A fixed structure is shaped like a pier and equipped on an upper part with a slide guide. A length of the pier extends from land to a point at sea where the depth is such as to allow boats to be transported to float. A trolley slides along the pier. At least two arms extend from the trolley transverse to the pier. The arms are equipped with independent hoisting devices for harnessing the boats. The arms also have a sliding counterweight and are equipped with devices which indicate the horizontal position of the arms and control the movement of the sliding counterweight. The arms are also equipped with commands with which the hoisting devices and other motorized parts of the machine are controlled.