At the other end of the scale comes the shipowner who finds the bulk-carrier too limited in the number of trades in which it can engage; for these the naval architects produced several variations. The first idea took advantage of the small amount of space needed for iron ore which could allow for a considerable amount of additional capacity and still be within more or less the same overall dimensions as a bulk carrier.  That spare space was designed as tankage and so the Ore/Oil carrier came into being.  This enables the shipowner to work in whichever trade – ore or oil – which shows the best return at any one time.  Occasionally the dual role allows the owner to overcome the bugbear of any specialised ship, the long return ballast passage, by loading one way with ore and then only a short ballast run top load another way with oil, or vice versa. As designers learnt more about the behaviour of large ships, helped sadly by some tragic casualties it became possible for bulk-carriers to trade with some holds empty without the risk of the ship breaking up and thus was designed and built the 0B0 – Ore/Bulk/Oil carrier. An interesting combination carrier to emerge took into consideration the basic problem of international shipping which is that so much trade needs different types of ships for exports (raw materials) from imports (manufactured goods).  The Con-Bulker (Container/Bulk-carrier) met this problem head on.  It took no great break-through in design techniques, simply commercial foresight and some container fittings in the bottom of the holds and on the deck of an otherwise normal bulk-carrier. The subsequent sophistication of the container trades has resulted in limited use of these types today. These vessels are the modern successors of the “conventional” cargo ships of 20 years ago, and designed to be able to take bulk cargo, bagged cargo, containers and general cargo with equal ease and, if necessary, at the same time.  They exist within the narrow size range of 5,000 to 25,000 dwt.  They usually have two decks (tweendecker), large hatches and with some of the cargo gear of sufficient capacity to lift containers and other heavy pieces of cargo (30/35 tons SWL).  There have been few new buildings of this type in recent years as in the main trades they have been replaced by container or Ro-Ro vessels.