To help understand the evolution of seminal research areas over time, we also complete a dynamic co-citation analysis
covering all articles in the nine seminal clusters. The evolution is graphically illustrated in Fig. 4. The size of a circle represents
the PageRank of the article and hence the larger the size of a circle, the more highly-cited and prestigious the corresponding
article.
Cluster 1, general port/maritime/ship emissions, was the first to shape followed by a slow evolution since the year 2000.
Cluster 2 (air-emission focus), cluster 3 (eco-efficiency), and cluster 6 (supply chain focus) have been the fastest growing
clusters during the past decade. These results are not surprising given the supply chain management research focus of many
universities and research institutes and global eco-efficiency and emission reduction trends. Clusters 7 and 9 are the youngest
among all, which may also explain why they are still disconnected from other research areas. These observations reinforce
the need for more integrated multidisciplinary research that connects ship construction/technology research with
other research clusters. We expect research in this domain to grow during the next decade or so.