improving performance - reverse bulbous bow principle, amplify wave near stern

Discussion in 'Hydrodynamics and Aerodynamics' started by vignesh, Oct 6, 2011.

  1. vignesh
    Joined: Jan 2011
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    vignesh Junior Member

    Is it possible to reverse my bulbous bow principle,ie, can i by any means amplify the wave near the stern [increasing the crest amplitude] thereby i can use the increase in pressure to assist the ships motion.
     
  2. Petros
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    Petros Senior Member

    Do I understand your question correctly? You think putting a bulb at the rear to create a stern wave will improve performance?

    It seems to me the bulb would create both more form drag and skin friction drag, and that energy to form that stern wave has to come from the engine of the ship. I do not see how you can come out ahead on that calculation.
     
  3. cyclops2
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    cyclops2 Senior Member

    Cheap actual test setup.

    Build a 8' long scale HULL model of a freighter model. Ballast it to the scale water line. Put a small screw eye into the bow JUST above the water line. A canoe with a electric trolling motor is also needed.
    Clamp a 1" x 1" stick across the bow of the canoe. Add 2 very low friction ball bearing pullies to the stick. Attach one end of about 20# test braided fish line to the screweye in the model. The other end goes to a sensitive scale that can read the drag of the freighter when it is NOT NEAR the canoe bow wave. NEVER NEVER allow the tow line to drag in the water while towing test are being done!!!!!!!!!!!!!!! A wet line will NOT EFFECT results. Keep the distance between the bow & stick about 2' to 3'.

    Get the idea ?

    Then add whatever SCALE changes to the freighter you want to.

    Is it as good as a $ 1,000,000 test tank ?

    Better. Because you can pick SCALE... WIND SPEED & DIRECTION for more scale data on drags created by scale sized waves. You pick the day when winds are making scale sized waves.

    Yes, we can not use scale sized molecules of water. But neither does a expensive test tank either

    Absolute accuracy? NO. But percent of gain or loss in efficiency WILL BE RECORDED. That will be enough reason to move up to larger ..radio controlled.. scale models for more accurate data.

    S
     
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  4. Submarine Tom

    Submarine Tom Previous Member

    In a word: No

    -Tom
     
  5. cyclops2
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    cyclops2 Senior Member

    Not sure?

    Try it.

    There are always oddities in every science. Who thought adding a bulbous nose would be a improvment.

    Test your idea.
     
  6. vignesh
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    vignesh Junior Member

    The fact is increase in pressure will definitly assist the motion.
     
  7. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    No, it cannot work in the way you have imagined it.
    However, it could work in another sense - to use a stern bulb for the creation of a wave system which would then interact with the wave system of the main hull, in order to obtain a partial wave cancellation. A total wave drag could then be decreased, but the total frictional drag would certainly be increased. The success of the operation will depend on the net balance between the two opposite effects.
    Cheers
     
    Last edited: Oct 12, 2011
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  8. vignesh
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    vignesh Junior Member

    i would like to summarize

    Is there by any means i can increase the pressure distribution at the stern region and use that increase in pressure to assist the ship's motion thereby i reduce the power required to propel the ship.

    to summarize ,does increase in pressure in the stern region helps me to reduce the resistance?

    I guess stern wedge ,apart from trim optimisation ,can help ships forward motion ,by above principle .correct me if i m wrong.
     
  9. J Feenstra
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    J Feenstra Junior Member

    you can increase pressure by using a propellor....
     
  10. vignesh
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    vignesh Junior Member

    well ,i am talking about adding upto propeller thrust .

    Infact,propeller action reduces the pressure in the stern side .
     
  11. Tim B
    Joined: Jan 2003
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    Tim B Senior Member

    Moving water takes energy. Moving water upwards takes lots of energy. QED, increasing wave height requires [more] power.

    And bulbous bows change the effective ship length, but they only work at a certain speed. At other speeds they will make things worse.

    Tim B.
     
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  12. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Consider the effect of an adverse pressure gradient (pressure increasing in the direction of flow) on flow attachment/separation. An adverse pressure gradient can lead to separation which reduces pressure in the separated region and increases drag. Reshaping a hull so that the pressure increase near the stern may result in separation and increased drag.
     
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  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    You could also look at boundary layer suction (or blowing) near the stern.
    Whether the energy expended in removing the fluid, or accelerating it is worthwhile, is a difficult question.
     
  14. Tackwise
    Joined: Mar 2010
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    Tackwise Member

    Yes an increase of pressure in the stern region can reduce the overall resistance of the ship.

    Simply put: The pressure resistance of the ship is the resistance force the ship 'feels' from the pressure at the bow area minus the pressure force the ship ‘feels’ at the stern area.

    Increasing the stern pressure can therefore reduce the pressure resistance. Note that only the pressure resistance is reduced, as other resistance and propulsion efficiency factors may have increased depending on the chosen method of pressure increase!

    One possibility of pressure increase at the stern (depending vessel type/speed etc, etc) can be accomplished with a stern shape that brings the water flow to nearly a full stop. Turn side of such a shape is the possibility of separation of the flow, which will generally increase the resistance instead of reducing it.
     
    Last edited: Mar 15, 2012

  15. vignesh
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    vignesh Junior Member

    Yes ,I was thinking about modification of stern lines ,yes i was aware of the seperation resistance and also flow to the propeller .I m doing a study in it .
     
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