Cut-in wind speed is the minimum wind speed at which a turbine begins generating electricity. H Nordic maritime turbines cut in at 2 m/s -- a barely perceptible breeze. On land, the difference between a 2 m/s and a 3.5 m/s cut-in speed matters at the margins. At sea, because a vessel's own forward motion creates continuous apparent wind, that difference can mean the gap between generating for 95% of a voyage and generating for 60%.
Below cut-in speed, the aerodynamic forces on the turbine blades are insufficient to overcome drivetrain friction and begin rotation. The turbine remains stationary. No electricity is generated.
Above cut-in speed, the rotor begins turning and the generator starts producing current. Output rises steeply from cut-in up to rated wind speed, where it plateaus at the turbine's rated power. The shape of this power curve -- particularly the low-wind performance -- determines how much energy is captured in the many hours when wind is present but below rated speed.
Most conventional small wind turbines have cut-in speeds of 3.0 to 4.0 m/s. The difference between 2 m/s and 3.5 m/s sounds small. In practice it determines whether a turbine generates in the light-wind conditions that make up a large fraction of any wind speed distribution.
A vessel underway generates its own wind. A ship cruising at 10 knots (approximately 5.1 m/s) creates 5 m/s of apparent wind from its own forward motion in completely calm air. Even at 5 knots -- slow for most commercial vessels -- forward motion alone produces 2.6 m/s of apparent wind at the bow.
This means H Nordic turbines are generating whenever the vessel is underway in conditions above absolute dead calm. The 2 m/s cut-in threshold is crossed by vessel motion alone.
On a commercial vessel with 6,500 operating hours per year, the turbines are below cut-in speed for only the brief periods of very slow manoeuvring in port and during the rarest calms at anchor. The effective capacity factor is correspondingly higher than for a comparable land installation.
Forward motion gives a floor. The venturi effect -- the acceleration of airflow over the vessel's deck created by hull and superstructure geometry -- raises it further.
At 3 m/s ambient wind, the effective airflow at turbine positions on a typical cargo vessel reaches 9 m/s or more. Combine that with 5 m/s of apparent wind from forward motion and the turbines at those positions are seeing 12 to 14 m/s of effective wind speed -- well above rated output for the 50 and 100 kW models.
Even in conditions significantly below 3 m/s ambient, the combination of forward motion and partial venturi effect keeps most turbine positions above cut-in.
The 2 m/s cut-in speed is the turbine's physical threshold -- the wind speed at which the rotor can begin turning. But conventional grid-connected systems impose a second, higher threshold: the minimum wind speed at which the grid synchronisation electronics will engage.
This synchronisation floor is typically 3 to 4 m/s for grid-tied inverters -- higher than the turbine's physical cut-in. It means that even when the rotor could turn, the system does not generate because the inverter has not yet connected.
H Nordic's DC-direct architecture does not have this constraint. Power routes from the DC generator through a DC/DC converter directly to the integrated battery storage. There is no grid to synchronise with, and no minimum inverter threshold. Generation begins at the physical cut-in speed of 2 m/s and every unit of energy produced is captured immediately.
For a vessel where apparent wind is frequently 3 to 5 m/s from forward motion alone, this difference is material. It is the difference between capturing all light-wind generation or losing it to an inverter synchronisation floor.
See also: How the venturi effect works on vessels · Turbines at anchor and in port
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