SA-VPP power catamaran

Discussion in 'Software' started by Dolfiman, Jun 26, 2021.

  1. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    I propose you a spreadsheet application for the estimation of the drag and of the power to install for a power catamaran, for speeds in the Froude range 0,3 to 1,0. Conditions of validity are :
    ** Length Beam ratio Lw/Bw > 7
    ** Length Displacement ratio Lw/Dc^(1/3) > 6,5
    (Length, Beam, Displacement here considered for one hull)
    ** Space Length S/Lw ratio 0,3 to 0,5

    It is a free and open source speadsheet application, developed on a support itself free and widespread (Open Office Calc 4.0.1) : to open and use an ods file, you have to download Open Office or Libre office according to : ODS File Extension - What is an ods file and how do I open an ods file? | OpenTheFile http://www.openthefile.net/extension/ods

    Also attached is the User guide, including in annex information about the sources and the computation involved in this prediction tool.

    By hoping this can be helpful for your projects,
     

    Attached Files:

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  2. Saqa
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    Saqa Senior Member

    Thank you
     
  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I am eager to try it, but have been awful busy.
     
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  4. Saqa
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    Saqa Senior Member

    As well, thanks for your efforts to share a tool that may help others help themselves. Nothing wrong with some show of appreciation either ;)
     
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  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Yes, many thanks for this, Dolfiman.
     
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  6. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Thanks Dolfiman! I have yet to read the instructions and previous post and understand what method this uses, but can this be expanded to speeds of freude 0.2? I'm looking very low speeds like 6 knots for ultra low power demands (a larger solar catamaran).
     
  7. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    I have to search for other model tests data and/or reliable formulation for the Froude < 0,3 range together with slender hulls of Lw/Bw > 7 and catamaran arrangement. Presently, I can just propose you an extension of the computation towards Froude zero by continuity of the formulations (here attached), perhaps useful for a preliminary approach of the orders of magnitude.
     

    Attached Files:

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  8. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    I propose you this new version 1.1 here attached for a better estimation of the residuary drag in the low Froude range < 0,4 . For that purpose, I use the Holtrop-Mennen formulation for a monohull, named « Rw-A » in their second paper (in the pdf attached), recommended for Froude < 0,4 and also valid for the cases of high L/B > 7 . A smooth transition between this formulation and the former one is automatically programmed from Fn 0,25 to 0,40, as illustrated in the User guide, in order to have a prediction continuity for Froude 0 to 1. And I add also the Holtrop-Mennen formulation « R Tr » for the drag of immersed rear transom if any.

    For the factor K of extra drag due to interhulls interference, within Froude range 0,2 to 0,4 the experimental results show a lot of humps and hollows, including whith large spacing ratio S/L (see Fig. 47 to 60 in the Molland paper attached), a variety of cases which are really challenging to represent finely with a formulation. So the approach here adopted is to be conservative and to cover as much as possible the humps as the goal is oriented towards the estimation of the necessary power to install.
     

    Attached Files:

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  9. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    @Dolfiman
    Thank you for your work and for sharing it.
    When I read your papers I didn't understand the meaning of "Mg" or "mg" in the "Dr/Mg (%)" quantity. As Dr is a force Mg should be a force also to gain a dimensionless fraction to express in percent.
    I looked several time to find an explanation but didn't find any. Could you help please?
     
  10. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    Thanks for your interest for that kind of work, this encourages me for further development.
    M is the mass and g the gravity acceleration, so Mg is a force in Newton like it is for Dr the residuary drag. And of course, Mg should correspond to the boat displacement considered for the speed prediction.
    Within SA-VPP Power Catamaran, the displacement of each hull is the input data, named Disp. We consider first the residuary drag of each hull, with using M (kg) = Disp * Rho to estimate Dr with the proposed adimensional curves. And secondly, the total residuary drag is : 2 Dr (1 + K) , K being the amplification factor estimated for the wave drag interference.
     
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  11. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    Thank you very much @Dolfiman. I was a blockhead as it seems. Because Drag is noted as a two letter entity Dr I didn't consider Mg as the product m * g.

    I appreciate your papers very much.
     
    Last edited: Nov 25, 2021 at 1:26 PM
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