Abstract: An apparatus for measuring and displaying the speed of a craft, especially of a sailboard, comprises on one hand an immersed rotator (1) equipped with a permanent magnet and attached to the craft (40) and on the other hand a measuring device mounted on the craft. The measuring device comprises a coil having a very large number of turns, at least one operational amplifier and a frequency meter for determining the frequency of rotation of the magnetic field produced by the magnet of the rotator (1). A sealed box (30) encloses the entire measuring device, its liquid crystal display unit and photovoltaic cells for its power supply. The apparatus is particularly adapted to be removably mounted on a sailboard or a light dinghy.