A mounting device for mounting a self-steering system on the stern (40) of a boat at a predefined angle determined from the stern configuration and a desired inclination of the self-steering system, comprises a supporting structure (10) for retaining and connecting the self-steering system with a pair of mounting brackets (20A, 20B) for retaining and connecting the supporting structure (10) with the stern of a boat (40), in which the mounting brackets (20A, 20B) have two first elongated holes (21, 22) extending along at least one approximately circular line for clamping the supporting structure (10) fast with at least four threaded bolts or the like such that its angle can be adjusted. In order to reduce the number of components specially tailored for actual sterns and for the desired installation position to least two first elongated holes (31, 32) extending essentially transverse to the long direction (L) of the plate-like adaptor piece and "congruent" with the elongated holes (21, 22) of the connecting bracket and, in a second longitudinal area (39), at least two second elongated holes (42A, 42B) extending essentially in the long direction (L) and arranged essentially parallel with each other.