I have taken a look at what should be the best section for an asymmetric bidirectional rudder.
It threw up a few surprises. The best I could come up is a double ended version of a NACA 07 series:
http://www.rickwill.bigpondhosting.com/Double_End_NACA07.png
At an Re# of 10E6 and aspect ratio of 4 it has an L/D of 27. It has flow separation on the trailing edge at all angle of operation that reduces at higher Re#. But it is no more than 3% of the chord in the normal operating range. The lowest Cd is 0.004.
By comparison a NACA0012 section at same Re# and AR has a best L/D of 17.7. The in-line Cd is actually higher than the asymmetric section at 0.009. One advantage is that it does not have flow separation until about 7 degrees AoA. It would be designed to work at about 4 degrees for nest L/D.
What surprised me is that the large diameter nose actually has better lift, less flow separation and lower minimum Cd than the pointy nose/tail version. The reason becomes apparent when you look at the pressure profile for the respective sections. Notably the 07 series was designed for a flat pressure profile. The NACA 16 series was a development that factored in air compression for higher speed subsonic flight. However not a factor for water although cavitation could need consideration.
This idea has a lot of upside with the one downside of flow separation and possible resulting vibration. There could be some benefit roughing up the nose/tail to help the flow stay attached - could be tested with a strip of sandpaper glued in place down the edges.
Rick
