Nick wrote originally; >> Primary stability has almost nothing to do with the cross sectional >> shape of a boat. Chines, no chines, makes no difference. It is the >> shape of the water plane and the height of the center of gravity that >> will determine initial stability. This sounded pretty strong to me so I ran some tests of my own and got different results. Nick sent me his data and it revealed that Nick had varied the CG in each test hull. This seemed like a no-no to me. When one wants to determine the effect of one variable the usual practice is to hold all other variables constant especially major variables like CG. Suppose, for example, some one said that increasing waterline beam had no effect on initial stability. Most would laugh because experience tells us something much different. Suppose, however, that this person showed us data revealing no change in initial stability - results he got by changing the CG with each change in beam. I suspect we would quickly point out the error of his ways. Whether one thinks it matters a lot or not is another issue. I suspect that the hard chine and round bilge proponents think it does. There is another issue. On Nick's "V" bottom boat he put the chine below the waterline. At small angles of heel the in and out wedges of volume are the same as the box shaped boat which helps explain why he got little or no difference. In my test I set the chine just above the waterline giving a different righting arm by 5.2%. Most boats will fit some where in between these extremes. More recently Nick wrote; > And as a practical matter most kayaks have fairly similar water plane > shapes, what changes most >significantly is the waterline width. I wondered about that too so I picked out four boats (not completely at random) and found that they had waterplane coefficients that varied from 0.674 to 0.609. Not sure what "fairly similar" means so maybe this much variation doesn't matter. "Fairly" similar sounds a bit "mushy" to me though. Of course beam does have more impact but does that mean you ignore the waterplane? Does any of this matter to paddlers? I don't know because I don't know how accurately paddlers can detect differences in stability. From a design standpoint a boat is composed of many small things as well as a few large things melded together to create a distinct boat. I recall Matt telling me once how he tweaked and tweaked his designs to reach what he considered the right boat. I bet some people would say he was just nit-picking but if you talk to the people who own his boats you will get a much different response. I suspect the same of Nick's boats and their owners. The real question is "Does all this matter here at Paddlewise, in the Nit-picking capital of paddle sport? :-) Cheers John Winters *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Two important designers, on opposite coasts of North America, having a polite debate about their approaches to a specific problem. Where else but on paddlewise? Thanks John and Nick, Craig Jungers Royal City, WA On 5/14/05, John Winters <jdwinters_at_eastlink.ca> wrote: > The real question is "Does all this matter here at Paddlewise, in the > Nit-picking capital of paddle sport? :-) *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
> I wondered about that too so I picked out four boats (not completely at > random) and found that they had waterplane coefficients that varied from > 0.674 to 0.609. Not sure what "fairly similar" means so maybe this much > variation doesn't matter. "Fairly" similar sounds a bit "mushy" to me > though. Of course beam does have more impact but does that mean you ignore > the waterplane? I would still consider Nick to be "correct". If you define primary stability to be the slope of the stability curve at 0 degrees (i.e. the slope between 0 degrees and (0 + Small Delta) degrees) it is a function of the shape of the water plane and more importantly the waterline width and the height of the center of gravity. You can hold the center of gravity constant (distance to the surface of the water or, if you prefer, the bottom of the boat. The latter will assist the narrower and V bottom boats a little). Then it is a function of the width and the shape of the waterplane. Now, as a designer, try the following. Increase the waterline width of a design (any design) by 1 inch. Now try and attain the same increase in primary stability by changing the waterplane. You cannot do it for all intents and purposes. The impact of that 1 inch is SO great and the impact of changing the waterplane shape within the "family" of shapes seen in commercial designs does not approach it. Certainly if the waterplane coefficients are between .6 and .7 If you think that one inch is a lot, you will find that it applies to 1 cm as well for all intents and purposes. Therefore, I would state that there is no commercial kayak out there with any arbitrary waterline width (X) that has a "primary stability" greater than any other commercial kayak with a waterline width of (X+1). And that is irrespective of shape of the cross-section or the waterplane. You can expand this statement to any design that is not "truly bizarre" So play with your curved and squared off and V cross-sections to your hearts content. As for "secondary stability"since that has no "scientific" definition, I cannot make the same statement. But the effect of width on the entire curve of stability is so paramount that again it is hard (impossible?) to find any "commercial-like" design where the stability curve for 0 to 45 degrees will be greater at ANY point than a commercial-like design with a waterline width that is 1 inch greater. The other important factor that comes into play as you move out the stability curve is the degree of flare of the "out of water" portion of the design. This can be roughly estimated by considering the width of the seam. So I would state that NO design X with a waterline width 1 inch greater than another design Y when the seam width of X is equal or greater than Y's will have a stability curve that is less than Y's at any point between 0 and 45 degrees. I would invite people with computers to try this out with whatever program they prefer. Bearboat Pro is available for free on the web and it will calculate stability curves. Submit your examples... I can conceive that the slope of the stability curve could impact something that might be perceived as "secondary" stability. If you are pushing on a door with 20 lbs of force and someone on the other side is pushing with roughly equal force and you push the door open another inch and suddenly the other person is pushing with a force of 10 lbs then you might "fall into" the room. And the same might occur if you are pushing with 40 lbs of force and suddenly the push back is 25 lbs. So the steepness of the reverse slope of the stability curve after you have reached the hump might be a definition of "secondary" stability. But my saying so makes no difference because their is no universal agreement. As I noted in a previous post, I believe that the shape of the cross-section DOES have an impact on the perception of stability at 0 degrees of tilt because of dynamic resistance to having the kayak rotate around its long axis. But this factor does not appear in a stability curve and is neglected in the literature that I have read. Also, in the real world, as kayaks are being flung around by waves and currents, I think that the shape of the cross-section (particularly flare) IS important in affecting one's ability to remain upright. There are dynamic forces that the viscosity of water imposes on the kayak. As the debate is currently framed with an actual definition of primary stability out there, reviews that talk about "poor" or "weak" primary stability are a little weird in that they say no more than one can basically judge by knowing the waterline width. Sea Kayaker reviews contain the numerical data so why would the reviewer be making these statements about the primary stability at all? I have also read about designs with poor initial stability but good secondary stability to the point that it is almost a clichi. But where are the reviews about kayaks with good primary stability but poor secondary stability? What are they comparing themselves to? *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. 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> The real question is "Does all this matter here at Paddlewise, in the > Nit-picking capital of paddle sport? :-) > > Cheers > > John Winters As I have been reading this discussion I think the one factor that is hard to put in numbers is paddler experience. Having paddled whiterwater and sea kayaks since 1975 my concept of what feels stable (initial or secondary) is going to be much different than some one just starting. I paddle an Arluk III which is 18 feet long and 21 inches wide and to me it is a "stable feeling" boat. Most beginners find this boat "too tippy". I recognize the importance of technical design numbers and tweeking these in designing boats. However when experienced boaters paddle our muscles are contanting working our balance. Paddle long enough and we don't even notice how much our muscles are working to keep us in balance. So to the designers keep up the good work and for the rest of us keep on paddling. Bob *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On Sat, 14 May 2005 10:42:46 -0800, "Bob Carter" <revkayak_at_aptalaska.net> said: > Paddle long enough and we don't even notice how much our muscles > are working to keep us in balance. Having spent a long time on the water yesterday this can be taken further. What feels stable early in the day may feel unpleasantly tippy as those "well trained" balancing muscles fatigue, and we fade to novice balancing skills.... -- Kirk Olsen *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
My responses on this have largely been pushed by advertising hype I read and conversations I hear. It is quite common to hear something to effect of "With it's AAAA chine this boat has XXXX stability." or "I paddled X hard chine boat and Y soft chine boat and the soft chine boat was much more ZZZZ". My goal is for people to take these broad sweeping statements with a grain of salt. They usually completely ignore important design characteristics that are much more important. It is possible to design a waterplane with stability considerations in mind. I suspect John does, as do I. Cross sectional shape is also an important design parameter which effects the apparent stability of a boat, and we both consider it as we design. But, when people compare the stability of the chined boat and the round bottomed boat, there is a sad tendency to attribute all the apparent performance differences to the most apparent design differences. Boat X and Y may have quite different waterline widths and lengths. One may be heavily rockered and the other with a straight keel. One has a high seat the other a low one. There are lots of characteristics effecting the stability, yet people tend to focus on chines for some reason. For marketers trying to sell boats, this is really convenient. Talking about "hard chines" or "multi-chined" (an almost completely irrelevant term) is a nice strategy to make purchasers feel like they are in the know about esoteric design terms - "Oh, yes, hard-chined, yea, that's good, yes mm-hm." It is obscure enough that non-kayakers don't know what it is, but a quick lesson makes it easy to recognize. And then the marketer can quickly attach any attribute they want to the chine, depending on who they are trying to sell to, and nobody really knows enough to argue with them. If I say my hard-chined boat has good initial stability because of the chines, as long as I made the boat wide enough to feel stable to the uninformed, they will think that I am a genius in my subtle use of good chine configuration and never notice that all I did was make the boat wide. There are lots of good reasons to go with hard chines or a round bottom. Some of them relate to stability. But most of the other reasons are hard for an uninformed paddler to detect on a pond during a 2 minute paddle at a local demo day. People are able to get a quick feel for the stability comfort level, so for a lazy sales rep, who also may not know that much about kayak design, talking about chine shape is an easy way to talk about the design. Unfortunately, people give the sales rep. too much credence and keep repeating what he told them. Next time a sales rep says "The chine shape of this boat gives it good initial stability." say to him: "Chines have nothing to do with initial stability. The shape of the waterplane is more important. " and see what he says. On May 14, 2005, at 8:15 AM, John Winters wrote: > > >> And as a practical matter most kayaks have fairly similar water >> plane shapes, what changes most >> significantly is the waterline width. >> > > Does any of this matter to paddlers? I don't know because I don't > know how accurately paddlers can detect differences in stability. > From a design standpoint a boat is composed of many small things as > well as a few large things melded together to create a distinct > boat. I recall Matt telling me once how he tweaked and tweaked his > designs to reach what he considered the right boat. I bet some > people would say he was just nit-picking but if you talk to the > people who own his boats you will get a much different response. I > suspect the same of Nick's boats and their owners. > > The real question is "Does all this matter here at Paddlewise, in > the Nit-picking capital of paddle sport? :-) *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On May 16, 2005, at 9:06 AM, Nick Schade wrote: > There are lots of good reasons to go with hard chines or a round > bottom. I admit to being befuddled by the technicalities. Therefore I must put my trust in Mother Nature. I will stick with round until someone can show me a fish or aquatic mammal with hard chines. :-) Michael in Tallahassee Strive for mediocrity. Remember, if no child gets ahead, no child will be left behind. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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