power catamaran speed/resistence prediction

Discussion in 'Boat Design' started by fede, Dec 11, 2006.

  1. fede
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    fede Senior Member

    Hi everyone, I'm struggling trying to find a speed/resistence prediction method for power catamarans (planing hard chine)
    Could you suggest one ???or direct me to a forum thread dealing with the same argument (searched but found nothing)
     
  2. alpamis34
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    alpamis34 Senior Member

    If you have the computer modelled hull,I suugest you to use hullspeed (a programme of maxsurf family) though it s gonna give you pretty reasonable resullts.

    The other method is the ITTC method where you can find the Cf with the formulae given but you would the nhave to adopt a form factor for finding out the viscous coefficient.For the wave resistance there are some methods that would help you to predict the result which basically rely on the potential flow theory.
     
  3. fede
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    fede Senior Member

    Thanks Alpamis, I'm familiar with ittc method but isn't that for displacment mode?
    I need a planing analysys.
    I don't believe hullspeed is agood one for this job too,since it's a savitsky based planing analysys wich I'm not shure how could apply to a CAT.
     
  4. yipster
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    yipster designer

    not such an easy question fede, for graphs on fast cats check jimboat
     
  5. fede
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    fede Senior Member

    Thanks Yipster,this is a 57 ferry supposed to cruise at 19/20 knots ,not a fast Cat as those discused by the tunnel boat book,I'm posting the lines in another thread in a while.
     
  6. alpamis34
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    alpamis34 Senior Member

    Also for planing hulls I'd suggest you to use Power prediction programme of Wolfson Unit (the research centre based in University of Southampton,UK where I did my MSc) You can download the demo version of the programme from this link
    www.wumtia.soton.ac.uk/
     
  7. fede
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    fede Senior Member

    Hi Alpamis, the Wolfson program that I did not know is a great program, it's a simple one that looks very well done.
    Only thing....it does not have a specific CAT prediction method, just plain savitsky.
    It's not clear for me if Savitsky is usable also for cats, maybe calculating resistence for one hull and then apllying it to the 2 demihulls with a correction factor that takes into account the interference of the 2 hulls and the different wetted amount of surface that is typical of cats.
    Possible that nobody on this forum has a clue for this ???
     
  8. alpamis34
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    alpamis34 Senior Member

    That is the most reasonable way of calculating the drag of demi-hulls .But the matter is the interference effects of hulls among each.If you can make an assumption like adding 5-10 percent to the final result that you deduced from the formulaes, that would help you. But if you want accuracy, you 'd better have it run in the towing tank.Cheers
     
  9. Jimboat
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    Jimboat Senior Member

    Fede - the AeroMarine Research software "Tunnel Boat Design Program" uses algorithms for planing designs at all velocities (above planing speeds), includes symetrical hull hydrodynamic interference (for 2 or 3 hulls) and wetted surface analysis for catamaran hulls and/or modified hulls with centerpod designs. Also accounts for aerodynamic contributions, if applicable. (Also has features to analyze vee-hull designs).

    Call if you'd like to discuss.
     
  10. fede
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    fede Senior Member

    Thanks Jimboat, I was considering your software...
     
  11. rkpshenoy
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    rkpshenoy student

    Hey to make matters more complicated even I am stuck at one stage of my project.(Design of passenger catamaran)
    After fixing the main dimensions of the cat i got a body plan which was geometrically similar my cat. Its CB is given as 0.695
    But after i have modelled the demihull using maxsurf i found that its CB is 0.37 as i expected with the hull form not very full and V.
    Some one help me. this is my autocad design of bodyplan.
    lines3.JPG
    The body plan appeared in a paper "Experimental Study on the influence of hull spacing of hard chine catamaran motions"
    Can anybody help me with the body plan.
     
  12. Pericles
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    Pericles Senior Member

    Far from wanting to put dent in your happiness guys, but I think you may just be trying to re-invent the wheel. If you check the achievements of Malcolm Tennant at his site, you will see how he has very successfully addressed the issues relating to fast power catamarans.

    http://www.tennantdesign.co.nz/

    Pericles
     

  13. fede
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    fede Senior Member

    Pericles...am I missing something ? I'm still looking for Cat resistence prediciotn method...should I quit deisgn becaus of mr Tennant achievements ?
    I myself am proud to be an "achiever",my new 60 cat hull has behaved perfectly during tank tests,24.5 knots @ 24 tons and with 900 hp total,in perfect balance with project CG.
    Nice but pointless... I'm still looking for a cat prediction method and still want to improve myself,but more than everything still want to design cats !
     
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