mechanical gearboxes

[Guest 000]

What's the deal with mechanical gearboxes by which I mean the little lever actuated cone boxes typically found on yachts in the 30-35hp range?

As far as I can see, the loading on the box is pretty constant as are the revs which only vary by a few hundred rpm's, yet almost everyone I know has had gearbox troubles at some time. In fact the ZF box used to be known as the 'dollar an hour gearbox.' It cost you a thousand bucks and was good for a thousand hours. They've gone up hugely in price recently so I suppose now they are the two dollars an hour box.

Compare that with automotive gearboxes which are multi stage,operating under varying rpm and loading and I don't know anyone whose automotive box has failed.

Are marine mechanical boxes just inherently unreliable?

[Island Time 1,293]

I think they mostly suffer selection issues. This is often operator error - fwd - neutral - reverse or vice versa with no pause in neutral so the engine is doing too many revs when it engages, shock loading the cone clutch. It's not a clutch plate/pressure plate like a car, as that would be too big, and there is no manual clutch - all in the interest of miniaturisation.  Selection issues have certainly been the main problem with my Volvo saildrive unit, although I'll admit it has about 10,000 hours!! Had 2 rebuilds in that time though...

[wheels 544]

Yes it's the cone clutch that tends to be the problem. If you think about it, it's the clutch that goes on Car as well. The reason these boxes use such a means of engaging drive is because they don't have a clutch like a Car has that we need to engage. There are various designs across the manufacturers of course.
The new owners of ZM have upgraded the design slightly. It will be interesting to see how this works out in hrs. It will likely be the reason for the price hike. It may well still be the dollar and hr box, by being $2/hr/2000hrs.

[Guest 000]

I think they mostly suffer selection issues.

Yes, and gearbox lever travel is also important - ensuring that the cockpit gear lever moves the box lever by the full amount. In this regard I wish the ahead gear would engage with a clunk like the astern gear does. We did have one occasion where one of us must have accidently bumped the gear lever out of full engagement. It was still propelling the boat but didn't do the box much good, I suspect.

[erice 732]

you could put a little sticker, or paint a mark, at the full engagement position 

[Guest 000]

I was thinking maybe a bit of 3mm cord from the top of the gear lever to a plastic jam cleat. The lever is quite vulnerable to being bumped when we move around in the cockpit.

[Island Time 1,293]

So the gear selector is not the throttle too?

[Guest 000]

No, got an antiquated two-lever system.

[Island Time 1,293]

Hmm unusual. I'd not be really happy tying the lever. Another issue in case of emergency.

[Farrari 4]

Can you swap over the control from a two lever system to a single?  I had this done on my Tracker.

[Guest 000]

Yes, thought about it.. But that's a capex that would require approval from the Keeper of the Purse.