Higher Cp at higher speeds equates to less resistance?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Dec 1, 2020.

  1. TANSL
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    TANSL Senior Member

    I think that the prismatic coefficient in any case should be defined by the area of the maximum section, not the area of the mid-section (Lpp / 2). Doing it this second way would take all the meaning out of this coefficient.
    It is totally impossible to move volume and at the same time "maintain the desired shape of the vessel", if the displacement is kept constant.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Your Fn is too high for a classic double ender!

    Why the obsession with a classic scull/ racing 8 type hull?
     
  3. DCockey
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    DCockey Senior Member

    So we agree. As I said in the earlier post I prefer using maximum cross-sectional are.
    When moving displacement towards the ends if the maximum cross-sectional area is reduced and not kept constant then the shape may be kept more similar to the original shape.
     
  4. TANSL
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    TANSL Senior Member

    @DCockey, yes, it seems that we can agree on the fundamentals, however, it is not a matter of preferring one way or another to define the Cp, but of adopting the correct definition. In addition, I am talking about not changing the value of the maximum section (its area), so that the Cp does not change, but simply changing its position. That makes the hull shapes change, without a doubt, creating a similar hull, as long as we define when two hulls are similar and when they are no longer similar. But, well, we agree.
     
  5. fastsailing
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    fastsailing Senior Member

    No.
    Lower Cp => less wetted area when all cross sectional shapes are kept the same. And a higher maximum cross sectional area.
    Very high CP => maximum wetted area for a given displacement and a given length, but minimum max cross sectional area.
     
  6. BlueBell
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    BlueBell "Whatever..."

    Dustman,

    You refer to rowing sculls.

    FYI
    Rowing sculls are:
    Single 27'
    Double 34'
    LOA

    They are designed for one speed and a limited load variance.

    Minimize your wake and you've minimized your form drag.
    But how with target speeds varying from 0 - 20?
     
  7. dustman
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    dustman Senior Member

    I am striving to understand the principles involved first then finalize the design. I am after a general shape that will perform well for my uses. I can scale it up or down to achieve the needed displacement. If I am sailing, I'm imagining I'll be spending most of my time between 6 and 9 knots. A slightly shorter and pudgier version of the double scull hull shape should perform well at these speeds. It seems like a hull that tapers horizontally aft would have low pressure affecting it in that plane, whereas the other way would tend to pull it down, especially at higher speeds.
     
  8. dustman
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    dustman Senior Member

    I know the length of various sculls, the double has the overall approximate shape I am after, I would take that shape and fatten and shorten it for the speeds I will spending the most time at. Yes, they are optimized for a certain speed, every hull has an optimum speed. As far as I'm aware the doubles operate at around 10-11 knots for elite rowers. The reason I chose that shape is because it is designed for maximum efficiency at and around it's design speed, every watt counts when human powered.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Great...

    BUT...you're falling into the trap of focusing on an absolute, rather than the objective.
    The objective is the design, holistically....thus, first of all BEFORE you even start thinking/imagining shapes, you need to 'design' the boat.

    So, you must do a general arrangement...then the weights....

    This is not a powered boat... not a sailing boat... but a human powered (via rowing/peddling) boat...is that correct??
     
  10. dustman
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    dustman Senior Member

    It will be sail powered(though I'm quite tempted to make it a power cat to simplify everything, but I don't want to guzzle fuel when I don't have to).

    As I believe I stated before the primary objective is to minimize drag, thus minimizing power requirements, thus minimizing weight, materials, sail area, structural requirements, thus minimizing cost. The design can be easily modified to balance the boat, and my accommodation/storage requirements are minimal so can be moved fore aft as needed. The only thing I really need is to have enough overall length and beam to be safe for island hopping in the bahamas. If the design ends up to be seaworthy I'll be building a scaled up version for larger crossings.

    "Your Fn is too high for a classic double ender!" I'm not sure I fully understand what the Fn is, or what you mean by it being too high. Is Fn related to cp, or to length, or both?
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Cart before the horse!!
    You cannot minimise anything, until you have a matured design at hand to investigate.
    Other wise, all you're doing is self flagellating about numbers... and numbers that mean what in the wider scheme of things???

    Fn = Froude number.

    Look at the Fn of sculls (and their objective) and then compare it with your preferred speed range....

    Unless you start to draw up and design the layout and understand your SOR to ahve something to 'focus on'... this thread, and your understanding of minimal drag will be an endlessly chasing circle ad infinitum....
     
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  12. bajansailor
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    bajansailor Marine Surveyor

    Dustman - as always, somebody else has pretty much had the same ideas as you, and has actually designed and built some boats that are probably coming close to what you have in mind.
    This is @Richard Woods - he posts on here occasionally, and he knows a fair bit about small cats, both power and sail.
    Here are his small sailing cats in the range 18' - 25' :
    Sailing Catamarans - First Choose a Design http://sailingcatamarans.com/index.php/designs-2/2-catamarans-under-25ft
    And his small power cats :
    Sailing Catamarans - First Choose a Design http://sailingcatamarans.com/index.php/designs-2/6-powercats

    Note that even the most basic / simple cats do not have an L/B ratio anywhere near 24 - they are all much lower. If you really want to go island hopping in the Bahamas, I think that you will need a bit more displacement than what you are proposing.
    Please do have a look at Richard's wee power cats - they are VERY efficient.
     
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  13. dustman
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    dustman Senior Member

    I mean I don't really know what the Froude number is exactly. I have read about it over and over and for some reason it doesn't compute.

    Alright, I'll make some drawings. All I wanted out of this thread is theoretical concepts and how they relate to different hull characteristics. I'll take that and do what I will with it. I do have a pretty mature design in my head, that has gone through hundreds of iterations and quite a bit of calculation.
     
  14. dustman
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    dustman Senior Member

    Thank you, I have looked at some of his designs and they had influence on my ideas. Does he have something where he talks in depth about the concepts behind his designs?
     

  15. DCockey
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    DCockey Senior Member

    Froude number, Fn, is the non-dimensional form of the Speed Length Ratio. SLR = Speed in knots / (Square root of Length in feet)

    Fn = Speed / (Square root (g * Length)) with speed, g (gavitational constant) and length in consistent units. Feet, feet per sec and feet per sec squared work. Meters, meters per second and meters per second squared work. Knots, feet and feet per second squared do not work.

    Or start with the traditional Speed Length Ratio and multiply SLR by 0.297 to obtain the Froude number.

    I've done many designs in my head which I was convinced would work only to discover a problem, sometimes a major problem, when I put the geometry on paper, in CAD or a physical build.
     
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