Resistance calculation of a "very" hard chine catamaran

Discussion in 'Multihulls' started by lausl, Nov 17, 2020.

  1. lausl
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    lausl carpe diem yacht design

    In the early design concept stage the client is investigating into : Mono or cat, a lot is in favour of mono, less height etc. But what would be the performance.
    LOA 15m
    BOA 4m
    Displ. 15t
    Hull width 1.1m
    I work with ORCA3d for resistance predicion, but for cats with that aspect ratio it would not work.
    Does anybody have a reliable method of calculating and deliver a guess or knows a collegue that perform such calculations, thank you.
    upload_2020-11-17_17-43-29.png
     
  2. Alexander Peter Bromley
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    Alexander Peter Bromley Junior Member

    If you can get hold of or know someone who has Maxsurf you can run your resistance there.
     
  3. fallguy
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    fallguy Senior Member

    F409F7D6-3924-4577-B6D4-554F181359DE.jpeg

    Why would you make hull sides vertical?

    For dynamic loading, the boat is basically all reserve or none, depending upon the persepctive.

    And then for wave interference; the hulls are probably closer than needed.

    I am not expert. Just a casual lover of boats.

    And a builder of a Wood's cat.

    See the chine/angle.
     
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  4. lausl
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    lausl carpe diem yacht design

    THew Yaerd has already settled for the dimensions, it is intended to cruise the canals and rivers, bridge heigts etc. are more relevant than other nput data. The yard choose a shou box design.
     
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  5. Ad Hoc
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    Ad Hoc Naval Architect

    Lausl,

    You may find the attached of use for you.
    Good luck.
     

    Attached Files:

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  6. bajansailor
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    bajansailor Marine Surveyor

    Lausl, re the performance of the cat, has your client specified a minimum service speed that is required?
    And will this be on canals and rivers in Germany / Europe?
    I know that rivers like the Rhine can flow quite fast in the spring time - will this be a factor regarding the minimum speed required while motoring up river?
     
  7. lausl
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    lausl carpe diem yacht design

    Thank you for that precious Info, Dr. Mueller Graf was a vivid speaker and attendee at our Yacht Design Symposia in Hamburg
     
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  8. lausl
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    lausl carpe diem yacht design

    Cruising Speed will be 6-8 km/h top speed 14 - so quite slow, still good to know the power needed - and get some data to compare design variants
     
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  9. Ad Hoc
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    Ad Hoc Naval Architect

    Your speeds, if written correctly indicate a max speed of just 7.6 knots.
    With this speed and the length of your boat, that's a Fn of just around 0.32 depending on actual Lwl.

    This is too low to be considering a hard chine hull - unless for ease of construction.

    You should consider a simple round bilge hull form....
     
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  10. DCockey
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    DCockey Senior Member

    Ad Hoc, given the size and other characteristics of the proposed vessel how much do you think the details of hull shape would affect total resistance at 7.6 knots? Would a hard chine hull have significantly higher total resistance (more than 10%) than a round bottom hull?
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    This is an interesting question and raises the debate from theoretical/academic exercise to a practical aspect of design.

    The LD ratio is modest, at best at around 5.8 - 6.0 depending on the actual Lwl. Thus this would suggest there will be some gains using a round bilge compared to that of a hard chine. How much, well that is the key part in the decision making process of design.

    Since if the difference is just say 2-3%...why bother changing..since hard chine is easier to build.
    But could we could also state that for around a difference of 10%...??

    Now 10% sounds significant, and it is. However, in the whole context of this boat, is it worth it?

    You would need to compare the round bilge v hard chine actual EHP data as a true like for like.
    Whilst the round bilge will be less, is it "less" enough to change the engine selection from say a 50HP motor to say a 40HP motor?

    Ultimately it is this type of design decision that dictates.
    Because no matter what the numbers show, ignoring those that like to highlight differences down to decimal places as a metric of variance, if the difference changes an engine selection option, then yes - as a cost/financial saving can be made. If it doesn't, then why bother?
    One must look holistically at the design rather than any individual aspect which when taken out of context appears 'significant'... but in the whole scheme of the design, plays a minor role.

    Does that answer your question?
     
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  12. Mr Efficiency
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    Mr Efficiency Senior Member

    You would have to query why a catamaran at all, a simpler monohull vessel seems well able to fulfil the requirements.
     
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  13. bajansailor
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    bajansailor Marine Surveyor

    Re the powercat in my avatar, she has two chines and the hulls are assymetric (as if a monohull was sliced along the centreline and then separated) - she is 15 m. LOA and cruises along at about 8 knots, and yes, a round bilge (and symmetric) hull would be more efficient at these speeds - but it was much easier to build her (in aluminium) with chines and assymetric hulls, and she still comfortably exceeded my initial expectations re speed when she was launched (16 knots with 2 x 70 hp O/B motors).
     
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  14. Ad Hoc
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    Ad Hoc Naval Architect

    Exactly... a case in point!

    Pure numbers of EHP of hard chine v round bilge is just one element of the long chain of events in design.
     
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  15. lausl
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    lausl carpe diem yacht design

    It is indeed the other perspective that governs the design here: Its not physics or going down or rather up the design spiral. The yard made first contact with the client and convinced him to build a cat - for whatever reason, roll period when at anchor etc..... But all the design input parameters, speed, headroom, bridge clearance height etc, point strictly towards a monohull, a very simple barge style cosy ship, right out from a nice parisian peniche picture. I raised that delicate point very carfully, lets see!
     
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