Vernacular voyaging canoe design

Discussion in 'Multihulls' started by dsigned, Aug 28, 2017.

  1. dsigned
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    dsigned O.R.C. Hunter

    I grew up overseas in SE Asia (Vietnam, but my parents moved to the Philippines after I graduated high school), and developed an appreciation for there often being more than one way to skin a cat (not sure if pun intended).

    In reflecting on Western/European multihull (specifically catamaran design), the aspect ratio (or length/beam ratio) of Polynesian voyaging canoes is significantly different than that of "modern" catamarans, as the prismatic coefficient appears to be as well.

    I took the dimensions of Hokulea from here:
    http://archive.hokulea.com/images/canoes/hokulea_plan.gif

    and did some back of the napkin (Excel) calcs that put the prismatic coefficient somewhere between .87 and 1 and the length to beam ratio at 4:1, with a slenderness ratio somewhere between 10 and 11.

    I'm taking my information from here:
    Catamaran Design formulas http://catamaransite.com/catamaran_hull_design_formulas.html

    and the comment on having a high length to beam ratio is that you can carry "less sail." This seems to me to be a non-sequitur, as the amount of sail one could carry would only be less if all other dimensions were equal. But in the case of a wider beam, it's very unlikely that all other dimensions would be equal.

    So my observation is this:
    it seems to me that "modern" cats have a bias towards smaller length to beam ratios. My guess is that this is a result of racing regulations that favor a shorter boat versus a longer one (a longer boat being more likely to be shunted into a higher category) and perhaps berthing that is based on overall length?

    The only other consideration I'm aware of is wave drag between the hulls, which I haven't calculated for Hokulea.
     
  2. gonzo
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    gonzo Senior Member

    Are the length/beam ratios you are referring to for each hull? In the post you also mention wave drag between hulls, which may mean the length/beam has a different context.
     
  3. dsigned
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    dsigned O.R.C. Hunter

    No, the ratio of the length at the waterline to the beam between hulls.
     
  4. hump101
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    hump101 Senior Member

    The length/beam ratio in modern sailing cats is primarily about balancing longitudinal (pitch) stability with transverse (roll) stability. Modern materials allow this balance to be optimised as the structure can be made strong enough to cope with the loads that result from wide beam. Historically this was not the case.
     
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  5. dsigned
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    dsigned O.R.C. Hunter

    With respect, this seems to me to be a rationalization rather than an actual explanation. Firstly, it assumes that longitudinal stability and transverse stability are inter-related in a way that is mutually exclusive. But this seems to me to not be the case at all, as the former is largely a function of the condition of the surf (e.g. flat vs. large waves) whereas the latter is a function of how great the force acting on the sail is. While these things are related (higher winds equate -- generally-- to both larger waves and stronger force on the sail), the size of the sail is directly proportional to the force, regardless of the wind conditions. In other words, one could choose a given sail area, design for the appropriate righting moment, and then make the hulls extremely long, with practically no penalty in straight line performance. Making the hulls longer doesn't affect the righting moment at all.

    The second reason that this doesn't seem a terribly compelling explanation to me is that if there was indeed some necessary tradeoff between roll stability and pitch stability, one would expect a drastic (or at the very least, noticeable) difference in length/width in inland cats vs ocean going cats. I can't tell that this is the case, except perhaps in very large cats.

    Modern materials allow this balance to be optimised as the structure can be made strong enough to cope with the loads that result from wide beam.

    I definitely think there's something to modern materials being stronger than lashed together tree trunks, but it also seems to me to be a situation where it's assumed that can implies should. I closed the tab I had open with the formulas for calculating structural loads, but generally, a wider cat is going to require greater structural reinforcement. And while this is certainly possible with modern materials, you're paying a significant weight (plywood/steel) or cost (carbon composite) penalty to have the strength.

    I suspect that the train of thought here is that with a rotational torque, being able to apply the torque on a longer lever gives you "free" torque -- it multiplies the torque applied at the point of rotation without requiring greater force (weight) to be applied. But again, this seems to be an assumption that could very easily be misleading, as the forces on the structure would scale non-linearly with added width (thereby increasing weight or cost non-linearly), especially if we're building living quarters in the middle. There would also be the temptation to increase the weight of the living quarters in the middle (by making them larger).

    I apologize if I'm coming across as presumptuous. I am going to defend this idea as vigorously as I can, but that's as much for dialectic reasons as it is my being married to the idea. Philosophy background, sorry.

    EDIT: Perhaps you could point me to calculations that are used?
     
  6. gonzo
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    gonzo Senior Member

    If you want to defend you idea, the first step is to clarify your terms. For example, you used length/beam in two different contexts in post #1. On post #5 you are making claims for torque that contradict every current knowledge in physics and engineering. This leads to your claim of a non-linear forces and cost increases. Torque is always a rotational system, otherwise is wouldn't be torque. However, for structural purposes, the moment should be used on calculations. Perhaps brushing up in basic statics and dynamics will help your philosophical pursuit.
     
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  7. Angélique
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    Angélique aka Angel (only by name)

    Wharram chooses for a smaller over all beam compared to most of today's Cat designers, and they explain why they do so . . .

    Wharram ---> How We Design ---> Introduction to Catamaran Stability ---> Catamaran Stability

    I don't know of and have never heard or read of any capsize of a Wharram Cat constructed according to the plans, but they also choose for smaller and lower sails compared to the majority of today's Cat designers.

    Wharram based his Cat designs from the 50's on research into ancient Polynesian boat design, and today's Wharram Cats are still very much in that style I believe.

    The Polynesian Catamaran Association is (was?) a club of people interested in Wharram designs, and they have magazine archives of The Sea People and The Sailorman.
     
    Last edited: Aug 28, 2017
  8. DCockey
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    DCockey Senior Member

    How are you defining prismatic coefficient? The standard definition is volume displacement / (maximum submerged cross-sectional area x waterline length). Using that definition the maximum possible value is 1, and that is only with a constant cross-section hull.
     
  9. dsigned
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    dsigned O.R.C. Hunter

    Sorry, it's been some years since my statics and dynamics classes. However, in the context of the catamaran beam, both terms apply. But yes, from a static perspective I suppose I was using torque where moment ought to have been used.

    As far as my usage of beam/length: I think you should re-read it. I only refer to it in the context of the beam of the overall vessel, not of the individual hulls. I did refer to the prismatic coefficient, which uses the beam for the hull, but I didn't refer to the beam of a hull there. In any case, the ambiguity is hardly my creation: I took the terms directly from the linked site.

    And insofar as moment: maybe you could clarify what you mean by

    You are making claims for torque that contradict every current knowledge in physics and engineering.​

    I'm not claiming that the relationship between the moment and the distance scales non-linearly in the general case, but in this case. If I remember correctly, it scales linearly as long as we can neglect the weight of the structure. But I am assuming that we are not going to neglect the weight of the structure, both because the reinforcement adds non-trivial weight, and because of the way we distribute payload across the hulls.
     

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  10. dsigned
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    dsigned O.R.C. Hunter

    *Facepalm*. Welp, that's what I get for doing calcs late at night. I forgot to take the half of the the displacement. Corrected value (assuming roughly triangular hull shape) is .507
     

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  11. dsigned
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    dsigned O.R.C. Hunter


    I hadn't seen that page, thank you! I'll read it and see how much of it I can grok. Do you know of any (charitable) responses to this from other multihull designers? The article (on first glance) seems to suggest that the short length to overall beam ratio is simply convention, which may be the case, but this doesn't seem to be the consensus view among nautical architects, as far as I can tell.


    I'm somewhat familiar with Wharram's designs, but they seems to be primarily geared towards building on a budget, and seem to have a bad reputation among other catamaran designers (ostensibly Wharram doesn't have any professional training as a nautical architect?). I don't necessarily mean that they're "bad", but since he does so many things differently (and there are so many variations on the type), I'm not sure which variables are affecting what.

    Finally, "smaller and lower" compared to similar displacement cats? It seems like the comparison metrics used might need to be overall beam or cost. I kind of wish there was a race somewhat like OSTAR, except with a payload requirement (e.g. 1000kg) and a cost requirement (e.g. $50000).
     
    Last edited: Aug 28, 2017
  12. gonzo
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    gonzo Senior Member


    How does the length/beam ratio of a single hull affect the wave drag between hulls?
     
  13. Angélique
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    Angélique aka Angel (only by name)

    Only about what held up James' acceptance as a serious Multihull Designer . . . :rolleyes:


    [​IMG]

    ‘‘ Jim Brown, American designer of the ‘Searunner’ Trimarans, and Canadian Multihull Sailor and Professional TV Cameraman Scott Brown, came to interview James on board Spirit of Gaia.

    ‘‘ You know, James, what held up your acceptance as a serious Multihull Designer was all the naked girls that run around on your boats.’’ - (Jim Brown)


    << Sorry, but I believe I can't show here the top right corner photo, whose sight 'in the flesh' made Jim say so . . . ;) >>

    [​IMG]

    Jim and Scott were very impressed with Spirit of Gaia:

    “Gee, she’s a real Ship, man.” - “See how she moves in a breath of wind.” - “Look at the power in those sails.” - “God, she is beautiful.” - (Jim Brown) ’’
     
    Last edited: Aug 28, 2017
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  14. garydierking
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    garydierking Senior Member

    Now we know why there are no books on how to design a multihull.
     

  15. dsigned
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    dsigned O.R.C. Hunter

    As in they keep getting put in the "adult" section and no one sees them?
     
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