Marine Gearbox

[Danny_Labrador]

I don't know much about marine gearboxes, (I know all about automotive ones), the sort that might be on the back of a Volvo MD 21.

Any good web sites ? the bit I don't understand is the clutch - how is it able to go from forward to reverse with little pause ?

If you had symptom where from time to time the engine revs go sharply up and then die down a again after a few seconds would you think that this was the gearbox ?

 

[saltwater_gypsy]

In the simplest gearboxes there is no clutch just dogs which transfer the drive from the "forward" shaft to the "reverse" shaft with a good old CLUNK!!
In more sophisticated gearboxes there are two sets of multi-plate clutches which do the same thing.
A friend of mine bought an old trawler in which the forward drive eventually failed because the plates had turned to mush. Undeterred he completed the 20mile trip back to his mooring in reverse at a steady four knots causing some consternation!!!.
Anyway, once he replaced the clutch plates he was back in business.
This could be your problem ie worn clutch plates. Have you checked the oil level recently?
Nigel Calder has a good section in his book"Boatowners Mechanical and Elecrical Manual"

 

[Phoenix of Hamble]

Also, some older gearboxes are quite picky about the type of oil used, and this can cause significant drive/clutch problems.... so check first!

 

[Guest]

Some marine gearboxes have flat plates, some have cones which take-up more smoothly. For reverse rotation, some older types employ a band, but most modern units have a layshaft. One significant difference between an automotive and a marine gearbox is the location of the clutch - as you know on a vehicle it's situated between the engine and gearbox, whereas in a marine setup it's located inside the gearbox itself.


Just realised that I've got a diagram of a NEWAGE-PRM 80 (mechanical gearbox) handy:

Drive from the input shaft is taken to the reverse gear (coloured GREEN) via the reverse idler gear (coloured BLUE), and to the forward gear (coloured RED), so that the forward and reverse gears rotate in opposite directions. These are permanently in mesh, and rotate freely on the output shaft. Direction is then selected by the cone/ clutch assembly (coloured ORANGE) which slides fore and aft on a splined area of the output shaft.

The clutch mating surfaces are believed to be made from sintered bronze, with the selector cone being pressed hard against the required gear by a shoe (not shown) which is connected to an external selector lever.

Hope this info is useful.