I didn't have much time when I wrote my last post on this topic so left out a lot. Some of you may find this interesting Most articles about sea kayak stability have a diagram showing a wide kayak that heels to follow the slope of the wave and a narrow kayak that remains more upright. The text with the diagram usually suggests that the same stability that makes a boat feel comfortable in flat water contributes to capsizing in waves. Unfortunately this often misleads readers. The water molecules in a non breaking wave travel roughly in circular orbits around the center of the wave. This results in centrifugal force that, in conjunction with normal gravitational force produces an apparent gravitational force acting normal to the wave surface. Some call this the "local" gravity and I am indebted to Bruce Winterbon for the term "apparent gravity" which makes more sense to me. The combined forces cause the "shape" of the wave. A blindfolded paddler in a boat lying parallel to the wave will not sense any heeling moment (although they may sense some motion) even though the wave surface may have a significant slope. This causes a problem for paddlers. Even though they may sense no heel they perceive heel visually by observing the horizon. In response they heel the boat into the wave and in so doing actually create a capsizing moment where none existed. You can test this phenomenon best by observing a plumb bob against the horizon while sitting still in a life raft. I believe that this phenomenon may have contributed to capsizes caused by what some researchers called "kayak angst" suffered by Inuit who paddled for long periods of time. The paddler would sense heel visually, correct against the apparent gravity and capsize. No doubt modern paddlers could have the same problem. Once the wave breaks, the situation alters as the rotational motion of the water molecules changes to translational motion which can cause a capsizing moment proportional to the righting arm of the boat. Note the term righting arm rather than righting moment. Righting moment is the product of the righting arm and displacement and acts to orient the boat to the surface. The righting arm is a function of the boat's shape and center of gravity without consideration for displacement. Of two boats with the same righting arm, the heavier will have more stability or righting moment. This difference between righting arm and righting moment leads to another interesting and sometimes confusing aspect of stability. Due to the centrifugal force, the apparent gravitational force varies with location on the wave. For example, the apparent gravitational force on the trough exceeds that of the force at the crest. Ocean sailors may have some familiarity with this for boats heel more on the crests than in the troughs (more than the variable wind force causes). I have read some suggestions that, given a large enough and steep enough wave, that the centrifugal force could counteract all gravitational force resulting in no righting moment even while fully upright. This explains the greater capsize vulnerability of boats in breaking beam seas and why a strong brace becomes such a useful weapon in avoiding capsize for any boat. So, returning to my earlier comment about the diagrams in magazines etc., a more appropriate diagram would show the two boats in breaking seas not smooth seas and the explanation should point out that the increased capsize moment only applies to breaking seas and/or confusion resulting from horizon and apparent gravity providing confusing signals to the paddler about her orientation. John Winters *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Dave wrote: (SNIP) > > Once the wave breaks, the situation alters as the rotational motion of the > > water molecules changes to translational motion which can cause a capsizing > > moment proportional to the righting arm of the boat. Note the term righting > > arm rather than righting moment. Righting moment is the product of the > > righting arm and displacement and acts to orient the boat to the surface. > > The righting arm is a function of the boat's shape and center of gravity > > without consideration for displacement. Of two boats with the same righting > > arm, the heavier will have more stability or righting moment. > > Not following you here, John. Are you thinking of a **fully broken** wave, > which pushes a wall of soup horizontally towards the paddler? I don't think > that's what you mean. As soon as the wave breaks the rotational motion in the top of the wave breaks down and becomes translational. Once the motion of the boat becomes translational gravity acts "normally" (i.e. as on a surfer) and the boat begins to plunge towards the trough. At that point the action on the boat becomes rotational and one leans into the wave to remain upright. The forces vary with the type of breaking wave. For example, a plunging breaker has higher velocities and more volume in the breaking portion than a spilling breaker. We most often see plunging breakers breaking on shore and spilling breakers in open water. Open water plunging breakers may create the most danger for paddlers possibly because paddlers don't expect them. I suspect most paddlers have little trouble with spilling breakers on small waves up to four feet or so. Once they start getting big even a spilling breaker can cause problems for inexperienced paddlers. In an ad for a safety device the picture shows the boat in a two foot spilling breaker. Sort of like those pictures of rescues done in calm water. Everything seems easy under the right circumstances. > > I am thinking you mean a wave **in the act of breaking** -- the basis for the > following comments: In my limited experience, the breaking part of the wave > seems to hammer my up-wave side downward -- "catching" that edge of the boat, > as it were -- and attempting to heel the boat into the breaking wave face. Of > course, once this happens, my upper body leans toward the breaking face, and my > paddle brace goes out, forming a stable configuration as I slide shoreward > along with the wave, in the wall of soup of a fully broken wave. Yes. Once the wave has broken the water begins to fall down the face. at that point you would experience a downward component. One could or should differentiate between "breaking" and "broken" waves. In open water as opposed to surf the broken wave doesn't seem to cause much aggravation. In suspect that surf soup becomes a whole 'nother problem due to the interaction with the bottom. > > > This difference between righting arm and righting moment leads to another > > interesting and sometimes confusing aspect of stability. Due to the > > centrifugal force, the apparent gravitational force varies with location on > > the wave. For example, the apparent gravitational force on the trough > > exceeds that of the force at the crest. Ocean sailors may have some > > familiarity with this for boats heel more on the crests than in the troughs > > (more than the variable wind force causes). > > Wait. Isn't this difference in "apparent" gravitational force more due to the > "free fall" nature of the boat's motion as the crest passes underneath, > contrasted with the wave's acceleration of the boat upwards as the hull passes > the trough? In other words, this is similar to the loss of apparent gravity I > experienced as a kid when my dad would take our old '48 Ford over a bump, and > as the car fell underneath my young tush, I "felt" less upward force from the > car seat -- a not yet free fall situation, but less force between me and the > car seat. Or, perhaps the effect I describe supplements the one you describe? Supplements it. Elevators, swings, etc. produce the same effect. Froude made a device to test this using something called a "Scotch yoke" (I had to look this up in a book on mechanical linkages) that mimics the motion on a wave. Damned clever those Victorians. If I recall correctly NASA mimicked zero gravity this way but in airplanes. Flew them up high and then let the bottom drop out. Makes my stomach turn and perhaps may help to explain why we get sea sick on big smooth waves. Cheers, John Winters Web site address http://home.ican.net/~735769 *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Onen question considering the stability on non-broken waves: The theory says that the local gravity acts perpendicular to the surface of the water, ad said previously in this discussion. If this is correct, then how is it possible to surf on a non-broken wave? My idea of a surfing situation has always been that there does exist a gravity component in the direction of movement of the boat. This gravity component should accelerate the boat. Or is it just that the rotation movement if the water results in a kind of "local flow" on the waves surface that diminishes the relative velocity of the boat with respect to the water? I am puzzled.... Greetings, Merijn Wijnen ****************************** Merijn Wijnen Vinkenhofje 8 5613 CN Eindhoven The Netherlands Tel.: 040-2939991 (job: 040-2650539) Fax: same as tel., call before sending or try twice E-mail: Home: merijn_at_music.demon.nl Job: m.wijnen_at_ind.tno.nl Web-site: http://www.music.demon.nl *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Merijin wrote: > The theory says that the local gravity acts perpendicular to the surface of the water, ad said previously in > this discussion. > If this is correct, then how is it possible to surf on a non-broken wave? My idea of a surfing situation has > always been that there does exist a gravity component in the direction of movement of the boat. This > gravity component should accelerate the boat. > Or is it just that the rotation movement if the water results in a kind of "local flow" on the waves surface that > diminishes the relative velocity of the boat with respect to the water? > I am puzzled.... Understandable. Doesn't seem intuitive. In the discussion on stability we confined ourselves to boats traveling across the face of the wave and not with it hence the concern over athwartships righting arms and moments. When a boat travels with the wave we must add the forward speed of the boat and its related forces. The net effects of dynamic lift, buoyancy, drag (as the boat moves through the water) produces a force tangential to the face of the wave. If the resulting equilibrium speed is the same as that of the wave the surfing boat remains on the wave but still moves fast relative to the water molecules in the wave. "Weight" always acts "down" and the resultant of weight and centripetal force act towards the center of the non breaking wave. Maintaining equilibrium speed generally requires using a fair amount of energy and one has to paddle pretty hard to surf non breaking waves. If you do not paddle you cannot surf non breaking waves. Breaking waves present a different problem and one has to work to avoid being swept ahead at the translational speed of the breaking wave crest which exceed twice the wave speed (or, if into these things, work to maintain an ideal position on the breaking wave face. You may have noticed how short surfing lasts in deep water waves. Because waves come in different sizes moving at different speeds the large surfable waves resulting from the merging of two or more waves disappear as the faster wave moves past and the wave size diminishes right out from under you. Cheers, John Winters Web site address http://home.ican.net/~735769 *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced/forwarded outside PaddleWise without author's permission Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
John wrote; (SNIP) > > Okay. I assume that means that the tests were done on flatwater (i.e. no > waves or current). Always the case which is why they are called static curves. > If that variable remains constant then what Nick is suggesting is that the > height of the center of gravity is the primary cause for the differences > in initial stability. (SNIP) Nick and I have no disagreement there. The issue we are discussing is the effect of shape. > When plotting the stability curves are the seat height and paddler height > considered as variables? Always. That is why in doing comparisons of parameters we would like to reduce the number of variables to see the true (or as nearly true as possible) effect of the parameter under consideration. > For someone just learning to kayak they may not have developed the muscle > control to make the subtle weight shifts so a boat with high initial > stability so it's going to be more forgiving if they lean a bit too far. > (SNIPS) That is always the problem - fitting the boat to the paddler. Some believe the paddler should learn to fit the boat. Some believe the boat should fit the paddler. Some believe the boat should urge the paddler to learn more. Some believe that not all paddlers want to learn more. If I had the answer I would be rich. Well, maybe not rich. Just better off. 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/ ***************************************************************************
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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