Shipping companies focused on fuel efficiency and emissions reduction may be overlooking a hidden source of cost when operating in rough seas that ClassNK researchers say can quietly erode performance, waste fuel, and damage engine components.
A new technical study from ClassNK’s research institute captures the way a persistent mismatch between commanded and actual main engine speeds - commonly referred to as a “speed gap” - can lead ships to burn significantly more fuel than necessary in heavy weather. The phenomenon, though well known to engineers in principle, has not previously been quantified or clearly connected to real-world fuel losses and maintenance implications.
“Even with speed reduction strategies in place, if the commanded engine speed remains higher than what the main engine can realistically achieve under current weather conditions, the governor will continue injecting excess fuel in an attempt to close the gap without success,” explains lead researcher Dr. Yuzhong Song.
The research suggests fuel wastage as high as 10–12% could occur during periods of inclement weather, depending on the extent and duration of the mismatch. On the Asia–US West Coast route alone, ships may experience such conditions for over 900 hours per year, notes.
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Analyzing onboard data from a Panamax bulk carrier as well as AIS and wave hindcast data across multiple trade routes, the modeling suggested that a vessel operating on the US East Coast-Europe route could waste up to 114 metric tons of fuel annually. This is equivalent to more than US $70,000 at current VLSFO prices being lost due to engine speed orders not being adequately adjusted for rough conditions.
Main engines are subject to torque limits enforced by the fuel rack limiter, ClassNK notes, which restricts how much fuel can be injected per cycle to avoid overloading. This limit defines the Maximum Attainable Main Engine Speed (MAMES) under specific sea conditions. When crews set engine speed orders above this threshold, whether intentionally or for reasons of habit, the system’s mechanical response is an unnecessary injection of extra fuel.
Beyond the economic penalty, ClassNK highlights a serious operational risk. When an engine defaults to acceleration to cover the speed gap, the air-fuel ratio may drop due to turbocharger inertia, leading to incomplete combustion. This can cause soot fouling on turbine blades and nozzles, resulting in vibration, component wear, and eventual turbocharger failure.
Despite the severity of the findings, the proposed solution is strikingly simple. ClassNK suggests monitoring the engine speed gap in real-time and reducing the commanded speed until it disappears. Doing so eliminates both wasted fuel and excess mechanical failures. ClassNK encourages ship operators to review past voyage data to identify where gaps have occurred, and to build these insights into crew training and voyage planning.
With carbon-neutral fuels set to remain significantly more expensive than conventional HFO, eliminating all causes of waste will be crucial to decarbonization goals and commercial success, the class society notes.
