> It does highlight the merit
> of rigidly linking the rudders with a reversing linkage so any
> instability in the moment is balanced out directly through the
> linkage.

i don't understand how such a linkage helps with the instability.

if a fore-aft symmetric rudder turns right and the pivot point is aft of the hydrodynamic center of effort (which it would be were the shaft at mid-chord), positive feedback makes it want to turn more to the right. if it turns left, then more to the left. as simulated by mr. speer:

http://www.basiliscus.com/ProaSections/Paper/Moments4.gif

if you connect the rudders to countersteer, which is what i presume you mean by a reverse linkage, then the front one turning right causes the back on to turn left via the linkage. positive feedback within each rudder still exists though, so the front one will want to turn even more right, and the back even more left. nothing about that linkage stops the positive feedback. in fact, it just reinforces it. or am i missing something?

turns out mr. speer has previously described what i suggested earlier: "One option is to place the pivot point toward one edge [instead of at mid-chord], at or ahead of the hydrodynamic center. This results in a surface which is stable when traveling in one direction and even more highly unstable when traveling in the opposite direction. When traveling in the unstable direction, the surface might be firmly fixed in position and act as the keel. A second surface, with the pivot towards the other end, would then serve as the rudder. The two surfaces would exchange roles for the opposite tack. This requires that the surfaces and their linkages have very high torsional stiffness and strength." (from http://www.basiliscus.com/ProaSections/Paper/ProaSections.htm)

rick of course would want the stationary rudder firmly fixed at an angle to the keel such that it prevents leeway. and i would suggest that on visionarry and harry, which already have two steering wheels, the easiest way to do this would be to have the wheels control the rudders independently, with no linkage, and for each have a locking mechanism. to shunt one would then simply center the rudder currently being used to steer, lock it in position, walk around to the other one, unlock and resume steering.

mr. speer's analysis does not have a publication date, but is in a folder created in 2001. i wonder whether there has been any more development since. in particular i wonder whether any boats have been built with such rudders and what control system they are using. that search is next on my long "to do" list...

ben arthur
weta #358, "gray matter"
ithaca, new york