Cutlass Bearing Lubrication

Discussion in 'Inboards' started by viking north, Mar 29, 2013.

  1. viking north
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    viking north VINLAND

    I know this must have been previously covered but I can't seem to dig it up. Re. Cutlass bearings, Is there some formula or chart that recommends the volume of water required verses area to be lubricated. The present set up on my build/conversion is lubricated by use of infeed scoops built into a large cutlass bearing assembly that bolts onto and extends far behind the keel.(approx. 8 to 10 in.) I would like to eliminate this by installing another smaller dia. shaft tube/cutlass bearing inside the existing tube. The new tube and bearing setup would allow the cutlass bearing to be housed inside and flush with the back face of the keel. This would the prop and the rudder shaft to be installed some 6in. closer to the end of the keel, reducing what would have been a rather long shoe extending from the skeg in which is mounted the lower rudder bearing. My intention is to lubricate the new cutlass with a water supply fed into the new tubes inboard end. Just need some idea of what dia. hose will provide enough volume for proper lubrication of a 1.5 X 6 in cutlass.
    P.S. The new setup would still allow good clean water flow to the prop.
     
    Last edited: Mar 30, 2013
  2. gonzo
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    gonzo Senior Member

    From what I read, you are installing a smaller diameter shaft. Also, if you move the propeller forward, the diameter may need to be reduced. Have you done the calculations for that? Also, what is the reason to install a pump fed cutlass instead of the usual way it was designed for?
     
  3. michael pierzga
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    michael pierzga Senior Member

    Dont know the water supply needs. Preferably its naturally fed.

    Blind cutlass bearings are troublesome. The typical big mistake is failure to bleed the bearing when the boat is launched. An air lock forms, the bearing stays dry and the cutlass burns up in ten minutes.

    Many boats overcome this by injecting engine cooling water into the cutlass.
     
  4. viking north
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    viking north VINLAND

    Gonzo, Photos show the origional bolt on cutlass bearing unit extending some 8in. beyond the end of the keel. This would place the end of the shaft/prop nut back an extra 4 to 5 inches or approx 12 to 13 inches behind the end of the existing keel. Allowing say 3 to 4 in. spacing between the end of the shaft/prop nut and the to be installed rudder tube/shaft, this will place the lower rudder bearing some 17in. plus behind the existing keel end. The lower rudder bearing will be housed in a stainless U channel shaped shoe that runs from the bottom of the keel back some 17 plus inches to facilitate fore said bearing. In the final build this channel will actually run back from the bottom of the to be installed skeg but the idea and distances are the same. I am playing around with the idea of eliminating the existing bolt on cutlass assembly and replacing it with a set up using the existing tube or a new smaller tube sleeved that will allow the cutlass bearing to be installed into the keel. This in turn will allow me to install the new rudder tube/shaft/lower bearing all things considered, approx. 6 in. closer to the back of the keel. This results in a 6 in. reduction in length, along with a corresponding reduction in stresses applied upon the skeg by the U channel shoe that supports the lower rudder bearing. (less length less transverse leverage). However by eliminating the origional bolt on cutlass unit and installing the cutlass in the tube I have lost the lubricating water supply. I can't simply drill supply holes into the sides of the keel and into the tube forward of the cutlass as there is a space between the keel skin and the tubes wall. The water would simply flood the hull. My only option is to inject the lubricating water at the inboard end of the tube. Michael I like the idea of feeding it from the engine cooling pump but my only concern is if the standard cooling pump on my 3 cyl. Yanmar could supply both. The problem is I have no idea of how much is required to lubricate the cutlass. I've read all the specs on the Thordon bearings but other than rating their SXL line as requiring low lubrication nothing is mentioned on recommended lubricating water volume.
     

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    Last edited: Mar 30, 2013
  5. CDK
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    CDK retired engineer

    The pump in the Yanmar draws water from a thru hull nearby. You can install a T there and feed water to the stern tube, the low pressure in front of the prop and the operating principle of the cutlass bearing will ensure there is sufficient flow.
     
  6. viking north
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    viking north VINLAND

    CDK, I was wondering about that, many of the rotary seals used in lue of traditional stuffing boxes make use of a similar set up for lubrication and cooling. Typically using a 1/4 clear plastic tube. I had not thought of the suction created by the prop, and the action between the cutlass and the shaft. This will certainly add to the same created by the forward movement of the hull, I.E. (the old fishermans automatic bailing principle) of which alone, I had doubt would be enough. Assuming the total suction created would move some 75% of the water volume thru the cutlass as is created by the existing scoops, If i measure the intake scoop areas on the existing cutlass assembly it should give me a basic idea of feed supply sizing to work with. If anyone feels i am off on my guestimates here please fire back. Now having confidence that this modified cutlass set up will work I will have a closer look at the spacing dynamics of the keel to prop, the prop to rudder, rudder tube location,shaft quadrant and tiller will all fit within the proposed design :). Fun and games -- Thanks again everyone for your responses and adding to this old guys education ---Geo.
     
  7. gonzo
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    gonzo Senior Member

    If there is a gap and water from a hole will fill the hull, it wouldn't be too difficult to fill it with resin.
     
  8. viking north
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    viking north VINLAND

    Hard to describe it without a photo but it's a dam hard spot to get at. One of the negatives of working with an existing assembly in a conversion build. Your idea is certainly another option that I thought about but shied away from due to the existing tube being metal. I was worried about the reliability of the resin maintaining it's bonding thruout the expansion contraction cycles of the brass tube. I have another 3 to 4 months before I have to make a final decision on this but it's starting to look like the injection of water on the inboad tube end is my only option if I am to make this modification. I'm certainly going to have another go at somehow getting scoop type feeds at the cutlass location as I am also not happy about more clamps, hoses and fittings below the waterline required by the alternative.
     
  9. gonzo
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    gonzo Senior Member

    A 1/4"l bypass hose from the water feed or one of the block drains should be enough. That is what dripless seals get installed with.
     
  10. viking north
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    viking north VINLAND

    Gonzo, CDK and I were conversing about that very approach . I measured up the size of the scoop intakes on the origional cutlass unit and they measure out to 1/2 in. sq. each. Considering the orig. engine and drive system was the equivalent of a tug boat setup, with my little 27hp. diesel I should get away with say a 1/2 in. I.D. supply hose feeding into the tube. Better to have too much than too little :).
     
  11. JSL
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    JSL Senior Member

    That skeg appears to have a wide & non faired trailing edge and could really compromise propeller efficiency. If you get heavy vibration when running it could be cavitation.... unless the propeller is a considerable distance aft of the skeg.
    Your speed could also suffer. Years ago we 'faired' the skeg on a 40' sailboat and the max speed went from 4.5 knots to 8 knots.
     
  12. viking north
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    viking north VINLAND

    JSL -- thats not the skeg, thats the origional keel of the surfboat/lifeboat hull being converted to a motorsailer. However your observation is correct in that it is a little on the wide side (3,1/4 in.) and will require some fairing to get clean water flow to the prop. This whole idea of repositioning the cutlass is still in the gathering of info stage that is subject to many tradeoffs -- fairing being one.
     
  13. gonzo
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    gonzo Senior Member

    If there is a gap right forward of the end of the skeg, fairing it will make a hole. However, that may be the perfect opportunity to fill the void and use a regular installation with holes forward of the bearing.
     
  14. viking north
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    viking north VINLAND

    Gonzo I'm with you on that option, I'm hoping once I get a closer look at it all I might be able to clean out all the foam thats now filling the void below, around and on top of the existing Cadillac of a shaft tube, refill that void with Seacast Foam, drill two 1/4 in. supply holes (one each side ) just ahead of the newly located cutlass. That failing ( leaks) I can simply plug the drilled supply holes (diving gear and temp. cedar plugs for the season) and it's on to the CDK/Viking route --injection at the inboard end of the tube. Now if all the other affected design parameters are within reason we got ourself a ball game :)
     

  15. gonzo
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    gonzo Senior Member

    You can glass in small pipes that will be probably better to prevent water from coming in. Otherwise, you could make a larger hole after pouring the foam, fill it with resin/fibers mix and then drill holes.
     
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