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.

> Ok, you finally forced me to go pull out my engineering handbook
> and study a bit. Here's it is in short:

> For a flat plate, which a kayak rudder usually is, the lift generated
> by a change in the angle of attack falls off faster with a high aspect
> ratio then it does with a lower aspect ratio. Ie, the longer and skinnier
> the rudder, the worse it works for steering. The "burble point"
> [where lift abruptly drops] being about 38 degrees angle of attack
> for an aspect ratio of 1, while only 25 degrees for an aspect ratio
> of 9. This means a higher aspect ratio rudder is far more likely to
> not turn a boat, especially if the rudder is thrown hard over. Efficiency
> is great, but not if it means the loss of steering control as a result.
>

> I also extrapolated the data out for a G-438 airfoil (symetrical).
> Roughly the same.

> Yes, a high aspect ratio is more efficient then a lower aspect
> ratio, but (and it's a big important but) only when the angle of attack
> is zero. This high efficiency is significant for the keel, daggar
> board, skeg, leeboard or whatever straightline low attack angle device
> is being used. Not so as much for the rudder which works by not having
> an attack angle of zero.

> This is bolstered by my observation of rudder designs and most
> of the design recommendations for them that I have seen from boat
> designers, who usually suggest an aspect ratio of between 1 and 3
> for boat rudders.