Fuel Alone Cannot Define Economic Speed: A Parametric Model of Hull Fouling and Engine Ageing in Indonesian Coastal Cargo Operations

Abstract

Fuel represents the dominant variable cost in ship operation, and the selection of sailing speed is the principal lever available to an operator seeking to control it. This study develops a transparent parametric model of fuel consumption and economic speed for a representative Indonesian coastal general cargo vessel, and uses it to quantify how hull fouling and engine ageing alter both. The model combines a cubic speed-power relationship with a constant auxiliary load, and derives the cost-optimal speed by minimising total cost per nautical mile. Parameters are taken from published literature rather than from measurement, and the study is therefore explicitly a simulation rather than an empirical trial. Two configurations were compared: a new vessel with a specific fuel oil consumption of 185 g/kWh and a clean hull, and an aged vessel with 205 g/kWh and 25% added resistance. The model predicts a fuel penalty attributable to ageing of between 27.9% and 36.4% across the range 8 to 15 knots, averaging 33.3%, and a reduction in cost-optimal speed from 10.22 to 9.17 knots. A methodological finding of wider consequence also emerges: minimising fuel per nautical mile alone yields an optimum of 4.61 knots, which is operationally meaningless, demonstrating that economic speed cannot be defined on fuel consumption alone but requires the vessel time cost to be represented explicitly. Sensitivity analysis shows the optimum is more responsive to time cost than to fouling severity.