-----Original Message----- From: 735769 <735769_at_ican.net> <SNIP> >I was also asked about the oft quoted 10% increase in drag caused by >rudders. I wasn't present so can't really comment on the validity of this >Before doing so or before accepting it as gospel I would want to know the >rudder area, the area of the boat, if the rudder vibrated or wobbled in its >head, if the rudder was locked fore and aft, what kind of surface finish it >had, what kind of section shape it had, what kind of edge treatment it had >and if the results reasonably conform to the resistance one would calculate >using the normally used formulas for calculating resistance. > >That information would help in determining the accuracy of the tank results >and to what degree one can apply the results to all rudders. <SNIP> Okay, here is what I know about that. I was at the towing tank the day before observing the tests and trying to talk the testers to run a rudder down test (and also to try an old scratched up Boat which I had vs. a brand new one with a shiny bottom which they had already tested). I was told they were there to compare boats not to do experiments. However the next day they did test a boat with a rudder at 3 knots. It was either the Eddyline Wind Dancer or the Current Designs Pisces. When I asked much later, my informant had forgotten which one it was. It probably doesn't matter much because both had very similar rudders at the time. The rudder was a long narrow (high aspect ratio) cut from either 1/8" aluminum or if the Eddyline probably about 3/16" thick and of fiberglass with only the edges rounded (not shaped). Both were smooth and flat sided. The rudder was left free to drag behind (not fixed in place). I have not ever seen this kind of rudder cavitate (however the fixed fins on some skegs cavitate so bad you can feel the vibration through the kayak). The data on the kayaks is in the Spring 1987 issue of Sea Kayaker (but it may be in error--there is evidence some of it doesn't agree with Nautilus calculations--but in the case of the Wind Dancer Nautilus numbers exist). I don't know the rudder area but they were probably about 10 to 20% bigger than a Feathercraft single rudder in wetted area. After finding 10% more drag than the 3 knot test with the rudder up the testers thought there must have made some mistake since doing the wetted surface calculations suggested a lot less drag. They ran the test again and again got 10% more drag. All the other retests that were done also were within 1% error. They still didn't believe the results and never published them. The editor at the time was John Dowd (a very big fan of rudders--who once likened a skeg to deodorant and a rudder to perfume). Maybe that's why I am so allergic to perfume. Unfortunately I had taken my video camera home with me or we could check the tapes to see if there was any noticeable cavitation (but I doubt it because the rudder was not fixed so unlikely to get into a resonance. During the previous tests the lower aspect ratio rudder on the Eskimo 18-6 added 8% to the drag but these tests had been very flawed for several reasons from a poor sensor to the lack of turbulence induction so the results varied widely on retests. My guess is that there is more drag than the wetted surface would indicate because of the blunter entry and flat sides all the surface was in turbulent flow and because of the deep draft of the rudder most of the water it was slicing through had not been set in motion yet by the hull so the water was flowing relatively faster over the rudder surface than over much of the rest of the kayak where drag further forward set the water in the boundary layer in motion. This is only a guess and may be way off base. Now here is a case where we might be able to use data from centerboards or better yet surface piercing foils that probably already exists. Anybody have access to this kind of data? Matt Broze http://www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Sun Oct 03 1999 - 20:23:41 PDT
This archive was generated by hypermail 2.4.0 : Thu Aug 21 2025 - 16:30:14 PDT