Round full blunt bow on a cruising catamaran

Discussion in 'Multihulls' started by valery gaulin, Jan 20, 2017.

  1. valery gaulin
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    valery gaulin Junior Member

    @Tom151.

    Why do i need such diplacement? I am more after the most space in a 30 foot X 14 foot catamaran than the displacement itself. Basically for more comfort than speed.

    I was also thinking about a 30foot X 12foot Dutch Tjalk type of sailboat as an option.

    Therefore I am looking at the most room for a 30foot sailboat or multihull with a very shallow draft to cruise the great lake, the great loop, The ICW, the Caraïbes and down the south america cost.

    I like Marina and very shallow bay to ancor. Lenght is the price for the marina and draft is for shallow bay. I am thinking half time in a Marina and the other half at ancor.

    Now would a catamaran with high volume hull sail and be a better compromise than a Sailing Dutch Tjalk???

    This is what I am trying to compare.

    I am not after speed, and not thinking of crossing an ocean.

    PS: Gemini Catamaran seam to be a good option for my program, but i like to complicate things and do what most people would not try or do. Sometimes it gives me good result trying unorthodoxe solution but other times it fail greatly. But this is how we learn most.
     
  2. valery gaulin
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    valery gaulin Junior Member

    @ Tom 151

    I fogot, Transat mini 6.5 when they are on plane it is on the aft part of the hull, basically like any planning boat, but the mini 6.5 are not always on plane if the wind is not there. Therfore this is the actual compromise, If there is no wind I think that the blunt bow sailboat dont perform as well as the other sailboat but as soon as the wind pick up they start to outperform the traditional mini 6.5 hull shape. Therefore overall the round blunt bow would outperform if it is in strong trade winds.

    Anyway we are talking about racing which is not my intended program.
     
  3. valery gaulin
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    valery gaulin Junior Member

    Anyway my idea of blunt round bow can't a worst idea than A flilybridge on a sailing catamaran like the Leopard 58!!! This flybridge things seams to be really bad and it is mainstream!!!
     
  4. upchurchmr
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    upchurchmr Senior Member

    Apples and oranges.

    Why don't you just make a barge or a sailing scow?
     
  5. valery gaulin
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    valery gaulin Junior Member

  6. redreuben
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    redreuben redreuben

    6.5

    Aren't the 6.5's an optimised shape for sailing downwind ? ie not very versatile ?
     
  7. valery gaulin
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    valery gaulin Junior Member

    I think you are right, But more precisely it is optimised to use the trade winds.
     
  8. upchurchmr
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    upchurchmr Senior Member

    Trade winds tells you less than downwind, IMO.

    You can do a hell of a reach in the tradewinds.
     
  9. DennisRB
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    DennisRB Senior Member

    This is a mistake I often see people make. They want say the space of a 35 footer on a 30 footer, thinking it will cost the same as a 30 footer. The only thing that will cost the same in this case is the dock fees, and depending on your marina this may not even be the case.

    Certainly a 30 footer ballooned out to have as much space as a 35 footer will cost closer to a 35footer than a 30 footer to build and fit out. It will be the most expensive 30 footer imaginable. So why not just add a little more hull and have a boat which looks and sails a lot better?

    IMO the idea is always a bad one unless you have external governing reasons to why the boat can't be longer as its not a cheap way to build a boat. The displacement to length ratio is the overriding factor when it comes to hull resistance so longer is better for performance and comfort.

    If you do indeed have an external factor governing the size that is not just cost (as this is not a rational reason when the expectation is as much space as a bigger boat) then I think the best boat to fit your box rule is something like this. A boat like this is almost a square box with rounded edges so wastes the least space possible within that box rule. It will have much more space and weight carrying capability compared to your cucumber hulled cat.


    [​IMG]
     
  10. daiquiri
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    daiquiri Engineering and Design

  11. upchurchmr
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    upchurchmr Senior Member

    The picture above shows exactly why you should not have blunt bows.
    Thanks for making the point.

    There is no wave action at all and yet you have a huge bow wave with lots of energy being lost.
     
  12. daiquiri
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    daiquiri Engineering and Design

    The boat in the picture is the production version of Teamwork Evolution, which has has won the Transat (trans-Atlantic) race in 2011, arriving before all the others, classic (point-bow) sailboats: http://www.yachtsandyachting.com/news/160185/David-Raison-wins-Transat-650

    https://www.youtube.com/watch?v=AgBxZDaKnQ4
     
  13. upchurchmr
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    upchurchmr Senior Member

    I'm obviously not keeping up.

    The write up seemed to indicate he did well in the Doldrums.
    So is this optimized for light winds/ low waves which might have matched the conditions?

    Pity you can't always pick your conditions.
     
  14. valery gaulin
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    valery gaulin Junior Member

    This video show exactly that I am not crazy! Round blunt bow can be a good option for a catamaran. The video shows that yes I does push water, that create lost of energy but I think that in a pointy bow we don't see the underwater flow seperation from a sharp poiny bow. What you don't see you don't know!!!

    Anyway the conclusion is that the blunt bow is not that bad of an idea and should be tried amd tested on a scale model to see the kind of behavior it could give to a catamaran.

    When I am done with my other project I will try to make a model to see the behavior.

    On the other hand if someone as a finite element analysis software the simulate flow around a blunt round bow hull shape let me know I would like to try to run an analysis. Or maybe someone knows of a free software?
     

  15. bjn
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    bjn Junior Member

    Nice thread, haha!
    I had not seen this thread when I started mine:
    http://www.boatdesign.net/forums/multihulls/why-does-multihulls-have-pointy-bows-57133.html

    I guess it has always been the first thought, to make it pointy, when trying to design something with low drag. To make it cut the fluid like an axe!

    When in reality it's better to put a large radius in front, and design to avoid the wake in the back?

    [​IMG]
    [​IMG]

    Could this be the big revolution in boat design? :D
     
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