Carbon intensity of ship emissions is a cornerstone of contemporary regulatory actions, with measurable targets of reduction being enforced in the coming decade. Short term measures to achieve them include voyage optimization. Therefore, the VISIR ship routing model was upgraded for computing least-CO
routes depending on ocean analysis products related to waves and sea currents. The speed loss in waves and the CO
emission rate of a medium size Ro-Pax ship were obtained from a coupled command-bridge engine-room simulator. The geographical and topological features of least-CO
routes resulting from VISIR were characterised by means of various types of isolines. A case study in the Adriatic Sea leads to bundles of optimal routes with significant spatial diversions even on short-sea routes. The carbon intensity savings were compared to the CO
savings, highlighting also their dependence on both route lengthening and fractional engine load. For a case study in winter, carbon intensity reductions up to 11% were computed with respect to least-distance routes between the same couple of ports of call. This is promising, as a reduction of this magnitude represents a significant amount of the carbon intensity curbing target required at International level.
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