folding multihull hull design

Discussion in 'Multihulls' started by grob, Oct 8, 2002.

  1. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Are the foil section symetrical? If they produce lift in opposite directions the resulting forces cancel out. However, they produce a lot of drag. Another thing to consider, is that wave resistance is not addessed by NACA foils. A hull is in two mediums, which creates a completely different environment. NACA foils, at anything but the slowest speeds create a huge wave resistance. Also, they handle poorly. As you correctly pointed they are aerofoils, not hull forms. A solution to the folding system is to have forward and rear sections that end in a flat. They can be joined to create a longer waterline. This gives a higher hull speed potential. The four sections can be identical. Another advantage is that there can be locks between the forward and aft sections so it takes a lot of the stresses from the pipe frame.
     
  2. grob
    Joined: Oct 2002
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    grob www.windknife.com

    Is this opinion or do you have some evidence for this statement, most surface piercing rudders are NACA profiles at the waterline, if they are slow, create large wave drag and handle poorly why are they used?

    I have been looking for any research on wave drag and hull (or rudder/foil) shapes, and have found nothing other than those that use simple ratios, that seem to tell us that hulls go just as quick going backwards or forwards, which to me is absurd.
     
  3. gonzo
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    gonzo Senior Member

    A rudder has a small area in the water/air boundary compared with a hull. Therefore the wave resistance is negligible. A hull is coming in and out of the water continuosly so it's submerged section changes. Also, the boundary area of the rudders is in the water exit from the hull. It is mainly eddies, which changes the conditions. Another problem that NACA foils don't adress is cavitation. They are designed as airfoils. At high speeds water behaves almost as a solid forward and a gas aft.
     
  4. grob
    Joined: Oct 2002
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    grob www.windknife.com

    Sorry to press you Gonzo, but I am looking for some information on what creates wave drag, How do you know about wave drag on NACA foils?
     

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

    gonzo Senior Member

    Any object moving through the water surface creates waves. The length of these waves is proportional to the speed. The other aspect of wave forming to consider is wave shape. A blunt form will create a huge bow wave. At very low speeds it is efficient. Think of a supertanker doing fifteen knots, one third of it hull speed. Compare it to a thirty foot sailboat at seven knots. The first has water flowing along the sides and the stern with several small waves forming. The second has only one wave between the bow and the stern with a big through in the middle. NACA foils were not designed for this surface turbulence. Also this description is only for flat water. Enter the sea waves: the vessel stars pitching, rolling and yawing. The hull shape is continuosly changing. The NACA foils only work well if the water flow starts at the blunt edge and leaves cleanly at the sharp edge. Proper hull designs account for the changing conditions. In other words, they are a compromise that works well all around.
     
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