A sail ship leverages the keel’s resistance to turning moments to allow a wind crossing a sail at an angle to tack, achieving a speed greater than the driving wind. But is it possible at all for an airship to also translate turning moment energy into forward energy? Such that a keel surface and sail surfaces can result in a velocity greater than the driving wind?
I realize we do not have any sail powered airships today. It does seem impossible except in the special case of sailing on a thermal boundary layer.