Slipping Prop

[ostra4]

I have a yamaha 4 stroke 2.5 outboard made in 2003 , its not had much use and been fine till last few days when the engine revs rise but no increase in speed,very much like a slipping clutch on a car .At low revs its fine .There is a split pin through the propellor nut . any thoughts anyone ,?

 

[NickRobinson]

Have you removed the prop and looked to see if the shear pin is intact?
Some OBs, usually bigger, have a bonded rubber cush drive instead of/as well as a shear pin. When the bonding gives way you get those symptoms.

 

[macd]

The split pin stops the prop falling off the drive shaft. There's usually also a shear pin, which fails when the torque on the prop might otherwise damage it, such as with a line around the prop or whan clouting the bottom. Sounds like your shear pin has failed. You can buy an original one, or find the relatively recent thread about home-made ones.

It's always worth carrying a spare shear-pin, typically lashed to the steering handle with insulating tape.

 

[duncan99210]

Just looked at the workshop manual for the engine. It doesn't have a shear pin. The prop shaft is splined and the prop has a rubber cushion mounting with splines to match the shaft. What I think has happened to the OPs engine is that, at some point in time, his prop has had a meeting with something solid. The join between the prop, the rubber cushion and the splined bush has now broken down so that the shaft is turning but the prop is slipping round.
Three fixes for this. Expensive one is a new prop. Cheaper is take prop to repair guy who may be able to reattach runner cushion. Cheapest is to drill a hole in the prop boss to take a self tapper, this will work fine for a time but will break in the end.

 

[ostra4]

Thank you for those very quick responses .Iv had gusts on our boat this week and yesterday was first time I noticed the slipping prop but as Id just dropped crew off I really wanted a relaxing evening and will check out the prop issue properly today armed with the information you folk have sent .

 

[Eeyore]

Yep.......no shear pin in this engine. Prop has bonded rubber bush inside the boss and they fail after a while. I have seen repairs with two self tappers through the boss into the rubber bush, but the only solution is a new prop!

 

[ostra4]

Just an update , took prop off this morning and drilled hole for self tapper as suggested , problem solved so now Im happy ,well happy ish to buy a new propellor , thanks for the advice every one .