Yanmar 30GM shaft drive gearbox transmission

[dunedin]

Recently had the engine serviced and a new shaft seal fitted. All seemed good. Engine a Yanmar 30GM circa year 2000, used mainly for manoeuvring

However have noticed an unusual whine from the transmission or shaft at circa 1,500 to 2,000 rpm when going ahead. Also heard the same noise when sailing with the shaft spinning before we engaged reverse (which locks shaft and prop folds).
Then coming into the berth yesterday, possibly going a bit faster than normal due to crosswind (perhaps 1.5 knots) and tried to move out of forward and engage reverse had no reverse thrust. Full reverse and didn't stop. (fortunately got spring on and rope stopped us so more drama than sister this time)
Clearly must have gone from forward gear to neutral - otherwise full reverse throttle would be full ahead and very nasty (seen that done when gear cable broke and a boat went up onto pontoon). Was like i had pressed red neutral button accidentally, but don't think that is likely.
Later after sorting out berthing I engaged reverse gently and worked fine - lots of reverse power (now safely secured)

Any ideas for explanations of the whine, and could it be related to the failure to reverse ?

Also good links to an idiots guide / non engineers explanation of the Yanmar gearbox ?

I am unlikely to repair myself, but need some clues as to where to start. SWMBO is naturally very worried about using again until understood / fixed. Thanks

 

[earlybird]

The Yanmar gearbox has 2 sets of cone clutches, forward and astern. These have a self-servo action; ie the more torque that is transmitted, the harder they grip.
The first check is that the selector arm on the gearbox is reliably moving fully and equally either side of neutral, the Yanmar manual specifies 35 mm either way, 70mm total at the cable attachment point.
The cable connection to the lever is a spring joint, does that seem in good repair, with no excess play?
Is the gearbox oil level OK?
Can't think of any more simple checks, more advanced adjustment or general wear assessment is a bit more specialised, so hope this helps.

 

[Gargleblaster]

I a year ago tightened up my Deep Seal Seal as it had been leaking a bit. The tightening certainly stopped the leak but introduced a whine, which has not gone away. I live with the whine as it is better than the leak which of course introduced water into the boat when the seacock was closed.

I have had a Yanmar gearbox twice stick in gear when the linkage came off. Very embarrassing and one time causing a lot of damage but not at all similar to your description of what happened.

 

[awol]

If the seal was replaced, the shaft has been out of the coupling. Unlikely scenario is coupling not tight enough so that shaft moves back in reverse but forward pushes it in again. Folding propeller messing about? Good luck!

 

[dunedin]

Thanks all. Some great pointers to investigate when next at the boat

 

[dunedin]

So an update. The engineer who did the shaft seal replacement took a look at this today.

Gearbox oil level fine, shaft attachment fine, gear linkage fine. But he too found when first tried it lacked stern thrust - but being safely tied up could check and prop-shaft turning away in reverse, so reckons must be a prop issue.

Prop is a Gori 2-blade folder - and never had any issues in 6 years. Also boat just re-floated so should be super clean (we polished the prop when out) - but I guess could have picked up debris.

I could imagine the prop failing to open when first used, ie when reversing out of the berth after not being used (I do tend to try to check opening before heading off if been away for a while - but never found an issue). What I can't understand is how could have an issue on coming back in - prop must be open to have motored in.
So how could a Gori folder which has been excellent in reverse suddenly go from hero to zero ?

Co-incidentally just bought a cheap underwater USB endoscope camera so will give a look at the prop with this to see if any clues