The S.51 was a single-seat sesquiplane flying boat. Its 224-kilowatt (300-horsepower) Hispano-Suiza 8A V8 engine, mounted on two N struts above the hull and below the upper wing, drove a two-bladed propeller in a pusher configuration. The inclined interplane struts were mounted in a V configuration. The lower wings had small stabilizing floats mounted on inclined struts so that they hung below and outboard of the outer tips of the lower wing.
Italy entered the S.51 in the 1922 Schneider Trophy race along with two Macchi M.17 flying boats in competition with a British Supermarine Sea Lion II flying boat. The race was held at Naples on 12 August.
The S.51 capsized in an accident during the seaworthiness trials before the race. Righted by its crew, it completed the race, piloted by Alessandro Passaleva but could only take second place, with the Sea Lion a comfortable 2 min 22 s ahead. The course required 13 laps, a total distance of 370 km (230 mi), over which the S.51 averaged 229.57 km/h (142.65 mph).