A trim control system for automatically controlling a trim angle of a trimmable marine propulsion device with respect to a marine vessel, the trim control system comprising: a memory that stores a plurality of trim profiles, each trim profile defining a unique relationship between a plurality of vessel speeds and respective trim angles of the marine propulsion device with respect to the marine vessel, wherein the unique relationship defined by each trim profile is one of: a calibrated relationship developed by determining optimal trim angles for a particular marine propulsion device powering a particular marine vessel at a number of tested vessel speeds and at a number of different conditions, including at least one of under a number of tested loading conditions and with a number of tested propellers, and wherein each calibrated relationship defining a respective trim profile is defined by data from a different marine propulsion device and marine vessel combination; and developed by modifying a nominal relationship between a number of vessel speeds and a number of corresponding trim angles, wherein the nominal relationship is a calibrated relationship developed by determining optimal trim angles for a particular marine propulsion device powering a particular marine vessel at a number of tested vessel speeds; a user interface (62, 66) including an input device (62, 70) that allows a user to select one of the plurality of trim profiles from the memory so as to specify an aggressiveness of trim angle relative to vessel speed; and a controller that thereafter determines a setpoint trim angle corresponding to a measured vessel speed according to the selected trim profile; wherein the trim control system thereafter positions the marine propulsion device at the setpoint trim angle. a memory that stores a plurality of trim profiles, each trim profile defining a unique relationship between a plurality of vessel speeds and respective trim angles of the marine propulsion device with respect to the marine vessel, wherein the unique relationship defined by each trim profile is one of: a calibrated relationship developed by determining optimal trim angles for a particular marine propulsion device powering a particular marine vessel at a number of tested vessel speeds and at a number of different conditions, including at least one of under a number of tested loading conditions and with a number of tested propellers, and wherein each calibrated relationship defining a respective trim profile is defined by data from a different marine propulsion device and marine vessel combination; and developed by modifying a nominal relationship between a number of vessel speeds and a number of corresponding trim angles, wherein the nominal relationship is a calibrated relationship developed by determining optimal trim angles for a particular marine propulsion device powering a particular marine vessel at a number of tested vessel speeds; a calibrated relationship developed by determining optimal trim angles for a particular marine propulsion device powering a particular marine vessel at a number of tested vessel speeds and at a number of different conditions, including at least one of under a number of tested loading conditions and with a number of tested propellers, and wherein each calibrated relationship defining a respective trim profile is defined by data from a different marine propulsion device and marine vessel combination; and developed by modifying a nominal relationship between a number of vessel speeds and a number of corresponding trim angles, wherein the nominal relationship is a calibrated relationship developed by determining optimal trim angles for a particular marine propulsion device powering a particular marine vessel at a number of tested vessel speeds; a user interface (62, 66) including an input device (62, 70) that allows a user to select one of the plurality of trim profiles from the memory so as to specify an aggressiveness of trim angle relative to vessel speed; and a controller that thereafter determines a setpoint trim angle corresponding to a measured vessel speed according to the selected trim profile; wherein the trim control system thereafter positions the marine propulsion device at the setpoint trim angle.