over-revving 1998 115HP

[oceanpotion]

After trolling for kingfish all morning at about 800-1200RPM I kicked my motor up 2500RPM to go through the inlet. The motor revved up to 3500RPM with no increase in speed. It ran fine at 1800-2000RPM so it made back onto the trailer without having to bother anyone. As soon as the motor hit 2500RPM it would revv up to either 3500-4000 on the tach without any increase in speed or power. Its a 1998 Johnson 115HP Im going to put the spare prop on and see if it is a bad hub.. any ideas what else it could be?

Thanks

 

[wilde1j]

Re: over-revving 1998 115HP

Prop slipping:
1. Mark inner and outer hubs with indelible marker
2. Run boat to planning speed
3. Compare marks ? if they?ve moved relative to each other, hub is slipping
4. Remove prop
5. Take it to a propeller shop and have it rehubbed

 

[oceanpotion]

Re: over-revving 1998 115HP

I put my spare prop on and backed it under water at the ramp. Both props over revved at 2500rpm on the trailer..

 

[wilde1j]

Re: over-revving 1998 115HP

Maybe both are spun.

 

[oceanpotion]

Re: over-revving 1998 115HP

both props were spun ..
thanks

 

[oceanpotion]

Re: over-revving 1998 115HP

1998 Johnson 115HP 15 X 17 RH ALUMINUM PROPS

Now prop 3 started to 'spin' .. erratically
same thing as the first 2 propellers, when the tach hit 2500 it revved itself up to nearly 4000 with no change in forward motion.

we think that the brand new prop and the used prop that the mechanic sold me were both encountering cavitation caused by air off an excessive sea weed build up on a self-installed thru hull fitting used for a livewell. the thru hull was mounted less than 12" from the stern and the sea weed has been thick for the past week in this area.

the mechanic went for a ride in the intracoastal on my boat and told me to run it up while he listened. the propeller didnt slip or anything. when the mechanic turned the boat sharply on plane he could make it slip out and completely overrev.

so he told me to remove the thru hull, patch the hole, and move it to a more suitable area. suggested hydrofoil stabilizers to prevent air from entering above propeller. he also suggested rearranging the boat heavier in the stern.

its just weird that all three props spun out at exactly 2500 rpm must be the maximum rpm cavitating?