One thing to note on vessels with stabilzers, a little trick, when boat is hauled for service, once you have the fins on exact center, is to make a permanent and prominent mark on the hull (we weld a bead on alloy and steel hulls) at the trailing point of the fins. This will give you a permanent reference point. Later, wether in or out of the water, gives you the ability to mechanically center the fin to the mark, and then adjust your electronic sensors to re-zero in the event of fin movement on the shaft or sensor failure. This saves the necessity to re-haul for the adjustment. This also lets a diver check center in a matter of seconds, and can then be easily re adjusted.