A rub rail assembly for mounting on a surface of a vessel includes a rigid portion having a mounting side facing the surface of the vessel, a wear side facing away from the surface of the vessel. The bearing includes an axial bore for receiving the piston, wherein the annular barrel-shaped bearing is designed for balancing the forces applied to the piston by the piston drive shaft symmetrically about the centerline of the piston. The two light processing units are positioned on two neighboring corners of the transparent substrate, and each of the two light processing units includes a light source and a detector.