Wind propulsion technologies, ships and routes
Introduction
The renewal of the use of wind for the propulsion of commercial vessels requires an adaptation of the technologies to the operational constraints of commercial vessels. Similarly, aerodynamic performance can be reviewed in the light of advances in fluid mechanics, of advanced computational capabilities now available to engineers and of automation possibilities.
These constraints and opportunities have led WAPS providers to offer a wide variety of technologies. From an aerodynamic perspective, the levers of performance are:
Increase in system area: this possibility is generally limited by the constraints of acceptable air draft on vessels depending on their operation (bridges, cranes,…) and by negative interaction effects.
The increase in maximum lift, which translates into force density per unit of surface: it is the means of increasing performance that leads to the greatest variety of systems (asymmetric wings, multi-elements, active systems,…).
The increase in apparent wind speed: this is the principle of dynamic kites which fly at high speeds to increase traction forces.
There is a cost associated with this search for performance that makes it difficult to determine which system has the best value for money.
One of the important specificities of wind propulsion, especially compared to the technologies available in aeronautics, is the need for reversible wing sections that can work with wind coming from either side, port or starboard.
Wind propulsion technologies
Soft-Sail
Soft-Sails encompass all the so-called «classic» soft sails, widely used in traditional navigation. These thin wing section sails are made from flexible materials, the wing section naturally bends on the right side under wind pressure. The sail is usually attached to an external mast or cable.
This technology is probably the one that has the largest performance interval according to the deformations of the fabric, the possible trimming, the wind grip of the rigs… Our default coefficients are given for the rather correct quality of the typical sail of a modern pleasure boat.
CRAIN has the background, the tools and the experience to study the aerodynamics of any type of sail.
Symmetric Wing Sail
Symmectric wing sails have a symmetric wing section. They can be used on both tack without any geometry modification, reducing the need for mechanical systems. They usually show a good lift to drag ratio, but the maximum lift coefficient is limited by the lack of camber.
Two Element Wing Sail
The section of theses wings consists of two articulated elements of symmetrical section. This solution allows increasing the camber of the section and therefore the CLmax while being reversible. The rear element rotates, on either side, with respect to the front element.
The two elements are spaced by a slot or the rear element can be a flap that deforms the traling edge of the section. The angle between the two elements can be adjusted according to the navigation conditions encountered.
Wind tunnel test results from the America’s Cup are available in the scientific literature. They allow to deduce the aerodynamic performance of the wings based on this concept. However, there are many possible configurations and the aerodynamic coefficients depend significantly on them.
On request, we can establish the aerodynamic coefficients for a given configuration.
Note: Front-back symmetrical camber There are also wings equipped with thick cambered section, symmetrical front-rear. The front and the rear edges are swapped according to the side from which the wind comes, like the large square lighthouse sailboats of yesteryear. Performance is between the symmetrical section and the two element section.
High Lift Cambered Wing Sail
These wings have sections made up of two or three symmetrical or asymmetrical section elements developed specifically for the purpose of wind propulsion. These sections are designed to achieve high lift coefficients. Sections with asymmetrical elements are easily reversible and require additionnal mechanical systems. Results from wind tunnel tests carried out in the context of Formula 1 are available in the scientific literature. They allow to deduce the aerodynamic performance of the wings based on this concept. However, there are many possible configurations and the aerodynamic coefficients depend significantly on them.
On request, we can establish the aerodynamic coefficients for a given configuration.
Flettner Rotor
Flettner Rotors are circular section cylinders that use the Magnus effect to generate a propulsive force. When the rotor is rotated, the air flow is accelerated by friction on one side and slowed down on the other. This difference in speed generates a pressure difference and that generates a strong lift.
This lift increases with the speed of rotation but is practically limited by the mechanics and by the power required for the rotation which increases very quickly.
The rotor is an active system, therefore a supply of energy is necessary to ensure the rotation of the system.
The aerodynamic performance of the rotors at the scale of the propulsion system is not well known. The most recent available wind tunnel tests show that the maximum lift does not vary with rotor size but that the lift to drag ratio degrades. The power required for rotation has been measured for the mid-section of the wing but the effect of the end discs on this power is not known.
On the other hand, the flow around the cylinder is very complex and has no aerodynamic equivalent that could be used to extrapolate the behaviour of the rotor to the size of the systems.
The CRAIN rotor aerodynamics coefficients are based on the largest scale tests and the aerodynamic experience of the CRAIN.
Suction Wing
The Suction Wing has a thick section, with a rear flap, which allows to achieve a very important lift coefficients thanks to an aspiration system that delays the flow separation around the section and accelerates the flow on the leading edge. Due to this high aerodynamic force, the system size can be reduced compared to other systems.
Suction is typically done by a fan located inside the wing, so it is an active system that requires energy input. On request, we can establish the aerodynamic coefficients for a different configuration.
The CRAIN has recently carried out extensive wind tunnel tests for this system. The proposed aerodynamic coefficients are based on the results of these tests for a flap and suction configuration. On request, we can establish the aerodynamic coefficients for a different configuration.
Dynamic kite
The dynamic kite consists of a wing designed to fly continously in altitude, where the wind is stronger, being held at the ship by one or more lines. The wing is driven to move at high speed on a eight shape trajectory without any input of external energy. The speed of movement increases dramatically the apparent wind on the wing and therefore the aerodynamic forces.
The aerodynamic characteristics of the wing are similar to those of a conventional wing, and it is only the dynamic behaviour of the system that allows it to achieve significant propulsive performance with a moderate surface.
In order to determine the potential savings of this system, CRAIN has developed a simulation tool that reproduces kite dynamics. The performance offered is based on this model.
Several parameters are involved in the performance of dynamic kites. The main ones are:
The lift to drag ratio of the kite. The better it will be the faster the kite will have the ability to fly and thus to increase the traction force. However, the speed could cause some control problems and challenge the resistance of the traction cables.
The elevation of the kite over the sea. The more horizontal is the wing, the more propulsive are the effort. However, the wing must maintain sufficient altitude to fly safely.
The lift coefficient relative to the apparent wind of the ship are very high but the lift to drag ratio is low. The kite is so far, a very typical system for large angles of apparent wind (sailing downwind).
In order to maintain realistic operating conditions, the lift to drag ratio, the tensile strength and the elevation angle are limited in SIMWAPS.
The proposed parameters and imitations are based on information provided by kite designers.
On request, we can digitally simulate a specific configuration.
Influence of ship
The ship is defined in SimWAPS by the following parameters, all of which are available when the user selects the option “Custom Ship”. When the user selects a specific vessel type, these parameters are calculated from the length of the vessel:
Speed of the ship: has a great influence on the energy production of the wind propulsion because the power developed by the WAPS is the product of the speed by the propulsive force. On the other hand, the speed of the ship is used in the calculation of the apparent wind.
Ship length: is used in SimWAPS to calculate the lateral surface of the ship which is involved in the calculation of the additional resistance due to drift as well as to evaluate the distance between the different WAPS devices in the calculation of the effect of interaction between WAPS devices. This parameter is also used to define other parameters when a ship type has been specified. Length is also used to assess ship displacement for stability criteria.
Vessel beam: used to calculate the distance between different devices of a WAPS installation. Beam is also used in the stability criterion.
Draught of the vessel: is used in SimWAPS to calculate the lateral surface of the vessel which is used in the calculation of the additional resistance due to drift mainly in the calculation of the hull resistance related to the lateral force generated by the propeller propulsion. The draft is also used for stability and excessive drift criteria.
Upper deck height: allows the user to set the height of the bottom of the wind propulsion system. True wind speed is corrected to this height.
When a vessel type is selected, two parameters are available:
Overall length [m]
Operating speed [kts]
The different validity ranges and default values are presented in “Appendix 2: Validity ranges and default values of the various parameters”.
The choice of vessel type and the adjustment of these two parameters make it possible to propose the other parameters of the vessel. The selected values are shown on the result page.
Influence of the operating route
The energy production of a WAPS depends heavily on wind speed and direction. In order to assess the average energy production on the vessel’s operating routes, the energy production for each wind angle and speed is combined with the wind statistics for the route. SimWAPS therefore offers different routes for which the wind statistics were calculated using the ERA5 product of the European Center for Medium Range Weather Forecast (ECMWF).
SimWAPS propose the weather statistics of the IMO Global Wind Matrix, derived over the main routes of shipping. SimWAPS provide also weather statistics over the individual routes that have been used to derive the statistics over the IMO Global Wind Matrix.
Weather statitics of the IMO Global Wind Matrix is coming from the IMO publication whereas weather statistics on individual routes, and return, have been calculated by CRAIN user ERA5 Re-Analysis from the ECMWF.
SimWAPS evaluates WAPS energy production on the direct route.
Weather routing, which adjusts the ship’s speed and course to take advantage of weather conditions in an optimal manner, can significantly increase the energy output of a WAPS.
CRAIN has its own routing tool adapted to commercial vessels equipped with WAPS and is able to determine the impact of weather routing on energy savings.
The following table shows the available route into SimWAPS and the return trip is also available. The True Wind Speed is obviously identical on the outbound trip as well on the return trip. True Wind Angle is assumed to be symmetrical, so in a range from 0° (headwind) to 180° (downwind). The sum of averaged true wind angle on the outbound trip and on the return trip is 180°.
Route |
Average TWS |
Average TWA (outbound / return) |
|---|---|---|
IMO Global Wind Matrix |
13.5 kt |
90° |
Sao Paulo -> Celtic Sea |
13.2 kt |
78° / 102° |
Sao Paulo -> Houston |
13.4 kt |
122° / 58° |
Sao Paulo -> Durban |
16.7 kt |
113° / 67° |
Shanghai -> Seattle |
16.1 kt |
106° / 74° |
Shanghai -> Singapore |
13.4 kt |
104° / 76° |
Celtic Sea -> New-York |
13.9 kt |
80° / 100° |
Suez -> Gibraltar |
11.7 kt |
73° / 107° |
Suez -> Singapore |
11.0 kt |
99° / 81° |
Gibraltar -> Houston |
13.7 kt |
147° / 33° |
Gibraltar -> Durban |
12.6 kt |
72° / 108° |
Panama -> Seattle |
10.8 kt |
70° / 110° |
Singapore -> Durban |
12.4 kt |
99° / 81° |