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Best tandem kayak rudder options.

No, for your and kayakwriter's purposes where delayed stall and higher efficiency is required and your additional specific need for less drag - a flat plate will not be better. this graph from above gives the rough idea:

[However at lesser requirements for low speed and lower duty [less requirement for high lift and low drag] the flat plate at low speeds is not too shabby.]

"it's all about the Reynolds Number" is just a short quip which doesn't cover all the issues involved. Total drag of a system in water with gravity is a function of viscous forces and wavemaking forces. The viscous forces are characterized by the Reynolds Number which in simple terms is a ratio of intertial forces to viscous forces [the surface of the body to the water] and the [Froude Number] wavemaking forces that characterize the ratio of inertial forces to gravitational forces [the shape pushing water out of the way].

Both the Reynolds Number and Froude Number are characterized as 'speeds', but they are both dimensionless numbers and their real use is in normalizing similar shapes to any size to a scaled speed.

Anyway, it's best to refer to wikipedia or other reference to actually see what it's all about and where I get everything incorrect.
A lot of pictures from mick_allen showing poor design.

Rudder pedals - make the entire pedal move, not just the top. Put the hinge level with the ankle. Wriggle your foot and you will see the top goes forward as the heel goes back. Now make the lines a little loose and push as hard as you want on the pedal without bending your ankle. Total "windvaning" with feet pushing hard against the pedals.

The yacht daggerboard rudder isn't the same as the kayak one as the yacht blade does not end upon deck for stowing.

The last picture of a "sliding pinhead rudder" is just poor design. My lower support is just about touching the water. See the second message in page two of this thread.
Yes, I should have referenced your sliding T-head rudder - it's a much more robust approach with solidly welded and attached rotating structure and only one cartridge that completely encloses the width of the blade. Altho there is a similar unsupported distance to the waterline, everything is so robustly constructed that distance is not really an issue. The only drawback is that I'm not aware if it is commercially available.

This is what Sandy is referring to:

I think where I made the mistake is that previous NZ blogs and even the referred one always included the other rudder as an example but not as well executed
. . . but loose steering or loose steering pedals is loose control.
Yes, I should have referenced your sliding T-head rudder - The only drawback is that I'm not aware if it is commercially available.
It was commercially available at one timer as I was producing them for an NZ manufacturer. Not many but definitely commercially.
A kayak manufacturer at the north end of this country also produced a version which went on their kayaks.
Note - it was not patented and the designer wanted it open to use. That The Finns sort of patented something a decade later and the Yanks a decade after that does not stop people copying it as those two patents don't cover the concept or my version.

. . . but loose steering or loose steering pedals is loose control.
Exactly and that is what is being asked for, the rudder floating at the most neutral point giving minimal drag until steering is wanted.
two patents don't cover the concept
They couldn't as the concept - of the sliding retracting rudder - not your T-head version - has been utilized thousands of times on southeast asian junks and their related vessels. Here's a chinese one of the Song Dynasty 1125AD - it even appears to have a vestigial rotator, or at least a sliding rotation axis:

The Finnish one, Kayaksport of Finland, when queried, offered and sent a copy of their patent. It was very little like the item they were bulding. I had corresponded with the American manufacturers, Sea-Lect Designs, and when asked what was actually patented, I go a very "loud" silence. Their one had 70 parts. Counting every last nut, bolt and washer I couldn't figure that out.