By Michael (updated)
We thought it was going to be simple, but the obvious moving suspects are happy and healthy. The Volvo MD3B is connected to the gearbox / reversing gear through a rubber coupling. We hoped the thud-thud-thud preceding the spin-up of the motor at failure was the sound of the coupling coming apart. (Note that the boat was in reverse, and had not shifted during the decomposition.) Both the coupling and cone linings are in fine condition. We still cannot explain the torque-less spinning of the prop and the ability to stop the shaft with a bare hand. The cause of failure is unknown, but judging by the paperclips and baling wire used as cotter pins, this was not the gearbox’s first rodeo. There are a few parts that don’t look at all like the stockers in the shop manual. The gearbox is apparently a common problem, since the motors seem to last forever, the combination of weak link, obsolete parts and diminished population of spares means we wait weeks for replacement. Let’s not forget the special tools favored by Swedish tranny mechanics… Like tool 884490 needed to remove the outer seal of the MS. Also obsolete.
The coupling is one the few parts still in production, see item 21 on the attached schematic.
As for root cause, if I hadn’t checked personally, I’d have suspected a fouled prop. Didn’t see evidence of that… though it may be possible. The rubber coupling is there to protect the motor from even greater damage, and a 41-year-old chunk of rubber is likely to let go now and again. But no. Not the clutch either. So the search continues.
A few deep breaths, a socket set and some very long weekends…