I've read that if foldables or hardshells are partially filled with water then the sponsons make the foldable more stable. That makes sense from a physics point of view. The foldable would be underdamped and the hardshell very damped! I've not seen significant deformation in the Klepper expedition sponson when submerged so not convinced that sponson deformation improves stability. PeterO *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Peter wrote; >I've read that if foldables or hardshells are partially filled with water >then the sponsons make the foldable more stable. Only if the hard-shell does not have reserve buoyancy or a pod or sea sock. The flotation in concert with the shape provides stability. So far I have seen no support for the argument stability have seen a number of studies on this topic and performed some myself with no suggestion that the hull materials have any unique properties. >That makes sense from a >physics point of view. The foldable would be underdamped and the hardshell >very damped! I've not seen significant deformation in the Klepper expedition >sponson when submerged so not convinced that sponson deformation improves >stability. One has to include free surface effects in any discussion of stability in a flooded state and one can reduce free surface effects in boats of any construction materials. In all of these discussions one must compare apples and apples and not confuse the occasional Valencia with the Granny Smiths. :-) Peter also commented on energy absorption (in another post) of the flexible hull and this may have some merit. Typically, the kayaks have so little mass that the energy from a wave tends to sweep them along rather than doing serious damage unless the boat gets capsized and the wave comes crashing down on it. In this circumstance one has to factor in the thee capsizing moment of the boat. A boat with high initial stability tends to capsize more easily in a breaking beam sea than one with low initial stability. If everything but hull construction remained equal, one might find a difference. Not sure, if it would matter much. Perhaps one of you bored engineers could do the calculations. Cheers, John Winters Redwing Designs Specialists in Human Powered Watercraft http://home.ican.net/~735769/ *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
John Winters wrote: > > Peter wrote; > > >I've read that if foldables or hardshells are partially filled with water > >then the sponsons make the foldable more stable. > > Only if the hard-shell does not have reserve buoyancy or a pod or sea sock. > The flotation in concert with the shape provides stability. So far I have > seen no support for the argument stability have seen a number of studies on > this topic and performed some myself with no suggestion that the hull > materials have any unique properties. I am not sure I agree. I have paddled hardshells that were half filled with water in the cockpit area while having bulkheads and waterproof compartments on the ends fore and aft. The boats felt real unstable...that is one of the reasons why there is such a rush to pump a kayak out and why after a capsize in a hardshell with that configuration, paddlers often are prone to capsize again (although certainly nervousness, fatigue, fear, etc. stiffens the person up and contributes to the re-capsizing). In a folding kayak, with its flotation along its perimeter along the sides, does not seem to feel at all unstable under the same circumstances. It is critical however in a folding kayak, to have air bags in the end of the boat as well in order to reduce the amount of water that enters, which is true of any hardshell without bulkheads or plastic kayaks whose bulkheads generally are susceptible to leakage and blowout. Which bring me to something, John, that you said in your early posting, the one I kidded you were using to press you shameless attack on wearing life jackets (when not killing babies or signing suicide pacts) :-). You said "Many paddlers achieve this result by packing their boats with waterproof bags along the interior sides of their rigid boats." I have never seen anyone do that in a hardshell. Normally you have no way to tie in waterproof bags around you or along the sides on the insides. That would require tie-in points. I have never seen a production kayak come so equipped except one make...Bavaria brand hardshells made of fiberglass. They have or did have sponsons tied on the inside along the sides. The company glassed in little pieces of nylon twine to tie in the sponsons. These sponsons ran most of the length of the sides inside and were supplied by the same company that made them for Klepper, which is not surprising as both kayak companies are located in Rosenheim, Germany. I have never seen any other hardshell so equipped. Have you really? Is anyone on this list who paddles a hardshell jamming drybags bags around them on the inside along the sides? ralph diaz -- ----------------------------------------------------------------------- Ralph Diaz . . . Folding Kayaker newsletter PO Box 0754, New York, NY 10024 Tel: 212-724-5069; E-mail: rdiaz_at_ix.netcom.com "Where's your sea kayak?"----"It's in the bag." ----------------------------------------------------------------------- *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Ralph wrote; (SNIP) >> Only if the hard-shell does not have reserve buoyancy or a pod or sea sock. >> The flotation in concert with the shape provides stability. So far I have >> seen no support for the argument stability have seen a number of studies on >> this topic and performed some myself with no suggestion that the hull >> materials have any unique properties. > >I am not sure I agree. I have paddled hardshells that were half filled >with water in the cockpit area while having bulkheads and waterproof >compartments on the ends fore and aft. This provides a good example of what I spoke about regarding apples and fruits of another color. The stability in a flooded boat largely depends upon athwartships flotation. The point I tried to make had to do with how boatsa with similar floatation had similar stability in the flooded sate regardless of hull materials. End tanks etc. do not add appreciably to flooded stability particularly with large free surface effects in the cockpit area. My discussion had to do with whether construction materials affected flooded stability (I am not sure they do to any noticeable degree) not whether the location of buoyancy affected flooded stability for that should I take as a given.. > >Which bring me to something, John, that you said in your early posting, >the one I kidded you were using to press you shameless attack on wearing >life jackets (when not killing babies or signing suicide pacts) :-). >You said "Many paddlers achieve this >result by packing their boats with waterproof bags along the interior >sides >of their rigid boats." I have never seen anyone do that in a >hardshell. Perhaps this has to do with ignorance. I do this on my boats when I do not have a sea sock or pod. I use a multitude of small gear bags. Some boats, may lack enough beam for this kind of thing but foam sheathing glued to the hull sides to reduce volume also works. Reducing the floodable volume in the boat ios the objective. I doubt if most paddlers understand or consider the effects of free surface in their boats. Most production boats don't come with methods to attach side bags, sea socks, or pods and that seems to me a serious omission. Of course, if one uses a sea sock or pod the internal bags become redundant. I cannot say why builders of hardshells fail to recognize the importance of flooded stability. As Ralph points out, all they need do is look at folders to see them in use and apply the same principle. Please don't call internal buoyancy bags sponsons. Sponsons attach to the outside of the boat. Buoyancy bags and tanks attach inside. Cheers, John Winters Redwing Designs Specialists in Human Powered Watercraft http://home.ican.net/~735769/ *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
John Winters wrote: > >Which bring me to something, John, that you said in your early posting, "Many paddlers achieve this > >result by packing their boats with waterproof bags along the interior > >sides > >of their rigid boats." I have never seen anyone do that in a > >hardshell. > > Perhaps this has to do with ignorance. I do this on my boats when I do not > have a sea sock or pod. I use a multitude of small gear bags. Some boats, > may lack enough beam for this kind of thing but foam sheathing glued to the > hull sides to reduce volume also works. Reducing the floodable volume in > the boat ios the objective. I doubt if most paddlers understand or consider > the effects of free surface in their boats. Most production boats don't > come with methods to attach side bags, sea socks, or pods and that seems to > me a serious omission. > I cannot say why builders of hardshells fail to recognize the importance of > flooded stability. As Ralph points out, all they need do is look at folders > to see them in use and apply the same principle. Well, as I pointed out, Bavaria does or did that (I haven't seen their kayaks in awhile). All it takes is to glass in some nylon twine or webbing straps every 2 feet or so along the sides inside the kayak. Sponsons than can be added for that desired effect. I found very interesting your comments the relative instability of bouyancy located at the ends of the boat and the free surface inside the cockpit. > > Please don't call internal buoyancy bags sponsons. Sponsons attach to the > outside of the boat. Buoyancy bags and tanks attach inside. I would love to and Webster's dictionary (and I am sure all naval architecture glossaries of terms do too) but sometimes things get called something and the label sticks and we just have to start using the term in the corrupted way. :-) But I do appreciate that there is a distinction...anything to get away from the infamy the term has gotten because of your fellow Canadian. I want to return to the question of stability of buoyancy aids in folding kayaks. And then I am going to leave it because no one seems to be applying any real science here other than general, unspecified references to Archimedes. What I have to say comes from observation, which has to count for something. Many years ago, long before the idea of doing anything with folding kayaks ever even entered my mind, I observed what happened with folding kayaks in two situations that I can only attribute to something to do with the buoyancy aids inside their soft skins. In the first one, I was on a group paddling trip. I was directly alongside a couple in a double Klepper, when the fellow decided to stand up (he later said he wanted to stretch his legs). He wasn't very well coordinated and he tipped the kayak over. I was about 30 feet directly to the side of his Klepper and all I saw was black bottom and keel strips including the ones on both chines, i.e. the kayak was almost completely on its side. He fell out and the kayak righted itself. The woman in the front had only paddled once before and hadn't the faintest idea of what a bracing stroke was...so it wasn't she that righted the kayak. The kayak wound up right side up with the woman having a stunned look on her face. I am not sure how another kayak, a non-buoyancy aided flexible skin one, would behave in similar circumstances. Another observation, again from my pre-evangelistic folding kayak days. I was paddling a double Klepper with my wife in our first months of kayaking experience. We went out through surf off of Brighton Beach in NYC. I was fiddling with a rudder lifter that I had added to the kayak (they lacked them then and it was the first thing that I ever innovated for the kayak that eventually got me interested in writing about this particular species of kayaks) and did not pay heed to what was happening with the waves coming in on us. The kayak completely broached, so much so that our kayaking friends on the beach with many years of kayaking and kayaking teaching experience fully expected our Klepper to windowshade all the way back to the beach. It didn't. While the wave side of the kayak rose way up and we were well over, something kicked in, without any bracing whatsoever by either of us...my wife didn't know how to brace and both my hands were on my rudder lifter line. Will folding kayaks flip? Of course they can and do. But it takes a lot, and my observation tells me that the bouyancy aids inside are kicking in at some point to some degree enough to say that the phenomenon does exist. ralph diaz -- ----------------------------------------------------------------------- Ralph Diaz . . . Folding Kayaker newsletter PO Box 0754, New York, NY 10024 Tel: 212-724-5069; E-mail: rdiaz_at_ix.netcom.com "Where's your sea kayak?"----"It's in the bag." ----------------------------------------------------------------------- *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
In a message dated 4/12/99 2:28:26 AM Pacific Daylight Time, rdiaz_at_ix.netcom.com writes: << Will folding kayaks flip? Of course they can and do. But it takes a lot, and my observation tells me that the bouyancy aids inside are kicking in at some point to some degree enough to say that the phenomenon does exist. >> Sounds fun, Ralph. :-) I respect the depth and breadth of your experience, but nothing *inside* a dry boat will right you from a sharp heel (well, maybe a coupla hundred pounds of lead along the gunnels:-)). Not the sponsons (sorry -- a buoyancy aid is a British PFD / life jacket) or your tackle box or the PFD you're not wearing. The external shape of the chine area of the boat is responsible for that function. The sponsoned side of some folders looks much more like a liferaft than any hard-shell kayak I've ever seen, having a short-radius convex shape. Perhaps that "P" shape on the side does the trick, but I still think it is, in the case of the Klepper, also due to lots of beam. I don't dispute that sponsoned boats have more secondary stability. My Khats-S does, but as I stated before, I have still seen no evidence or convincing argument that it is not due solely to the hull shape. I don't mean to down-talk folders, but I think accuracy in making claims is important. Folders are not super stable because they're folders or because they're skin boats. The original non-spononed Khatsalano is very tender, as are the other non-folding, Greenland and Aleutian skin boats that I have paddled. In fact, I've paddled both the plywood and skin Greenland yaks built from Christopher Cunningham's _Sea Kayaker_ plans, and the skin boat was *way* more tender than the hard-shell. Does that prove anything? No, because the widths and shapes of the two kayaks are not quite the same. I appreciate all the input on this subject, especially from the boat designers. I need to go back and reread the posts about the flooding question, because I always assumed that the sponson would increase edge buoyancy and thus righting moment, beyond that contributed by hull shape, when flooded. I reasoned that, since there was water surrounding the bottom and sides of the chamber, the chamber is displacing water toward the center and ends of the craft. Should this not add buoyancy and righting moment to the edge of the craft, or does the entrapped air know its inside the boat and refuse to do its duty? I would love to see a clear analysis of this. Matt, John, guys? Inquiring minds of limited reasoning (mine) need to know. Thanks Harold So. Cal. *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Ralph wrote; (SNIP) > >I would love to and Webster's dictionary (and I am sure all naval >architecture glossaries of terms do too) but sometimes things get called >something and the label sticks and we just have to start using the term >in the corrupted way. :-) But that don't make it right. I will go away and pout ;-( >But I do appreciate that there is a distinction...anything to get away >from the infamy the term has gotten because of your fellow Canadian. OH YES. > >I want to return to the question of stability of buoyancy aids in >folding kayaks. And then I am going to leave it because no one seems to >be applying any real science here other than general, unspecified >references to Archimedes. What I have to say comes from observation, >which has to count for something. It does but observation can deceive as well as enlighten otherwise magicians could never make a living. (SNIP) >The kayak wound up right >side up with the woman having a stunned look on her face. I am not sure >how another kayak, a non-buoyancy aided flexible skin one, would behave >in similar circumstances. Assuming the same shape and circumstances a hard-shell would probably have done the same thing. I suspect the standee pushed against the hull creating a righting moment as he fell similar to what happens in a canoe when a paddler falls out. Most people fall out of canoes and the canoe ends up floating away right side up even with another paddler aboard.. (SNIP) >It didn't. While the wave >side of the kayak rose way up and we were well over, something kicked >in, without any bracing whatsoever by either of us...my wife didn't know >how to brace and both my hands were on my rudder lifter line. >Will folding kayaks flip? Of course they can and do. But it takes a >lot, and my observation tells me that the bouyancy aids inside are >kicking in at some point to some degree enough to say that the >phenomenon does exist. Possibly the wave action at the right time or instinctive movements on your part. I don't know since I didn't observe it first hand but to say that the air tanks inside a boat affect stability in the unflooded condition contradicts the laws of Newtonian Physics. While what you observed certainly did occur, one has to recognise that the laws of physics do not change just because a boat can fold up. Water is basically very stupid and cannot tell if a boat is an aircraft carrier or a kayak. Unlike justice, water really is blind and treats all that float in it with equality. To put this kind of thing on a scientific plane I would gladly enter the offsets of any folder that anyone wishes to send me into my computer and calculate the stability both in an unflexed rigid and in a flexed mode to see what happens. I did it today with a "folder " I designed and the unflexed boat had greater stability. Keep in mind that my program can calculate both flooded and intact stability so we have a lot of flexibility. Cheers, John Winters Redwing Designs Specialists in Human Powered Watercraft http://home.ican.net/~735769/ *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
-(SNIP) >I appreciate all the input on this subject, especially from the boat >designers. I need to go back and reread the posts about the flooding >question, because I always assumed that the sponson would increase edge >buoyancy and thus righting moment, beyond that contributed by hull shape, >when flooded. I reasoned that, since there was water surrounding the bottom >and sides of the chamber, the chamber is displacing water toward the center >and ends of the craft. Should this not add buoyancy and righting moment to >the edge of the craft, or does the entrapped air know its inside the boat and >refuse to do its duty? I would love to see a clear analysis of this. Matt, >John, guys? Inquiring minds of limited reasoning (mine) need to know. I hope this helps. First, consider a boat heeling say 10 degrees. As it heels the center of buoyancy (CB) shifts outboard. If the Center of Gravity (CG) remains constant and the CB lies outside a line drawn vertically through the CG a righting moment results that acts to bring the boat back to level. Now, take the paddler out and replace his weight with water (Ignore for the time being the relative heights of the center of gravity) . As the boat heels the water flows downhill seeking its own level and shifting the center of gravity thus reducing or even wiping out the righting moment created by the shifting center of buoyancy and the boat now has less stability Now, add internal buoyancy tanks down the side. If the center of gravity remains fixed as in the first example the righting moment will equal the righting moment of the first example regardless of the presence of the tanks since the tanks do not affect the shift in the CB. Now replace the paddler with water of the same weight and heel the boat. This time the water cannot flow as far downhill and seeks its own level closer to the centerline of the boat. If the tanks encompass enough volume, the CB will lie outside the new center of gravity providing a positive righting moment. Of course, the tanks might also reduce the amount of water to enter the boat with a further positive benefit. Now let us repeat the example but this time we will use a bulkhead running fore and aft at the same location as the air tanks. The bulkhead though will have 3/8" holes in it. As the boat heels the water will flow more slowly through the holes than if unconstrained and the boat will actually have more stability even though there are no sealed tanks. Now we will replace the water with ball bearings equal in weight to our paddler or water. This time when the boat heels the ball bearings are held in a similar position to the water in the boat with tanks. The result is a boat with the same stability as a tanked boat. By this experiment we can see that the stability effect of tanks has to do with its restriction of water flow in the boat . The material of the boat or tanks matters not so long as it does its job in restricting the flow of water to the down side of the boat. Anti roll tanks on ships use the principle of the holed bulkhead to reduce roll in ships. Now, imagine another boat. with no water in it that has "x"righting moments. The righting moment is the product of the righting arm (distance for the vertical projection of the CG and the CB. If we add water and it sloshes to the low side we get both a shift in weight and the effect of the shift (inertia). This we call free surface effect. The farther the water has to travel the greater its inertia and so we can see how restricting its travel reduces the capsizing moment of water in the boat. I have simplified this enormously but hope it helps. Entire books devoted to stability and damage stability in particular have been written. Cheers, John Winters Redwing Designs Specialists in Human Powered Watercraft http://home.ican.net/~735769/ *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
In a message dated 4/12/99 5:40:59 PM Pacific Daylight Time, 735769_at_ican.net writes: << I have simplified this enormously but hope it helps. Entire books devoted to stability and damage stability in particular have been written. >> Yes, you did, and it helped greatly. My thinking was right, but my mind was just tongue-tied. :-) Thanks John. Harold *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
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