Canoe "Stability Factor" w/ Freeship

Discussion in 'Stability' started by Tim Hall, May 12, 2011.

  1. Tim Hall
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    Tim Hall Junior Member

    I have a rough design worked out for a solo canoe, and would like to use Steve Killing's "Stability Factor" to get an idea of where I stand with the stability.

    ...load the canoe to...250 lbs for a single and then heel it to 15 degrees. Once in this attitude the stability is related to the distance from the center of gravity (which I set at the waterline for all canoes as a base point) and the metacenter. The metacenter is the point (usually about 12" above the waterline) where the vertical line through the center of buoyancy intersects the centerline. Once the metacenter at 15 degrees has been determined, the stability factor is simply the metacentric height in inches divided by 12.24 (for kayaks use 7.50) times 100.

    I've tinkered with Freeship a tiny bit, but some of the concepts and nomenclature still elude me. Can anyone tell me if Freeship will give the information I need to make the above described calculation before I go through the trouble of making offsets only to find out this is the wrong tool?

    Thanks for any help,

    Tim
     
  2. gonzo
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    gonzo Senior Member

  3. Tim Hall
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    Tim Hall Junior Member

    Hmmm, well I was curious, but couldn't find anything about stability testing on their website.

    Actually I'd like to use Killing's method as a benchmark since there are designs with these figures attached for me to compare to. Otherwise numbers from another system have little meaning to me.
     
  4. daiquiri
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    daiquiri Engineering and Design

    I don't get this:
    "the stability is related to the distance from the center of gravity (which I set at the waterline for all canoes as a base point) and the metacenter."

    What does the bolded part mean? That he loads 250 lbs at the WL level or that he conventionally assumes the CoG is at WL, regardless of where it actually is?
     
  5. Tim Hall
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    Tim Hall Junior Member

    It seems I once saw an illustration of what Steve Killing was doing, but I can't seem to hunt it down now. I think what he means is if you strike a vertical line up from CoB that intersects through the centerline of the vessel, the distance between the waterline (at the heeled angle) and intersection is what you're looking for.

    Killing states that this is not a foolproof method, and of course the perception of "stability" (really the word "comfort" is more accurate) in a paddle craft is extremely subjective. It's really supposed to be a measure of how advanced of a paddler you need to be to feel comfortable in the particular vessel.
     
  6. Tim Hall
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    Tim Hall Junior Member



    No, I'm sure what he's talking about is the actual waterline @ 250lbs displacement and 15 degrees heel. For tandem canoes he loads the vessel to 400lbs. This is all irregardless of design displacement or design waterline.
     
  7. Tim Hall
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    Tim Hall Junior Member

  8. DCockey
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    DCockey Senior Member

    His verbage which implies the CG is at the waterline is confusing. The stability index is based on metacenter height above the waterline, not GZ, so the location of the CG doesn't matter. My guess is the software he uses outputs GZ so the simpliest way for him to calculate the metacenter height above the waterline is to set CG at the waterline.
     
  9. gonzo
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    gonzo Senior Member

    For a canoe, the crew provides the main stability factor. Calculations for the hull alone are not too useful.
     
  10. Tim Hall
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    Tim Hall Junior Member

    That's why it's important to get an idea of what kind of paddling skill is required for a particular hull. Reread post #5, if you missed the purpose of the calculation. Are you telling me there is no empirical stability distinction between a sprint canoe and a whitewater canoe?

    I don't understand what any of this has to do with my original question. What's useful about this?
     
  11. DCockey
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    DCockey Senior Member

    Sea Kayaker magazine calculates and publishes stabilty curves for the kayaks they test. Information about the data they publish and their thoughts on it's meaning is here: http://www.seakayakermag.com/PDFs/Kayak_Reviews_Info_0609.pdf
    This PDF is the data from one of Sea Kayaker's reviews for a kayak with reasonably high stability: http://www.seakayakermag.com/2010/Oct10/BorealisXL_Tspecs.pdf
    This one has less stability: http://www.seakayakermag.com/2010/April10/Alchemy_Dagger_Tspecs.pdf
    It's true that the abilities of the kayaker/canoeist has a lot to do with how comfortable they will be in a given kayak or canoe and whether they will stay upright. But there does appear to be a good correlation between static stability and the amount of ability needed to stay upright and be comfortable. That's about all that Steve Killing claims for the index he uses. It is simplier to comprehend than the stability curves which Sea Kayaker publishes.
     
  12. Tim Hall
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    Tim Hall Junior Member

    DCockey, thanks for the links...I've managed to now find data on other kayaks from their website. I still can't find a good body of stability data on canoes specifically, but the kayak info is a very good starting point.
     
  13. gonzo
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    gonzo Senior Member

    The paddling skill necessary for a certain canoe is not limited by its empty stability. If you do calculations by the load method, it is deceiving, because the weights are in the bottom. You need to add the crew to the stabitily calcualtions. For example, how well braced are your knees so you can hang over the sides and keep it from capsizing. Wave action is very important too. Another aspect to take into consideration, is the difference in stability with trim. Some canoes become very unstable when you load the ends. That often happens when approaching a dock or climbing from the water.
     
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  14. DCockey
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    DCockey Senior Member

    I'm not aware of anyone suggesting using calculations of the stability of a canoe when empty.

    Killing / Bear Mountain Canoe use the metacenter height at 15 degrees heel (not righting moment or similar) and rescale it for presentation purposes. From the Bear Mountain website in Tim's link above:
    These stability factors are calculated for canoes at a common displacement of 400 lbs (except singles at 250 lbs).
    The values are the metacentric height at 15 degrees of heel with:
    12.24 taken as 100 percent for canoes

    They believe there is good correlation between metacenter height at 15 degrees / "stability factor" and skill level needed to feel "comfortable" in a canoe or kayak.
     

  15. gonzo
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    gonzo Senior Member

    Read post 10. Tim says he doesn't understand why my statement that calculations for the hull alone are not useful.
     
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