Complex sentence: "Turnaround" visits generate more revenue for the port and city than stop-offs and the restriction was designed to minimize unfair competition with other ports that had been built with private funding, particularly Southampton.
Split into simpler sentences: Turnaround visits generate more revenue for the port and city than port-of-calls. The reason for the strange restriction was that it was to minimize unfair competition with other ports that had been built with private funding, particularly Southampton.