"Does anyone have any feel for how much load such tethers must take?" In the case of paddle tethers it must take whatever load the paddle itself can take. With either paddle or personal tethers the loads will equal whatever force is required to suddenly accellerate the mass of your body, moving against water resistance, pulled by a lunging object weighing 100 to 200 pounds (for a loaded single). The lunging kayak could have hundreds of pounds of water pressure acting on it. Because of the hammer-blow like nature of the forces that might act on this tether if you and your boat are thrown in differrent directions I would make a wild guess at thousands of pounds of force (momentarily). I would not be surprised if the forces involved can be similar to the forces involved in leader falls. Certainly it would not be overdoing it to use gear as strong as climbers use to make a personal tether. Sailors, expecting to be dragged through the water by a multi-ton boat, use very strong tethers. This kind of tether could snap a light paddle shaft in half easily. Does this sound right to someone who knows more specifics about the physics of this problem? Nick Lyle *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
Whitesavage & Lyle wrote: > > "Does anyone have any feel for how much load such tethers must take?" [snip] > With either paddle or personal tethers the loads will equal whatever > force is required to suddenly accellerate the mass of your body, moving > against water resistance, pulled by a lunging object weighing 100 to 200 > pounds (for a loaded single). The lunging kayak could have hundreds of > pounds of water pressure acting on it. Because of the hammer-blow like > nature of the forces that might act on this tether if you and your boat > are thrown in differrent directions I would make a wild guess at > thousands of pounds of force (momentarily). I would not be surprised if > the forces involved can be similar to the forces involved in leader > falls. Certainly it would not be overdoing it to use gear as strong as > climbers use to make a personal tether. Sailors, expecting to be > dragged through the water by a multi-ton boat, use very strong tethers. > This kind of tether could snap a light paddle shaft in half easily. > > Does this sound right to someone who knows more specifics about the > physics of this problem? Well, yes and no. In a leader fall, the climber will have fallen a distance twice the distance from the last point of protection to the climber before he/she falls, before the rope begins to arrest his/her descent. Because modern climbing ropes elongate as they are loaded, the total energy of the falling climber is dissipated over quite a bit of rope elongation. This has the effect of reducing the peak forces generated in the system, a feature older ropes (Goldline, manila, etc.) did not have. There is nothing comparable to this amount of energy (the falling climber) in a yakker-paddle boat system under the likely scenarios sea kayakers would encounter in water (excluding big surf). If you used a modern climbing rope as part of the leash system, it would perform similarly to help reduce the peak forces. In addition, if you are in a water environment, and your body is *not fixed* (anchored) to something solid, then your body will respond to the tug of a leash by moving, thus spreading out the dissipation of the energy of the (moving) boat over more time, and making the peak forces much less than those in a leader fall, in which the belayer's end of the rope is "anchored" through a belaying device. ("Anchored" except in the sense that the belaying device is designed to allow more rope to slide through it when the force reaches a specified figure, making the belayer's end of the rope *not* rigidly anchored.) Nick, I suspect "thousands of pounds of force" is probably way too high an estimate even for the force generated in a worst-case "over the falls" scenario for a surf kayaker, because there is nothing rigid to "anchor" the yak or the paddler to, and the kayaker can only fall a few feet. Is "over the falls" the scenario you are envisioning, or do you have something else in mind? In any event, I believe it is widely agreed that leashes on kayaks used in **big** surf constitute a death wish (by entrapment or strangulation), and that leashes in even moderate to small surf are a very bad idea, *for kayakers.* Board surfers, of course, use bungie systems on their ankles in big surf, and do OK, because the dynamics of the surfer-bungie-board system are much different than the dynamics of a kayak-bungie-paddler system. I believe most of us are concerned with the forces generated if we capsize in a tide rip or in gnarly wind waves. Under these conditions, the yakker-paddle-boat system will not generate anything like the forces in a climbing rope in a leader fall. Mainly, we are dealing with the forces when somebody capsizes or falls out of the boat. This is because the yak is never moving faster than a few knots, and the yakker falls only a foot or two into the water, so falling overboard can not generate huge forces, unlike with the falling climber, who might be travelling 50 - 60 mph (!!) in a leader fall before the climbing rope begins to arrest his/her fall. Finally, a bungie or "telephone cord" leash, such as those discussed for a tether or paddle leash, will elongate lots at very low load (way less than a hundred pounds of tension), further reducing the peak force. OTOH, if a leash of wire cable is used, all bets are off -- wire does not elongate. -- Dave Kruger Astoria, OR *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************
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