Max weight of shaft/prop on cutlass bearing

Discussion in 'Inboards' started by kaakre, May 30, 2008.

  1. kaakre
    Joined: May 2008
    Posts: 5
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    Location: St Thomas, USVI

    kaakre Junior Member

    Am refitting and old steel ketch. Deleted the old grease filled prop shaft system in favor of a cutlass bearing and a prop shaft seal in the seven foot tube. Will pump water into the tube for lubrication of the PSS and Cutlass bearing.

    It has about a 12 foot by 2 1/2 inch prop shaft. Weight of the shaft is about 140 pounds and the prop is about 60 pounds. The support of the overall shaft and prop weight comes from the forward transmission flange connection, a bearing block 2 feet aft of that connection and the cutlass bearing at the stern. In short. the majority of the weight of the shaft and prop are likely carried by the cutlass bearing.

    The question is-- how much shaft/prop weight can a cutlass bearing support without excessive wear or deforming? The cutlass bearing is about 3 1/8 by 2 1/2 by 10 inches. This info may be on a website somewhere but I have not found it.

    Thanks,

    Keith
     
  2. mydauphin
    Joined: Apr 2007
    Posts: 2,164
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    Location: Florida

    mydauphin Senior Member

    Surprise no one has answer your question. It may be more difficult to answer than you think. Because when prop is spinning, it is generating lift, drag and may be weighting nothing in the water. The more rpms the less pressure on bottom of bearing.

    Try this experiment, get a flexible small flexible wire about 3/16, put in drill. hold it place with straw. Notice end of straw will get straighter the more speed.
     
  3. Tigawave
    Joined: Sep 2007
    Posts: 39
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    Location: Buckland

    Tigawave Junior Member

    The water film should support the shaft if the bearing is long enough.

    You are correct in being concerned though, at start up and run down a soft material like a rubber bearing will allow the shaft to sag. Most of the wear will be during this period as the shaft runs on the bearing material. Even a small deflection at eth bearing can result in large shaft deflections.

    You could look at a composite bearing material which offers more mechanical support and generally run tighter tolerances on the shaft. They do not defrom under pressure and have better dry running properties than rubber.
     
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