Quick Water Ballast Question

Discussion in 'Hydrodynamics and Aerodynamics' started by Southern Cross, Oct 11, 2017 at 1:55 PM.

  1. latestarter
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    latestarter Senior Member

    Often when we are at cross purposes it is because of your education and experience with larger boats and I think from first principles and about small craft such as in this case, trailer - sailer boats that use water ballast to be as light as possible for moving and trailing.
    I can see why the phrase is confusing, I was making 2 points, the first was what theory to use to analyse what is happening to an Elvstrom bailer, the second was that possibly millions have been sold and if you go fast enough they will bail out a boat.
    Luckily I do not use a translator but I hate predictive text it never suggests the word I want. :)
     
  2. Joakim
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    Joakim Senior Member

    You will get suction with a bailer or a scoop. The suction will be about 0.5*rho*V^2 depending on the shape and location. So at 6 knots you can get the tank level 45 cm below the waterline and starting at the waterline you will have a 4500 Pa pressure difference pushing the water out. You will likely have at least 1.5*rho*V_outflow^2 pressure drop. With that the V_outflow would be 1.7 m/s (SQRT(V_boat^2/3)). If the scoop has ID of 50 mm, the water mass flow would be 3.3 kg/s. With a 20 mm*20 mm bailer opening the mass flow would be 0.7 kg/s. When the tank level drops lower the flow becomes slower. At 30 cm below the waterline you would have only 1500 Pa left, which results to 1.0 m/s.
     
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  3. DCockey
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    DCockey Senior Member

    Bernoulli's equation applies to incompressible fluids. A change in speed of the fluid results in a change in pressure of the fluid.
     
  4. TANSL
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    TANSL Senior Member

    I agree with you, Bernoulli equation applies to incompressible fluids. But, can you tell me at what point in my answers to this thread I have spoken of Bernoulli's theorem? Thank you. The only thing I have tried to do is make understand the OP that the Venturi's effect was not applicable to his problem. I'm wrong?. Thanks again.
     
    Last edited: Oct 13, 2017 at 6:22 AM
  5. latestarter
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    latestarter Senior Member

    Very helpful post.
    scoop links to a photo of a device, what is its purpose?
     
    Last edited: Oct 13, 2017 at 11:22 AM
  6. DCockey
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    DCockey Senior Member

    You didn't speak about Bernoulli directly. My comment was in reply to your statement (which I quoted in my reply):
    That statement appears to be saying that the pressure depends only on the depth of the orifice below the surface because water is "incompressible". That is incorrect. My comment about Bernoulli's equation was a short hand way of saying that the pressure in a fluid also depends on the speed of the fluid, even for incompressible fluids. This is true both inside a venturi and elsewhere. The pressure difference due to speed is not negligible at for many situations involving boats. It is the reason self bailers work.
     
  7. TANSL
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    TANSL Senior Member

    DCockey, please read carefully all that I have written. I do not say what you suppose but what I have written, everyone can check it because everything is written. What I wanted to say, I repeat, is that the Venturi effect was not applicable to this case, a watter ballast tank with a hole of communication to the sea and I have said that there may be other overpressures or depressions but, imo, they are negligible IN THE CASE described by the OP (or as I understand it)
    Read again, please, with more attention and less prejudice what I have written. Thank you.
     
    Last edited: Oct 13, 2017 at 4:16 PM
  8. Southern Cross
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    Southern Cross Senior Member

    Thanks everyone. Didn't mean to cause a stir.
    As an avid dinghy sailor/racer as a kid, I am very familiar with the scupper mentioned above.
    I was wondering why a racing sailboat yacht had to hand bail a ballast tank after failing to transfer the water to the lee tank prior to the tack. After the tack, that lee tank became the windward tank etc etc.
    It was argued that a tank on the leeward side could not be emptied because it was below the water line.
    After studying some photos of the boat it appears that their is a single "common" drain for all discharge at the transom a few inches above the waterline close to the centerline. I could not find any evidence of scuppers which are quite common on high performance offshore racing boats. Thus the reason why the tank had to be hand bailed.
    Thanks again.
     
  9. TANSL
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    TANSL Senior Member

    I do not know if I have helped you in anything but that was my intention. Otherwise, do not worry, in this forum some of us are very flippant;).
     
  10. Joakim
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    Joakim Senior Member

    Scoop is used to fill and empty water ballast. Normally it is closed so that the hull remains flat and the thru hull is closed. Then you can push the scoop so that the hole in it faces forward or backward under the hull. The former will cause about 0.5*rho*V^2 over pressure and the latter same amount of under pressure. This makes it possible to fill above waterline (or faster) and empty below waterline (or faster), but it needs speed to work well.
     
  11. latestarter
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    latestarter Senior Member

    Thank you Joakim

    It would be helpful if you explain to us how you think self bailers work.
     
  12. TANSL
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    TANSL Senior Member

    A hole, an opening, an open transom, through which the water escapes. I'm wrong?. The potential energy of water is transformed into kinetic energy, the speed of water increases and escapes from the cockpit. The depression created by the hull on the way forward could help empty the water. There is no doubt that the movement of the boat also helps empty the cockpit. In the case of a hole, you have to check that its diameter is enough for the cockpit to empty in a minimum time that I do not know what it is, because what they ask, the rules I have used, is that the hole has a minimal net area. I do not know if I can explain something else, I do not know, frankly, in which direction your question is going.
    For more information you can consult, for example: ISO 11812: 2001 Small craft - Watertight cockpits and quick-draining cockpits.
    Anything else?. I will be glad to answer you, if it is within my knowledge and experience. At your entire disposal.
     
  13. Southern Cross
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    Southern Cross Senior Member

    Thank you Joakim. Like I said, I could not see any evidence of scoop on the underside of these boats. A drain/fill combo scoop is quite common on the offshore boats these days.
     
  14. Southern Cross
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    Southern Cross Senior Member

    Hi Tansl. Take a look at the picture of the scupper again. It doesn't work in the same way a thru hole or open transom work. On a sailboat, open transom's work primarily because the cockpit floor is slightly angled down going aft - gravity.
     

  15. latestarter
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    latestarter Senior Member

    You are describing a self-draining cockpit where the floor of the cockpit is above the water level.
    The point of a self bailer is that it removes water from the boat that is below the water level.
    Look at the picture beside "At Masters level...... Teaching https://collaborate.plymouth.ac.uk/sites/designflow/Pages/People.aspx
    The colours show changes in pressure.
     
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