The resultant force that generates Thrust on a wind propulsor comprises of the wind pressure exerted perpendicular to the wind propulsor’s projected surface area and/or the aerodynamic force generated due to the pressure distribution difference around the wind propulsor shape.

For any direction of the incoming to the wind propulsor, a specific Lift (L) and Draft (D) force is developed, the geometrical vector sum of which results in a Thrust force direction which propels the ship.

###### Thrust force (F) from wind direction (V) – *Image courtesy of M. Fakiolas*

The wind is comprised of air masses of a specific density (ρ) which varies with atmospheric pressure and temperature, flowing (u2) over the surface (S) of a specific coefficient of lift (CL) wind propulsor at specific relative angle to the wind propulsor z axis orientation, thus generating a Lift force (L) perpendicular to the surface geometric reference line.

###### Lift force equation

The lift coefficient (CL) relates to the ability of a wind propulsor geometry to generate and/or mechanical design or features (i.e. such as rotational speed for a flettner rotor or fan aspiration for a suction wing sail) to generate Lift.

The drag force is also dependent on the wing sail geometry and mechanical features while different shapes have different drag coefficients.

The variation of the angle of attack of the wind direction on a given wind propulsor has a direct impact on the lift coefficient and drag coefficient, and hence the lift, drag and thrust generation capability of a wind propulsor. For every wind propulsor geometry and function, there is always an optimal angle of attack that provides the maximum coefficient of lift.

The performance and efficiency of every wind propulsor type, size and technology is primarily determined from 4 factors:

- Surface area
- Relative velocity: wind speed and ship speed related
- Lift coefficient: surface geometry/sail mechanism/positioning related
- Drag coefficient: surface geometry/sail mechanism/positioning related

*– Information courtesy of Konstantinos Fakiolas’ book ‘Wind Propulsion Principles’, Edition 1 –*