Cp. changes when heeled.

Discussion in 'Hydrodynamics and Aerodynamics' started by frank smith, Dec 7, 2013.

  1. frank smith
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    frank smith Senior Member

    How much consideration is given to changes in cp when the boat is heeled.
    In looking at some sharpie type hulls with a high cp, it occurs to me that it would change when the boat was heeled.

    F
     
  2. TANSL
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    TANSL Senior Member

    When the ship is tilted, the midship area and waterline length change, and change the trim. Therefore it would be rare that the Cp not change.
     
  3. philSweet
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    philSweet Senior Member

    What sort of sharpie type hulls do you mean? Now-a-days, it seems anything with a single chine is a sharpie. Traditional work boats were low powered and low speed and had low Cp's. Traditional sharpie hulls could maintain a very narrow variance of Cp with huge changes in displacement. That could reasonably be a definition of a sharpie. Since they rarely could exceed 10 degrees of heel without flooding, the effect of heel was not a big issue for them. They were long and skinny and didn't trim much with heel. The chine line was constructed so that the Cp was not going to change much. Later sharpies had a bit of deadrise or bow to their stern quarters which helped as well.

    One thing worth mentioning - just because the Cp doesn't change, it doesn't mean that the distribution of volume doesn't change. There are many ways to get any one Cp value. You can design a boat so that the midship section area doesn't change with heel, the waterline doesn't change with heel, yet the other stations' areas do change with heel. Same Cp, different volume distributions. You can only infer so much from Cp. And when you start considering the volume distributions of heeling boats, I think you are pretty much at the limit.
     
  4. TANSL
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    TANSL Senior Member

    Since Frank is not talking about small or large changes but only changes, I think that the Cp changes in all cases except when the hull is shaped like a cylinder, like a can of diesel.
     
  5. frank smith
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    frank smith Senior Member

    Thanks for the answers, I guess like every thing else in boat design. It is related to other variables.

    F
     
  6. TANSL
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    TANSL Senior Member

    In fact, as you know, Cp is only related to the displacement, the area of ​​the main section and waterline length.
    By varying the angle of heel, displacement does not change. The other two quantities do change and there indeed much influence the shapes of the boat.
    But this is obvious, no need to explain.
     
  7. frank smith
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    frank smith Senior Member

    I think of it as distribution of the Disp. and it relationship to wave creation and attachment of the boundary layer at differing speeds. A round section hull would have the least change in Cp when heeled I think.
     
  8. daiquiri
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    daiquiri Engineering and Design

    The Cp generally changes as the boat heels, and it is taken into account during the design stage. In fact, usually you don't want the same Cp when going upwind and downwind (for example) because the attainable speeds are very different in two cases. The hull shape has to reflect that fact and to be adapted accordingly and separately for various points of sailing.
    Cheers
     
  9. frank smith
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    frank smith Senior Member

    I would assume a higher Cp downwind. Partly because of that change in trim. That makes sense when looking at the scow mini's.
     
  10. philSweet
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    philSweet Senior Member


  11. frank smith
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    frank smith Senior Member

    Thank for the link, I ran across it years ago, but didnt really read it.
     
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