Stern tube?

Discussion in 'Hydrodynamics and Aerodynamics' started by Bilgerat, Jun 8, 2011.

  1. Bilgerat
    Joined: Feb 2004
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    Location: Puget Sound, WA

    Bilgerat New Member

    I've just recently installed a new larger stern tube in a 40" IOR sailboat. My old stern tube was ground off flush with the hull. My new one, I've left a couple inches of the tube sticking down.

    Will this matter? I've actually read somewhere that it's better to cover the whole prop shaft due to it's rotation causes resistance. How true that is, I don't know!

    But should I grind my new tube off flush with the hull or can I leave it as is for alignment assistance, if needed?

    .
     

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  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    On a powerboat that is correct. When you are sailing, probably most of the time, the larger tube will create more drag.
     
  3. tunnels

    tunnels Previous Member

    With anything on the under side of a yacht do some fairing and stream lining !!
    If you have a tube see if you can work it to a foil shape! If the water flow is smoother then naturally is will be better! some shapes are better than others so have a look at some foil shapes and see which one fits your tube best !!. SMOOTH FLOW WITHOUT CAUSING TURBULANCE IS WHAT ITS REALLY ALL ABOUT . :D
    fIT THE SHAFT IN AND FAIR THE END OF THE TUBE TO A CLOSER AND BETTER LINE THAN WHAT YOU HAVE THERE NOW !!
     
  4. Bilgerat
    Joined: Feb 2004
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    Location: Puget Sound, WA

    Bilgerat New Member

    That's what I'm trying to get at, which is going to cavitate/vortex worse? (spell checker does't recognize Cavitate :rolleyes: ) The shaft is 1-1/8" and the tube ID is 1-3/4" with a 1/4" wall.

    So, is the water going to flow evenly around the end of a squared off tube or vortex back into the tube? vs. Is the water going to vortex back in the angled tube less or more then the squared off tube.
    As for a foil, how would make a foil out of a tube other then sharpening the trailing edge?
     
  5. tunnels

    tunnels Previous Member

    You need to find a foil the right shape and size ad cut the shape out of thin ply and place it over the tube onto the hull and then use you imagination from there . Trailing edge is just a part its got to be the whole shape leading and trailing i would also cut the tube at the same angle as the hull not just square off . Ting will beging to look better with the tube cut !!. :D
     
  6. viking north
    Joined: Dec 2010
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    Location: Newfoundland & Nova Scotia

    viking north VINLAND

    In comparison to the turbulance from even a folding prop I wouldn'y worry about it. As long as you don't have a bugle shaped affair. Taper the circumfrence end edges inward to blend in with your cutless bearing (if there is one at this location) if not with the shaft. The resultant turbulance will be so minor as to basically be non existant in comparison to the strut and prop. ---Geo.
     

  7. ABoatGuy
    Joined: Aug 2004
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    Location: LeftCoast

    ABoatGuy Member

    Reality check - this is an old IOR boat, not an Americas Cup contender. There is about as much drag in that stern tube as dragging your hand in the water. I agree with Viking North - I wouldn't worry about it.

    That said, the man asked a question and the forum answered it!!

    Smiling
     
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