Complex sentence: It was not until 1820 that an American entrepreneur commissioned the construction of a true steamship, the "Robert Fulton", for service between New York and Havana, Cuba, but although this vessel apparently proved a commercial success, she spawned no imitators.
Split into simpler sentences: It was not until 1820 that an American entrepreneur commissioned the construction of a true steamship, the "Robert Fulton", for service between New York and Havana, Cuba. Although this vessel apparently proved a commercial success, she spawned no imitators.