Tumblehome and buoyancy in a solo style canadian canoe

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

  1. gonzo
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    gonzo Senior Member

    Actually, a hull with a lot of flare will have more stability. As the boat heels, the flared part of the hull submerges and creates a moment opposing the heeling force. The tumblehome is convenient to lean over the side and makes paddling easier though.
     
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  2. TANSL
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    TANSL Senior Member

    There is no reason to think that "a hull with a lot of flare will have more stability", I would even say that, thinking about the value of the GZ at small angles, the opposite could happen.
    We must take care of the expressions and be careful with the terms so that it does not seem that we do not know what we are talking about. A moment can never be opposed to a force, it can be opposed, if properly applied, to a heeling moment.
     
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  3. bajansailor
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    bajansailor Marine Surveyor

    Tansl, your passion for semantics is getting wearisome, especially so as they are always aimed at Gonzo.
    Everybody knows what Gonzo means in his post above.
    OK, so Gonzo should have said heeling moment rather than heeling force - no big deal.

    And I think that one can reasonably make a general statement to the effect that a hull with a lot of flare will have more (reserve) stability than a hull with no flare - but this is just a broad generalisation (there will be the odd exception I am sure).
     
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  4. TANSL
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    TANSL Senior Member

    We agree on that. I think I was saying something similar to what you say here. Fortunately there is always a charitable soul who explains that a fairly large misconception does not matter. It is not an obsession with semantics, it is asking for a bit of rigor when expressing concepts. Using the wrong word is not the same as getting the concepts wrong. If this is not important to you, then what do you want me to tell you, I will tell you that it seems wrong to me.
    Regarding stability, I still think that my statement is correct, although it could be wrong, and I am sure that you, who prove to be quite kind and conciliatory, will give me the necessary arguments to get me out of my error, without semantic nuances.
     
  5. Dolfiman
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    Dolfiman Senior Member

    I initiated a preliminary design to illustrate the asymetric hull idea, for a Solo Canoe 14' with a design displacement of 122 kg (light weight 22 kg + loading 100 kg). The two half hulls share the same keel line and the same sheer line. The asymetry of the sections lead to a starboard half-Bwl = 0,398 m (the side where the paddler should sit ) and to port half-Bwl = 0,304 m, so a total Bwl = 0,711 m (28''). Although the asymetry looks impressive in the cross sections, the waterlines of the bottom view remain fairly streamlined. I pay attention to maintain tumblehome shape on the Starboard side and a sufficient bilge on Port side to preserve a righting moment energy on both sides. **
    I have not finished the stability issue, I have to add columns of calculus in my Gene-Hull Canoe application to deal with the Y (tranverse) values involved in the center of buoyancy and of weight. I expect that the Y of buoyancy center at design displacement can be around 10 cm on Starboard, so for a paddler of 80 kg with 20 kg of equipement/food/water put on port side, that could be a sit at about Y 20 cm on Starboard (midway between axis and sheer line).

    ** For the stability in relation with the tumblehome shape, I had done a specific investigation in this old thread here below, pdf document "Examples ...". To appreciate the "dynamic stability", I compared the righting moment energies up to 20° of heel (a relevant threshold before the risk of water intake in the case of a canoe open body), i.e. the area under the GZ curve crossed the mass. And the tumblehome shape gives more stability re. this criteria (see pages 15 and 16).
    Gene-Hull VE Canoe 2,4 | Boat Design Net
     

    Attached Files:

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  6. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    Lol
    Aren't you glad you asked, Seth?!

    Enjoy your Canadian style canoe and stop thinking about it.
    As a fellow Engineer, I ask you: "Are we blessed or cursed with the ability to reason?"
     
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  7. seth godin
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    seth godin Junior Member

    This is great work, I'm transfixed by what you're doing here.

    Thanks for sharing it. I appreciate the insight.
     
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  8. fallguy
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    fallguy Senior Member

    Lots of good discussion.

    It is not tumblehome, per se, that delivers the stability

    Consider two canoes, one with half the beam of the other, but both the same amount of tumblehome. The narrow beam canoe appears to have less tumblehome; despite it being the same amount.

    The wider canoe is ultimately more stable, but not forever. Many people died in jon boats in the last hundred years. And they are wide and they capsize. Tumblehome would not have helped I'd say. The chine type (and length) was more relevant. Tansl's remark and Gonzo both missed this bit. And despite the bow shots, I think the moments are a big deal.

    As a novice, but a guy with a Gilpatrick Laker canoe, I can attest to the fact, the boat has tumblehome and almost impossible to tip over. But the reason is that the tumblehome requires a roundish chine.

    Easy to walk in late and summarize and comment, so I apologize in advance or late to wiser men.

    I think if you consider the wide hull and tumblehome, the round chine becomes a factor and the boat becomes very, very difficult to turn over, especially when the chine gets further and further out in the middle.

    If you remove the round part of the hull, what happens? The boat develops an absolute tipping point. If you make the boat wider and the boat gets abeam, one side of the boat is up and the other down.

    Perhaps wiser men can speak to the issue, but I always felt the round chine forced to create tumblehome on my canoe made it very, very hard to flip. The hard chine boat below I'd not step into, but they could have the same waterplane and the same beams. (Apologies for the grammar and chicken/egg stuff)





    51AEA650-F1BA-466E-AF49-32FB3D138A70.png
     
  9. seth godin
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    seth godin Junior Member

    yes indeed. In my untutored inner monologue, the chine/curve you're talking about is part of how I think of the tumblehome.

    I think that this insight, combined with @Dolfiman's fabulous asymmetrical innovation is my going forward plan, at least until I figure out more by doing.

    thanks all.
     
  10. philSweet
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    philSweet Senior Member

    This depends on what you want to keep constant. If the level underwater hull is kept constant, and the displacement is kept constant, then yes, this is often true, but freeboard and angle of submersion needs to be looked at. In general, you will need more surface area, freeboard, and hull weight to see significant gains. These also would tend to increase the maximum load carrying capacity.

    If you look hard at the condition of maximum load and minimum required rm at maximum heel, the minimum weight hull won't have flare. But this won't be the best hull for more moderate conditions, which is where you are 99% of the time.

    So you need to study extreme conditions very carefully, then optimize for performance at more typical conditions within those constraints. The classical canoe shape is optimized for low power cargo carrying efficiency. Interestingly, light racy canoes have less tumblehome than cargo canoes.
     
  11. Dolfiman
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    Dolfiman Senior Member

    I have finished the stability study with the asymetric hull and to better illustrate and evaluate this alternative, a comparaison is made with its symetrical equivalent hull at same Length, Keel line and Sheer line (beam and free boards), weight and (most important) with same waterline beam at design displacement (0,711 m / 28'' for D 122 kg) and tumblehome shape above the water.

    For the comparison, I considered a 80 Kg paddler sit slightly decentering on the starboard side, of an Y value in order that both, at equilibrium :
    • the asymetric hull is upright,
    • the symetric one has already a heel angle of ~ 15° , as it can be seen on the videos you have posted on another thread,
    In my example attached, an Y = 7,1 cm corresponds to the upright equilibrium of the asymetric hull and gives exactly 16,35° of heel angle for the symetric equivalent one. This decentering means that the paddler is at 33 cm instead of 40 cm of the sheer line for the paddling.

    Another important point for the relevance of the computation is how we take into account the weight vector of the paddler when the boat is outside its equilibrium. From what I have seen on the videos, I assumed that the paddler can maintain the trunk of his body approximatively vertical whatever the boat heel angle in the usual range (± 20°), as a basic instinct to maintain his stability. So, for the stability computation, that means a body weight vector origin (i.e. the pivoting point of this compensation) slightly above the bench. Here attached, I assumed that such point is at Z ~ 20 cm.

    With these assumptions (Paddler weight 80 kg, Y 7,1 cm, Z 20 cm + X at 1,90 m giving no trim), I computed the GZ curve and the minimum freeboard evolution. The asymetric hull option shows a quasi symetrical curve on both sides while the symetric hull option, as predicted, shows poor reserve of stability and of freeboard on its wrong side where it is already heeled at equilibrium.

    The « gain » of decentering is 7 cm can be judged low and don't worth the asymetric option, although it is spectacular that such low value can already gives an heel 16° with the symetric one (Y 4,6 cm > 10° heel , Y 2,4 cm > 5° heel). But the asymetric hull does not prevent you to also adopt more decentering up to say 15° heel angle, this option leads to a decentering of 12,4 cm, so a paddler at 28 cm from the sheer line instead of 40 cm.

    When considering the paddler 80 Kg + its 20kg of camping equipment put on port side (the displacement then at its design value 122 kg), at upright equilibrium the paddler can be decentering at Y 14,5 cm, i.e. the paddler at 26 cm instead of 40 cm to the sheer line. In all cases, its worth to have jerrycans of water to be put on port side, that will give a more comfortable canoeing.

    More explanations, data and figures in the document attached.
     

    Attached Files:

  12. fallguy
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    fallguy Senior Member

    @Dolfiman

    I read thru your work briefly and have a few observations to share.

    1. I don't quite understand comparing asym and not asym/tumblehome. I would prefer to compare a canoe with and a canoe without.

    2. The tumblehome, I realized, through your work, prevents the paddler from getting too far offcenter; something I had never considered. In a heeling moment, the shifting of the paddler's mass toward the lower level is greater in the 'open' boat. The tumblehome prevents this somewhat in that boat.

    3. I'm not sure I fully understand the summary because I am confused over using the asym approach. i.e.-if the boat heels toward the 'open' boat. Can you quantify the work and say the boat with tumblehome is X% more stable? Because the tumblehome has more freeboard at heel? Or because the tumblehome prevents the paddler from getting too far out? Or some bit of both?

    Or because each boat, forgetting the paddler, has a different righting moment (Gz) at various angles of heel which is caused by a larger? more dynamic? center of buoyancy in the boat with tumblehome?

    sorry if I am a pest, but I think Gz is at play some
     
  13. Dolfiman
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    Dolfiman Senior Member

    Dear Dan, thanks for your interest and questions, my tentative answers for these notions not easy to summarize :

    >>> As mentioned, I have already done such comparison in another thread two years ago, here attached the document + a 2 pages abstract with main results, less arid to read. Tumblehome more exactly allows a higher rate of waterline beam enlargement with either an extra load or an heel angle, effect which can also be obtained with a hard chine.

    >>> For me the paddler effort to maintain his posture as vertical as possible for both the stability (his own stability, boat stability) and the paddling efficiency is dependent of the heel and is possible up to a certain moderate heel angle (more or less 20° I suppose ?). Beyond, you could no longer react correctly and a capsize can occured rapidly. Here, the asymetric hull just allows the decentering paddler (if it is a must for solo canoeing) to start from an upright equilibrium, which ease such corrections on both sides, you recover the symetrical feeling. Tumblehome, as any waterline beam enlargement higher rate, by giving more GZ due to the center of buoyancy transversal move, requires less GZ due to paddler weight posture correction.

    >>> In the example here attached, I give % based on the righting moment energy : +23% (at design load) to + 59% (at extra load 270 kg), and it is neutral at light load (70kg).
    >>> Freeboard is another issue : the benefice of a better righting moment energy is real as long as, at heel 20° for example, you still have a sufficient margin from the rsik of water intake. So, the designer concern should be the remaining freeboard at say 20° both sides, to disconnect the water intake risk to the dynamic stability issue. In my asymetric hull approach, I have 13,5 cm freeboard at heel 20° starboard side and 10,3 cm at heel 20° port side.
     

    Attached Files:

  14. Dolfiman
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    Dolfiman Senior Member

    ... and the gondola at rest, showing their heel angle and their asymetry. To note that this asymetry was introduce quite later on, during the 19th century, although the boat existed centuries ago. For those who have the opportunity to visit Venice, I recommend you to visit the Squero di San Trovaso :
    Visita lo Squero di San Trovaso - Venezia - Venice - by Geomaps (squerosantrovaso.com)
    gondole-venise.jpg
     
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  15. fallguy
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    fallguy Senior Member

    such great work to put my gutcheck into reality... France is lucky to have you

    I did not realize the gondola was as such; haven't bee there.
     
    Last edited: Aug 23, 2021
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