Through Hull Discharge Design

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

  1. SuperPiper
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    SuperPiper Men With Little Boats . .

    [​IMG]

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

    Cockpit Drain.JPG

    There is still 17" of ice on Mazinaw Lake, but the temperatures have been creeping up, so it's time to start some spring projects.

    The photo (looking forward) shows the cockpit drain discharge under the centre of the hull. The objective is to soften the trailing edge of the hole. Currently, it's about as hydrodynamic as a cheese grater.

    The first step will be to eliminate the centre strake from the hole to the transom. The strake is about 1/4" deep. It's function was probably to add stiffness into the hull, similar to the creases in the hood of your car.

    More to follow . . .
     
  11. SuperPiper
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    SuperPiper Men With Little Boats . .

    Cockpit Drain 2.JPG Divot Filled.JPG

    The crease that was the inside of the strake has been filled with 24 layers of 6oz glass and epoxy.
     
  12. Yellowjacket
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    Yellowjacket Senior Member

    The NACA inlet was specifically designed to have a good inlet recovery as well as the ability to reject most foreign objects flying by. It was intended as an inlet for aircraft that would allow stuff in the air (like rocks or other heavy items) to go past the inlet. The NACA inlet relies on two vortices that are created by the sides of the ramp to keep the flow attached as it goes down the ramp into the inlet. The inlet recovery is pretty good (around a 3% loss) but it is not something that you would use as an inlet for a waterjet or really for anything on a boat. The problem with such an inlet is that even though the losses are small, the inlet has a good bit of distortion and you would never want that kind of inlet distortion on an inlet on a boat, where a simple flush inlet (like you see on waterjets is appropriate.

    NACA also looked into how to eject cooling air from a cowling to minimize drag and maximize thrust. That part of the equation is actually pretty simple. So long as the flow coming out of your outlet is turned to lie long the surface you are going to do just fine.

    The type of discharge that you've sketched in post 2 will result in higher drag and won't suck much flow out of the system since, while flush, it will result in a big bump in the surface flow if flow is discharged with any pressure. If you're relying on the lower pressure in the flow to pull flow out if that discharge not much is going to flow out of it.

    The key to a discharge is to have a large (much larger than is shown in post 2) downstream radius and have a upstream surface that displaces the flow over top and basically discharges the flow along the surface.

    The turbine trailing edge treatments are designed to eject the air from inside the blade and at the same time cool the trailing edge so that it doesn't burn off. The shaping for those surfaces is very dependent on the pressure delivered to that specific stage. For instance, the first stage turbine has cooling air pressure that is closer to the stage exit pressure than the second stage where the pressure differential is much greater and there is a need to diffuse the flow as it leaves the blade. You didn't mention what pressure was available to drive the flow out or if it was being drawn out by the passing flow. Two different animals as far as drag/thrust is concerned.
     
  13. SuperPiper
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    SuperPiper Men With Little Boats . .

    Great background on inlets and outlets in a fluid stream, Yellowjacket. All good information, for sure.

    This project involves a cockpit drain. The only pressure is the head from a couple inches of water standing in the tube. The objective of the project is to reduce the drag created by water flowing across this outlet. Primarily, a radius will be added to the trailing edge of the hole to make it more hydrodynamic. I'm somewhat surprised that there isn't a similar treatment to centreboard trunks on dinghies. We seem to be venturing into virgin territory with this project.

    Here is a confusing diagram showing the proposed radius (flow from right to left). I've opted to use a standard NACA0012 shape. The most significant point of the diagram is that 69 layers of glass will need to be added into the crotch of the tube-to-hull joint to accommodate the radius. With the help of an electric heater, the environment inside the lazarette will be suitable for epoxy work this afternoon. Updates to follow.

    NACA 0012.jpeg
     
  14. SuperPiper
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    SuperPiper Men With Little Boats . .

    The epoxy work got completed during milder temperatures and with the help of an electric heater. You would expect that the sanding could be completed in any weather. But no! My breath was condensing on the inside of the cold hull and creating a paste with the dust. The paste clogged the sandpaper and rendered it useless. I waited for warmer weather and used the electric heater to boost the climate in the lazarette. Here is a photo of the drain inside the hull. Yes, that is more than 70 layers of 6 oz. glass.

    Cockpit Drain Profile.JPG
     

  15. SuperPiper
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    SuperPiper Men With Little Boats . .

    A spell of spring weather has allowed a little more work on this project. Check the photos. The strake has been removed from the cockpit drain aft. The next step is to carve a trailing edge radius as discussed above. Any watch-outs, or recommendations?
    Strake Removed 1.JPG Strake Removed 2.JPG Strake Removed 3.JPG
     
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