Thoughts on differntial Cp foreward/aft

Discussion in 'Boat Design' started by Tim Hall, Apr 29, 2011.

  1. Tim Hall
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    Tim Hall Junior Member

    I'm designing a canoe/kayak (somewhere in between) hull, and recently in revising my section curves I noticed my Cp forward of midship was significantly lower than aft.

    I suppose what this tells me is the hull is fuller astern, which is intended. But I'm wondering if there are any other implications to this.

    Anyone have any thoughts on the merits/faults of having this differential?
     
  2. ancient kayaker
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    ancient kayaker aka Terry Haines

    Can you post the lines?
     
  3. Tim Hall
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    Tim Hall Junior Member

    Lemme format the drawing...
     
  4. Tim Hall
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    Tim Hall Junior Member

    You'll have to excuse the drafting conventions...I'm new to this. The shear line looks odd in profile because that's the point of tumblehome - haven't designed the topside yet.
     

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  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    At a quick glance, I'd say the rocker forward is probably responsible for most of the difference. Extra buoyancy aft will shift the CoG aft a little. I don't have the knowledge to analyse it, but it looks like a nice, fairly conventional hull that should paddle easily and be fairly agile despite the long keel line, provided you do not add an external keel.

    Any fore/aft asymmetry should mean that it will perform better forward than aft-wards so if fitted out as a double, it will not make a particularly good solo unless there is a third seat or one seat is movable. I don't think that will show up much in terms of speed vs paddling effort, but as a double, when paddled solo bass-ackwards as is commonly done, it may tend to crab.

    There is probably more canoe and kayak expertise and experience on the kayak forum at http://www.kayakforum.com/cgi-bin/Building/index.cgi/
     
  6. Tim Hall
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    Tim Hall Junior Member

    Thanks for the feedback. I was kinda hoping to hear something like that. I'm aiming for a sorta middle-of-road touring hull and trying to keep things simple. As I read through John Winters' "The shape of the Canoe" and pick up different bits and pieces from this forum and elsewhere, I'm making little changes.
     
  7. philSweet
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    philSweet Senior Member

    It helps to be absolutely sure how your software calculates Cp,Cpfwd, Cpaft. If it works by referencing your declared midship station rather than by finding the maximum cross section for itself, it is difficult to use it for comparisons unless the station layout is identical. If you can't figure out what it's doing, plot a displacement curve and work from that. Freeship in particular has this problem. The calculated Cps read high if the declared midsection is not the one with maximal area.
     
  8. Tim Hall
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    Tim Hall Junior Member

    Phil, I am using a displacement curve to make calculations at the moment. Good to know about Freeship though. I have a Simpson's rule spreadsheet another forum member was kind enough to share. Will be using all three methods for comparison.

    I drew the curve forward and aft of midship and let the computer calculate the areas separately.
     
  9. Tim Hall
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    Tim Hall Junior Member

    Actually I guess I should ask how others define midsection...or perhaps what it should be defined as.

    What I'm referring to is the section of maximum beam/area, which does not coincide with the longitudinal midpoint of the hull.
     
  10. ancient kayaker
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    ancient kayaker aka Terry Haines

    Phil - good point about using max station for Cp, a point that is easily missed. I hadn't noticed what FreeShip was doing until today.

    The displacement curve displayed by FreeShip only extends from the stem to midships, which is fine for the symmetrical canoe design I am working on, but how do I make FreeShip display the whole length?
     
  11. philSweet
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    philSweet Senior Member

    I think the tendency to have a higher Cp aft is often the result of the need to package the paddler a bit aft. On a short boat, you end up a bit aft to gain foot room. And in a longer boat, you may end up a bit aft to balance windage or to keep the beam as narrow as possible at the point of blade entry. This is kayak logic and is of doubtful value to a canoe. In a canoe, balancing out the windage is the biggest design issue since the windage of a canoe is several times greater than a that of a kayak. A completely symmetrical underwater profile and sheer goes a long way to accomplishing this. If it's a solo canoe, lower bows and part decking are effective. One other consideration- if you intend to portage the canoe in the usual fashion, you need a thwart at or just forward of the CG - which means you can't kneel there when paddling.

    Off topic- the proper way to portage- http://www.porchlight.ca/~aferg/home_htpac.html
     
  12. philSweet
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    philSweet Senior Member

    Terry, I've never seen that. If you generated the aft hull by a series of transforms on the front hull, you may experience some problems. Try running "check model" under tools.

    Lackenby transform is also based on declared midship station in FreeShip.

    also, if the aft stations are in a different layer, Make sure the hydrostatics switch is set in layer properties.
     
  13. philSweet
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    philSweet Senior Member

    Having driven this thread off topic, I guess I ought to at least try to answer the original question regarding the merits of a different fore and aft Cp. If you envision two hulls, one consisting of the fore section mirrored front to back and the other consisting of the aft section mirrored front to back, you can look up the ideal Cp of each based on length for each and the design speed. If the aft section is shorter, it should have a higher Cp. Since the range of Cp for paddlecraft is rather small, you need to establish your midship very accurately for this to be meaningful. With the hull lines fixed, moving the declared midship location 0.01LWL forward will decrease forward Cp by 1% and increase aft Cp by 1% (Cp new = 0.99, 1.01 times old Cp respectively.)

    If anyone one wants to play around with these sorts of distortion and their effects, you can load a symetrical hull into FreeShip and perform the following transforms.


    set midsection 3/4 aft (project, settings, main dim, mid)
    go to lackenby transform
    note Displacement, Cb, Cp, LCB.
    perform lackenby xform with factor of 0.98 applied to old Cp
    set midsection 1/2 lwl
    perform lackenby xform to restore displacement to its original value.
    save hull
    repeat ad nauseum to generate a series of more and more distorted hulls.
    plug into Michlet and compare.

    It takes a bit of practice, but you can map a hull onto a different displacement curve this way. Someone might want to set up a series of bump functions to automate the process (Leo):idea:;)
     
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  14. LP
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    LP Flying Boatman

    Terry,

    Can you post a screen shot or image save of your curve? What comes to mind immediately is the placement of your section cuts, but that doesn't really fit your description.
     

  15. LP
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    LP Flying Boatman

    Phil, could you embellish on this thought? I've been working a different thread and have some Cp parameters specified there, but would like comfirmation or refinement of those values if they are incorrect. It sounds like you know what works.

    http://www.boatdesign.net/forums/boat-design/kayak-design-parameters-37739.html

    I have them expressed in post #13. If you didn't mind posting on that thread, I would like to keep the information growing there.
     
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