Tumblehome and buoyancy in a solo style canadian canoe

Discussion in 'Hydrodynamics and Aerodynamics' started by seth godin, Aug 18, 2021.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    I must say, I find this thread rather peculiar....since almost all the comments are from non NAs. Lots of "feeling" going on ...:p
    The reason why I say this is that any NA doing their degree and reading their text books and attending their lectures can tell you the answer to this in heart beat.. :rolleyes: As it is part of understanding stability and how form effects stability.
    Yet non-NAs are still debating mute points.

    Thus to quote one, an endless and many text books:

    upload_2021-8-24_13-1-1.png

    upload_2021-8-24_13-1-18.png

    For those interested.. ref: PNA
     

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    Last edited: Aug 24, 2021
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  2. fallguy
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    fallguy Senior Member

    Well, the only reason all of us are speculating is the wise owls were absent for almost a week! Go and read my gutcheck post from Saturday 9:59 am. I said I don't think tumblehome is the reason.
     
  3. fallguy
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    fallguy Senior Member

    As a paddler, I can tell you I'd rather paddle a canoe with tumblehome than one that opens out on the top the opposite way with the same waterline beam.

    It is because the tumblehome keeps me centered and loads don't get wild.
     
  4. gonzo
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    gonzo Senior Member

    The tumblehome keeps you closer to the center, because of the geometry of paddling. Your hand and arm can stay lower and closer to the body with tumblehome than with flare.
     
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  5. philSweet
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    philSweet Senior Member

    Well, with canoes, stability is mostly in the mind of the paddler. The classical static stability isn't what's being considered. It's like talking about the stability of a motorcycle. It's all about a consistent and predictable response that lets the paddler maneuver deftly without upsetting things. One of the charts Dolfi posted shows how the RM curve is largely independent of load over a wide range of loads. So when transferring weight laterally when paddling, the boat always feels the same.

    And this stuff isn't taught to NA's. It isn't taught to anyone. Some canoes just paddle like crap no matter what you do. Their natural roll and pitch periods work against the solo paddler no matter what cadence, paddle choice, and trim ballast you use. These might be fine as a tandem, but designing a solo/tandem that works well at both is mostly an art form.

    Compare to a sharpie, which is a different long, narrow, low freeboard design. The sharpie is heavier, but more importantly, it isn't paddled. The propulsion system isn't bouncing around all the time. The well designed sharpie has constant RM at a given freeboard over a wide range of loads (so light load, 16 degrees of heel, 4 inches of freeboard has the same RM as heavy load, 5 degrees of heel, 4 inches of freeboard.) This makes it handy under sail.
     
    Last edited: Aug 24, 2021
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed… but relying solely on “feel” is delving into the realms of religion based engineering!

    Correct – dynamic stability.
    But to discuss dynamic stability one needs to understand and address the statical stability first. If the hull was wide and flat, with tumblehome, and another with less beam (more narrow) but with the same tumblehome, one shall exhibit better characteristics to the other. Whether you’re sitting on top of the hull or in the hull, also plays a part into this mix, for the dynamics. But it does not alter the statical stability of flared v tumblehome, it just exacerbates it.

    I beg to differ.
    This is basic stability and dynamic stability, regardless of hull form. These are the “tools” that NAs are taught to analyse any design/engineering problem and/or hull form at hand. Whether 1, 2, 3 or more hulls.

    That can be labelled at any boat, not just canoes!

    And natural periods, that is based upon the statical stability.

    And that’s the issue, it is just a measure of comparison between one ‘type’ of boat/hull to another.
    Some planning boats are great whilst others, porpoise and have crap rides to boot. Yet compare their statical stability curves and natural periods of motion would not necessarily suggest so. Every design is different and ergo every design shall have its own +ve’s and –ve’s. Nothing new there.
     
  7. fallguy
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    fallguy Senior Member

    Right, and so Cm doesn't move as far outboard and thus the righting arm is affected under load. So, tumblehome doesn't matter on a raw boat as Dolfiman's work shows, but under load, it helps.

    No?
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Cm ..??
     
  9. fallguy
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    fallguy Senior Member

    Center of mass which I believe is Center of gravity. The bulk of the load does not move too far outboard because the sheer forces the paddler closer to the center of the boat. Feel free to explain it from a learned perspective
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Correct - 'we' tend to use CoG/CG rather than Cm

    As Gonzo was hinting at it is all about the weight distribution but also, more importantly, for the dynamics, the location of the KG and the fluctuating location of KG during the stroke.

    Simplistically, the KG is at the red dot:

    upload_2021-8-26_10-42-6.png

    The stroke can be done from either side of the body; the other location shown by the red line.

    As soon as the paddle is in the water and then being sued to push the canoe fwd and on that extension of the upstroke of the paddle - it has water 'attached' to it. So there is now an addition of weight and at a higher KG shown by the second red dot:
    upload_2021-8-26_10-45-43.png

    Thus the KG rises, just like a crane lifting a weight, the KG moves up and athwartships/transversely to the blue dot:

    upload_2021-8-26_10-47-28.png

    A rise in KG and offset, shall effect the dynamics, especially of such a small hull.

    Having flare, means the outreach is greater, thus the blue dot moves further outboard, creating more stability issues.
    Whereas with flare, this transverse shift of the CoG is kept to a minimum. Which is why many try to keep their arms tight against their body when stroking.

    Does that make sense now?
     
  11. fallguy
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    fallguy Senior Member

    all I have been considering is the location of the paddler to the keel centerline

    tumblehome keeps the paddler closer vs an open boat which allows the paddler to move (more) athwart when the boat heels, which is sort of a revelation to me; despite a canoe with plenty

    tumblehome also forces a round chine versus a vee (generally), and the round chine behaves differently when heeling than a vee shaped chine...the vee,combined with no tumblehome or an 'open' boat has a better chance for the boat to go over because the paddler can move outboard and a boat with a sharp chine can go past a(the) tipping point easier

    if it were only an issue of beam, then not nearly as many men would have died in flat bottom skiffs and jon boats over the years.

    I don't profess to fully understand the mechanics, but beam alone is not it and tumblehome doesn't seem like it was done for aesthetics..
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    But that is common to any rounded v's Vee'd/chine hull, whether canoe, planing hull etc.

    But it is, when in relation to the the amount of buoyancy the hull has and the location of the KG and its dynamic KG.
    Just like any planing hull.

    As noted above... any text book will tell you this:

    upload_2021-8-26_11-37-31.png
     
  13. fallguy
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    fallguy Senior Member

    Forget about tumblehome for a moment and consider a load that is tied to the centerline of any boat.

    Then consider the same boat and the load is not tied.

    Which situation is more stable?

    All we are saying about tumblehome is it keeps the loads closer to Cg. And it forces a round chine. Touche'

    Wikipedia has an interesting bit on tumblehome. I read it. They cite Mather for this bit, but not sure it is accurately cited, unless keeping the paddler centered happens when easier to paddle?

    E93D92E2-8354-4C82-B961-FECDB3803ECC.png
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Depends how and where it is tied?
     

  15. TANSL
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    TANSL Senior Member

    A boat with a moving center of gravity is the perfect candidate for capsizing. Most ship capsizing are caused by cargo movements.
     
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