7 The global cargo ship network connected to many different ports (e.g. Antwerp). To compare the movements of cargo ships of different types, separate networks were generated for each of the three main ship types: container ships, bulk dry carriers, and oil tankers. Applying the network parameters introduced in the previous section to these three subnetworks reveals some broad-scale differences (see Table I). The network of container ships is densely clustered, C = 0 . 52, has a rather low mean degree, 〈 k 〉 = 32 . 44, and a large mean number of journeys (i.e. number of times any ship passes) per link, 〈 J 〉 = 24 . 26. The bulk dry carrier network, on the other hand, is less clustered, has a higher mean degree, and fewer journeys per link ( C = 0 . 43, 〈 k 〉 = 44 . 61, 〈 J 〉 = 4 . 65). For the oil tankers, we find intermediate values ( C = 0 . 44, 〈 k 〉 = 33 . 32, 〈 J 〉 = 5 . 07). Note that the mean degrees 〈 k 〉 of the subnetworks are substantially smaller than that of the full GCSN, indicating that different ship types use essentially the same ports but different connections. A similar tendency appears in the scaling of the link weight distributions (Fig. 2b). P ( w ) can be approximated as power laws for each network, but with different exponents µ . The container ships have the smallest exponent ( µ = 1 . 42) and bulk dry carriers the largest ( µ = 1 . 93) with oil tankers in between ( µ = 1 . 73). In contrast, the exponents for the distribution of node strength P ( s ) are nearly identical in all three subnetworks, η = 1 . 05, η = 1 . 13 and η = 1 . 01, respectively. These numbers give a first indication that different ship types move in distinctive pat- terns. Container ships typically follow set schedules visiting several ports in a fixed sequence along their way, thus providing regular services. Bulk dry carriers, by contrast, appear less predictable as they frequently change their routes on short notice depending on the current supply and demand of the goods they carry. The larger variety of origins and destinations in the bulk dry carrier network ( n = 616 ports, compared to n = 378 for container ships) explains the higher average degree and the smaller number of journeys for a given link. Oil tankers also follow short-term market trends, but, because they can only load oil and oil products, the number of possible destinations ( n = 505) is more limited than for bulk dry carriers. These differences are also underlined by the betweenness centralities of the three network IV. THE NETWORK LAYERS OF DIFFERENT SHIP TYPES