Nick wrote: >>>The Mariner XL does not have distinctive secondary by my proposed definition. http://www.guillemot-kayaks.com/MarinerXL.gif shows a replotting of the data, traced (crudely) from the web site and then put into Excel. The first derivative is just the slope of the curve at any point. The Magenta line shows this by taking a simple rise-over-run calculation. The Red line does the same thing again to get the rate of change of the slope of the curve.<<<< <snipped lots> Below is much of what my back channel response was to Nick (I'm on the digest and didn't realize he was posting this to the list because I thought the graphs wouldn't show up (but now I see why he posted them to the web rather than just send an attachment to me). To put this all in layman's terms, it looks to me that what the paddler feels is the stiffness (slope of the curve) and when that stiffness starts to decrease significantly is when the paddler is going to get nervous. This pushes the critical angles for "secondary stability" (or whatever we choose to call it) even further to the left than I had imagined (and I have been moving to the left from how I first looked at secondary stability on the curve progressively). First I thought the capsize point had some significance but then John Dawson's explanation convinced me to retreat to near maximum stability as the critical point. Then his explanation of how the test boats capsized uncontrollably beyond the maximum righting moment caused me to believe that the critical point was probably somewhere between the point of maximum stiffness and the maximum righting arm. In other words, if one moved beyond the break in the Pisces' curve's steep upward slope (where the stiffness drops off quickly) they will have gotten into an area (beyond an angle) where you couldn't trust that any momentum would be absorbed by the increasing stability. (If you read it, this is what I was trying to point out with the first ramp example where I suggested it would be hard to roll a ball bearing up and over the break in the Pisces curve without also going all the way over the top). The curves can be found in the XL review at http://www.marinerkayaks.com I think early on I was fooled by my own experience with how far I could lean a hull and still feel secure. I probably wasn't allowing enough for the effect of shifting my CG to keep it over the kayak. This is why I'd like to compare curves that include the shift in center of gravity that a kayaker can make with the existing static curves. I think they would show us better what the kayaker really feels out near the edges of comfort and by measuring the kayaks tilt then also see to what that angle relates to on the stability curve (corrected for CG shift). The ability to lean will probably mean that the area up to the original maximum or a little more are important to the stiffness (on the static stability curve). I think it is the rate of change with a further lean (from the existing balance between the healing and righting forces at a given lean angle) that is important to what the paddler senses. Now I'll comment on some other things that weren't in my e-mail to Nick. I like John's suggestion that the ratio of height at a given angle to the overall height of the curve (or some other ratio) might be valuable. One caveat, too much initial stability will mean that having a good feel of secondary stability is difficult to achieve because one has difficulty getting the kayak up to that angle of tilt without shifting some weight out to one side (over the water) to gain the leverage needed to tilt the stabile kayak in the first place. Thus you must first lower the stability of the kayak by leaning out over the water to get it to tip very far at all (and one result will be reaching the capsize point at a lower angle than might be indicated on the static stability curve). Even then this lean still takes a lot of effort with a loaded stable kayak. The same paddler in a much less initially stable kayak can lift a knee and at the same time shift his weight in the direction that makes the kayak more stable rather than less stable. The paddler in the stable kayak must at some point shift from the "lean to help the kayak to tip" mode to a "balancing the kayak on edge" mode. Probably this happens when the upward slope of the stability curve starts to decrease its rate of climb--this would be looking at the curve that takes into account how he had to shift his center of gravity to get the kayak to tip at all. At this point the kayaker will be able to find only a tenuous balance point between capsizing or having the kayak flop back down hard due to its high initial stability. The high force needed and the unfavorable body lean give this stable kayak a very insecure feel when leaned to higher angles. If anything slips (like the knee on the kneebrace) the kayak may right itself but the paddler may also fall overboard. Contrast this with the initially tippier kayak that was easy to tilt just by lifting a knee and that at the same time allowed the paddler to move his center of gravity in the direction that increases his stability right from the start (rather than have to try to switch the direction of his lean at some tenuous balance point later). If your knee slips with this less initially stable kayak you are still directly over the kayak and the righting moment simply rights the kayak below you. Your center of gravity will shift slightly lower but will not have to shift to the side to regain equilibrium. If you want to lean your kayak it seems pretty obvious it had better also be unstable enough initially that it is easy to do so. If it takes too much effort you just won't lean it much. A tippy kayak with little or no "secondary stability" will require better balance skills than one that is tippy at first but progressively stiffens up. The paddler learns they can trust that the initial looseness will be absorbed by the increasing stability at greater angles of heel and will catch the kayak before it tips too far. BTW the XL is probably the quintessential high secondary stability kayak to a paddler who "knows it when he feels it" so I'd look closely at that curve. I think you can find secondary stability on a static stability graph by looking for a relatively shallow but consistent rise in the stability curve (changes to the slope of the curve is probably what gets described as notchy, and humans appreciate consistency). The further to the right the point of maximum stability is the further you can lean such a kayak before feeling unstable (but this area of feeling unstable may still come well before the maximum righting moment is reached--probably it is where the rate of increase lessens significantly--like the break in the Pisces curve). If however, the curve is too shallow initially you won't pick up the feeling of secondary stability soon enough in the lean to give you a feeling of confidence even if the point of maximum righting moment is further to the right than the kayak with a slightly steeper curve. Essentially if a paddler can't easily tilt the boat far enough to finally feel the secondary stability kick in they will feel that the kayak is unstable even if it ultimately has good secondary stability for those willing to lean it that far. The best feel will come from a consistent slope that doesn't start to round off until at an angle just a little further than the paddler will want to lean it. Given curves with the same height of the maximum righting moment, if the maximum is reached too quickly the paddler might accidentally go beyond it and capsize (or have to brace), if the maximum is too far to the right the slope will be shallower and the stability won't be felt as much as it could have been had the slope been steeper. The higher part of the curve beyond that point will be wasted on most paddlers unwilling to lean far enough to take advantage of it. The kayak that is (or with a gear load becomes) too initially stable will not allow a strongly leaning paddler to feel secure for the reasons given above, but more likely the paddler won't lean it much at all because it is too much work. That paddler is therefore unable to enjoying the benefits that come from leaning the kayak. Oh, yes I disagree with whoever said a low backrest (or being free of the backrest) makes one feel more stable in waves. While there are several advantages to just high enough in a backrest (like rolling ease for a lay back type roll) I always feel far more in control in waves if I can use my spine against a firm backrest to hold the kayak level as I keep my body vertical over the kayak (if the kayak is not too stable to allow this). Now the difference may be that he uses a very stable kayak, and I use a less initially stable one. I think in a very stable kayak in chaotic seas, I would want to keep my head level and let the kayak rock as it wants below me while staying loose at the hips. In a tippy kayak I want to keep it level (as well as myself vertical above it). Awhile back, John Winters made the case for why a stable kayak being tipped by the swell would not destabilize a rigid paddler. While he is right about this I wonder if the resulting tipping side to side of the (stable kayak and paddler) unit (and the momentums that make the tipped paddler's seat of the pants feel be balanced--with his eyes closed) might not also give the paddler a disorientation that could lead to sea sickness or kayak angst as he tries to integrate the horizon with his seat of the pants (inner ears) feelings. Matt Broze http://www.marinerkayaks.com *************************************************************************** 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/ ***************************************************************************
Just to fill in with some of the back channel response to Matt's comments: I am not ready to eliminate the backside of the curve yet. Even if you go over the hump with some momentum, the backside of the curve is absorbing energy and offers at least the theoretical potential of slowing you down. Also if you do stop your momentum by bracing, the stability shown on the backside of the curve will be enough to right you without additional help from a brace. It may be a slow recovery, but it will happen as long as no other forces come to push you back down. The area under the curve from zero to the angle of heel is the potential energy built up that is ready to push the boat back upright. If boat reaches the point but is still moving, it still has kinetic energy left to absorb. If the area remaining on the curve is less than the kinetic energy of the rotating boat, it will go over, otherwise it will stop. A boat with a lot of area under the backside of the curve will be able to absorb a tipping hit better and will require less bracing energy to stop the tendency to capsize. This points to the backside of the curve being more important in dynamic situations where some capsizing impulse has to be absorbed. Nick >At 3:51 AM -0800 11/21/00, Matt Broze wrote: -snip- >To put this all in layman's terms, it looks to me that what the paddler >feels is >the stiffness (slope of the curve) and when that stiffness starts to >decrease significantly is when the paddler is going to get nervous. This >pushes the critical angles for "secondary stability" (or whatever we choose >to >call it) even further to the left than I had imagined (and I have been >moving >to the left from how I first looked at secondary stability on the curve >progressively). First I thought the capsize point had some significance but >then John Dawson's >explanation convinced me to retreat to near maximum stability as the >critical >point. Then his explanation of how the test boats capsized uncontrollably >beyond >the maximum righting moment caused me to believe that the critical point was >probably >somewhere between the point of maximum stiffness and the maximum righting >arm. In other words, if one moved beyond the break in the Pisces' curve's >steep upward slope (where the stiffness drops off quickly) they will have >gotten into an area (beyond an angle) where you couldn't trust that any >momentum would be absorbed by the increasing stability. (If you read it, >this >is what I was trying to point out with the first ramp example where I >suggested it would be hard to roll a ball bearing up and over the break in >the Pisces curve without also going all the way over the top). -- Nick Schade Guillemot Kayaks 824 Thompson St, Suite I Glastonbury, CT 06033 (860) 659-8847 Schade_at_guillemot-kayaks.com http://www.guillemot-kayaks.com/ >>>>"It's not just Art, It's a Craft!"<<<< *************************************************************************** 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 don't disagree that there might be some minor benefits to a capsizing paddler to have a large backside of the curve (or a even a flatter top of the curve that might provide a slight warning). What I'm saying is with regards to what an expert considers "secondary stability" in a kayak these things are irrelevant, not that they are irrelevant all the time and for all reasons. Further, I think an expert would prefer to keep the curve increasing at a constant or increasing rate up to a point out beyond where he wants to lean a kayak (without having to rely on a brace) rather than have the curve be steeper and then break at an angle less than the angle he wants to be able lean it (the steeper curve would have to break earlier--at the "risk" point--in order to flatten out the top of the curve and keep the same maximum righting moment). The designer's task is to find the shape that will give the most security for the paddler (the kayak is aimed at) and still keep the drag of that kayak as low as possible given that level of security. -----------original message-------- >>>>>>.Date: Tue, 21 Nov 2000 16:04:19 -0500 From: Nick Schade <schade_at_guillemot-kayaks.com> Subject: Re: [Paddlewise] 2ndry stability (and more) Just to fill in with some of the back channel response to Matt's comments: I am not ready to eliminate the backside of the curve yet. Even if you go over the hump with some momentum, the backside of the curve is absorbing energy and offers at least the theoretical potential of slowing you down. Also if you do stop your momentum by bracing, the stability shown on the backside of the curve will be enough to right you without additional help from a brace. It may be a slow recovery, but it will happen as long as no other forces come to push you back down. The area under the curve from zero to the angle of heel is the potential energy built up that is ready to push the boat back upright. If boat reaches the point but is still moving, it still has kinetic energy left to absorb. If the area remaining on the curve is less than the kinetic energy of the rotating boat, it will go over, otherwise it will stop. A boat with a lot of area under the backside of the curve will be able to absorb a tipping hit better and will require less bracing energy to stop the tendency to capsize. This points to the backside of the curve being more important in dynamic situations where some capsizing impulse has to be absorbed. Nick<<<<<<< Matt Broze http://www.marinerkayaks.com *************************************************************************** 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 can agree with that. If there is something quantifiable as "secondary" stability it happens at or before the highest point on the stability curve. At 11:05 PM -0800 11/22/00, Matt Broze wrote: >I don't disagree that there might be some minor benefits to a capsizing >paddler to have a large backside of the curve (or a even a flatter top of >the curve that might provide a slight warning). What I'm saying is with >regards to what an expert considers "secondary stability" in a kayak these >things are irrelevant, not that they are irrelevant all the time and for all >reasons. -- Nick Schade Guillemot Kayaks 824 Thompson St, Suite I Glastonbury, CT 06033 (860) 659-8847 Schade_at_guillemot-kayaks.com http://www.guillemot-kayaks.com/ >>>>"It's not just Art, It's a Craft!"<<<< *************************************************************************** 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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