Design of thru-hull underwater hole for least drag?

Discussion in 'Sailboats' started by psyklik, Mar 10, 2012.

  1. psyklik
    Joined: Feb 2012
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    psyklik New Member

    Tthis has probably been discussed previously. What is the best way to configure an underwater drain hole for least drag.

    I assume the up currect half of the circumference should have a sharp edge, and the down current half, a rounded edge.

    Any thoughts?

    ~psyklik
     
  2. hoytedow
    Joined: Sep 2009
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    hoytedow Carbon Based Life Form

    Is it a planing hull or a displacement hull? If it is a planing hull place it low on the transom if it will function there.
     
  3. Submarine Tom

    Submarine Tom Previous Member

    Exactly!

    -Tom
     
  4. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Typically drains discharge above the water line, and raw water intakes are below. There really isn't a reason to have drains below the waterline, since it increases back preassure, reduces pump efficiency, and adds another weak point in the hull that can leak.

    Intakes below the waterline design really depends on the system that is being serviced. If you have a scoop on a raw water intake it can either create preassure in the line while running if facing forward, or act as a siphon if refersed. Ever way the system being fed must be designed to handle it, and still have enough flow to work properly.

    This is most often a problem in engine intakes with debris covers that can reduce the inlet size enough to reduce the water stream enough to cause the engine to overheat.
     
  5. psyklik
    Joined: Feb 2012
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    psyklik New Member

    It's a displacement hull & unfortunately have no choice, as it is also where the cable exits for retracting the centerboard.
    Therefore my question "I assume the up current half of the circumference should have a sharp edge, and the down current half, a rounded edge." ?
     
  6. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Correct "in general"; use a chamfer radius of ~25% of hole diameter. To be quite correct, the shape depends on the flow out of the drain and the velocity ratios, but that's probably another story here....
     
  7. HakimKlunker
    Joined: Aug 2009
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    HakimKlunker Andreas der Juengere

    If you are not going for a world championship: You better look into efficient performance of the drain itself?
     

  8. psyklik
    Joined: Feb 2012
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    Location: Connecticut

    psyklik New Member

    Thanks guys!
     
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