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/ ***************************************************************************Received on Fri Sep 15 2000 - 09:24:06 PDT
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