08/31/2011. Three Model 37I O-1Es, s/n 29-319 to 29-321, were converted to YO-13C by the installation of 600 hp Curtiss V-1570 Conqueror twelve-cylinder liquid-cooled engines. Designation later changed to O-13C. One additional
O-13C, s/n 28-207, was created temporarily by replacing the supercharged SV-1570 engine of the YO-13D with the unsupercharged model of the O-13C.