There is an often repeated misconception that a rudder must be streamlined, "have an airfoil", "be profiled", et cetera. This is not true, most of the time. When, say, a sailing canoe is properly trimmed there should be no load on the rudder (one of the beauties of a yawl rig is that it is much easier to achive it with such a rig), as that means least drag from the rudder, no matter if it is "profiled", or a flat plate. A thin plate has the least drag of all, when not loaded. But during vigorous manouvering, a thick profiled rudder has an advantage, as it is not so easily stalled, but in that respect a non-vertical rudder is better than a vertical, just as the Concorde flew very well at low speed with its highly swept wing, it just needed a lot of power. So while a rudder might well be a flat plate with a rounded leading edge the leeboards/dagger board/centre board should definitely be profiled, as that is almost always sideways loaded, unless you're going straight down wind! Depending on speed and the power needed, the optimum width of the rudder varies, as power is a function of the area and speed squared, but the narrower it is the easier it will be to stall, especially if it is long, as the sharpness of the stall is a function of the rudder's AR, aspect ratio (the length divided by its width). You see very few thin plate rudders of high aspect ratio, as they are very prone to flutter (pouch's leeboards are made of thin plate and while short a tubby buzz (flutter) a lot! So at low speed a wide rudder is a real boost, while at high speed a long, narrow rudder is good enough - US tugs have multiple rudders that are far wider than long (AR < 1!) and Americas Cup boats have long and narrow (AR > 4) :-)! Yours, Tord *************************************************************************** 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/ ***************************************************************************
> Depending on speed and the power needed, the optimum width of the rudder > varies, as power is a function of the area and speed squared, but the > narrower it is the easier it will be to stall, especially if it is long, as > the sharpness of the stall is a function of the rudder's AR, aspect ratio > (the length divided by its width). You see very few thin plate rudders of > high aspect ratio, as they are very prone to flutter (pouch's leeboards are > made of thin plate and while short a tubby buzz (flutter) a lot! I suspect air-foil blade (or is it hydro-foil?) not only improves speed (can be deflected less due to additional lift), but also eliminates flutter. My Feathercraft aluminum foil double rudder (AR of wetted surface approx. 4) is 3/16" foil, and I never had any flutter. No flutter too on small flat 1/8" single blade of another kayak (AR about the same), but it has small area for such a thickness. I heard a flutter when increased area 4 times with the same 1/8" flat aluminum, and also increased AR to, may be, 5. The third one, 1/8" flat aluminum by Folbot, has AR no more than 2, and of course has no flutter, but one needs a post-and-yoke lever system of Folbot to turn such a blade. (or to modify it to make it balanced). > So at low speed a wide rudder is a real boost, while at high speed a > long, narrow rudder is good enough - US tugs have multiple rudders that are > far wider than long (AR < 1!) Those tugs use hydraulics and/or motors to turn their rudders; but you don't. That's why narrow high-aspect blade is preferable if you want to apply less efforts on pedals. Or heavily balanced blade, with 30-35% of area fore of the pivoting axis. *************************************************************************** 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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