Many liner shipping companies have a core fleet of owned ships, but for flexibility of operation, charter-in a proportion (in some cases the majority) of their fleet, in most cases on time charters.
When there is an upturn in trade, additional tonnage may be sought in the charter markets, and when there is a downturn, expired charters need not be renewed. Tonnage may also be chartered-in when an owned ship is out of service.
