Context: CREATE TABLE table_name_73 (tonnage INTEGER, type_of_ship VARCHAR, date_entered_service VARCHAR)
Question: what is the sum of tonnage when the type of ship is twin screw ro-ro motorship and the date entered service is 11 february 1983?
Answer: SELECT SUM(tonnage) FROM table_name_73 WHERE type_of_ship = "twin screw ro-ro motorship" AND date_entered_service = "11 february 1983"