With due respect, can I suggest that you ignore the posting in Motor Boat Chat as it unlikely to be the case. The BMC and the Morris Vedette Petrol engine had a there own gearbox and was long before the days when PRM and Hurth came on the market.
The mechanical gearbox on the Vedette had two adjustments, the first was for the three clutch toggle levers and the second was for the reverse brake band. Basically the first adjustment alters the amount of pressure applied to the multi plate clutch and the second increases the clamping effect of the brake band. If you find that the engien is revving up and the boat slows down then the toggles require to be adjusted, If the boat has no speed astern then it is the brake band.
One word of advice is to tie a piece of string onto the spanners before working in the gearbox, oily fingers can let spanners slip.
I still have the manual from my father BMC Vedette circa 1960 and if you would like a copy of the adjustment, please send me a PM with your e mail address or a fax number.
I bow, Sir, to your superior knowledge...../forums/images/icons/wink.gif
I did a search but couldn't find anything on Vedette gearboxes, only the engines.