In the mid-twenties, there was a major thrust to operate single-place, fighter-type aircraft from the catapult-equipped ships.  As a result, final development of the UO series brought an order on June 20, 1926, for 20 single-seat UO-3 airplanes with the Wright Whirlwind 220-hp J-5 engine.  These were catapult-launched, floatplane fighters armed with 7.62-mm, forward-firing, machine guns.  Before delivery in 1927, they were re-designated by the Navy as FU-1 to more accurately describe their mission. 

The standard observation two-seater UO was converted to a single seater and stiffened up to withstand the additional loads, including improved wings and a revised vertical tail.  It also incorporated the first service application of supercharging.  These aircraft used the Rootees model 3 supercharger capable of maintaining sea level pressures at the carburetor to the critical altitude of 15,000 feet.  These aircraft were the last wood and wire design emanating from World War I that were bought for the U.S. military.  They went to service simultaneously with the larger Boeing Packard FB-5.  This was a single seater with a 600-hp, water-cooled engine.  The squadron equipped with the Vought FU’s came into daily competition with this FB-5 squadron. It was a striking fact that anywhere above 10,000 feet the little FU was superior in speed, maneuverability, and climb to its competitor, which had approximately three times its sea-level horsepower.  Here again the influence of wing loading and supercharging on altitude performance were demonstrated in actual service.  VF-2, calling themselves the “Big Apes,” was the only squadron to receive the FU-1.  In due course, eighteen of the FU-1’s were converted to two-seat FU-2’s, performing utility and training duties when new fighters appeared on the horizon in 1928 Many of these excellent and well-built aircraft were serving in secondary roles into the early 1930’s.

When two new carriers, the U.S.S Lexington and U.S. S. Saratoga, came onto the scene, the “battleship fighter” concept was dropped, and the FU’s were modified to two-place trainers as FU-2’s. 

This rounded out their service in fleet support and training duty.  By 1933, the Navy UO/FU’s were retired, with the last Coast Guard UO-4 phasing out in 1935.