Pontoon 70hp revs high at full throttle

[firemanp31]

I have a 20 ft pontoon and i recently installed a 70 hp tohatsu. I used to have a 30 hp johnson. At full throttle both motors at times reved high for a second or two and I seemed to loose power. I Replaced the tank with a new one but it still does it. Any suggestions. The motor has no power tilt or trim.At less than full throttle it seems fine

 

[flashback]

Re: Pontoon 70hp revs high at full throttle

sounds to me as if you are experiencing cavitation. I never had a pontoon boat so i don,t know much about them. Do they plane? If not, then they would be considered a displacement hull and increasing power would not make the boat go a whole lot faster but the prop would start slipping. maybe a prop change could help......good luck

 

[bruoff]

Re: Pontoon 70hp revs high at full throttle

Hey P31, I raised my kids on an old Aloha 24'.It would cavitate under extreme turns and sometimes in rough water. I'm now back to pontooning on another 24'er. Try dropping the motor one pin hole toward the transom. That will usually remedy the problem. To respond to flashback's<br />comment. "They sorta' plane!" The toons will ride up on the water a "little bit"in response to more horsepower. Back on Kentucky lake I've seen 24'ers with 150's throwing a rooster tale that won't quit. Mine was powered by an Evinrude 70 and it had enough power to ski the kids.<br />Have a goodun'

 

[BillP]

Re: Pontoon 70hp revs high at full throttle

I agree with the others. It is likely cavitation, which seems to be commom on pontoon boats. I have a 20' pontoon with 50 hp. I can trim it down to reduce the problem. Weight up front makes it worse or when in big chop. RPMs are correct at top speed so changing props probably isn't the fix.I really think motor installions on pontoon boats need to be lower than normal. <br /><br />Also, I've been considering the installation of a Doelfin to help reduce cavitation. I put one on another boat (17' skiff w 70hp) and it helped cavitation at top speed when trimming up for speed...it made a HUGE difference.

 

[rickdb1boat]

Re: Pontoon 70hp revs high at full throttle

A Doelfin helps alot on a toon, as far as cavitation. I put one on mine years ago and have been very happy with it. Jumps up faster to speed and no noticable Cavitation. I can turn very hard and it still does'nt blow out. Love them! Give it a try and I think you will be suprised!

 

[flashback]

Re: Pontoon 70hp revs high at full throttle

after reading Tarpon Bob,s post, I tend to think he is right.. you are most likely driveing the bows down into the water at full throttle. by changing the angle on the motor you can improve on things.. the doel-fin will also help.....

 

[rickdb1boat]

Re: Pontoon 70hp revs high at full throttle

Actually, you may want to try it one pin hole OUT.. Why? Because the more you trim IN, the More the bow lowers and the Motor lifts, causing Cavitation. I know this sounds wierd, but I seen it happen on a relatives toon. When we trimed it in, it blewout like crazy. Then we tried one hole out from where it was in the beginning and it ran fine. Maybe different toons react differently!

 

[rickdb1boat]

Re: Pontoon 70hp revs high at full throttle

Tarpon Bob,<br />What prop are you running. Size and Pitch! I also have a '91 70 HP on a 20' toon. Just curious, as I am running a 14X13 Alumnimum! Also, what kind of speed are you getting, if you don't mind? Thanks,