Rick,

  I'm very excited by the work you're doing. 

  I know that not everyone shares my short-tacking or short-shunting needs, but for me, minimal effort and time are important.  While a cruiser out on the open water might not care about rotating the rudders 270 degrees on each shunt, and might prefer independent rudders, I'd drool over being able to use a whipstaff to control linked rudders that go to a max of perhaps 20 degrees.

  Have you done an analysis on the sections developed by Tom Speer?

    http://www.basiliscus.com/ProaSections/ProaIndex.html

  I don't know enough to be able to form an opinion; the only reason I bring them up is that a number of proa sites laud the Speer sections as the ultimate bidirectional foils.  It would be interesting to see how they perform with your software.

  Dennis: there's also some discussion of wing masts and rigid wing sails on the main site:

   http://www.tspeer.com/index.html

        - Mike


On 8/4/2010 5:26 AM, willoughby_rick wrote:

 

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