Vernacular voyaging canoe design

Discussion in 'Multihulls' started by dsigned, Aug 28, 2017.

  1. Angélique
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    Angélique aka Angel (only by name)

    Grand Rapids ?
     
  2. dsigned
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    dsigned O.R.C. Hunter

    Yeah. Calvin, although her family is from Wisconsin.
     
  3. Manfred.pech
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    Manfred.pech Senior Member

    A lot of tests have been done concerning interference between catamaran hulls under various conditions. For instance here: http://mararchief.tudelft.nl/file/35511/ .
    At low speeds there is not much increase in resistance. But at fr ~ 0.5 more than 30% depending on trim and sinkage. From my experience I would avoid too much gradient of curvature and reduce the wetted surface as much as possible. This can be done by hydrodynamic lift from an optimized waterflow around the hull. May be you can avoid the bow wave and stagnation point and the whole crap of whorls, vortices and eddies. :p

    [​IMG]

    More:
    http://www.maths.adelaide.edu.au/yvonne.stokes/Tuck/pdfiles/tuclaz.pdf

    https://www.researchgate.net/publication/263925728_URANS_Simulation_of_Catamaran_Interference

    catamaran interferences 30% drag - Google-Suche https://www.google.de/search?q=catamaran+interferences++30%25+drag&client=firefox-b&dcr=0&tbm=isch&imgil=KFvPAELRvckZQM%253A%253BLOOzR4LppJ640M%253Bhttp%25253A%25252F%25252Foffshoremechanics.asmedigitalcollection.asme.org%25252Farticle.aspx%25253Farticleid%2525253D1456674&source=iu&pf=m&fir=KFvPAELRvckZQM%253A%252CLOOzR4LppJ640M%252C_&usg=__EQezSYUhNUPXydLutavyN7F9Np4%3D&biw=1070&bih=588&ved=0ahUKEwiJwNCchJ3WAhWDvxQKHeWOAMIQyjcIMw&ei=AHS2WYnAPIP_UuWdgpAM#imgrc=KFvPAELRvckZQM:
     
  4. Angélique
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    Angélique aka Angel (only by name)

    Last edited: Sep 12, 2017
  5. dsigned
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    dsigned O.R.C. Hunter

    Is there something I'm missing about calculating Froude number for a multihull? Getting a Froude number in the .5 range for even the "wide" cat using the dimensioning comes out to about 3 knots. A froude number of .80 seems to equate to about 5 knots? If that were true, it seems it would be over the hull interference "hump" in about 10 mph of wind, but that seems unlikely to me, so I'm assuming I've made an error somewhere.
     

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  6. dsigned
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    dsigned O.R.C. Hunter


  7. tspeer
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    tspeer Senior Member

    I think for most sailing catamarans, the beam is wide enough that there is little hydrodynamic interaction between the hulls. The shift of wetted area and buoyancy from one hull to the other due the heeling moment from the rig is likely to be more important to the performance than the interference between the hulls. John Shuttleworth discusses the change in wetted area with heeling moment in this article. If there is a significant hydrodynamic interaction, you'd need to take into account the displacement of each hull as a function of speed - it's not going to be a simple relationship. You'd need to use something like Michlet to make many runs at a variety of different loading conditions to see how the drag shakes out over the speed range. A velocity prediction program (VPP) would also be quite useful for solving for the equilibrium loading conditions.

    As for the beam/length ratio to balance longitudinal, lateral and diagonal stability, John Shuttleworth also addresses this through his stability indices. I highly recommend his article on Multihull Design Considerations for Seaworthiness.
     
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