Gearbox Clutches

[Sailfree]

On my Volvo 2040 saildrive the forward clutch went prematurely (2 seasons) such that boat went astern but no drive forward.

I had the gearbox replaced but the question on this forum reminded me that I don't really understand how the clutches work and why one would fail/wear out.

The boat was fitted with a feathering prop. Theoretically prop would feather or forced to feather by engaging reverse.

Could anyone explain to me or point me in the right direction of how the clutches work and speculate as to whether a feathering prop could contribute to the failure.

I am just about to fit a feathering prop to new Jeanneau with yanmar engine/gearebox.

Many thanks

 

[Ruthie]

Hello, Sailfree.

The only reference I had lying around is Nigel Calder's "Boatowner's meachanical and electrical manual" which has a short section on transmissions, which will give you an overview. Not much substitute to actually looking at the stuff though, 'cos visualising how an epicylclic gearbox works is strangely tricky.

Putting it very simply: in forward gear, a clutch is engaged that connects the prop shaft directly to the engine drive shaft. In reverse, the forward clutch is disengaged, and a brake-band locks a geared hub that surrounds the drive shaft. Inside the hub, a set of epicylclic gears attached to the prop shaft then cause it to spin in the opposite direction to the engine shaft. There - clear as mud ;-)

However, I would very much doubt that your feathering prop had anything to do with gearbox failure, which (happily) is rare. Much more likely that the unit was defective in some way, and you were just unlucky.

 

[Sailfree]

Thanks - I will look at Nigels book you recommend.

I have stripped down and repaired car gearboxes including autos but never touched a boat one.

If anyone wants one dismantled (I can't promise to be able to rebuild it) send me a pm.

 

[charles_reed]

Most marine gearboxes operate with a servo cone-clutch or a dog clutch.
Some of the hydraulic gearboxes use an epicyclic band brake.

The Volvo gearbox is most probably a cone - the failure was most probably only due to insufficient movement on the linkage to engage it sufficiently for the servo effect to cut in.

Reversing the actuating cam can frequently solve the problem.
The wear is probably due to the forward gear being used more frequently than reverse.