Minamal Drag

Discussion in 'Hydrodynamics and Aerodynamics' started by Dan Cronk, Sep 9, 2017.

  1. Dan Cronk
    Joined: Sep 2017
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    Location: Sacramento

    Dan Cronk New Member

    As part of a science project I am helping my son design a solar powered boat that will be entered into a competion at the end of the project. As power is minamal, drag reduction is critical. Max speed is around 5 mph. We are considering a catamaran design. Are there any other designs ( such as kyack etc) that would offer less resistance? If catanaran is the best design, what would be the best shape of the pontoons.
    Thanks
    Dan
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    A catamaran is the likely winner, and the key is to keep the two hulls slender, around 12 times longer than their breadth. But, and there are always "buts", you will need to be mindful of weight. Have the stern tapering in like a canoe.
     
  3. David Cooper
    Joined: Jan 2015
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    Location: Scotland

    David Cooper Junior Member

    I'm not convinced that a catamaran would be a lower drag solution. Where are all the catamaran kayaks if that's the case? I've been trying to find information on this for a long time, but all the people providing numbers seems to be interested only in maximum hull speed rather than minimising drag at lower speeds for maximum efficiency. In a discussion at Sailing Anarchy about trimarans vs. cats, it became clear that trimarans are faster at lower speeds because they have lower skin friction (because they act almost as monohulls with only a small amount of extra drag from one ama which is resting lightly on the water), and the cat guys were saying that they had to be doing somewhere around 15 knots before the advantages reversed when racing against monos. With smaller boats, that speed would come down too, but would it really be as low as 5mph?

    I'd like to see numbers giving clear answers on this, setting out the best length to beam ratios for slow speeds with the same displacement each time. A semisphere may be fastest at very low speed because it minimises skin drag and there's next to no induced drag, so there should be a clear progression from there to longer, narrower shapes as the speed goes up. For different amounts of displacement, conversion factors would be applied to scale things up. I'm really surprised that it's so hard to find such a table anywhere.
     
  4. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Dan,
    I can help you, I won an event like this with my design.
    Private Message me and I'll give you my email.
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Without knowing the rules of the competition, not a lot can be said.
     
  6. Dan Cronk
    Joined: Sep 2017
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    Location: Sacramento

    Dan Cronk New Member

    Hi Bluebell, Thankyou for your reply. I would be very interested in your design. Dan@cronkcorp.com
     
  7. markmal
    Joined: Nov 2012
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    Location: Toronto

    markmal Junior Member

    If the boat is solar powered, that means that it should carry quite a large solar panel. Monohull could be more efficient, but it needs to be wide enough, or something should be added to it just to keep balance.

    BTW drag for monohulls can be calculated, using different methods with different degree of precision in software like FreeShip.

    Also catamarans or trimarans can be designed the way that waves from multiple hulls compensate each other minimizing wave drag at certain speed(s).
     
  8. David Cooper
    Joined: Jan 2015
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    David Cooper Junior Member

    Thanks for the tip - I'll give Free!ship a go and see if my struggling computer can handle it.
     
  9. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    What variables are known?
    Like weight for example.
     

  10. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Bad luck, the OP's policy is "minamal" information, seemingly.
     
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