As I mentioned earlier, I think most rudders and skegs could use some improvement. Many skegs fit rather loosely in their trunks and have rough or irregular edges. I believe the vibration (singing skeg or rudder) that some develop usually results (not sure that other causes of vibration apply to kayaks given their low speeds) from vortices shed off the trailing edge. Sailors cured this problem by shaping the leading and trailing edges carefully. In all my years of boat building and sailing I never found a case of singing rudder or centerboard that reshaping the edges could not cure. In general one wants a leading radius of around 1.5% of the chord length for rudders. I suspect the same would apply to skegs where the boat has a tendency toward a lot of leeway (in excess of 7 degrees). Sharp or squared off at an angle trailing edges work nicely. Of course, one should have a rigid rudder head. To me the floppy rudder heads cause a lot of problems including wear an tear on the system and poor response. Both skegs and rudders work best when they have proper section shape. The ubiquitous (among sailors) NACA 0006 and 0012 airfoils seem to work just fine. If they didn't a sailboat race would sound like a giant kazoo band engaging in a game of bumper cars. Improper shaping (mostly flat plates) causes most of the ventilation that increases drag in a big way. Nicely rounded leading edges and a fuller foil like the NACA 0012 just about eliminate it for all but the most aggressive turning actions. Back when I was racing 505's I experimented with an NACA a section supplied by a friend at NASA and despite all reasonable efforts I could not make that rudder ventilate. The section was the same as that used on the U-2 spy plane wing and designed for high lift. I believe (but won't swear to it) that the section was designated NACA 2006. I have modified flat plate rudders by gluing wood on each side and then shaping the blade. A little varnish and it looks rather sexy and works better too. I prefer rudders with stops that prohibit turning the rudder more than 20 degrees. This reduces the possibility of ventilation considerably. If one uses the rudder more like a trim tab and set it at an angle to hold a course rather than wiggling it back and forth all the time it seems to work quite well (someone mentioned that earlier but I cannot recall who). Interestingly the Inuit seemed to have used their rudders this way. They had control lines running around the front of the cockpit that they must have adjusted and left since one can't paddle and handle lines at the same time. Can learn a lot from those Inuit. Managed to dig out the paper on the International sailing canoe tests. For those who want to get it search for Tanner, T. , Full Scale Tank Tests of an International 10 Sq. Meter Class Canoe, Royal Institution of Naval Architects, 1960, Unfortunately they printed the graphs in such a small scale that taking information off results in lots of error not to mention the error due to normal distortions of copies etc. Doing the best I could for 5 degrees of leeway I got a 1.8% increase in drag at 2 knots, 11.4% increase at 3 knots and 14% increase at 4 knots. Now, don't go off and quote me on those saying that John Winters said that Blah blah blah % blah% blah% !!!!!! First off, even Tanner admitted to some problems in fairing the data and my take-off certainly won't win any awards for precision. In any case, the graphs do suggest that leeway can result in considerable added resistance and that it may even pay to design boats with small keels or hull shapes that resist leeway. Lots of options here and fun for designers, builders, and advertising copy writers. Cheers, John Winters Redwing Designs Web site address, 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.paddlewise.net/ ***************************************************************************Received on Wed Oct 06 1999 - 06:43:29 PDT
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