Ah-ha! (Not the Swedish pop band, the Swedish gear box)

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…

7 thoughts on “Ah-ha! (Not the Swedish pop band, the Swedish gear box)

    • Pending the actual surgery, fingers are crossed that we have located the issue. Shipping from Sweden may make it hard to justify a backup, but you are so right about Murphy!
      Thanks again for the note and the good wishes. Hope you are doing well!

  1. Hey all. Sounds like the issue. Only other cause might be the cone in the transmission wore out or smooothed out. Lost mine when I was pulling into a dock after a nice days sailing … all of the sudden I had only reverse gear and no forward.

    Don’t despair. It will get fixed. Glad you got some good sailing in first. Mine gave out about month (three weeks) after we bought the boat … the owner I bought it from was so honest that he helped me install the ‘new’ one for free. He was a diesel mechanic so the help was appreciated. Just one of those boat things.

    I think you have the same tranny I do. Anyway, get it fixed so I can see you sailing again. And try to remember that one definition of ‘cruising’ is fixing your boat in strange and exotic places all over the world. I think I got that right.

    • Michael spent all weekend in the engine room and learned a lot! He’s slowly taking things apart and hasn’t gotten to the bottom of the issue. Tranny comes off next.
      Have you had any issues since you fixed it? And did you find a replacement?
      Hope you both are well!

      • No issues since fixing nearly three years ago. I have a company email on the east coast if you need to replace. The sell you a used or a reconditioned (your choice) and you send them your old one as part of the exchange.

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