Originally Posted by R.R
oops...I forgot to mention that you can use the E836 for both boards and rudders with no problems at all. On the LR cats our rudders are just chopped down board sections and the section we use has a lot of similar characteristics to the E836


And this means that LR rudders are too large for its job. A NACA 0012 has a cLmax of about 1.2 at low Reynolds numbers and the Eppler section below 1. Hence the rudders could be 17% smaller and still give the same manouvreability. Apart from the reduced friction and profile drag of the reduced rudders you save some weight at the stern of the boat.
Just hijecked the plot from Tom Speer from his posting on boatdesign.net to show this important effect.

[Linked Image]

Looking at your diagrams I would think, that something with your calculation is wrong too. You should check, if you have selected the right parameters (Reynoldsnumber, Ncrit, etc.) for the calculation. Again Tom Speer's plot may give you some hints.

Cheers,

Klaus