question about foil of sailboats.
 

Dear all.

i was wondering why the new sailboats have keel profiles concave in the aft part. given all parametrs equal, my analysis using xfoil show better results (especially in terms of drag) of foils without concavity. those foils must be symmetric and are supposed to work at Re around 2 or 3 million at 3° aoa. attached you'll find the images of those keels.

thanks for your help

Sail,
On foils that are design for a large extent of laminar/transistional flow a primary issue is flow separation in the region of pressure recovery (adverse pressure gradient). To combat this, a rapid initial recovery where the boundary layer is still stable is used followed by a slower final recovery up to the trailing edge hence the rapid concave shape followed by the shallow region towards the trailing edge. I believe Principles of Yacht Design by Larson may contain a brief description of this. A more detailed description can be found in the 8th CSYS symposium paper "Keel Design For Low Viscous Drag" by Obara and van Dam. My guess is your results may be the result of bypass transistion due to improper free stream turbulence intensity if you are using a transistion model (e^n model).
Regards,
Dave

thanks for the references, i will look them up!