Sea State, Trim, and Specific Fuel Consumption in Short Sea Shipping: An Empirical Analysis Using Operational Voyage Data

Abstract

Trim optimization is widely promoted as a low-cost lever for reducing ship fuel consumption, yet most estimates of its benefit come from calm-water simulations or empirical models, leaving open whether the effect survives where sea state, wind and speed vary jointly with trim. This study analyzes 139 voyages of a Ro-Ro/Container vessel on a fixed interisland route (Puerto del Rosario–Las Palmas de Gran Canaria, Canary Islands), combining loading-computer hydrostatics, flow-meter fuel records, AIS-derived speed and course, and hourly sea-state observations. A naive comparison across trim bins reproduces the ≈4% saving commonly cited for moderate stern trim, but the difference is not significant (Welch t-test, p=0.80) and is no longer identifiable once significant wave height (Hs) and speed over ground are controlled for (p>0.7 in every specification). Over the interquartile trim range these operations explore, the 95% confidence interval for the trim effect spans −1.4% to +1.7% of specific consumption, excluding savings of the magnitude reported under controlled conditions. Trim is itself confounded with sea state: bow-down voyages occur, on average, in significantly higher sea states than moderate stern-trim voyages (p=0.009). Added resistance from wave height is significant in head seas (p<0.001) but indistinguishable from zero in beam seas (p=0.42). For this vessel and route, trim adjustments within the range routinely used are therefore secondary to weather-driven resistance, while the wider envelope simulated under controlled conditions remains beyond the resolution of operational data, with direct implications for how trim optimization tools are calibrated and how their recommendations are presented onboard.