I was happy to follow the request of my esteemed former teacher, Professor Dr. Szabo, to write a book on modern problems in my field as part of the collection "Engineering Science Library" that he edited. In the last ten to fifteen years, the lifting surface theory has been successfully applied to the theoretical treatment of the hydrodynamic problems encountered with propellers; the same applies to other flow machines and their components, such as blade grids. The standard theory of the single lifting surface has thus undergone a significant and interesting expansion and enrichment; for the treatment of a lifting surface in the field of a specific propeller flow requires the adaptation of the solution to the various additional conditions; among these, in some cases, is also the B.