Doubt on resistance...URGENT!!!

Discussion in 'Software' started by souravsandesh, Oct 13, 2014.

  1. souravsandesh
    Joined: Aug 2014
    Posts: 8
    Likes: 1, Points: 0, Legacy Rep: 33
    Location: INDIA

    souravsandesh Junior Member

    Hello,

    I am doing a final year project on a seater GRP catamaran.

    I calculated the resistance for a demi hull (as am not able to compute for both the hulls in RHINO due to some error) using RHINO with the help of a plug in named ORCA 3d.

    I got it about 1399 N and effective power of 5.8 Kw. My vessel's speed is 8 knots.
    My vessel length is 18 m..breadth demi hull is 1.75 m..overall 6 m breadth. hull depth of 1.3 m.
    When i showed the report to my guide he was complaining about very low resistance. No matter how much i try that's the maximum i get. Is the resistance really that low?? Do i have to calculate for both hulls so that there could be some difference??...please help me....its urgent!!

    the rest of the parameters like block co efficient, prismatic coefficient etc. all the hydrostatic data for a demi hull and as well as for both hulls are in the excel sheets that are attached. The first one is for demi hull (sheet 27 in that excel sheet) and the second one for both hulls.

    P.S am not able to attach any documents due to exceed of maximum quota. I dono how to delete them. Please help me with this as well.

    My block coeff for demi hull is 0.586 Prismatic coeff is 0.806 and midship section co eff is 0.722. Vessel is designed for a draft of 0.5 m with a total displacement of 18.44 tonne...9.2 tonne for single hull.

    Thank you.
     
  2. Leo Lazauskas
    Joined: Jan 2002
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    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    You could try Michlet to get a rough estimate of the total resistance and the
    resistance components. That should give you an idea if your method is giving
    a reasonable answer

    By the way, it doesn't matter how urgent your question is, or how many
    exclamation marks you use, people will answer you if they have time and feel
    like helping you with your homework or university project.
     
  3. Alik
    Joined: Jul 2003
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    Location: Thailand

    Alik Senior Member

    If You use Rhino/ORCA, likely that it was done by Holtrop method. This method is not suitable for catamaran demihulls and it might be out or the parameters of the method, so it will freak out. I am sure hull shapes of catamarans were not included in Holtrop's regression!

    With such hull spacing, You will get major effect from interference of wave systems of the demihulls. It can increase resistance at hump by 40%. This means You can NOT estimate the resistance by doubling resistance of demihull.

    The problem of today's education is 'software user mentality'. You find he soft, press the key and expect the answer. Not thinking about is the tool is correct and what is inside black box.

    For Your catamaran, I would recommend Muller-Graf method that You can find and program Yourself in Excel. Another options is Molland series, but this is for round bilge cats. We always use 2-3 methods together so make sure that there is no mistake, and final selection of method for power estimate is based on experience. Great reference would be new Molland's book 'Ship resistance and propulsion'...
     

  4. Hampus
    Joined: Jan 2010
    Posts: 49
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    Location: Sweden

    Hampus Junior Member

    Try calculating it manually for a comparison. With 18 meter hulls and 8 knots a large portion of your resistance will be frictional. Either do an approximation of the length of your wetted surface or divide it into bands and calculate avarage length, Reynolds number, skin friction coefficient and resistance for each band. A very rough approximation is that frictional resistance would be around 1/3 of total resistance.
     
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