Customs preventive control requires port clearance before departure, ensuring duties secured and permitting boarding and inspection. Customs preventive control requires a written order of port clearance before any vessel or aircraft may leave a Customs station, granted only when the proper officer is satisfied that formalities are complete and duties or securities are paid. Preventive Officers may board and inspect craft, obtain declarations and crew lists, and conduct selective rummaging and examinations based on vessel history, origin, and intelligence to detect undeclared or unmanifested goods and prevent unauthorized removals.