I've seen that on rudders that had a flat profile instead of an airfoil. The shape causes turbulent flow off the rudder which makes the rudder shudder as the flow leaves the blade. The fix is to put a NACA airfoil cross section onto the rudder.

Another possibility, if the rudder already has a NACA airfoil, is that the daggerboard or centerboard is the problem and upstream turbulence is impinging on the rudder. In that case the upstream board should be shuddering, too.

Either way, try making sure that all your appendages have smooth symmetric airfoil shapes.

Laszlo

...or, maybe she's just frightened by all that high speed which gives her the shivers?  Or, maybe it's me with the shivers; just warmed up to -6 F here in central Ohio, with the wind blowing snowdrifts up against my front door.  <;-)

.....Michael

Merry Christmas Eve Eve!

   Laszlo, Great feed back. Would not have thought of the dagger board as contributing. OK, I'll check and see if there is somehting that is obvious on those two.

Having built a few wood/epoxy/ cloth SJ21 rudders, I second Lazlo's comments regarding the foil shape contributing to the vibration.  In particular, pay close attention to the final shape of the last inch or so of the trailing edge.  It's needs to be as fair as possible on each side, but most important is to have the trailing edge itself "squared off" with a final thickness around 3/16" +/- thick...your don't want the edge coming to a point, and it's better not to have it rounded either...try to make the back edge a fairly flat/square edge.  It's OK to ease the corners themselves, but don't round over the entire edge.  The fairly sharp, flat edge helps to reduce the exiting turbulence and will reduce the vibrations.

Mike