## 4 Discussion
### Application to the Optimal Ship Routing Algorithm  
Ship weather routing, which involves suggesting low-emission routes, holds significant potential for contributing to the decarbonization of maritime transport [38]. The graph-search VISIR-2 model (discoVerIng Safe and effclient Routes) ([[https://zenodo.org/records/10960842](https://zenodo.org/records/10960842)]([https://zenodo.org/records/10960842](https://zenodo.org/records/10960842)), last accessed online 28 October 2024) incorporates several innovations: refined velocity composition with currents and leeway; no restrictions on vessel type provided the performance curve is available; a cartographic projection, rapid coast intersection screening via a K-dimensional tree; and a least-CO2 emissions algorithm with dynamic graph edge weights demonstrating quasi-linear computational performance [39]. Numerical experiments conducted with VISIR-2 in the Mediterranean Sea throughout 2022 [40] utilized Metocean analysis fields and report that for a 125 m ferry, CO\({}_{2}\) savings were calculated at 2% CO\({}_{2}\) compared to the least-distance route on average and up to 49% savings on about 10 days per year with dire weather conditions. VISIR-2 integrates meteorology, oceanography, ocean engineering, and computer science to evaluate the impact of ship routing on decarbonization efforts within the shipping industry. For our case study, Figure 7a demonstrates an indicative example in the Cretan Sea, connecting port of Heraklion to Chania port, utilizing operational downscaled forecasts of \(1\times 1\) km grid NEMO and WWIII simulations for 5 July 2024. The three alternate routes minimizing distance, time, and CO\({}_{2}\) emissions are shown, along with the time-averaged significant wave height and direction, as well as the sea surface circulation speed and direction throughout the duration of the trip. As is shown, the minimum CO\({}_{2}\) route is southern, utilizing the anticyclonic pattern of the currents during the trip from Heraklion (HER) to Chania (CHA). The route minimizing CO\({}_{2}\) calculated 0.8% less emissions compared to the least distance route, translating in total saving for the trip of approximately 52 kg of CO\({}_{2}\). The example provided is a proof of concept, and it is noted that these results were obtained during non-extreme weather conditions. Additionally, the routes that the higher-resolution models were implemented in were limited to the region where the models were run and constrained by computational cost, which currently do not include the open sea that would have higher waves and a more significant CO\({}_{2}\) savings [39]. This application is freely available in Iliad Marketplace ([[https://ocean-twin.eu/marketplace/product/ship-routing](https://ocean-twin.eu/marketplace/product/ship-routing)]([https://ocean-twin.eu/marketplace/product/ship-routing](https://ocean-twin.eu/marketplace/product/ship-routing)), last accessed online 28 October 2024) with relevant details and links for the prospective end-user to both download forecast data and access application packages that can be adapted for replication of the digital twin application.  
Figure 7: _Cont._