Thanks DougR,
I think you have hit the nail on the
head probably.
The flex coupling was supplied new with the engine/gearbox.
Beta say a flexible coupling "is strongly recommended in almost every case".
I would have thought that my arrangement was very common on this sort of boat size/ displacement/horsepower.
I did not fit the coupling originally because Volvo-penta say do not use a flexible coupling with their rubber cone shaft-seal. Interestingly R&D told me the other day that volvo penta refer only to their own flex coupling here because it does not guarantee concentricity between the two half couplings as do R&D and I think probably others which register into the half couplings.
About a year ago with 1300 hours up the ATF started leaking out the back of the
gearbox; not out the
shaft seal but out along the spline/thread where the output coupling slides onto the output shaft.
I had it "fixed" twice by mechanics, both fixes lasted only a few hours.
I then fixed it myself using the
sealant (Threebond 1211) recommended by Twindisc Aust and was really patting myself on the back until it started leaking again after about 350 hours!
Out of frustration I decided to try fitting the flex coupling, hoping it might stop this leakage.
I think I will do as you suggest and just take it out, which is a pity because up to 1900
rpm it did improve NVH.
R&D suggested similar to you that a half cutlass brg at the front end of the shaft log may help; that the length of shaft unsupported was a little longer than recommended, and the shaft may be "whipping".
Could such a brg just be pushed into the shaft log?
The LAST question is does anybody know how to stop these gearboxes leaking
oil out the output shaft spline.
Thanks for your responses.