Two 4 kW turbines, running at a 30% capacity factor for 6,500 hours, produce roughly 15,600 kWh of electricity -- enough to displace approximately 4.6 tonnes of diesel. Stena Line recorded 80 to 90 tonnes of annual fuel saving from the Stena Jutlandica installation. The gap between 4.6 tonnes and 85 tonnes demands an explanation.
The arithmetic is straightforward. Two turbines rated at 4 kW each give 8 kW of installed capacity. Applying a realistic maritime capacity factor of 30% -- accounting for anchor time, port calls, and variable wind -- gives an average output of 2.4 kW.
Over 6,500 operating hours: 2.4 kW × 6,500 hours = 15,600 kWh.
At a diesel generator consumption rate of 0.295 kg per kWh, that displaces 4,602 kg -- approximately 4.6 tonnes of diesel per year.
At USD 700 per tonne, that is USD 3,220 per year in direct fuel savings from electricity generation alone. Useful -- but not remotely close to the 80 to 90 tonnes Stena recorded.
Ships moving through air experience a phenomenon called vortex shedding. As airflow passes around the blunt edges and corners of the superstructure, it separates from the surface and rolls into rotating vortices -- essentially turbulent eddies trailing off the structure. These vortices create aerodynamic drag, and overcoming that drag requires additional propulsion energy.
Vortex shedding is well documented in fluid dynamics. It occurs on any blunt body moving through a fluid -- from bridge pylons to tall buildings to ship superstructures. On a ferry like the Stena Jutlandica, with its large flat superstructure faces, the vortex-driven component of total aerodynamic resistance is significant.
Turbines mounted at the superstructure edges -- precisely the points where vortex shedding initiates -- disrupt the process. Instead of the airflow rolling into resistance-generating turbulence, it is intercepted by the rotor. The vortex does not form, or forms much less vigorously. The result is a reduction in aerodynamic drag that the propulsion system no longer has to overcome.
This drag reduction -- not the electricity generated -- is the most credible explanation for the bulk of the Stena Jutlandica saving.
The relationship between direct generation saving and drag reduction saving is not fixed -- it depends on vessel geometry, route wind conditions, and superstructure design. But the Stena Jutlandica result suggests the drag reduction effect can be an order of magnitude larger than the direct generation saving for the turbine sizes used.
This is counterintuitive. A 4 kW turbine is small. But its effect on the airflow around the superstructure is not proportional to its rated power -- it depends on its position relative to the vortex shedding points. A small turbine in exactly the right place can disrupt a vortex that is generating far more resistance than the turbine's electrical output would imply.
The IMO Energy Efficiency Design Index framework measures total fuel consumption, not just electrical generation. A drag reduction that lowers propulsion fuel consumption appears directly in this measurement -- it is real fuel not burned, not a theoretical offset.
H Nordic turbines in the 50 to 100 kW range are substantially larger than the 4 kW units on the Stena Jutlandica. They generate far more electricity directly, and they occupy proportionally more of the superstructure edge -- the zone where vortex disruption is most effective.
The fuel savings figures published in the H Nordic model cover direct electricity generation only. Aerodynamic drag reduction is excluded from the table -- deliberately, because it is difficult to calculate without a vessel-specific aerodynamic assessment. It is a real and material additional saving, not a theoretical bonus.
For many vessel geometries -- particularly car carriers and RoRo vessels with large flat superstructure faces -- the drag reduction contribution may exceed the direct generation saving, consistent with the Stena Jutlandica pattern.
See also: How the venturi effect works on vessels · Fuel savings methodology
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