This is a response to an earlier post but that is the drawback of getting the digest. Nick wrote; > Lets take a really weird example: A box shaped hull with two fully > submerged pontoons well down and out to the side. I really good example of what happens below the surface. Which also shows up in Nick's "V" bottom and Box shapes. However, it is the change at the surface that matters and if the hull shape is such that a change occurs at the surface then you get the differences I got in my examples. Robert wrote; > But for it to be a scientific concept it needs some kind of definition > that > people can agree on. How true. If one states that shape has no effect then it must have no effect. If it can bge shwn that it does have an effect then you have a different ball game and need to restate the theory. Maybe to, " Of the factors affecting initia stability CG and beam have the greatest effect and waterplane area and underwater shape have lesser effects. Of course, I subscribe to Karl Poppers definition of a scientific theory. If a peson does not agree with that then we won't have much of a discussion. :-) > It is my contention that kayaks are VERY different from normal craft because > the user shifts his/her weight constantly and this represents a > substantial > part of the weight of the craft. Stability curves are based on the concept > of the paddler sitting rigidly in the boat as it heels which does not > happen > in real life. Actually the idea of shifting weights is not unique to kayaks. Naval architects have to deal with shifting weights in their designs as well. when designing for shifts in cargo (like fliuids) and the effects of damageand sometimes these are rapid and severe. Kayaks are unique, however, in that the shifts are intentional and, apparently, metaphysical in nature :-) You can, with a little effort calculate the stability at any angle of heel with any shift in paddler CG. I don't do it much anymore because I think ("THINK" is the operative word. I certainly don't know) that I can tell what my target paddler would like in stability characteristics. > My contention is that the square cross-section kayak feels initially more > stable than the round kayak because there is much more dynamic resistance > to > the boat rotating around its long axis. It is NOT because of its stability > curve which at 0-2 degrees might be identical to a boat with a rounded > hull. That is certainly one reason. There is a difference between roll resistance and stability, however. The one measures the buoyant forces vs gravitational forces and the other measures the resistance to rotational motion. The two are interrelated but are often treated as separate. You can calculate the roll resistance (period) but I don't know if anyone does in the kayak world. (Having said that I just know some one will tell me how they not only do cross curves of stbility for every boat but roll period studies as well for nboth male and female paddlers) Robert later wrote; > 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. OK, so if I find one you will retract? :-) The issue here was not one of degree but whether shape has any impact on initial stability. It's a nit but then science (and paddlers) picks at nits all the time. > 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? Good point. As I mentioned earlier the hard part is fitting the stability characteristics to the paddlers perceptions. However, once you know how a particular paddler responds to a boat with specific stability characteristics you have a good idea how to design the boat. In this case, however.the discussion started about effects not degree of effects. When I used numbers it is to show that an effect exists. If we shift the discussion to the importance of degrees of effect we have real problems because people react differently. I would contend that people can compare the stability curve to their experience and draw useful conclusions. I can't prove that but from a practical viewpoint designers do it all the time even if it is unconcisious. I design my boats based on paddler responses to prototypes and comparing that to the stability curves and then make alterations to suit. I know of at least two happy customers so one can't say the method is a complete failure. :-) Others may use a different method with equal, better or maybe even worse results. 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/ ***************************************************************************
> How true. If one states that shape has no effect then it must have no > effect. If it can bge shwn that it does have an effect then you have a > different ball game and need to restate the theory. Maybe to, " Of the > factors affecting initia stability CG and beam have the greatest effect and > waterplane area and underwater shape have lesser effects. As I understand stability curves, PRIMARY stability is almost entirely a function of the waterplane (one of which parameters is its width) assuming the center of gravity above the water surface is held constant. The other factor is perhaps the flare of the sides of the kayak at the waterline. If there is a substantial angle, it makes a difference in the calculation of primary stability. (Basically the equations compare the waterplane at 0 degrees of tilt with the waterplane at very small delta degrees of tilt.) Most commercial boats (when loaded) have fairly vertical sides at the waterline. So the calculations would have to be for a loaded boat to prove my point. Some 30lb boat with a V bottom might have a substantial angle at the waterline if it was unloaded (just perched on the water) but the commercial boats that I see do not have much angle at the waterline when loaded. I could design a cross-section that had a major flare right at the loaded waterline but I do not see such boats "out there". Any major flare tends to greatly increase the seam width. I would say that the cross-section that is under the water and stays underwater when the boat is heeled a fraction of a degree has no effect on the PRIMARY stability. The equations do not "care" whether that is a square shape or a round shape or a V shape or some bizarre shape. I thought that was Nick's message and I would agree with him. So I would continue to maintain that the underwater shape (that stays underwater as the kayak heels a fraction of a degree) has no influence on the primary stability. It just does not show up in the equations. (But, to reiterate, I think that it can have an influence on initial stability as perceived by the paddler -- roll resistance or whatever you want to call it.) *************************************************************************** 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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