75 Mercury gearbox probs

[Paora]

So, I've started to cruise on sunny saturday morning with my 1990 75HP Mercury motor on the back of my 18 ft inflatable Boat. All of a suddent the motor suddenly revs out, as if it jumped out of gear. The boat only travels at 6 or so miles an hour now. Any ideas what happened. Know where I can download any diagrams of dismantling instructions for this motor?<br />Any help would be greatly appreciated.

 

[Trent]

Re: 75 Mercury gearbox probs

Most likely you have a spun prop hub. You cant really look at it and tell. Put your spare on and check it or take it to a prop shop and they can check the hub Or you can mark the hub and go run it and come back and see if the mark moved.

 

[Paora]

Re: 75 Mercury gearbox probs

Forgive my apparent ingnorance. I think we may have a different name for differing parts (cos I'm from the South Pacific) what is, and where is the hub?

 

[Franki]

Re: 75 Mercury gearbox probs

your prop is the outer shell with the blades attached.<br /><br />The hub is the center piece that the body of the prop is attached to, there is usually a rubber sleeve pressed between the two..<br /><br />if that has deteriorated or damages, the prob can spin on the hub and as a result you will either not go anywhere, or you'll go places but rather slowly..<br /><br /><br />rgds<br /><br />Frank

 

[Paora]

Re: 75 Mercury gearbox probs

Thanks team, checked it out, its certainly the hub, its worn and broken. Are they repairable or do you simply replace the whole prop?

 

[Trent]

Re: 75 Mercury gearbox probs

Yes they are repairable. A propeller shop can press a new hub into it.

 

[Franki]

Re: 75 Mercury gearbox probs

This brings me to a question I have wondered about for awhile..<br /><br />my prop has a chip missing from one blade, its only about 4 mm deep, and about 1cm long.... I was wondering how much extra vibration I am getting from this and is it bad to run the motor with a chip like that?<br /><br />also, I am told it can be fixed easily by a prop machinist.. they can weld up the gapp and machine it even with where it should be..<br /><br />my question is... will the heat of the weld change the strenght of the prop?? is it likely to get brittle in that area?<br /><br />Is there any reason I should replace the prop as opposed to repairing it? <br /><br />rgds<br /><br />Frank

 

[dmessy]

Re: 75 Mercury gearbox probs

By the time you replace the hub assembly and repair the prop, you'll have the cost( or very close to) of a new aluminum prop. Do yourself a favor and get a new one.

 

[wippb]

Re: 75 Mercury gearbox probs

I have taken props that are big time damaged into a dealer and sometimes they will exchange the prop for a reconditoned one for a lot less money then a new one.