the sliding arm 6 of a known linear potentiometer 7 a housing 32 encases the potentiometer 7 and is journalled for rotation about the arm 2 a known vane wheel 13 can be secured to the shaft of a dynamo 14 the vane wheel which is driven by the wind can be used to induce an output voltage from the dynamo 14 and thereby measure wind speed this information can be electrically fed to nautical instruments on the ship in a known manner the potentiometer 7 is electrically connected via a current carrying wire 8 to an electrical signal generator 9 the electrical signal generator sends a signal through the current carrying conductor 10 in the hull of the ship to the rudder 11 and trim tab 12 immersed in the water under the hull the generator 9 comprises a voltage source divided into two essentially equal parts 15a and 15b the voltage sources are connected in series via the conductor 8 as shown in fig1 and 2 the voltages are connected on either side of the resistive element 16 of the potentiometer 7 located in the vane assembly 1 the generator 9 further comprises a second control potentiometer 17 connected across the potentiometer 7 the potentiometer 17 can be calibrated with indicia showing degrees of angularity not shown a third potentiometer 18 is secured within the trim tab assembly 12 and is electrically connected in parallel to the potentiometer 7 the signal generator 9 also comprises a first differential amplifier 19 which can be any known signal amplifying means the inputs 19a and 19b of which are connected to the slide arms not.