Primary stability considerations in kayaks

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

  1. cthippo
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    cthippo Senior Member

    Having just about finished up the Raptor, I'm trying to assimilate the lessons learned for my next design.

    One of these was that I need more primary stability than most kayakers at my skill level because I do a lot of photography from my boat. This requires me to take both hands off the paddle and focus through the viewfinder, which makes it hard to keep an unstable boat still. On the other hand, I'd like to keep resistance down so that I''m not killing myself trying to get to where I want to take the picture.

    So, here's the question:

    What are the factors in hull design that contribute to primary stability?

    Obviously beam and height make a difference. A low center of gravity helps, as does a hard chine hullform and a higher prismatic coefficient. Am I missing anything?

    I'm thinking that a boxier cross section with a more angular lines amidships will help. Is this correct?

    Thoughts
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Beam and VCG.
     
  3. Petros
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    Petros Senior Member

    If you make the hull longer, without changing anything else, you will increase the stablity because you will displace more water as the hull rolls. This also has the effect of increasing your finess ratio, and will reduce drag somewhat (your skin friction goes up a bit, but your cross sectional area is not affected). The only draw back is a longer water line length would be more difficult to turn.

    So keep the CG low (seat low in the hull), make it long and thin, you will be nice and stable.
     
  4. alan white
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    alan white Senior Member

    A medium length kayak (approx. 14 ft) will paddle easier at low to moderate speeds than a longer kayak due to wetted surface being lower relative to the displacement. Top speed will be lower however, once you pour on the power.
    A flat and wide bottom is the most stable in the upright position. When leaned over to a certain degree, the hull gradually becomes less stable due to the bottom shape becoming vee-like.
    The flat bottom also is slower than the rounded bottom. This is due to greater area in contact with the water. The rounded bottom approaches the shortest distance from side to side under the hull (a bowl has less surface area than a box for the volume it holds).
    In fact, most kayaks now available to the general public have quite flat bottoms anyway. People try out the (round-bottomed) kayaks paddled by very experienced paddlers and find them very tippy. As a result, the market has wised up and now most kayaks are not very efficient in the hands of a seasoned paddler.
    As a photographer, the stability you want could be possible with a wide and flat hull but it's also possible to rig up a pole with a pair of inflated (small mooring ball) floats on the ends. Set crosswise, the rig only needs to be about 5 ft long to be ultra stable when used.
    A bit of cleverness and you could keep the rig on the aft deck and aset it up in a few seconds.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Sorry, but that is nonsense.

    Stability, transverse stability, since we are talking about rolling, is related to the inertia of the water plane area, which is proportional to the beam.

    So what governs this...beam. It is the I = LB^3/12, the B^3 is the figure.

    Thus if you have say a boat 5m long and 1m wide, the inertia is roughly 5 x1^3/12 = 0.42.

    If you doubled the length, the inertia is roughly 10 x 1^3/12 = 0.83 (twice as much)
    If you doubled the beam, the inertia is roughly 5 x 2^3/12 = 3.33 {8 times as much ie (B-new/B-old)^3]

    Thus, in numbers a simple demonstration of increase in beam.

    Stability is related to beam NOT length.

    You can see the effect of increasing beam here:

    stability - beam increase.jpg

    And if you're still not convinced here is an excerpt from Basic Ship Theory, for the doubting Thomas':

    stab beam-length.jpg
     
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  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    You could try playing around with Michlet which is a free program.
    It will calculate BMT and GMT for a hull with a supplied KG.
    You can easily try out some kayak hull shapes and vary one parameter at a time to see the impact on the initial stability and the total resistance.
     
  7. J Feenstra
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    J Feenstra Junior Member

    the choice of hard chine or round bilge has more to do with sea states. So a sea Kayak may have a Round bilge, and a mountain lake kayak can have a hard chine.

    For a stable fast kayak I would suggest laminating a keel, for roll resistance.
     
  8. cthippo
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    cthippo Senior Member

    Thanks for the input, everyone!

    Another lesson learned from the Raptor is that I think 25" is the correct beam for me. The Raptor is 26" and is almost comfortable without padding. Within that given beam, I'm working on finding a happy medium between stability and speed.

    In this case it's also related to construction technique. I've built both skin on frame and cedar strip boats, and I prefer the cedar strip method. While it's certainly possible to do a hard chine in cedar strip (TX River Rat did some nice ones), it's not ideal. On the other hand, I could do some smaller radius bends that would act like hard chines.

    Are you thinking of a keel in terms of weight and moment arm or in terms of hydrodynamic resistance, more like strakes? I think the former would tend to be weight prohibitive (it still has to go on my shoulder) but I've been thinking about the latter as a possibility

    I found the original Freeship drawing which gives some idea of the hullform. I'm thinking something like the second picture may work better for the next one. It would be a higher block coefficient and more wetted area, but a bit less draft. I think
     

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  9. J Feenstra
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    J Feenstra Junior Member

    I was thinking in terms of the hydrodynamic resistance. A lot of ships have bilge keels for adding resistance. you can install the keel on the centre lines or on the bilges. I would install them on the bilges, so you can approach the shore easier than with a large keel.
     
  10. johneck
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    johneck Senior Member

    In kayaks stability is not quite as simple as calculating the waterplane inertia divided by the displacement as it would be for a large vertical sided ship. The waterplane changes very quickly with changes in dispalcement and heel. So the shape of the sides and the amount of freeboard before the deck edge goes underwater all play into the stability and the feeling of stable vs tippy. There is also a difference in how the boat feels in waves vs in calm conditions. Typical wavelengths of ocean or large bay waves are much,much greater than the kayaks beam, so a boat that feels very stable in flat water will feel like it wants to tip over when a wave passes underneath. There is also a significant influence in the feeling of stabiltiy depending on how "connected" your rear end and thighs are to the boat since you probably weigh 3X as much as your boat. None of these would matter in a ship but do matter in kayak.
     
  11. Fanie
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    Fanie Fanie

    Cthippo, if I was you I'd find an old hobie cat and put a trolling motor on it. Ditch the sail if you don't need it. Depending on the trolling motor and the battery you get, you should have a 4 to 6 hours of riding around on it. You could even add a small outboard if you want to travel longer distances faster.

    The hobies are around 2m wide - stable enough to move around freely. The trolling motor can move you around quietly and the petrol motor is good for long trips.

    The other option is to get another canoe and mount them side by side with a little bridge between them. I still like the idea of the hobie more. The 2nd hand hobie should be less expensive than building a new canoe or kayak.
     
  12. gonzo
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    gonzo Senior Member

    A longer hull will have more stability if the beam stays the same. Think about it this way: Say you need 1 unit of weight at the gunwale to capsize the kayak. If you were to fasten two kayaks rigidly end to end, you would need 2 units of weight to capsize them.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    As noted above, utter nonsense.

    Stability is about the ability of a vessel once disturbed to resort itself back to its equilibrium position. With regards to Rolling, this is transverse stability. This is a function of its beam, not its length. Length is longitudinal stability, i.e. Pitching.

    So, please explain where length comes into the derivation of transverse stability, a typ. diagram showing how transverse stability is calculated, reproduced for your ref:

    stability diagram.jpg
     
  14. gonzo
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    gonzo Senior Member

    A vessel of more length with the same beam will need proportionally more weight at the same distance from the center to incline at the same angle. Can you quote my complete post and then indicate what part you didn't understand?
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Here above, if that makes you feel better :confused:

    Whether I quote a portion or all, it is referring your statement about length, which is utter nonsense.

    If you are unable to understand the basics of how transverse stability is calculated, not postulated, but calculated, i.e. what actually governs the resorting force to return a vessel back to its equilibrium, there is little point in explaining again as it shall go over your head again.

    However, just for info again, highlighted in the hope you may read and understand it:

    stability-length change.jpg

    So, what is it you fail to understand here with regards to an increase in length? (Like above, from "Basic Ship Theory" & Rawson & Tupper)

    Happy to explain further if you do not, but if you continue with a closed mind attitude, there is little point in going around in circles.
     
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