Apparatus for measuring the instantaneous distance between the stern of a vessel and a fixed structure such as quay, a jetty or a landing stage

The apparatus includes transmitter-receiver units (10, 20) for emitting acoustic signals in the form of beams (F1, F2) directed rearwardly of the stern and for receiving acoustic signals reflected by the fixed structure (B, C). The number of transmitter-receivers and the beam angles of the respective beams are determined so as to include substantially the entire area behind the stern in their field of view. Each transmitter-receiver includes a transducer for converting input electrical signals into output acoustic signals and for converting reflected acoustic signals into output electrical signals which are supplied to an electronic processor unit. On the basis of electrical input and output signals, the processor unit determines the instantaneous distance between the vessel and the fixed structure. A visual and/or acoustic signalling device is connected to the processor unit for emitting a visual and/or acoustic signal in real time indicating the instantaneous distance determined.