myself I had a co. I know the added weight would be a concern but was a big difference. I am wondering if something is wrong.

My rt188 has a 2015 Mercury Pro XS 115hp 2 stroke

[Tim Kelly]

Try running solo and see if it still has the same issues. If it does, put the old prop on and re-test. Your set up is about what seems to work for most people, so you need to eliminate some of the variables to see whether the problem is with the motor or the set up.

[jbh3]

Added weight is DEFINITELY a factor. Try removing one plug at a time from your new prop to improve hole shot. Set the boat up like you will run it MOST of the time - i.e. light or heavy.

[Way2slow]

Adding weight and changing props are pretty good candidates for the problem.

Also, I think I saw a post were you connected a battery wrong or something and made smoke come out of the motor.

 

You need to back up and chase one rabbit at the time.  Chase too many at once and you loose everything.

First, I would go back to my original setup and see if the performance went back to what you had before you started making your changes and the sending up smoke signals. 

If everything is good there and performs the way it did, then you have a known good starting point.   If not, then your smoke job may have caused some hidden damage.

Assuming things go back to as before, now make the changes that adds the weight, see how it performs and how much things change. 

Once you have that, put the new prop back on and try it.  It's very possible with your new setup, you have too much prop.  As mentioned also, take the plugs out and try it, that should help hole shot.

[BrianinMD]

The smoke was not me

[Way2slow]

Ok,  It still possible your motor may have developed an issue affecting performance. 

That's why you should still go back to your original setup and verify it performs the same as it did before.  Then make one change at the time. 

Most likely, it's going to come down to having too much prop for you new setup, but that needs to be verified.  Opening the vents should help hole shot, but shouldn't do much to increase WOT RPM. 

 

This is why you should always make your modifications one at the time and before messing with a the prop.  One of the first things people want to do is change to a performance prop for that "I want more speed".  Well performance props are a little more sensitive to setup so when you install one and then make changes afterwards, it's a good chance you will be buying another one.   I see people all the time buy a new performance prop, then  decide they need to install an adjustable jack plate when the procedure should be just the reverse.  The prop should be one of the last pieces for the puzzle because of how much all the other stuff affects it's performance.   

Aluminum props are a little more forgiving on setup because they are actually somewhat self tuning.  The blades on aluminum props have a tendency to flex under a load, so the more you load one up they can actually loose some pitch.  That's why they are junk for any type of performance application.

[dwtaylor]

I would start with experimenting with the plugs in the prop.  Removing the plugs allow exhaust gas to escape to the blades of the prop.  This cavitation allows the prop to spool up because the extra "air" around the blade allows the prop to turn with less torque required and can turn at a faster RPM as a result.

 

No setup is 100% identical but the issues you state make me want to point to those pvs plugs as your first thing to change.

[Way2slow]

Actually the way it works, when starting off, the back pressure created by the water causes some of the exhaust to vent out the holes, creating a controlled cavitation (controlled by the size of the hole).  This lets the motor turn more rpm and get into it's peak torque band (about 4,500 rpm is best) to help get it on plane.  Once on plane and moving forward at some speed, the back pressure is gone and the exhaust quits going out the holes and the cavitation goes away. unless you over trim and the you get a different type cavitation.  So the prop holes should have no affect on WOT rpm. Pitch and a couple other factors control that.