Primary stability considerations in kayaks

Discussion in 'Stability' started by cthippo, Feb 28, 2012.

  1. Petros
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    Petros Senior Member

    Ad hoc,

    With all do respect, "nonsense"? You completely missed the point, and you proved mine. His request was not to just make it more stable, but to do it WITHOUT increasing drag. If you double the width of course you increase stablity, but what does it do to the drag?

    Making it longer, without increasing the bean, will increase stablity (as you proved) without increasing the drag very much at all.

    What is nonsense about that?

    Sure, make it stable by making it a big rectangular box. Ever try paddling that through the water? You can not even efficiently paddle a wide hull anyway, you have to reach way to the side to get the paddle in the water (that is why I hate factory kayaks, way too wide: 21" inches is the max for me, preferably less).

    Keeping cross section and bean the same, and making it longer will increase stablity without much affect on drag. waterline length goes up, finess ratio goes up, slight increase in wetted area, net affect: increased stablity with little to no affect on drag.
     
  2. alan white
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    alan white Senior Member

    Gonzo made a valid point describing two boats end to end. In simple terms, lengthening a boat without any other increases elsewhere, the percentage increase should equal the length increase providing the only change is to the length; not the width, height or general shape.
    The example was apt and very easy to understand.
    You're not saying that example is inaccurate, are you?
    BTW, beam at the waterline is the significant figure, usually less than the gunwale beam by a couple of inches at least. mine is 23" at the sheer and about 18-19" at the waterline.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Oh??..here is the question again:

    Thus, a reply based upon that question.

    If you wish to digress into powering/resistance etc, fine, that is your prerogative, but doesn't address the prime question of stability. You're inference is on "design" per se, not stability.

    Design is a compromise of all the conflicting arguments. The stability is just one.

    Forgive me, but i though the question is about 1 boat, not 2??

    How is Chippo going to put 2 kayaks end to end to make the stablity of his kayak better?
     
  4. gonzo
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    gonzo Senior Member

    Come on Ad Hoc, you really can't visualize two boat attached end to end as a simil to a longer boat?
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Yes I can, but that wasn’t the question.

    If you have 2 kayaks joined, that is not the same as one long hull.

    Since if you have the displacement the same, keeping the beam the same, the draft must reduce. Thus the KB is reduced, which means the BM is reduced, which means the stability lever GZ is also reduced.

    If you increase the length, but keep beam and draft constant (say like 2 hull joined together), the KB is unchanged. BUT, the displacement has increased. So, BM is equal to (I/V), I =- waterplane inertia and the V = volume. So if the waterplane beam is the same, but has a doubling of length, L, the waterplane inertia has increase by a factor of “L” (I = L x B^3/12). However, the volume has also increased, by a factor of L also, L x V = L.V.

    So with I increased by L and V increased by L, the result, as noted in the citing, the 2 “L” cancel each other out, thus BM remains unchanged. BM = I x (L)/ V x(L) = I/V, i.e. no change.

    The excellent series 64 hulls produced in the ‘40-50s after WWII were addressing the resistance and seakeeping aspects of long slender hulls. Both were improved greatly. But guess what, their stability …transverse stability (rolling)…was very poor. Because increasing the length has no effect….the longer you go again as noted by Tupper/Rawson, the KG generally increases, thus the stability gets worse, NOT better. Which is why the Series 64 hull form never went beyond the test tank, for Frigates etc. as intended.
     
  6. alan white
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    alan white Senior Member

    How is Chippo going to put 2 kayaks end to end to make the stablity of his kayak better?
    Don't be ridiculous. He will do it with his mind. In your mind, you can "make" things that represent other things. Otherwise, you might say dumb things like, "A boat is not a B and an O and an A and a T!
    A boat isn't a part of the alphabet! You can't just connect four letters together and jump in and paddle!"
     
  7. J Feenstra
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    J Feenstra Junior Member


    so in light of this information I would say, take yoga lessons or play wii fit. :D
    those exercises help you gain an understanding in how the body's TCG can switch.

    increase in length won't work, I agree with Ad hoc
     
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  8. Fanie
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    Fanie Fanie

    LOL.

    I agree with Gonzo, a longer flat bottom boat will be more stable simply beacause the draft will become less for the same weight. The more weight you load in it, the less stable it will become.

    Can validate it too, the short bath tub like ducklings they use to take fishing lines in is short, but sits deep in the water under weight, and is not very stable. The 4.5m indian canoe I had was the same width, it was more stable even with three people in it. A 10m canoe will be too stable :D

    The problem would be to lengthen a canoe... it will mean more drag and less easy to turn.

    Can also say I had different kayaks here a while back I tested on the pool. The shorter ones are less stable than the longer ones, I ended up buying the longest two they make, more weight for length, less stable. No two ways about it,
     
  9. Fanie
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    Fanie Fanie

    You can calculate it as well if you make a drawing of the shape, then add the waterline for different weights and calculate the righting moments at different angles. The shape also determines if the righting moment increases with heel as well, which is why kayaks and canoes have rounded sides. Of course over a certain angle the righting moment quickly falls off and you capsize - which is why canoe's are not V-shaped.

    I can add - do not confuse with the COG in a hull where you have lots of reserve booyancy and the dead weight is placed in the center of the hull to increase righting moment. Canoe's don't have much reserve booyancy - or width.
     
  10. Submarine Tom

    Submarine Tom Previous Member

    This answer is so easy you all missed it.

    Get a lighter camera!

    -Tom
     
  11. Petros
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    Petros Senior Member

    Ad Hoc,

    Perhaps you missed this part of his question:

    You will not meet Cthippo's needs by ignoring his requirements. Just making the hull wider only makes it more stable, but not keep resistance down.
     
  12. NoEyeDeer
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    NoEyeDeer Senior Member

    Your own post contradicts your assertion about beam. :)

    Sure, increasing beam is the fastest way to increase stability and is usually the best option. However, these two sentences contradict each other:

    In other words, you know perfectly well that I is proportional to the product of length AND the cube of beam. Telling people that length is irrelevant is technically incorrect and is going to cause confusion. You can already see the confusion in this thread.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Not if you read the whole post, as noted above.
    Since the first post is addressing waterplane inertia ONLY.

    No they don't. I think you need to read and reread both posts then you'll realise why. In the post above (#20) I explain further. Stability is NOT just the waterplane inertia, and so referring to the imaginary doubling the length of a hull, as if 2 hulls, ie 2 times it displacement. The above is explaining BM, which is a function of "I" AND "V". The important aspect is the "V", volume, or displacement (which was not addressed in the first post for clarity). Thus, if you were to do the whole calculation, it has zero effect.

    The purpose of the first post was to keep it simple. Since it is very very clear, that no matter how many times it is explained to people, and with graphs and even citations from well established text books (since why take my word for it), far too many people simply do not understand how to calculate stability and what effects it and what it really means.

    Thus the clarification, after a digression, above. There is no contradiction, save for your own understanding of each question and its answer ;)
     
  14. gonzo
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    gonzo Senior Member

    If you claim that a longer hull of equal beam does not have more stability, it contradicts reality.
     

  15. Submarine Tom

    Submarine Tom Previous Member

    Maybe this has more to do with how one defines stabilty?

    -Tom
     
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