A sail propulsion system for boats and the like comprises at least one mast (A) and a jib (F) controlled by sheets and secured to the head of the mast (A), as well as at least two mainsails (RS, RD) rove on two shrouds (S; SR) located one to the right and one to the left of the mast (A) and arranged each on a boom (B) mounted on one of said shrouds (S; SR). This sail system has a greater sail surface with respect to the conventional set of sails consisting of a single mainsail and the relevant jib, whereby with the same wind the sail thrust is greater, moreover the jib (F) and the leeward mainsail meet the wind like a single sail whose efficiency is greater than the efficiency of the two separate sails present in conventional equipments, and finally when sailing before the wind the sail surface has its center of thrust located on the boat axis so as to make the sailing more stable and less prone to rightward or leftward deviations of the bow, thus increasing safety and allowing to dispense with the use of special sails when sailing before the wind, such as the gennaker or the spinnaker.