catamaran resistance

Discussion in 'Multihulls' started by quyetvtt46, May 27, 2014.

  1. quyetvtt46
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    quyetvtt46 Junior Member

    hi everybody!
    i am calculating propeller of a low speed catamaran but all methods which i have don't suit. can you give me advice? thank you so much!
    note: my boat lines in attached file.
     

    Attached Files:

  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    You could try OpenFOAM, but you will need a fair bit of computer power.
    http://www.openfoam.com/
    See also:
    http://openfoamwiki.net/index.php/Sig_Ship_Hydrodynamics
    Some validations can be found at:
    http://openfoamwiki.net/index.php/SIG_Ship_Hydrodynamics/_Validation_test_cases
     
  3. Rastapop
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    Rastapop Naval Architect

    Sorry for going off topic, but I just had to say looking at your image made me think that's a very strange cat hull - what's it to be used for?
     
  4. quyetvtt46
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    quyetvtt46 Junior Member

    right, it is the strange hull. this catamaran is the working boat whose speed is about 8 knots. in my opinion, with low speed, hull is not the most important.
    because of this hull, i can not find the suitable method to calculate resistance.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Bp-Delta charts will suffice.
     
  6. quyetvtt46
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    quyetvtt46 Junior Member

    thanks Leo Lazauskas for helping!
    because I do not have a lot of time for this project, I must find a simple method for calculating. I think, your method which you gave me is rather complex. i really have not understand it.
     
  7. quyetvtt46
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    quyetvtt46 Junior Member

    thanks Ad Hoc!
    I have a problem with calculating its resistance. have you got great idea?
     
  8. TANSL
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    TANSL Senior Member

    I do not know what your (very special!) circumstances but never heard a designer say "I will not do some calculations because I have no time".Frankly, I do not understand what you want to do with your design to say something like that. That does not say anything good about you, imo.
    Now if the propeller and the power required not care about, then you need not ask for advice, as at the beginning of this thread.
     
  9. daiquiri
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    daiquiri Engineering and Design

    Correct me if I am wrong, but I am reading (though barely) the following data in your picture:
    LOA = 14 m
    LWL = 12 m
    b = 6.4 m
    D = 2.0 m
    If these numbers are correct, than in my opinion (and at first sight) 7.5 kts would be perhaps a more reasonable target speed due to the rather particular (or should I say "odd"?) hull shape and volume distribution in the stern area, which looks very draggy.
    Could you post a picture with better resolution, or a summary of vessel's main dimensions and weight?

    And yes, TANSL is right. It looks like a pretty odd situation here, where you are designing a research cat but have no time to calculate resistance properly. It could head both you and that vessel into a sea... of troubles.

    Cheers
     
  10. gonzo
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    gonzo Senior Member

    The separation of the hulls is also critical on calculating resistance on multihulls. The bow wave, turbulence a the stern and wave slamming on the bridge can add a considerable amount of drag. They also may contribute to other things to consider. For example, in quarter seas, whether a propeller will come out of the water.
     
  11. TANSL
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    TANSL Senior Member

    Gonzo, although not the subject of this thread it would be interesting if you explain what is the "wave slamming on the bridge" and how it affects the resistance of the hulls. Thanks for your kindness
     
  12. quyetvtt46
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    quyetvtt46 Junior Member

    yes, my circumstance is quite special. in fact, this catamaran are being designed by other company, but They employ my company to design propeller for it. They required us to do it in very short time. although, I calculated it I feel unreliably, because my methods do not entirely suit vessel. so, I opened this thread.
     
  13. quyetvtt46
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    quyetvtt46 Junior Member

    that are exact number.
    b DemiHull = 2,7 m
    distance 2 DemiHull = 1m
    draft = 1,2 m
    D = 2 m
    CB DemiHull = 0,66

    This catamaran use main engine with 55 kW in power and 1800 rpm in revolution, gear ratio = 3,5 : 1. I predicted its speed about 8 knot which i feel really unreliably. Why did you find out 7,5 knot?
     
  14. daiquiri
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    daiquiri Engineering and Design

    I guess that 1m hull separation is a typo. 10 m, perhaps?

    7.5 kts (or a tad more) is my gut feeling by seeing your hull lines, but of course I might be wrong. The upwards-curved buttocks near the stern rise with a too-steep slope, imho. The buttocks should have a much more gentle transition from the maximum draft to the bottom of the transom. The way you have done it, imo, you might have a significant flow separation and/or increased wave drag. Hence, your speed will be reduced, for a given engine power. Or your engine might have to be oversized to get a desired speed of 8-8.5 kts.
     

  15. TANSL
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    TANSL Senior Member

    I fear, daiquiri, that quyetvtt46 not have time for all these trifles.
     
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