The Sperry Gyrofin Stabilizer consists of two actively controlled underwater fins, projecting from opposite sides of the ship near its center. The fins, controlled continuously and automatically, co-operate in applying roll stabilizing moments to the ship. Stabilizing moments are ordered by the stabilization system to counteract disturbing moments applied to the ship by the wave action. For descriptive purposes, the stabilization system has two major divisions, the control system which orders the proper stabilizing moment, and the stabilizer fin machinery units which apply the ordered stabilizing moment to the ship. The stabilizer control deduces the disturbing moment being applied to the ship from measurements of the ship's response to those disturbances. Sensing instruments detect and measure several functions of the ship's roll motion and supply the basic data needed to compute roll stabilizing moments. A computer then orders each stabilizier fin machinery unit to apply a fin lift force to the ship which will produce a stabilizing moment equal and opposite to the disturbing moment. Although responding independently to ordered stabilizing moments, the stabilizer machinery units co-operate to apply fin angles which produce equal lift forces. Stabilizing moment applied is, therefore, in the form of a pure couple. The desirable equality of fin lift forces in the Sperry stabilizer is assured first by the equality of fin lift orders to each fin, and second by the "lift Control" feature. With lift Control, each fin servo control system compares the measured actual fin lift with the ordered lift and tilts the fin to maintain continuous equality between actual lift and ordered lift. Two equipments of the Sperry Gyrofin Stabilizer have been installed in passengers "Mariposa" and "Montery", displacements of approximately 20, 000 tons, speeds of about 20 knots, and it is reported that the sea trials were very successful.