Spinning prop

[Birtie]

Hi everyone we have a snapdragon 747 ,with 20hp inboard engine, yesterday we lost propulsion, got recovered to Totnes boat yard ,lifted out,propeller is free spinning on shaft ,not had chance to take prop off yet anyone any ideas on what to expect ?

 

[Sticky Fingers]

Lost the shaft key?

 

[Birtie]

Not sure how it is connected to shaft can't see a groove for shaft key

 

[CliveF]

Lost shaft key or the shear pin has gone Just depends which method was used. Only way to find out is take the prop off and go from there.

 

[Birtie]

Not sure how it is connected to shaft can't see a groove for shaft key

 

[duncanmack]

There is a woodruff key on mine.

 

[Fr J Hackett]

I once chartered a Vancouver 32 which lost the shaft key halfway through the charter, I made it back by every time the prop lost drive I put it into reverse when the prop slid back up the shaft and locked on the taper but sailing would periodically push it back down. Made for an interesting week as I either prepared to anchor or enter a harbour.

 

[NormanS]

I once chartered a Vancouver 32 which lost the shaft key halfway through the charter, I made it back by every time the prop lost drive I put it into reverse when the prop slid back up the shaft and locked on the taper but sailing would periodically push it back down. Made for an interesting week as I either prepared to anchor or enter a harbour.

Sorry, but that doesn't make any sense. If you go ahead, the prop is pushed onto the taper, and conversely when you go astern, the prop is pulling the boat, and therefore trying to come off the taper.

 

[Fr J Hackett]

Sorry, but that doesn't make any sense. If you go ahead, the prop is pushed onto the taper, and conversely when you go astern, the prop is pulling the boat, and therefore trying to come off the taper.

That's what happened. I think it's the opposite if it were not the prop would always be in contact with the taper and you wouldn't know it had lost the key.

 

[bikedaft]

Sorry, but that doesn't make any sense. If you go ahead, the prop is pushed onto the taper, and conversely when you go astern, the prop is pulling the boat, and therefore trying to come off the taper.

maybe LH prop or vice versa?

when ours went was the opposite, no astern.

 

[bikedaft]

and it was the woodruff key, sheared along its length.

 

[PCUK]

Not sure how it is connected to shaft can't see a groove for shaft key

If the woodruff key sheared, it may look as though it is part of the shaft as the spinning prop' will have polished it.

 

[Birtie]

Fingers x , unfortunately can't get back to boat yard till Sunday,to get prop off, would you know if it has likely damaged inside of prop ?

 

[Pye_End]

Insufficient tension on prop nut. How is the nut locked?

 

[SteveG]

Had same problem many years ago. Previous owner was obviously a plonker who fitted a brass woodruff key, maybe thinking it was a bronze one. Key dissolved away leaving a thin brass framework in the corners of the groove. Luckily I did not lose the prop. Made a bronze key, but then the nut split into two as I tightened it - wrong material again! Local marina engineer made me a new bronze one on the spot for £5 before the tide came in. Problem solved.

 

[NormanS]

maybe LH prop or vice versa?

when ours went was the opposite, no astern.

No matter whether RH or LH prop. When driving ahead, the propeller is pushing the boat, and is therefore pushing onto the taper on the shaft.

 

[VicS]

Not sure how it is connected to shaft can't see a groove for shaft key

If it is a Woodruff there is just a short machined slot in the shaft , which you wont see but there will be a key way the full length of the bore in the prop, however obscured by the nut but you shout see it at the forward end if theres nothing obscuring that.

If the prop is on a taper with a Woodruff key the nut must be tightened to the correct torque so that the drive is transmitted by the taper , not by the key. If the prop is not tight on the taper the key will very likely be sheared.

 

[Fr J Hackett]

No matter whether RH or LH prop. When driving ahead, the propeller is pushing the boat, and is therefore pushing onto the taper on the shaft.

It will attempt to push the boat forward but there will be a reactive force against and in the opposite direction which will if unsecured will force the prop down the taper. Try it remove the locking nut on the end of your prop shaft and go for a short motor.

 

[NormanS]

It will attempt to push the boat forward but there will be a reactive force against and in the opposite direction which will if unsecured will force the prop down the taper. Try it remove the locking nut on the end of your prop shaft and go for a short motor.

Rather, I would suggest that you do it. Don't go astern, 'cos if you do, you'll lose your prop.

 

[Daydream believer]

A few props have a rubber bush inside them as an electrolytic cut off. It also works as a sheer section if the prop catches a line etc to prevent damage to the drive. If the op has had issues with this over the years without knowing it may have sheared years ago. The idea being that one can still get in to port . But if one carries on motoring it will finally let go completely and the prop will just spin.