Tumblehome and buoyancy in a solo style canadian canoe

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

  1. seth godin
    Joined: Aug 2021
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    seth godin Junior Member

    Thanks to all for the warm welcome. Here's the first question I'm wrestling with:

    In a normal canoe, used in a normal fashion, the tumblehome doesn't do much, because it's out of the water. Only in waves or rocky moments does it provide any utility in terms of stability or keeping water out: (images in cross section--this is the front or rear view, not the side view)

    buoyancy.001.jpeg

    But when the paddler is solo and heeling the canoe, several forces are at work:

    buoyancy.002.jpeg

    My theory is this:

    The tumblehome combines with the steep sides of the boat to create a pocket for air, which is buoyant.

    And the width of the canoe creates a sort of outrigger, where the raised part of the boat is pushing down with leverage.

    This combination of buoyancy (on the right) and gravity (on the left) leaves the paddler feeling stable and secure, and it's in this groove that a canoe feels special.

    Every metal, fiberglass or plastic boat I've paddled hasn't felt this way at all, and I'm assuming it's because of the lack of tumblehome.

    On the other hand, a canvas/cedar/ribbed canoe has given me a really great experience, even in rough waters.

    Am I imagining the forces at work here?

    What are more intentional ways of creating the same magic?

    Thanks all. Appreciate any insights!
     
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  2. patzefran
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    patzefran patzefran

    You are right on the principle : when the canoe is flat, around zero rolling angle, the roll stability is minimum as the roll restoring moment curve is flat near zero around zero roll angle.
    In this configuration, the weight force and the flotation force are aligned and acting center of symmetry of the canoe. Around some higher equilibrium roll angle (when the paddler moves ouward together the center of gravity) the configuration assymmetry results in much steeper curve of roll restoring moment and give a good roll stability .
     
  3. BlueBell
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    BlueBell . . . . .

    No.
    And to me it doesn't really matter what forces are at work because there is an equilibrium.
    Not an overly stable one but one that can be maintained by a skilled paddler.
    It seems pretty deliberate to me.
    Do you mean in hull design? Nothing. It works just fine the way it is, no?

    I'm not clear what you're after here.
     
  4. seth godin
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    seth godin Junior Member

    yes, it works. I'm thrilled at how well it's been working for hundreds of years.

    but design is about improvement and understanding. I'm hunching that SOF could create a lighter, more resilient solo boat, perhaps with chines, or an asymmetrical shape or a bigger rocker or who knows...

    The same way few people paddle a birch bark canoe today...
     
  5. Kayakmarathon
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    Kayakmarathon Senior Member

    A SOF canoe will be lighter, but will flex considerably more than canvas over wood construction. A SOF lighter hull will also have less roll moment. The paddler will have to apply a lighter and faster counter-torque to maintain stability. The stiffer hull design and construction from 100 years ago also give the paddler a better sense of what the boat and water are doing. I would spend more time becoming one with the canoe than one with the CAD package.
     
  6. DCockey
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    DCockey Senior Member

    Where is the pocket of air - inside or outside of the hull?

    If the pocket of air is outside the hull what keeps it from rising to the surface of the water?

    If the hull plus canoeist is stable without any intervention by the canoeist then the center of gravity is directly above the center of buoyancy. (In principal the center of gravity could be below the center of buoyancy such as may be possible in a boat with a deep ballast keel but that is not the situation for a canoe.)

    Stability is due to the center of buoyancy moving more quickly to the side then the center of gravity when the canoe heels.
     
  7. seth godin
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    seth godin Junior Member

    thanks for this. A lot to think about.

    I can't tell for sure, but I'm pretty sure that the boat wouldn't be stable in the heeled position if you put a cadaver or a sleeping person in position--there's a lot of core work and body adjustment involved.

    And no, the air is inside the hull.

    What I'm trying to wrap my arms around is exactly WHY it feels so stable and powerful to be in the pocket of a heel. It took me years to feel it, and when I teach it, if I'm lucky, I can get someone into that groove in thirty minutes, and then they have it forever.

    But in a 'tippy' canoe, it's much more difficult to find.
     
  8. DCockey
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    DCockey Senior Member

    Buoyancy only depends on the shape and volume of the displaced water. What is inside that volume has zero effect on buoyancy.
     
  9. BlueBell
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    BlueBell . . . . .

    Hence, the "skilled paddler". How can you not be sure about this?

    Because it creates an equilibrium, a balance of all the forces you're looking to define.

    Research vessel stability and you'll see all the forces involved, statically.
    Then you can start studying dynamic vessel stability.
    I'm not convinced this is going to help you define "The Sweet Spot", but it'll be educational.
     
  10. seth godin
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    seth godin Junior Member

    A cork is buoyant and a similarly-sized piece of lead is not. Because if you displace the water with something like air or low specific gravity, it floats, right?

    The purpose of my illustration was to show that the shape of the boat creates a steep pocket of air that gives that side of the boat more buoyancy. In the boat, I feel like I'm driving a bubble of air ever deeper into the water, and the bubble pushes back.

    Sorry if I'm confusing the issue...
     
    Last edited: Aug 19, 2021
  11. tlouth7
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    tlouth7 Senior Member

    I think tumblehome is not the primary driver here. Wide beam at or slightly above the [horizontal] waterline creates the stability as the boat heels. Tumblehome simply reduces the beam on deck which makes for easier paddling.

    If you image the most extreme tumblehome cross section of a trapezium then the submerged volume could actually move in the wrong direction, giving negative righting moment.
     
    Last edited: Aug 19, 2021
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  12. DCockey
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    DCockey Senior Member

    The buoyant force of the water acting on the outside of the hull is completely independent of what is inside the hull. The buoyant force of the water acting on the outside of the hull depends only on the size and shape of the portion of the hull which is immersed in the water.

    The gravitational force acting on the hull depends on the weight and center of gravity of the hull. It is independent of size and shape of the portion of the hull which is immersed in the water.
     
  13. Dolfiman
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    Dolfiman Senior Member

    If your goal is always solo canoe and paddling the same one side, the most adapted hull can be an assymetric one in volume, i.e. with more immersed volume on the side you are for paddling, without changing a priori the beam at sheer line to not change the ergonomy of the paddling. A similar approach is for the hull of the Venice gondola, she is assymetric (but here with different beam port/starboard) because the gondoliere is always on the same side. At rest without loading, a gondola heels at about 15°-20°, this angle gives the limit of the exercise.
    IMG_4945 (2).JPG
     
  14. seth godin
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    seth godin Junior Member

    this is a big deal, thank you for helping me think about it this way
     

  15. seth godin
    Joined: Aug 2021
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    seth godin Junior Member

    wow
    yes, of course.

    a gondola.

    magic.
     
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