Kruger/Krüger and Borgen

How do I go about figuring out what ship someone came over on. My Great Great Grandparents Immigrated in 1864 but have no clue on the ship and all. what Can I do Their names are Wilhelm Friman Koren Christie Krüger ( He dropped the Umlots when in states) and his wife Tore Torsdatter (Borgen) Krüger ( we knew here as Thora) and theirDaughter Hannah Severine Kruger [?] Thank you