FORCE 75 H.P performance

LLS858

Recruit
Joined
Aug 15, 2010
Messages
4
i have a force 75 hp on a pontoon boat .first ten minutes in water ,runs fine ,above half throttle while riding along then rpms go way up like something is slipping i have replaced prop with brand new prop,still same problem please help !
 

roscoe

Supreme Mariner
Joined
Oct 30, 2002
Messages
21,842
Re: FORCE 75 H.P performance

Welcome to iboats.

If the prop was slipping, the engine rpm's would climb, and you would be slowing down while the engine was speeding up.
 

87blf125

Petty Officer 3rd Class
Joined
Jul 23, 2009
Messages
98
Re: FORCE 75 H.P performance

when does it happen? just goin straight or when turning? once it starts does it keep doin it? don't know if it is the right word but cavatation " air through prop" my boat kida did the same thing when i put a hydrodoil on it and it lifted the back of the boat up so much that my prop was sucking some air and the rpms would go up like it was slipping. plus i also seen a guy that didn't know he had the spacer that goes behind the prop on and put another on on top of the old one and his sounded like what you are discribing.
 

roscoe

Supreme Mariner
Joined
Oct 30, 2002
Messages
21,842
Re: FORCE 75 H.P performance

Really doubt he has a Cavitation problem, not with that motor, and the props that are available for it.
 

87blf125

Petty Officer 3rd Class
Joined
Jul 23, 2009
Messages
98
Re: FORCE 75 H.P performance

sorry roscoe but please reread his post his rpms are going way up
 

87blf125

Petty Officer 3rd Class
Joined
Jul 23, 2009
Messages
98
Re: FORCE 75 H.P performance

please read the post by stevetedder " cavitation/aerateing problem" posted on the pontoon forum on june 13th 2010
 

roscoe

Supreme Mariner
Joined
Oct 30, 2002
Messages
21,842
Re: FORCE 75 H.P performance

My apologies. For some reason, my brain saw the words "going away", as in falling off. My mistake. Brain fart.

As for the cavitation or ventilation problem, it was not resolved in the post by stevetedder. So we don't know what his problem was.

The truth is that cavitation rarely affects performance in this way.

Ventilation does, but on a pontoon, 98% of them, the motor is mounted so low that half the leg is under water. And the boat doesn't lean as much as on a v hull. So ventilation is unlikely to occur unless the motor is mounted unusually high, or the operator has trimmed the motor too high - causing a blow out.

I guess we really need more info, such as how large the pontoon is, what rpm's he is running, and how low the motor is mounted in relation to the water level. Also the prop pitch.

There is a slight chance that the new prop had a spun hub, or that the pitch is too steep causing it to spin, but LLS858 seems to have ruled that out.

But ventilation doesn't explain that he can run the boat without problem for 10 minutes, then the trouble starts.
 

87blf125

Petty Officer 3rd Class
Joined
Jul 23, 2009
Messages
98
Re: FORCE 75 H.P performance

true on the needing more info roscoe
 

LLS858

Recruit
Joined
Aug 15, 2010
Messages
4
Re: FORCE 75 H.P performance

prop pitch is 8, i talked to a local marine dealer in parts: he said the pitch is appropriate, i have also changed lower unit oil too with a synthetic oil. in my repair manual, it shows a clutch dog on prop shaft. could this be slipping after oil has warmed up ?
 

roscoe

Supreme Mariner
Joined
Oct 30, 2002
Messages
21,842
Re: FORCE 75 H.P performance

Not unless it is making lots of noise and stopping completely.

The clutchdog is a a large square tooth "gear", either its in, or its out.
Anything in between and the corners are banging into each other making a racket. sample photo below.

22663-2-johnson-outboard-clutch-dog-47.95.jpg




Yep, prop sounds about right.
 
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