Wingboat Design

Discussion in 'Boat Design' started by MasterBlaster, May 25, 2003.

  1. Franklin
    Joined: Jun 2005
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    Franklin Junior Member

    Stability And Supercavitation

    I think some of the interest in WIG boats also stems from overcoming the stability problems associated with boats traveling at high speed. A hybrid approach could be incorporating a submerged hull or appendages based on supercavitating concepts.

    http://www.ctechdefense.com/speed.html

    2509 West 19th Street, Port Angeles, WA 98363
    PHONE (360) 452-2275 * FAX (360) 452-2297 * info@ctech.esdcorp.com

    Underwater Speed Limits
    (Following is the full text submitted to Scientific American. The published version was shortened for editorial reasons.)

    "There appear to be two conceptual ways of approaching supercavitation. The generally accepted one derives from propeller cavitation theory and holds that the water is essentially boiled by dropping its pressure via abrupt acceleration. This creates a source of gaseous water vapor which creates the cavitation bubble. It is generally assumed that the cavitation bubble is filled with this water vapor. Indeed, in low speed (say torpedoes) supercavitation applications the cavity size is usually enhanced with ventilation gases. This fits well with the understanding that gas creates the bubble in the first place and appears to work well within that context. It also fits comfortably in the general framework of marine engineering.

    Last September, at an ONR sponsored Supercavitation Conference, Dr. Kirschner (of Anteon Corporation) and I were discussing the idea of a theoretical speed limit for supercavitating objects, assuming material strength issues could be overcome. As previously mentioned, conventional wisdom holds that the cavity is created by the water vapor and therefore, at some speed, the volumetric rate at which vapor can be generated will become insufficient to support the formation of a cavitation bubble which will clear the body. In other words, at some velocity the rate at which the water boils will become insufficient to fill the volume of the "hole" in the water created by the passage of the projectile and the cavity will collapse.

    For whatever reason, I have a different mental picture of how the bubble is created, perhaps due to my background in hypersonics in graduate school. In that field discontinuities and rarified flows are encountered in the course of normal business. I do not know if anyone else shares this view but Dr. Kirschner and I have discussed it at some length. In any case, I believe the process is fundamentally one of momentum transfer. The cavitator, be it a disk or cone or whatever, imparts a significant radial velocity (relative to the axis of flight) to the water it comes in contact with. In effect the water is thrown violently to the side. It therefore has a high radial momentum that is resisted by the pressure of the water around it. This pressure serves to slow its radial velocity and will bring it to a stop over a finite time. The accepted definition of cavitation number is compatible with this idea. In the meantime, assuming a circularly symmetric cavitator, a round "hole" has been created in the water. What is in this hole, other than the projectile? I believe it is a vacuum, at least initially. Of course the water on the interior face of the bubble begins to boil, but it can only boil so fast, even in a hard vacuum. At slow velocities the rate of boiling can create a fairly decent partial pressure of water vapor in the cavity. In the limit case, as velocity increases, the pressure inside the cavity in the vicinity of the projectile will go to zero. Eventually the pressure acting on the water will reverse its radial velocity and cause the cavity to close. However, the projectile will be long gone by that point. If this approach is correct then, except for finding a material to withstand the steady state stagnation pressure, there may be no hydrodynamic upper limit to the velocity of a supercavitating body.

    In any case, perhaps there is room for both viewpoints. In fact, they may very well be opposite sides of the same theoretical coin. I would certainly be interested to know what other people in the field thought of this approach. Perhaps it would provide an interesting topic of discussion within the article?"
     
  2. yipster
    Joined: Oct 2002
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    yipster designer

    "there may be no hydrodynamic upper limit to the velocity of a supercavitating body"
    hmm, really?
     
  3. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    Drag due to lift goes as (K L^2) / (pi/2 rho V^2 span^2). Assuming the airspeed and the speed through the water are comparable, water's higher density (rho) actually reduces the drag due to lift, because it's always more efficient to accelerate a large amount of fluid by a little rather than impart a greater acceleration to a smaller amount of fluid to produce the same force.

    Where the difference comes is in the span loading. If one sets out to support the weight using aerodynamic lift, one typically picks a much larger wing span than the width of the hull. But even then, it takes a wing span 15 - 20 times the width of the wetted planing area to have the same drag due to lift.

    A less important effect is the effect of the water's surface. The free surface doubles the induced drag of the planing surface compared to a deeply submerged hydrofoil of the same span. At the same time, the water's surface reduces the induced drag of the wing operating in close proximity to the surface. The combination of these two effects reduces the effect of the ratio of densities by by a factor of something like 2 to 3 (K in the equation above).

    See
    http://naca.larc.nasa.gov/reports/1958/naca-tn-4168/naca-tn-4168.pdf, Fig 16, which shows the equivalence between a planing surface and a shallow-running hydrofoil,
    and
    http://naca.larc.nasa.gov/reports/1955/naca-report-1232/naca-report-1232.pdf, which shows the drag due to lift in water.
     
    1 person likes this.
  4. Franklin
    Joined: Jun 2005
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    Franklin Junior Member

    Stability

    It might be possible to use small supercavitating appendages to stabilize the WIG boat hull or provide pitch and roll control.
     
  5. Franklin
    Joined: Jun 2005
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    Franklin Junior Member

    Supercavitation

    As long as you're looking at configurations like this, you could consider making the submerged hull a supercavitation body.

    http://www.deepangel.com/html/the_squall.html
     
    Last edited: Jul 2, 2005
  6. John David
    Joined: Jul 2004
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    John David Junior Member

    I remember reading the Scientific American Article with great interest. Now I fail to see how lift can be generated by a fully submerged super cavitating body. I think most of us have in mind surface craft that we can sit in and steer. Initially, at rest they are suported by bouyant forces.As fast boats speed up, the bouyancy is replaced by planning forces that lift the boat partially out of the water. What is the sequence of events that take place as a bouyant hull transforms into a super cavitating fully sub merged body? Are there surface piercing struts? I just can't visualize how this concept
    is applicable to surface craft. Then again, I said lots of great inventions wouldn't work either.
     
  7. Franklin
    Joined: Jun 2005
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    Franklin Junior Member

    Planing

    Supercavitating bodies don't sink because they're PLANING. They're inside the A BUBBLE. The boundary of that bubble is THE SURFACE OF THE WATER. The fact supercavitating torpedos DON'T SINK should have been you're first clue. Unlike surface bodies that can only plane in two dimensions, supercavitating bodies can plane in three dimensions. A boat connected by struts to small supercavitating appendages could use that ability to plane in three dimensions to control roll, pitch and yaw.
     
  8. John David
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    John David Junior Member

    Torpedos, the last time I looked, were basicaly neutrally bouyant.They dont need much lift. They don't sink anyway.Whether super cavitating or not.
    I thought the idea of super cavitation is to reduce friction drag and that it doesn't do anything for profile drag. Long slender bodies(like torpedos) might benifit from this idea. The best planning configurations have a high aspect ratio wetted area, ie the surface is wide and rather small longtitudanally.
    Thus fiction drag is low to begin with. If one could find an advantage, surface piercing struts would in my opinion, add more drag than saved.
    Returning to the core of the idea, are you saying that the boat would plane on the sheath of bubbles created by the cavitation? How high a pressure can this bubble sheath support? What is the pressure in the bubbles anyway?
    Why don't they get "squashed" by the relatively high pressure under the planning surface? If there are no answers to these questions please say so.
     
  9. Franklin
    Joined: Jun 2005
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    Franklin Junior Member

    Bubble

    How many times does this have to be explained to you? A supercavitating body is inside A BUBBLE, as in ONE BIG BUBBLE. It is not inside a "sheath of bubbles." Bouyancy is IRRELEVANT because a supercavitating body ISN'T FLOATING.

    Read this article and you might finally understand it.

    http://www.ocnus.net/cgi-bin/exec/view.cgi?archive=64&num=16623

    MEMORIZE this phrase in the article. "About two thirds of the way back from the nose are four spring-out cylinders angled toward the stern. Although they loosely resemble fins, these spring-tensioned skids actually support the aft end of the torpedo by allowing it to bounce off the inner cavity surface."
     
    Last edited: Jul 16, 2005
  10. Franklin
    Joined: Jun 2005
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    Location: Florida

    Franklin Junior Member

    Cavity

    "At slow velocities the rate of boiling can create a fairly decent partial pressure of water vapor in the cavity."

    "This creates a source of gaseous water vapor which creates the cavitation bubble."

    "Of course the water on the interior face of the bubble begins to boil, but it can only boil so fast, even in a hard vacuum."

    John David - NOTE THE ABOVE PHRASES from the article below. How could anyone possibly misinterpret this to mean "a sheath of bubbles"?

    http://www.ctechdefense.com/speed.html

    2509 West 19th Street, Port Angeles, WA 98363
    PHONE (360) 452-2275 * FAX (360) 452-2297 * info@ctech.esdcorp.com

    Underwater Speed Limits
    (Following is the full text submitted to Scientific American. The published version was shortened for editorial reasons.)

    "There appear to be two conceptual ways of approaching supercavitation. The generally accepted one derives from propeller cavitation theory and holds that the water is essentially boiled by dropping its pressure via abrupt acceleration. This creates a source of gaseous water vapor which creates the cavitation bubble. It is generally assumed that the cavitation bubble is filled with this water vapor. Indeed, in low speed (say torpedoes) supercavitation applications the cavity size is usually enhanced with ventilation gases. This fits well with the understanding that gas creates the bubble in the first place and appears to work well within that context. It also fits comfortably in the general framework of marine engineering.

    Last September, at an ONR sponsored Supercavitation Conference, Dr. Kirschner (of Anteon Corporation) and I were discussing the idea of a theoretical speed limit for supercavitating objects, assuming material strength issues could be overcome. As previously mentioned, conventional wisdom holds that the cavity is created by the water vapor and therefore, at some speed, the volumetric rate at which vapor can be generated will become insufficient to support the formation of a cavitation bubble which will clear the body. In other words, at some velocity the rate at which the water boils will become insufficient to fill the volume of the "hole" in the water created by the passage of the projectile and the cavity will collapse.

    For whatever reason, I have a different mental picture of how the bubble is created, perhaps due to my background in hypersonics in graduate school. In that field discontinuities and rarified flows are encountered in the course of normal business. I do not know if anyone else shares this view but Dr. Kirschner and I have discussed it at some length. In any case, I believe the process is fundamentally one of momentum transfer. The cavitator, be it a disk or cone or whatever, imparts a significant radial velocity (relative to the axis of flight) to the water it comes in contact with. In effect the water is thrown violently to the side. It therefore has a high radial momentum that is resisted by the pressure of the water around it. This pressure serves to slow its radial velocity and will bring it to a stop over a finite time. The accepted definition of cavitation number is compatible with this idea. In the meantime, assuming a circularly symmetric cavitator, a round "hole" has been created in the water. What is in this hole, other than the projectile? I believe it is a vacuum, at least initially. Of course the water on the interior face of the bubble begins to boil, but it can only boil so fast, even in a hard vacuum. At slow velocities the rate of boiling can create a fairly decent partial pressure of water vapor in the cavity. In the limit case, as velocity increases, the pressure inside the cavity in the vicinity of the projectile will go to zero. Eventually the pressure acting on the water will reverse its radial velocity and cause the cavity to close. However, the projectile will be long gone by that point. If this approach is correct then, except for finding a material to withstand the steady state stagnation pressure, there may be no hydrodynamic upper limit to the velocity of a supercavitating body.

    In any case, perhaps there is room for both viewpoints. In fact, they may very well be opposite sides of the same theoretical coin. I would certainly be interested to know what other people in the field thought of this approach. Perhaps it would provide an interesting topic of discussion within the article?"
     
  11. John David
    Joined: Jul 2004
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    Location: Long Island

    John David Junior Member

    I give up. I don"t understand the theory or your intended implementation for planning craft enough to comment further and you dont seem disposed to help me do so. I may not be able to verbalize it properly, but my gut feeling is, as hard as you try, you can't "shoe horn" this idea to do much for planning boats.
    It sounds like it may be the thing to increase the speed and/or range of torpedos etc.Do yourself a favor and stick with that.
     
  12. Franklin
    Joined: Jun 2005
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    Location: Florida

    Franklin Junior Member

    Supercavitation

    Planing on the inner surface of the cavity isn't a theory. It's what prevents The Shkval (Squall) torpedo from tailslapping -- bouncing around -- the interior of the cavity.

    The problem is you read the Scientific American article and couldn't even comprehend what the illustrations were showing, let alone what the text was saying. You came away from the article having no clue what was discussed were bodies surrounded by a cavity -- A VOID -- in the water.

    "About two thirds of the way back from the nose are four spring-out cylinders angled toward the stern. Although they loosely resemble fins, these spring-tensioned skids actually support the aft end of the torpedo by allowing it to bounce off the inner cavity surface."

    "At slow velocities the rate of boiling can create a fairly decent partial pressure of water vapor in the cavity."

    "This creates a source of gaseous water vapor which creates the cavitation bubble."

    "Of course the water on the interior face of the bubble begins to boil, but it can only boil so fast, even in a hard vacuum."
     
  13. John David
    Joined: Jul 2004
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    Location: Long Island

    John David Junior Member

    Franklin,
    I think this discussion (rel supercavitation) has gone well beyond the topic of this thread,namely boats that use air lift, especially WIG lift to enhance performance. Also note there are no other repliers than you and me. I will send you some additional comments via E mail.
     

  14. archnav.de
    Joined: Jun 2003
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    Location: Germany

    archnav.de Senior Member

    1 person likes this.
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