Higher RPM Prop Spinning

[garytally]

Hello...Brand new to forum and Mercury Outboards. I just acquired an old 1992 Mercury 175 Optimax (2 stroke of course). Put in water last weekend and ran great--up to about 45MPH on a heavy 22 foot deck boat. After running around about 15 minutes the prop appeared to start spinning on the shaft, but only at high RPMs. At low RPMs the motor runs and sounds fine. At higher RPM there is no "crunching" or other loud noise...just the apparent "spinning." Someone told me that there was a rubber connector that might have just deteriorated or I may have hit something that caused it to fail. Any ideas would be greatly appreciated.

 

[444]

Your prop has a rubber hub that connects the output shaft on your motor to the actual propeller. This hub is only a tension fit. The reason for this is it allows the prop to slip a bit if you hit a rock or something as to not transmit the full force of the impact into the output shaft and shear off your output shaft splines. Also serves to absorb harmonic resonances at speed. If you or the last guy hit something with that prop, the hub will begin to spin like this where you can get thrust at lower power levels but at higher power levels it will slip. Your options are replace the engine prop or have a new hub installed. Note that new hubs need a hydraulic press to install.

 

[garytally]

Excellent. Thank you Sir 444! So, just to confirm, I can EITHER replace the prop OR have a new hub installed? I actually have an extra prop so that might be easier. Thanks again.

 

[Dukedog]

check on flo-torque hub kit for your prop.... all tha v6 mercs went to this type hub many years ago... doesn't press in.. you just put it on yourself..............

what prop is on it now?

 

[Scott Danforth]

is your hub spinning, or is the motor ventilating. you stated that this is a new to you outboard.

to check your prop if that is spinning, take a sharpie marker, mark a line across your prop, hub, and prop nut. go for a boat ride where you get it to act up. come back. if the line moved between the hub and prop, its your prop hub spinning. if the line didnt move, your motor setup is wrong and your ventilating the prop.

 

[Texasmark]

Quickest and easiest test for you per the suggestions offered, would be to just stick on your replacement prop. If problem changes significantly or disappears, chase the prop. If not, chase something else and get back on here with more information about what you have and how it used to work and works currently sort of things.