What is route optimisation?

Route optimization or weather routing predicts the optimal conditions for efficient sailing with wind propulsion devices, taking into account the instant/prevailing ocean wind energy potential to ensure minimum fuel consumption. Because of the complexity of the application, a fully automated voyage optimization system coupled with the wind propulsion system and the main propulsion plant of the ship is needed. The resulting route may differ depending which wind propulsion device is installed, due to their different optimum wind conditions for maximum performance.


On board voyage optimization computers can be used in conjunction with accurate and real time weather forecast data, which will provide to the captain the best possible routing that would be optimal for ETA and for fuel consumption, or at least the best compromise of the latter.


Especially in large oceangoing voyages of over 3-5 days where the weather forecast uncertainty rises considerably, there can be methods to use stochastic optimization of the routes by combining real time weather data coming from such providers.


The application of wind propulsion technologies requires either more trained and competent captains or more integrated automation controlled ship power adjustments, wind propulsor functional parameters adjustments, weather routing and voyage optimization in a holistic way to ensure safety of navigation and reduction of fuel cost and CO2 emissions at all times.





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

What wind conditions are suitable for wind propulsion?

The most attractive wind/wave combination for an effective wind propulsion takes place between 5-8 Beaufort (BF) scale, and for some ships which sail around 10-12kn even in BF 4. The Global wind probability chart has also shown statistically that the higher range of average global wind speeds (probability over 10%) happens around 5-8 m/s, which corresponds to BF 4-5.


Therefore, the slower the ship sails, the better global wind utilization she achieves over time.


The probability of wind speed is widely scattered globally. These phenomena generate the following possible problems related to the wind propulsion utilization in the oceans:


  • Ship speed is critical for the wind propulsion energy harvesting capacity,
  • Wind blow direction may not coincide or be synchronized with wave direction,
  • Ocean currents may also act as an extra force to the hull of the ship, thus impacting either negatively or positively wind propulsion efficiency


Because of these factors, weather routing is also highly important for optimising the wind propulsion performance.


Global winds tendencies exist in local regions. For example, in Northern Europe and in specific in the Baltics and North Sea, the wind forces are more intense and more prevalent compared to the Mediterranean Sea, with the exception being the Aegean Sea in Greece and the Southern coast of France. Therefore, despite their possible smaller sea time utilization due to regional trading, small and medium size cargo and passenger ships may highly benefit from wind propulsion devices in certain regions such as North Sea and Baltic Sea due to their smaller size and required propulsion power.



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

What are soft sails and how do they work?

The origin of such a sail system for a commercial ship use is definitely derived from the yacht wind design experience and experimentation, with first square-rigged soft sail systems appearing since the 1960s.


One of the most reputable such introductions has been made from Dykstra naval architects who are specializing in yacht design and have developed the so called ‘Dynarig’ system which is an advanced automated deployed and trimming soft sail wind propulsion system.


The Dynarig system is comprised of free-standing masts where the soft sails are deployed on the rigidly attached curved yards, in a way to have no gaps between them as seen in traditional conventional soft sail system.


Operations for unfurling are done automatically from a remote control panel, and it takes about a minute to set a sail with setting one per mast at the same time, thus requiring about six minutes for the entire sail system to be hoisted. Rotation of the rig into the wind automatically if the ship starts to heel too much, with specific limits able to be set, connected to sensors such as anemometers.


With the Dynarig the ship can sail upwind at apparent wind angles between 33 and 36 degrees and managed also with change of mast direction through 100 to 105 degrees.


Soft sails need deck space for installing the mast, the foundation, and the rotating system with bearings. It is expected however that moving parts such as soft sail furling would need replacement depending on their operational use and the soft sail cloth as well.




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

What are soft wing sails and how do they work?

These wind propulsors are similar to the rigid wing sails that are described above, the difference is that they do not have a rigid wing coverage surface material but a more soft one, so that Furling and adjusting of variable camber of the wing profile is possible.


The material of the soft sail is normally made of composite (polyester sailcloth or similar) for endurance in a marine environment, having certain flexibility for being compressed and reefed appropriately.


When the wind becomes unfavourable then instead of retracting, the wing sail is reefed and furled to reduce their exposure to the wind.


The aspect ratios per wing can vary from heights of 30-40m and width from 10-15m, depending on the design and the available space on board.


The requirements for the soft wing sails regarding the installation are mainly similar to the rigid wing sails as described previously, it would be expected an extra auxiliary mechanism to be installed for the furling of the sail cloth, either that be electrical or hydraulic or even pneumatic depending on the provider.


One differentiation regarding maintenance compared to rigid wing sails would be expected to be the soft cloth strength and durability, since such would be related to the operational hours on wind force exposure but also to environmental conditions, such as sun radiation, heat, cold, ice and humidity.




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

What are rigid/hard wing sails and how do they work?

Rigid wing sails resemble classic sails but are comprised of rigid materials so that the sectional profile of the Sail is more stable and resembles an aircraft wing in the cross section.


Wing sails are mounted vertically on the main deck and/or forecastle of the ship and operate under the same aerodynamic lift principles as an aircraft wing. Each wing has a specific aspect ratio (height/width) and wing profile geometry so that an as high as possible aerodynamic Lift force is generated.


In adverse conditions or at berth, wing sails can be furled/reefed telescopically or otherwise to remove unnecessary drag forces or air draft. All systems are designed to work automatically, thus by adjusting their wing sails orientation depending on the anemometer readings, while in unfavourable wind conditions there is an automatic function for reefing or furling.


Normal sizes offered by the system providers so far relate to a Wing Sail height of 20-37 m and width 8-20 m – or other tailor-fit intermediate sizes, while in case of a solo Wing Sail installed in the forecastle can be a bit higher when in full deployment – i.e 50-55 m x 15 m and 20-25 m when reefed.


One issue with the wing sails systems, especially the ones which are equipped in more than 2 on board is related to how the IMO visibility and safe navigation rules are satisfied, which is still under review for case-by-case consideration at the moment.


Installation equipment other than the sail and structure includes an electro-hydraulic power pack for rotation, reefing and furling. A remote-control panel is usually installed on the bridge for system operation. Moving parts will need regular inspection, overhauling and maintenance especially of the wear and tear items (i.e. bearings, gaskets, etc).



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

What are towing kites and how do they work?

Towing kites are airborne paraglide-style devices attached to the bows that utilise wind thrust to pull a ship forward.


The main components of towing kites are:


  • The flying kite, which is similar to a paraglider wing being elliptical in shape and geometry,manufactured of high strength fabric material and is inflated with air,
  • The Control Pod, which is a mechanism that is connected between the towing rope and the Kite, and activates and controls the Kite motions through numerous suspension ropes connected to the Kite edges and interior surfaces,
  • A towing rope which is a high strength synthetic material rope which incorporates also inside the signal/supply cables for the control pod functioning,
  • A towing winch, which is a typical variable speed controlled electric-hydraulically operated marine type winch that releases or retracts the Kite at certain speeds, a reefing winch which is helping to guide the Kite on the docking position during retraction,
  • An electrically operated telescopic mast which is extended for Kite launching operations and retracted when Kite is landed/docked back,
  • Power pack needed for the electro-hydraulic parts operation and the Control Panels with relevant software and Bridge / Remote controls.




Diagram of AirSeas SEAWING towing kite / Image courtesy of AirSeas 



In contrast to other wind propulsion systems, the propulsion Kite is airborne and operates away from the ship structure at altitudes of over 150m. The Kite needs a minimum wind speed to lift-off and then it can operate at smaller than lift-off wind speed.


The optimal performance and efficiency corresponding to normal flight conditions are achieved when flying within the Power Zone which has an elevation angle of 10 to 35 degrees from the sea level. When the wind conditions become unfavourable (i.e. headwind) the Kite comes in the neutral/parking position where no forward thrust is generated.


The optimal towing force to the ship is when the wind direction is in reaching course (side tailwind), rather than in running course (full tailwind), because of the apparent wind speed benefit (i.e. in running course the ship’s speed is deducted from the wind speed, hence the useable wind energy is less).


In operation, the kite moves in a dynamic figure of 8 movement to increase the apparent wind of the kite to more than 10-fold than that of the vessel. This way, the kite maximizes the traction force by flying fast into the incoming wind while towing the ship. With a relatively much smaller surface area compared to other on-deck wind propulsors, it can generate multiple times of thrust because of the dynamic movement. Furthermore, they operate at heights where the wind speed is double that at sea level.


Sizes of kites range from 20 m2 Kites to 1,000 m2. Selection for each ship project usually refers mainly to the Kite’s rope design load in kN while the maximum strength is much higher for safety reasons.


The kite is designed to operate fully automatically, from the moment it is unstowed and docked on the retracted telescopic mast until the time it is deflated after being docked back to the mast prior to restowage. This means that the crew intervenes manually to only unstore and connect the kite to the mast and then again during undocking and storage. The whole deployment process takes about 15 minutes.


In case of a power blackout, the kite has a 30-minute self-redundancy until the emergency generator is engaged. The winch is also designed to retract the rope of the kite much faster than usual during emergency conditions, with special consideration given to avoid entanglement with the propeller. A final resort emergency system is a pneumatic rope cutter, disconnecting the kite totally from the vessel.


Towing kites have relatively little impact on heel and yaw angles on the ship and have a positive contribution to the course keeping of the ship and it requires much less corrections on the rudder and much less rudder action than deck mounted systems.


Installation is located on the forecastle of the vessel aligned to the centre and requires no drydocking. A steel platform may be necessary to elevate the kite foundation if there is no space.


Depending on the utilization time of the kite, and the quality of the components, the surface material and the towing rope may represent a variety of operating hours replacement, counted in few thousands of operating hours. The rest of the system is a conventional winch and telescopic mast system as usually found in other marine applications.




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

What are suction wings and how do they work?

Suction wings are wing sails with very large thickness and an in-built mechanical air suction mechanism.


Suction wing sails are currently installed on a number of general cargo vessels, bulk carriers and oil/chemical tankers.


Econowind Ventifoils on MV Ankie / Image courtesy of Econowind


Suction wing sails are comprised of the below main components:


  • Vertically installed wing incorporating the ventilator fan
  • Ventilator system for the fan
  • Folding mechanism (for folded systems)
  • Hydraulic system & powerpack
  • Control Panel and Electrical system


To control the airflow around the ‘thick’ foil-shape, a boundary layer suction is applied for which one or more Ventilators needs to be installed inside the suction wing profile. At the leading edge (the ‘nose’ of the egg-shaped cross section) the airflow is accelerated leading to very low pressure on the top-left side of the profile and all along the suction-side.


It is an artificial way to reduce the drag coefficient of the wing profile while keeping the lift coefficient high, even as high as up to 7-8 depending on the angle of attack and the suction efficiency. The sizes can vary between 10 and 36 m tall.


The suction wing sail can develop quite high lift forces without any self-rotation mechanism, with relatively compact dimensions. They are fully automated and can be foldable in case of unfavourable wind conditions or during cargo operations.


Suction wing sails can be installed either in a containerized form or with a flat-rack or fixed on the deck. When a containerized system is considered then it can be installed as a simple container unit by fixing the system with straps to the hatch cover and plug the system to a 400V/32A plug.


It is expected that certain maintenance will be needed on the main bearing, the ventilator fan, the flap motor and the hydraulics, with most components being familiar/similar to other usual ship equipment.




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

What are Flettner rotors and how do they work?

The wind propulsor type called ‘Flettner Rotor’ is a mechanically operated cylindrical sail installed vertically on the deck of the ship and rotates at certain speed range.

The name comes from the German aviation engineer and inventor Anton Flettner who experimented on the idea of a rotating cylinder generating lift force inside an air stream, tested in both marine and aircraft applications during the 1920’s.

Rotor Sails are a modernized version of the Flettner Rotors and are considered as mechanical sails, currently installed and operating in RoRo, RoPax, product tanker and bulk carrier ships.



Main Components


  • The Rotor Sail is mainly comprised of the below main components (Figure 3.1):
  • The cylindrical Rotor
  • The rotor Tower
  • The Foundation with the bearings
  • The Drive system (electric variable speed)
  • The wind sensors
  • The Power Control and Bridge operating panel




Flettner rotor on the Viking Grace / Source: D. Newman


The rotating cylinder is usually made of a laminated glass fibre and carbon fibre composite material.


The Rotors are utilizing a physical phenomenon called ‘Magnus effect’. When the rotor is rotating, a small boundary layer is formed around it, inducing a pressure differential between its opposite sides when it is exposed to an air flow (wind).


A resultant perpendicular to the wind flow force is generated and the maximum exploitation of the wind happens in beam reaching directions to the ship. The Rotor sail speed of rotation increases or reduces the thrust force magnitude.


The Rotors consume electrical power from the existing ship’s electric grid to rotate at their specified operating speed, usually 0 to 180 or 250 rpm. The power required for the rotor rotation can vary depending on the size of the rotor sail, the aspect ratio and the max RPM, ranging from 40 kW up to 160 kW. Usual dimensions of rotors are 18 to 35 m tall and 2 to 5 m in diameter.


The Rotor Sail normally operate in fully automatic mode, in a way that the operation starts and stops from the Bridge control panel. When the wind conditions are unfavourable then the Rotor Sail can either rotate more slowly to mitigate unfavourable drag or completely stop.


Some Rotor Sails can be tilted to decrease the air draught or moved along rails on deck to facilitate cargo handling operations.




Types of Flettner rotor (FR) / Source: D. Newman


When placing multiple Rotor Sails on deck, as far as practical and as far away from each other, with a general rule of thumb being that each Rotor should be located such that there is a minimum distance equalling the height of the rotor between adjacent rotor sails and nearby deck structures.


To install a Rotor Sail on deck, a foundation is needed to incorporate the housing needed for the main driver system and the lower bearings. A power supply and the remote-control panel on the bridge are required and auxiliary power to enable tilting-moving if necessary.


The material of the Rotor Sail is usually composite, thus the expected lifetime depending on the manufacturer material quality choices could be at or above 5 years, with some manufacturers reporting that with good quality material the design lifetime of the rotor composite can reach even up to 20 years.




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

How does thrust from the wind propel a vessel?

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 –