Kayak stability

Discussion in 'Stability' started by lost, Jul 30, 2017.

  1. lost
    Joined: Jul 2017
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    lost New Member

    I'm trying to work out what's going on with designing for initial kayak stability. I built a prototype kayak which came out with a beam of about 0.5m (20") (which was a bit narrower than intended but it uses origami plastic like an Oru Kayak so the shape wasn't controlled very well). On testing the secondary stability was fine, but the primary stability was horrible and it felt initially actively *unstable*, i.e. it wanted to lean over a bit to reach a stable position.

    I did some reading and built the measured prototype in Freeship, which came up with KMt = 25cm (at a draft of 0.11m which about what I eyeballed it to be and matched up in terms of calculated displacement and loading). I understand this is the height of the metacentre above the keel or z=0.

    A bit of googling suggested [1] the centre of mass of a seated figure, legs out in front, is at about 11" = 28cm, so probably actually bit higher for a kayaking position with knees and arms up. Given that is actually a bit *higher* than the calculated KMt for my prototype I felt that made sense of the prototype's initial stability issue.

    However I can't work out what sort of figure I should aim for when redesigning. I plugged the offsets for Yost's Sea Tour 15R into Freeship which gave a KMt = 26cm, i.e. basically the same as my prototype. Elsewhere ([2], [3], [4]) I've seen suggestions for canoes/kayaks of 23, 27, 28cm although it's never entirely clear whether this is height above keel or above centre of mass.

    Any suggestions please?

    [1] https://www.faa.gov/data_research/research/med_humanfacs/oamtechreports/1960s/media/AM62-14.pdf
    [2]
    Free Kayak Plan: Peabody Essex Museum Labrador Kayak 1867 http://www.paddlinglight.com/articles/free-kayak-and-canoe-plans/free-kayak-plan-peabody-essex-museum-labrador-kayak-1867/

    [3] Siskiwit Bay Multi-Chined Kayak Plans for Plywood Building • PaddlingLight.com http://www.paddlinglight.com/articles/free-kayak-and-canoe-plans/siskiwit-bay-multi-chined-kayak-plans-for-plywood-building/
    [4] Metacentric Height - FREE!Ship or Michlet https://www.boatdesign.net/threads/metacentric-height-free-ship-or-michlet.15582/


     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    A hollow along the centreline would give you more initial stability, but could be a nuisance, like the old tailshaft hump in rear-wheel drive cars.
     
  3. gonzo
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    gonzo Senior Member

    Those old kayaks were built around the owner. They fit like a shoe. Their initial stability is not great, and relies largely on the skill of the operator. I think you are looking at a kayak for more general use. Also, most kayaks had a flatter bottom than the one you show. Lessening the deadrise will give more initial stability. This book is really good to get ideas:
    https://www.amazon.com/Bark-Canoes-Boats-North-America/dp/1628737921
     
  4. upchurchmr
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    upchurchmr Senior Member

    You might go to a forum about kayaks.
    Kayak Building Forum - Message Index http://www.kayakforum.com/cgi-bin/Building/index.cgi
    Or try the following sites:
    Guillemot Kayaks - Small Boat Plans, Kits, Instruction and Handmade Wooden Boats http://www.guillemot-kayaks.com/guillemot/
    Laughing Loon Wooden Strip built Kayaks and Canoes -Wooden Kayaks Canoes Build a Boat, Boat plans, Wood kayak plans, wood canoe plans. Strip planked kayaks. Wood boat, Sea kayaks, Canoes, Wood Strip Boat Building Plans, and Beautiful Boats for Sale http://laughingloon.com/
    Wood Kayak Designs, Plans, and Kits by Redfish Kayaks http://redfishkayak.com/

    A little research there, talking to the owners of the sites, and comparison with existing kayaks.
    I have built several Yost kayaks, including the 15R and don't think that one is unstable - same conclusion you have I think.

    I'm not familiar with Freeship - are you calculating from a single section or the whole "as designed" kayak?
    One concern I'd have is that the plastic Oru style hull is not holding its shape in the water, reducing the width (at the waterline) and depth (in the ends) of the boat. Perhaps as you lean, the hull deflects asymmetrically actually reducing the stability even more dramatically.
     
  5. Heimfried
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    Heimfried Senior Member

    (Transversal) Metacentric height is defined as GM, the (initial) vertical distance between CoG and (apparent) Metacentre. If KMt is given, it is not the metacentric height, but the sum of KG (distance of Keel, z=0, and CoG) and GM.

    The CoG of the empty kayak has to be corrected, if the kayaker takes his or her place. The metacentric height changes also.

    If you are interested in playing a bit with change of stability, you can try this: Schwimmlage von Schwimmkörpern http://www.bootsphysik.de/rechner/bootxp.php
    First click button "English" top right, then check out the texts appearing if you click the "i"-buttons. This site provides not a real kayak hull form, but enables you to learn about similar hull forms. You are able to put in load (e. g. kayaker moves in) and look at the different behaviour of the boat (rithing lever curve and so on).
     
  6. Skyak
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    Skyak Senior Member


    Cool site for showing how standard stability calcs work.

    I think the original poster has a very different problem. Kayaks are not stable by any standard measure. Kayaks are actively balanced by hip motion in a small range of stability -any accurate boat design calcs will say the same -unstable.

    The standard metacenter value would be useful, but since his boat is made of thin corrugated plastic I question whether the boat is actually holding the design shape -I am pretty sure it is deflecting a significant amount. Compounding this problem, I suspect that there is significant deviation between the motion of his hip bone and the boat. I presume the corrugations run fore to aft -that material has roughly zero stiffness in the transverse direction.

    My suggestion for improvement would be to link a stiff seat that conforms to the shape of your butt to the vertical sides of the kayak and also to thigh braces. When you have this stiff seat installed, mount a camera on it and do some rolling motions with your hips. The camera will show the boat deflections you need to improve for stability.
     
  7. lost
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    lost New Member

    Deflections are a few mm at most. This is much heavier gauge than the stuff you see for signs and packaging. I was really surprised at how stiff it is - think thin ply and you'll have the right idea. I agree kayak stability isn't a normal boat stability problem but *all* I'm talking about here is the initial stability at small angles. Surely that must be influenced by the shape and therefore the metacentric height must provide some predictive capability?
     
  8. Heimfried
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    Heimfried Senior Member

    My kayak "experience" consists of a few hours kayaking long years ago. Sometimes I watch kayakers. In my opinion there is normally an initial stability for the system kayak plus kayaker. Otherwise they would capsize when having a rest at the kayak lean back without body move to balance the kayak.
     
  9. Heimfried
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    Heimfried Senior Member

    @lost

    While freeship is on my computer I'm not able to work with it. I assume you can put in the Mass of boat plus kayaker to look at the new waterline. Does it give you the new CoG if you add the weight of the kayaker?
     
  10. lost
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    lost New Member

    In freeship you put in the draught & it shows you the waterline if you want to see where it lies graphically. The hydrostatics calcs will give you displacements (and values for KMt, BWL etc etc etc) for a range of draughts. So not quite like your guess, but I think functionally similar.

    Skayak (nice name!) is right that kayaks are actively balanced by the user, but there is still a large variation in stability between designs. Some people use a "stability factor" to try to describe the apparent stability to prospective buyers but all this is is the metacentric height at 15deg heel scaled by a constant to give a "nice" number, although I've seen some different treatments of the centre of gravity for this calc, hence why I haven't bothered to try to reverse-engineer this for my problem (yet), hoping someone would be able to give me a KMt value for designs which they know feel ok.
     
  11. Heimfried
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    Heimfried Senior Member

    The KMt is not the real criterion of stability, it depends on the KG (CoG above z=0) if there will be a positive GM = (KM - KG) .
    In my opinion the KMt of a kayak design is only an indication, because the crucial GM depends on the CoG of the respective kayaker and the seat hight above K (z=0).
    Do you know how to calculate the shift of the CoG due to boarding by the kayaker given the weight and the CoG of the empty boat and the same of the kayaker?
     
  12. lost
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    lost New Member

    Heimfried thanks but I get that, I was explaining to you what Freeship takes as inputs and gives back, as you asked. In my first post I give what I estimate to be the KG the kayaker, which will dominate the CoG of the combination as the kayak is so light much lighter than me (~5kg vs ~70kg). So equivalently if someone can give me a target GM I can use the KG I've got to estimate what KM I should be aiming for. Either way is fine. My issue is that I don't know HOW +ve the GM needs to be for an "ok feel", especially because I estimate a known design would give me a value very close to my unacceptable prototype.

    Maybe I've confused things by starting with KMt in my first post, I don't know! Clearly KG isn't really a design variable in this case (unless I work on getting really big thighs, or ballast which isn't exactly the way I want to go).

    The most helpful info has been from Gonzo who suggested that maybe the stability of the known design wouldn't have been great either!
     
  13. Heimfried
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    Heimfried Senior Member

    I understand now, sorry.
     
  14. Skyak
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    Skyak Senior Member

    As I said, the metacenter height is a good number to rate the design stability. It is a bit dated -it's a circular approximation of the stability curve from pre-computer times. What you really want is the transverse shift in the center of buoyancy per degree of heel at your particular displacement -which any computer program can tell you. If you want to use the traditional methods you would do two models -one presuming that your weight is centered at the pivot point where you role your hips, and the other would be the higher true center of gravity. The stable range would be based on the later and the control would be based on the former.

    I have not seen corrugated plastic that is as stiff as ply so I can't comment there, but if it deflects a few millimeters it is contributing greatly to your loss of initial stability. Your hip bone is much narrower in the seat than you think. Bend over and feel for the bone peaks -then hold a piece of paper and grab it at those points. That is the width of the hard contact with the seat. Take half that distance and divide your "few mm" deflection by it -the arc sine of this ratio gives you the angle of error the deflection is causing. Does your kayak have thigh braces connected solidly to the sides? If you are still not convinced I would suggest you mount a video camera hard to the side of your kayak with a clear view of your hip bone (put a dot on it) -do the rolling motions through initial and secondary stability range -does your hip bone move relative to the side of the boat? Then there's your problem.

    20inch is not very wide for a kayak but there are certainly narrower. My Pax20 is 17" beam, my 14s are 24" max but 20 to 22" waterline beam. My 10 is 31" and is the only one with too much stability (mini sailboat project). If you want, I could put out some stability curves from comparable kayaks -what beam, length and prismatic coef?
     

  15. gonzo
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    gonzo Senior Member

    You hit the nail in the head. The CG of the person should always be aligned vertically over the center of the seat. I think that most computer programs assume the CG is not shifting.
     
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