Hello there Can you let me know how the primary and secondary stabilities of sea kayaks are measured? Thanks Selville *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
Selville wrote: >Can you let me know how the primary and secondary stabilities of sea kayaks >are measured? Now that is a good question! Primary stability is how hard the craft resists being tipped from the upright position. If you are looking a graph of stability (see the reviews pages of http://www.marinerkayaks.com or any kayak review at www.seakayakermag.com ) the steepness of the angle off of zero is a measure of the primary stability. Secondary stability is a lot harder to define (at least for kayakers). Most kayakers will be hard put to define it but will tell you they know it when they feel it. Sort of a seat-of-the-pants thing. Some have tried to use the total area under the curve but this doesn't correlate well with the "feel". One designer I know used to claim secondary stability doesn't exist and the kayak that is more stable initially will always be more stable at all angles of lean. I'm not sure if he still claims this--especially the latter claim-- since I showed him some stability graphs (Chinook and Puffin in the Winter 1986 issue of Sea Kayaker magazine) where the less initially stable kayak--the Puffin--had not only higher relative stability at higher angles of lean than the Chinook but also a higher maximum stability and a greater total area under the curve). Personally (not necessarily scientifically) I define secondary stability as how secure you feel when you have leaned the kayak well to one side. This makes it more relative to the primary stability. A kayak whose maximum stability is five times as high as its stability at 5 degrees of lean will "feel" more secondarily stable than a kayak with a much higher maximum stability that is only twice as stable as it was at 5 degrees. A kayak with less stability (height of the curve at a given angle of lean) can still feel a whole lot more secure while leaned. I'll make the comparison with rocking back in a rocking chair (low initial stability/high secondary stability) and compare that to rocking back in a regular four legged chair (high initial stability/high total stabilty/low feeling of security as you teeter at the balance point). It is hard to lean back on a standard chair (most of my high school teachers frowned on my practice of balancing my desks on their back legs when I was bored, especially when I would draw their attention to what I was doing when I would almost lose my balance backwards and in a desperate recovery crash back down loudly to the primary stable position. I don't remember ever going over backwards but that was always the risk I flirted with--a capsize to the rear?). Looking at the graphs I would define a kayak with good secondary stability as one with a relatively shallow angle off of zero (so you don't have to put a lot of energy into leaning it) but which has the point of maximum stability (the top of the curve) at a greater angle of lean (and maybe at a higher maximum point) than a kayak with less "secondary" stability. Even this doesn't totally account for the difference in "feel". A smooth progressive increase in stability out to near the maximum allows a trustworthy "feel". Any abrupt changes would be like putting a speed bump or flat spot on the rocking chair rockers. These are totally my own observations and guesses regarding secondary stability in kayaks and I make no claim to scientific validity or even at a valid definition of the term (which probably exists somewhere in Naval Architecture and someone will hopefully inform us of). I and most experienced paddlers prefer lower initial and good "secondary" stability. Easy to lean/secure while leaned. Then there is what I define as "dynamic stability". Which to me comes down to: "how does the kayak feel in rough seas?". Does it stay under you or is it always wanting to slide out to the side out from under you. Does it feel neutral, consistent, gyroscopic or are the waves knocking it around and rocking it. Are the motions smooth or jerky. Do two waves pick it up by the ends and make you feel like an off balanced turkey on a spit or does it maintain a comfortable secure stability in waves. Does the high initial stability mean the hull stays flat to the wave face leaving the paddler constantly bending at the waist to stay vertical or does a high center of gravity (higher seat usually) mean that even bending at the waist is not enough to compensate for the upside down pendulum action created by the high initial (flat to the wave face) stability. Imagine a wide flat canoe with its high sitting position in steep seas as the stable canoe is tilted by the wave the high seat is swung to the side, shifting the center of gravity to that side. Any attempt by the paddler(s) to swing their center of gravity back over the canoe will by an equal and opposite reaction also tilt the canoe further in the direction that it was already dangerously tilted towards. Luckily the canoeist may be able to get down on their knees to pray while also lowering their center of gravity (after all God helps those who help themselves doesn't she). (With John Winters on vacation who will come forward to defend the seaworthiness of canoes after another of my scurrilous attacks? I'm sure John would say to not go to sea in a too initially stable canoe.) Matt Broze http://www.marinerkayaks.com -----Original Message----- From: Julia Selby & Kevin Melville <selville_at_ozemail.com.au> To: PaddleWise_at_lists.intelenet.net <PaddleWise_at_lists.intelenet.net> Date: Tuesday, August 10, 1999 10:40 PM Subject: [Paddlewise] Primary and secondary stability *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
On Wed, 11 Aug 1999, Matt Broze wrote: > Selville wrote: > > >Can you let me know how the primary and secondary stabilities of sea kayaks > >are measured? > > Now that is a good question! I thought so, too. Hopefully you'll get around to answering it. Meanwhile, I enjoyed your dissertation, which taught me that what I thought I understood about the subject was wrong; I thought initial or primary stability was basically wave-surface-hugging strength. Is what you were saying, that primary is tendency to return to the vertical (not the water surface) when displaced from it a small amount, and secondary is the same, but the tendency is weaker and spread smoothly over a larger range of displacement? How IS it measured? (not "defined") With John Winters on vacation who will come > forward I don't mind that but I wish he wouldn't take the Professor with him. Slainte - e Elaine Harmon - eilidh_at_dc.seflin.org - eharmon_at_cs.miami.edu *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
It is best to forget theory and measure experientially by leaning different boats. In particular, try any Mariner boat and compare it to any boat of beam similar to the particular Mariner you are testing. The Mariner will have moderate initial stability but phenomenal secondary. Jerry *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
About a week ago (I've been away), Matt Broze wrote, [snip] >These are totally my own observations and guesses regarding secondary >stability in kayaks and I make no claim to scientific validity or even at a >valid definition of the term (which probably exists somewhere in Naval >Architecture and someone will hopefully inform us of). [snip] I looked in Gillmer and Johnson's _Introduction to Naval Architecture_ (which John W recommended to me as a good one-volume book, since I didn't want to spring for a better and more expensive three-volume one). I found several entries in the index for "initial stability", and no mention of "secondary" or "final" stability. My guess is that it isn't a real naval-architecture term, nor indeed anything well defined, but just an ambiguous term invented to mean "something other than initial stability." In that case, Matt's definition/description is as good as any, and IMO much better than most. Bruce Bruce Winterbon bwinterb_at_magma.ca http://magma.ca:80/~bwinterb Economics by mantra -- "communism", "capitalism", "free markets",... -- is a fraud. *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
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