Fixing the Yanmar SD-50 Cone Clutch. In Galapagos.

This is a more technical article for all the ones that are also „suffering“ from the Yanmar SD-50 cone clutch they may have installed in their vessels.

Our mechanic in Shelter Bay Marina, Panama simply didn’t check the clutches despite his invoice stating the contrary. So the surprise came in Galapagos, when our port engine failed to engage. A clear sign of a glazed over cone clutch, as it is frequently found in the Yanmar SD-50 that has not been checked and serviced every 500 hours of operation.

After we got over the mild stress induced in a crowded Isabella anchorage with strong winds from starboard, a reef on our port side and no operational port engine we diagnosed the cone clutch defect. The clutch will fail to engage (and thereby transmitting power from the engine to the propeller) whenever two surfaces therein are to smooth. In our case that happened due to the usual wear and most likely due to two incidents involving a line in the affected prop.

As we were very happy to use some very valuable resources from the internet concerning a repair outside a proper boat engine workshop, I decided to improve the respective manuals a bit and post it her. Hopefully, they will help others in their repairs. Yanmar Service in the US was also a great help to us. Whatever anyone may say about their SD-50 Saildrives: Their Service was world class. Thanks for that. Also a big thanks for El Tigre on Isabella. He runs the local car workshop and allowed us to use his vice. We also used some of his scap aluminum to use as brackets in the vice for protecting our clutch shaft from the iron vice brackets.

Please find the attached manual for repairing the Yanmar SD-50 Saildrive cone clutch here: Yanmar_SD50_Clutch_Repair_v03.

By the way: We did use a vice to open the cone clutch assembly. But we also met Laurent of the Catamaran „Sept à Vivre“. Laurent managend to repair / lap the clutch without a vice. He did so by applying lapping paste to the cones and lapping without taking the assembly apart. I guess he performed a very thouroughly cleansing of the unit after that procedure to make sure not to have the agressive paste in it after starting the engine. But the clutch works fine after the process. Thanks for sharing that, Laurent, I think it is a very cool improvisation.

Books: Yanmar SD-50 Service Manual
Georg Forster, Entdeckungsreise nach Tahiti und in die Südsee
U.B. Peter, Europa –  Warum wir sind, was wir sind
Neal Stephenson, Seveneves: A Novel

2 Gedanken zu „Fixing the Yanmar SD-50 Cone Clutch. In Galapagos.

  1. Darren

    hi we are having the same issue with sail drive slipping, I can get someone to pickup some spare parts and sail them to us, but can’t seem to find anyone to tell me the parts I need to replace when putting back together, ie new nut and copper washers? I have talked to yanmar in Panama and they have no idea, so I emailed yanmar in the states, all they do is refer me back to Panama, any advice you can give , I would appreciate.

    1. Fritze Beitragsautor

      Dear Darren,
      thank you for your comment and question. I am surprised, that yanmar can’t help as they are usually great in their support.

      Without knowing the exact status of your system (engine / sail drive and clutch), I can’t give 100% reliable advice. But lets try to find some answers.

      As long as your system has not run too many hours and has not been repaired many times, you would not need any spare parts.

      Slipping clutches unsually need spares (replacements of the cones) in case they have been lapped (made rough again as described in the article you have attached your comment to) several times.
      In this case, you might have removed so much material from the cones, that you would need new ones (they can not establish sufficient contact anymore).
      In any other case (usually the first three lappings) you should be fine without special spares.

      You do need lapping paste that you should be able to obtain in Panama (DIY store or maybe some car service shop).
      In case you chose to service your clutch by taking apart the assembly (as described here, there will be this one part, that might break (the locking safety nut at the top of the clutch assembly). In our case, we lifted the locking lug with a hammer and screw driver, this broke the lug and a part of the upper nut rim (wich holds the lug), but the base of the nut was OK and we could improvise without a spare (see pages 9 and 12). We chose cut a new grove the vertical shaft with a grinder that was positionend in a way that we could use the remaining upper ring of the nut to secure it again (by simply hammering it into the new grove). This setup allowed us to travel all the way from Galapagos to Turkey, where we replaced it with a new nut. You could also use loctite (red / strong) to secure the nut. In this case, I would always try to replace the improvised setup with a new one, as soon as possible (but thats just me not trusting loctite to much).

      Alternatively you can lap your system without disassembling your clutch. You will still have to pull it out of your SD50, but you could place some lapping paste between the cones and roughen up their surfaces by turning them as described on page 11 of the document above (only that you do this while the clutch is still in one piece). In this case you should very, very thoroughly clean the the assembly by it in Diesel / Gasoline or any other good solvent to remove all the grit from the lapping paste. You do not want any of the paste in your clutch or sail drive after the procedure.

      I hope I could help,

      all the best,

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