Hey, where'd my emoticon go?! I stuck a smiley face at the end of my first sentence to make sure it didn't sound huffy.

I do remember that angle of attack is through the water, and has nothing truly to do with boat lines. I also agree about the severity of the angle. The two times I can readily see generating high angles of attack are tacking and jibing, and emergency type manuevers. Bit they docking attempts or collision avoidance.

After pondering kayak rudders some more, and noting that a whole lot of that rudder unit is actually in the air and not in the water, I'd say the hydraulic aspect ratio is more 1:1 to 1:3. Still a pittifully small rudder for actuall ruddering.

Myself, I do plan on using a much larger rudder on my Millcreek when I finally build it. Can't remember, but I think by sail and hull size calculations I need a rudder with something like 1.5 to 2 square feet surface area for control. I was planning to go with a teardrop shape to achieve it. Going to mean a lot of air catching surface when its tipped up on deck. We'll see how badly that affects me in the wind then. If it proves to be a horrible problem I'll either rig up a way to flop the rudder over on its side when it's up on deck, or create a slot in the deck to lower it into.

Oh, the book I was using at the time was the Mechanical Engineers Handbood by Lionel S Marks. Circa 1930. Old as heck but really neat. Got lots of fascinating steam locomotive information. It's fun to compare against modern publications, just to see how much has not changed. Graphs of Cl against aspect ratio it did not have, just tables of that. I stuffed the tables into a spreadsheet so I could see graphs. I like pictures, find them easier to work with conceptually then stacks of numbers.

> Remember that the angle of attack (AOA) for a rudder is NOT the
> angle between the centerline of the boat and the mean chord line of
> the rudder. It is the angle between the rudder and the fluid flow.
> As the boat rotates, the AOA becomes significantly lower. I rarely
> turn a silboat rudder past 45 degrees, almost never beyond 60 degrees.
> If you adjust for the rotation of the boat, the AOA is rarely above,
> say, 30 to 40 degrees. For a flat plate (or symmetric airfoil) at
> zero AOA, the aspect ratio makes NO difference because no lift is
> being generated. So, as AOA increases, the higher aspect ratio will
> be "better", until a point where a lower aspect ratio becomes
> more effective. An aspect ratio (AR) of 3:1 is quite reasonable. 2:1
> is probably common, and is also what I suggested to use. I don't think
> I've ever seen a sailboat with 1:1 AR.

> Even within the "burble" zone, a rudder will have some
> effect, though partially diminished. I don't have the curve here in
> front of me, and don't recall seeing a curve of CL at high AOA vs
> AR. It sound quite interesting. What book do you use? There's a chance
> I may have it, and I'd like to see it so I can get a good mental picture.
>

> BTW, If any of you guys want an EXCELLENT book about aerodynamics
> (Hydrodynamics use the same principles) I'd HIGHLY reccomend Introduction
> To Flight by John D. Anderson Jr. It gives theory and derives key
> formulae, but is understandable to most people. If you have an engineering,
> science, or math education you'll find it easy reading and enjoyable,
> as well as straightforward and informative.