The interaction phenomenon causes two sails (the jib and the main sail) working together in an upwind condition to produce a larger lift force than a sum of lifts generated on the sails operating alone. In spite of the fact that this phenomenon is widely known to both sailors and sail designers, its reason is still regarded arguable. The aim of this work is to use the advantages of numerical methods to provide an exhaustive description of the aerodynamical mechanisms causing the sails to interact. In order to attain this aim, an adequate simulation methodology has been developed. The authors...