"Spinning out" effect??

[rhino_160]

Took the 22' Pontoon out the last couple weekends and seem to have an issue. The motor is a Suzuki 70 fuel injected and I can run 20mph at about 4400 RPMs. The issue is that every now and then at or near full throttle the motor will jump to 7000 RPMs but the MPH slows down. It's similar to a car doing a burnout so to speak, lots of power but not really going anywhere. I can throttle down and back up and it may or may not fix it. The only "fix" I have found is to put it in reverse then back in forward. It's like the propeller is turning faster than the water can be churned or the propeller is out of the water. It only does this every now and then. Rode for 4 days and did it about 5 times. Any ideas?

 

[MinUph]

If it clears after you put it in reverse its probably weed wrapped around the prop. Happens to me all the time in the rivers when there is a lot of crap in the water. Doesn't take much with my motor.

 

[Nauteal]

Rhino 160 - I had that same issue with my 22' pontoon with a 90hp Mercury last summer. The prop hub was basically worn-out. I ended up buying a new prop after running my boat, motor and my passenger load specs. through a "prop calculator." I used Mercury's "prop selector" to perform my search and ended up with even better performance than I had before. All you do is plug in your information as best you can and it will give you a range of pitch's to work in.

I believe my old prop was a 14 pitch. Although it had no markings. I ended up with a 13 pitch and I get a consistent 22mph out of my boat now. My boat and motor weigh in at 2200 pounds and my load w/boat runs between 2800 to 3500 pounds.

Even though you have a Suzuki, check it out just for understanding purposes if nothing else.
​http://www.mercurymarine.com/propellers/prop-selector/#

 

[MH Hawker]

Mark your hub and prop and look at the marks later to see if its slipping.

 

[dpeslinger]

Your prop is more than likely cavitating. (sucking air) run your trim a little lower and this will probably stop.

 

[BigDfromTN]

If putting it in reverse, Then back forward would lead me to believe you are getting some debris on the prop or lower unit.
If it is simple cavitation, or slippage. your throttling down and back up should fix it. Just my .02

 

[wgl333]

It is probably either slippage or "ventilation", not cavitation. Ventilation occurs when the prop is too high in the water and gets caught up in the prop wash, thus spinning in more air than water. When it occurs, try trimming the motor downward, giving the prop more water to bite in to.....my guess is it will correct the problem, or at least rule out ventilation as the problem.