The Mantis is a G-class ship. The core rule book states that G-class ships have an FTL of 20. A boosted displacement drive lowers the FTL rating with 1. So 6 drives gives minus 6 => 20 - 6 = 14.Not sure if its a bug but the Mantis class has boosted diplacement drive x6 but only has a ftl rating of 14 which seems way too hgh for that boost