Catamaran Wave Interference

Discussion in 'Boat Design' started by fallguy, Sep 6, 2019.

  1. rxcomposite
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    rxcomposite Senior Member

    Very nice work. Mine is an amateur compared to yours. I just find the least interference from the graph and find a hull (among many) that has the least resistance for the parameters given.

    I just can't open the spreadsheet. The PDF file I can.
     
  2. rxcomposite
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    rxcomposite Senior Member

    Stable hulls are usually wide beamed but hard to paddle. Olympic rowing shells are easy to paddle but tips over easily.
     
  3. Dolfiman
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    Dolfiman Senior Member

    Many thanks :) Here attached a copy in PDF format of the spreadsheet. If you have an Ipad, you can download the free application OOreader to open any Open Office document. Anyway, Open Office is itself a free software, I downloaded mine with Apache OpenOffice (just 3 codes provided by SMS, (for the french version ?), costing me 3 x 4,5 Euros) and that works well, all Gene-Hull versions are developed with it. Libre Office, also free, can also be used to open and use these .ods applications.
     

    Attached Files:

    fallguy likes this.
  4. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    I found another very interesting paper by E. O. Tuck and L. Lazauskas : "Optimum hull spacing of a family of multihulls" which gives the numerical computation of the wave drag within Michell's thin ship theory for a Wigley hull in both a mono and a cata configuration (also for other configs : trimaran, etc…, but I limited my investigation to the cata mode) :
    http://www.maths.adelaide.edu.au/yvonne.stokes/Tuck/pdfiles/tuclaz.pdf
    As the considered hull is "slender" (L/B >7) and that in the Froude range < 1,0 the shape of such hull is of second order importance with regard the ratios (in particular Lw/D^(1/3) for the wave drag), one can validly compare their numerical results with the model test series (document attached). Main conclusions :
    *** The slender monohull : curve shape and order of magnitude is quite good / model test series in Froude 0,35 to 1,0 range.
    *** Yet, the « wave drag reduction as beam squared » rule consubstantial to the Michell's thin ship theory appears optimistic. In adimensional format, that is equivalent to"Dr/Mg reduction as beam" as M is also reduced as beam, but the comparison from test results when considering for example a half beam versus a beam hull, shows a consistent 0,7 reduction of Dr/Mg (in Froude 0,45 - 1,0 range) instead of 0,5 from the theory.
    *** In cata mode, the wave (residuary) drag amplification factor one can derived from the Wigley hull computation with Michell's and a S/Lw of 0,47 is :
    - For Froude 0,35 to 0,5 : similar to the one of the model tests results, with factor peak ~ 0,22 (i.e. 22% extra drag)
    - For Fn > 0,6 : the factor vanished to zero (i.e. no interference) instead of 0,1 for the model tests (i.e. 10% extra drag)
     
  5. rxcomposite
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    rxcomposite Senior Member

    That is what I based my study from. My excel program generates a set of hulls with different parameters. I choose the one with the least resistance.

    Check also Leo and Tuck article Solar Powered Multihull same site. He has some model test and comparison of resistance for different configurations.
     
  6. fallguy
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    fallguy Senior Member

    So, for my boat...

    Is there a minimum or optimum hull separation?

    This discounts strength concerns.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Yup:

     

  8. fallguy
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    fallguy Senior Member

    So I am at ...

    3.81/9.504 = 0.401
     
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