Through Hull Discharge Design

Discussion in 'Sailboats' started by SuperPiper, Sep 24, 2017.

  1. SuperPiper
    Joined: Jan 2003
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    Location: North Of Lake Ontario

    SuperPiper Men With Little Boats . .

    [​IMG]

    This is the famous NACA inlet. Is there an equivalent development for outlets?
     
  2. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .

    Thru-Hull.jpeg

    Here is a sketch that may trigger some discussion. Is one of these 2 options the better trailing edge for the through hull? Is there an even better configuration?
     
  3. tspeer
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    tspeer Senior Member

    Yes - the Elvstrom self bailer.
    [​IMG]

    It depends a great deal on whether the discharge is intermittent or continuous. If it's intermittent, then what matters most is the drag when there is no flow because that will be the condition most of the time. For the intermittent case, I think only small improvements can be made to the typical flush through-hull. If the flow is continuous, then you'd want to turn the flow so it is discharged parallel to the hull instead of perpendicular to the hull. Best, of course would be to discharge through the transom.
     
  4. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .

    Tom, your replies are always concise and accurate. You are correct in assuming that the flow will be intermittent.

    I have been searching for examples of discharge openings into a flow stream. I thought perhaps something from turbine heat transfer could be reapplied, but that is for a continuous flow:

    [​IMG]



    So what I really need is to reduce the drag of the blunt trailing edge of the hole. Perhaps there is already a sailing example of a fix for this. How is the blunt trailing edge of a centreboard trunk treated? I have experienced first hand, water boiling up into the centreboard trunk of my dinghy due to the obstruction created by the back of the centreboard opening. Is there an example of a crease or a tapered dimple that would alleviate this drag?
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I ran into a similar problem in reverse a few years back and used a "sea chest" strainer. In my case I was designing a powerboat that would challenge a typical raw water strainer, plus I didn't (couldn't afford) the drag associated with the typical blister style of strainers. At the speeds this boat would run, these tend to dry out, in the bubble of air trapped just aft of the blister. The sea chest uses a flat plate with slots or holes (slots are more efficient and less draggy at speed), mounted flush and parallel with the flow. Above the strainer is a box, mine tapered to make a ram, but most are simply a box, which is the intake, though in your case, the exhaust/discharge. Facing the "other way" as yours would be, a low pressure area would be created just at the slots, literally sucking out the exhaust (water, whatever). After some quick flow tests, I found the slots should equal about 50% of the exposed surface area of the strainer. In other words, if you need 6 square inches of area for discharge/exhaust (whatever), just arrange your slots for this amount. I'll post an image when I find it. Mine was mounted on a relatively flat portion of the bottom, in the planing patch, though still inclined to the deadrise, if you can picture this.

    [​IMG]
    This is a flush mount Gem Lux, not slotted, but you should get the idea.
     
    Last edited: Sep 27, 2017
  6. SuperPiper
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    SuperPiper Men With Little Boats . .

    Elliptical! Of course. The fundamental shape of hydrodynamics. I totally overlooked the ellipse. Thanks, PAR.

    I'm still curious to know if there is a shape applied at the back of a centreboard trunk to eliminate the blunt edge. What are the high performance guys doing?
     
  7. nacrajon
    Joined: Jan 2009
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    nacrajon Junior Member

    I have used cowl and bullet designs on my surfski for intermittent discharge. They take trial and error to optimise the bullet position but they work very well and are low drag.
    images (1).jpg images.jpg
     
  8. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .

    I am intrigued.

    I'm not familiar with cowl and bullet, so I tried to google it. A lot of firearms sites popped up, AND, this Boat Design thread. Is there a more generic name for this type of discharge?

    Could the cowl and bullet be mathematically transformed until the cowl was flat and the bullet was a dimple? Then it would look similar to the sketch in Post #2 above.

    Comments? Ideas?

    What are the high performance guys doing at the back of their centreboard trunks?
     

  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Most board slots are covered with a membrane of some sort, so not a lot of turbulence is generated.
     
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