This study examines whether operational fuel intensity varies according to fuel type and prevailing weather conditions using an observational dataset of ship performance. Fuel intensity is measured as fuel consumption per unit distance travelled. The dataset comprises 1,440 observations collected from 120 vessels, with voyages categorised into calm, moderate, and stormy weather conditions. A two way analysis of variance is employed to assess both the main effects and interaction effects, followed by Welch t tests and Tukey post hoc analyses to explore differences within groups. The findings indicate a strong and statistically significant main effect of fuel type. Diesel based operations consistently demonstrate lower fuel intensity compared to heavy fuel oil operations across all weather categories. In contrast, weather conditions exhibit only a minor influence, and the interaction between fuel type and weather is not statistically significant. This suggests that the differences attributable to fuel type remain stable across varying sea states. Overall, the results highlight that decarbonisation strategies should focus primarily on fuel selection and fuel switching, while variations due to weather can be addressed through operational measures such as route planning and speed optimisation.