magnus effect sailboat

The Untold Myth of the Magnus Effect – How it is used in Boats and Sport

The Magnus effect is a physical phenomenon that occurs when a cylindrical shape spins and moves through a fluid. It is utilized in many different ways.

When you think of sailing boats, you will generally think of beautiful sculptured wooden pieces of art, with long white sails, dancing in wind of the ocean .

However, at one point and another, some engineers have and do make boats slightly differently. Sails were replaced with long rotating cylinders, and so the rotor ship was born. However, how did these boats work? Let’s talk about everything related to the Magnus effect.

Magnus Effect Explained

History behind the magnus effect, why do some boats use the magnus effect, examples of boats utilizing the magnus effect, magnus effect in sports, magnus effect experiment, utilizing the magnus effect for the future.

The Magnus effect is a fluid dynamics phenomenon that occurs when a cylindrical shape spins and has a velocity through a fluid. The rotational velocity of the cylindrical shape going through the fluid causes one side of the cylinder to have high pressure, with the other side having a low pressure. This exerts a perpendicular force on the cylindrical shape, to the velocity of the object through the fluid it is traveling through.

This works, in part, due to Bernoulli’s theorem on fluid dynamics: the faster the velocity of a fluid, the lower the pressure of the fluid.

When a cylinder moves through a fluid, it will have an equal velocity of fluid passing over it, on both sides. Introduce a rotational velocity to the cylinder, and the velocity of the fluid around each side of the cylinder changes:

• The side that is rotating in the same direction as the motion through the fluid has an increased fluid velocity – low pressure
• The side that is rotating against the direction of motion through the fluid has a decreased fluid velocity – high pressure

This occurs due to the interaction of the fluid environment and the surface of the cylindrical object in friction with the fluid.

Friction of the cylinder rotating with fluid → increased/decreased fluid velocity around each side of cylinder → perpendicular force to motion of travel through fluid

What is interesting regarding the Magnus effect is that the science of it has not been explored as in-depth as other areas of fluid dynamics, potentially due to the lack of capability of utilizing the physics behind it.

 Heinrich Gustav Magnus was the first person to describe the Magnus effect, back in 1852. Later, other physicists would describe the phenomenon, such as Isaac Newton and Benjamin Robins.

The Magnus effect was only truly industrialized in the 20th century by Anton Flettner, who utilized the experience of the likes of Albert Einstein to create the first ship that utilized the physical phenomenon with rotor sails. Hence, the name ‘Flettner rotor ship’ was born.

The Magnus effect was also used in World War II, by Barnes Wallis and the bouncing bomb he invented. It was made sure that the bomb had 500 rpm of backspin to give the cylindrical bomb lift to make it bounce several times over 700-800 meters.

Due to the nature of the Magnus effect, it was explored by some boats to replace sails and other forms of propulsion with rotating cylinders and still is. But, why?

• Efficiency – Combining a rotor sail with a typical propeller propulsion system, powered by a motor, can improve the efficiency of a vessel. The Viking Grace Ferry, actually estimated an improved efficiency of 6% from installing a rotor sail.
• Improved tacking course – The angle of tack is the direction of motion a sailed ship has to go, ‘zig-zagging’ due to the direction of the wind. Typically, sailing boats have to tack at an angle of 45 degrees to the wind. With sailing boats that utilize the Magnus effect, the angle of tacking can be greatly reduced down, to 20-30 degrees. This results in less severe ‘zig-zagging’.
• Quicker journeys – Due to an improved angle of tacking, the distance traveled is reduced, resulting in quicker and more efficient journeys.
• More robust – In heavy storms, sails can easily get damaged, and require maintenance throughout the journey. With rotor sails, they are much stronger and can withstand storms much better than sails, whilst also requiring less maintenance during the journey.

Owned by Germany’s Enercon GmbH, the cargo ship had four large rotor sails that generated the force from the Magnus effect. The boat was built in 2010 and, to accompany the Flettner rotors, had two propellors too.

Buckau Flettner Rotor Ship

Built by Anton Flettner, and even assisted by Albert Einstein, the Buckau was finished in 1924. It proved advantageous to sailed ships, in the sense that it could tack into the wind at 20-30 degrees, as opposed to sails at 45 degrees to the wind. The rotors were also less likely to be damaged in storms than the sails, adding to the reliability of the ship, powered by an electric motor.

Viking Grace Ferry Ship

A ferry ship with one 24 meter high rotor sail, the Viking Grace Ferry is relatively new compared to the above, finished in 2014. It is expected that the rotor sail will help save up to 6% of the fuel: the equivalent of €180,000 and 900 tones of carbon dioxide a year.

We’ve all seen those curling free-kicks from the likes of David Beckham, ‘bend it like Beckham’, and Lionel Messi. That’s done by the Magnus effect, from Beckham and Messi hitting the ball in such a way to provide a spin on the ball through the air.

Another great example is with Cristiano Ronaldo, that hits the ball with topspin. Instead of getting a sideways force on the ball to make it spin left/right, a topspin enables the ball to have a force exerted on it towards the ground. This ultimately allows the ball to dip very quickly, so Ronaldo’s free kicks are able to get up and over the walls very easily, whilst applying a lot of power.

Another example that uses the physics phenomenon is with another type of free-kick, by the likes of Gareth Bale and Cristiano Ronaldo again. This is where the ball is kicked in such a way, that the ball has a very little spin on it, if any, at all. This ultimately provides a very uncertain path of travel for the ball, since any brief spin on the ball will cause a such a force – if the spin changes ever so slightly in different directions, the motion of the ball can suddenly change too, making it even more difficult for the goalkeeper to predict the direction of travel.

Jabulani Ball – Germany 2006 World Cup

The Magnus effect was actually at fault during the Germany World Cup, where Adidas designed a ball, called the Jabulani ball, which had the seems stitched on the inside. This made the surface of the ball incredibly smooth, which made the ball much more unpredictable – the boundary layer of air influenced by the ball was reduced. This meant the effect of the Magnus effect was reduced, and natural variations in the ball/wind and other external factors had a greater influence. This is generally why you will see many balls in sports have a textured surface: to prevent such unpredicted behavior.

Other sports that utilize the Magnus effect include:

• Formula One – Using bodywork to direct the airflow away from the tires, not only due to the frontal area of the tires causing resistance but the high-pressure zones above the tires, caused by the Magnus effect.
• Baseball – Spinning the ball in different directions to put off the batsman.
• Tennis – Hitting a ball with topspin so that the ball dips quicker over the net. Slicing the ball allows for the ball to float more, giving more time for the player to recover/reposition.

Far and few experiments have been conducted on the Magnus effect to determine the physics behind it, and how the main variables involved with the Magnus effect affect the force exerted. Such variables include:

• Rotational velocity of the cylindrical object
• Surface friction coefficient of the cylindrical object
• Velocity of object through the fluid

Other variables can have an impact, including the fluid density and diameter of the cylindrical object.

Magnus Effect Equation

The Magnus Force’s equation is notorious online. For example, Wikipedia had an equation, which had been removed (as of 2013). This makes clear that to model the Magnus effect with an equation is an extremely difficult task. Nasa has attempted to model the Magnus effect through the following equation (for a cylindrical object):

F­m = Ժ4π²r²sv Magnus effect equation for a cylindrical object (source: Nasa )

• Ժ = fluid density.
• r = radius of the cylinder.
• s = rotational speed.
• v = fluid velocity.

However, this equation cannot be used because it does not take into account the surface coefficient of the cylinder which is a fundamental element to the Magnus effect: it is the interaction between the surface of the cylinder which will determine the size of the boundary layer which will affect the size of the force.

To look into this crudely, an experiment was conducted to see how the surface coefficient of friction of the cylinder affects the Magnus effect:

The cylindrical barrel will roll from a set displacement from the ramp into the water. As it enters the water it will, therefore, have a fixed velocity and rotational speed. The barrel, when spinning through the water, will produce a Magnus Force causing is to deflect to the left of the ramp (at the point of impact with the water).

Ultimately, although a crude experiment, it provides the basis that the Magnus effect equation, even supplied by NASA, is incomplete. There is still a lack of true clarity between the variables that affect the force exerted.

The Magnus effect works through creating a boundary layer /vortex of air around the cylinder which spins in sync with the cylinder. The problem is that with a smooth surface, the friction between the first layer of air and surface of the cylinder is minimal. Therefore, the boundary layer will be much smaller. The results, make clear, as the surface coefficient of friction increases, so does the boundary layer and Magnus force exerted.

The proof is in the pudding that the Magnus effect is a hard physical phenomenon to utilize in an effective way – the issue comes that there are cheaper alternative ways to gain propulsion or force.

The biggest issue surrounding the Magnus effect is the resistance it creates to achieve the force. Not only is the area of the cylindrical object generally on the larger side, but it also requires an external power source to rotate the object.

However, when installed in the correct manner, there is the hope of improving the efficiency of large ships. The Viking Grace Ferry is a prime example of this. However, without a firm grasp of the physical equation behind the Magnus effect, it is difficult o fully utilize it. For example, the surface coefficient of friction that the rotor ships use for their rotor sails is a question that clearly has not been explored, and, therefore, has no basis as to how much that can influence efficiency (instead of spinning the rotor quicker).

As well as this, the issue stems also with the upfront design, manufacturing, and testing cost. With the world moving towards becoming more eco-friendly, maybe the Magnus effect is something we will start to see on more and more on small and large ships alike…

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Anemoi is a global provider of proven wind technology, driving a more sustainable future for the shipping industry

What are Rotor Sails?

Rotor Sails, originally known as Flettner Rotors, are an energy saving technology for the shipping industry. These modern mechanical sails are comprised of tall cylinders which, when driven to spin, harness the renewable power of the wind to propel ships. As a result, this additional thrust significantly reduces fuel consumption and lowers harmful emissions entering our atmosphere.

Our mission

Every year the shipping industry releases almost a billion tonnes of CO 2 into the atmosphere. Like many other organisations in the industry, Anemoi would like to see this figure reduced dramatically and one day removed all together. To achieve this goal, industry parties need to join forces to develop and adopt emission reduction solutions.

Anemoi Rotor Sails can reduce emissions by up to 30% and, with our patented Deployment Systems, be installed on most merchant vessels. Our mission is to accelerate the maritime industry’s transition to zero emission shipping by delivering market-leading wind technology.

The history of Anemoi

Rising emissions.

Research & Development

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m/v Axios ‘Wind-Ready’ Installation

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A sustainable future of shipping

Anemoi Management Team

Kim Diederichsen Chief Executive Officer

Kim, who joined Anemoi in November 2019, has a proven track record of successfully commercialising businesses in the Marine, Oil and Gas industries. Kim was educated at the Fanø Marine Academy, where he was certified as a Master Mariner and went on to be a Chief Officer for a large ship management company. He then turned his focus to the commercial aspects of the industry, retraining at Aarhus University and moving into sales, business development and later into Chief Executive roles.

Specialising in business expansion, strategic development and opportunity identification, Kim utilises his Marine background to take a hands-on approach to leadership. He is pleased to be taking Anemoi on its journey to commercialisation, with an innovative, forward-looking product. As a Dane living in London, Kim enjoys showing his family the London sights as well as exploring the countryside with nature walks and visiting historical landmarks.

Clare Urmston Chief Financial Officer

Clare joined Anemoi in early 2020, bringing her extensive commercial knowledge from a range of industries. After attending the University of Sheffield, Clare formally trained with one of the big four accounting firms and has gone on to hold numerous executive and non-executive positions in the retail, manufacturing and healthcare sectors. Having not worked in the shipping industry before, Clare is enthusiastic about the new and exciting challenge that Anemoi poses. She is motivated by the collaborative, start-up culture and the huge opportunity to take the business from Research and Development to full commercialisation.

When she’s not in the business, Clare enjoys spending time with her family, property interiors and exercising. She has a keen interest in animal conservation and says that Anemoi’s eco-friendly mission is her opportunity to give back to nature.

Nick Contopoulos Chief Operating Officer

Nick is a Chartered Engineer with a degree in Engineering from the University of Exeter. He has over 18 years’ experience across multi-disciplinary engineering and shipping sectors, with a proven track record of delivering complex engineering projects in Europe and the Far East. Nick is one of the founding members of Anemoi which he took a lead role in setting up, following an established career working at a prestigious ship owning company and as a consultant at some of the world’s largest providers of professional and technical services.

Nick is extremely passionate about innovation and the environment, so delivering renewable wind propulsion solutions to the shipping and maritime sector is a perfect fit as the industry looks for sustainable ways to operate and lower its environmental impact on the natural world.

Luke McEwen Technical Director

Luke joined Anemoi as Technical Director in August 2020. He previously led a team of 30 engineers in a multinational design consultancy, where he gathered 20 years’ experience designing large, high-strength, lightweight structures in composite materials for marine and wind energy applications. Luke is a chartered engineer with a Masters degree in Engineering from Cambridge University.

Sailing is Luke’s passion; he has sailed around the world in a 35’ yacht and won several national and European championships in racing dinghies, so he understands the power of harnessing the wind at sea, as well as the unique challenges of the marine environment. Luke strongly believes that Rotor Sail technology can substantially reduce the environmental impact of shipping and is committed to helping Anemoi make that happen.

Liam Campbell Projects Director

Liam is an experienced maritime professional, having worked in senior and director positions in shipyards, classification societies and shipping companies. He joined Anemoi in early 2023 and brought a wealth of strategic growth and business transformation experience.

Liam has managed large organisations and ship newbuild and refit programmes in four different countries for both shipyards and owners. He was also previously responsible for selection of concepts and delivery of the energy efficiency programme for two major ship owners. Liam is a Naval Architecture and Offshore Engineering graduate from the University of Strathclyde, Glasgow, a Chartered Engineer and a Fellow of the Royal Institute of Naval Architects.

When he’s not delivering projects, Liam enjoys spending time with his family and friends, cycling and hill walking.

Anemoi Board of Directors

Dimitri goulandris chairman of the board.

Dimitri set up and runs The Cycladic Group, an investor in, and creator of businesses.  Founded in 2002 to invest capital on behalf of his family and other investors, the Group has invested in over 40 businesses and founded eight in the U.S., Europe, India, Africa and Latin America.

He previously worked in private equity for Whitney & Company and Morgan Stanley having graduated with an MA and BA in Electrical and Information Sciences from Cambridge University, an MBA from the Harvard Business School. He also successfully completed the Singularity University Executive Programme.

Dimitri is an active board member and investor to a number of other emerging companies, in addition to being actively involved in non-for-profit organisations. He is passionate about children’s mental health and learning differences and spends significant time focusing on this area both on a for-profit as well as a charitable basis. He lives in London with his wife, Christina, and their three children.

Alex Singer Non-Executive Director

Alex has served on Anemoi’s Board since 2017. He provides wealth management and investment advice as principal at Kallisto Wealth & Investments, a Multi-Family Office.

Alex is a Chartered Financial Analyst (CFA) charterholder and studied Philosophy and Neuroscience at Brown University, Biophysics at Johns Hopkins and holds an MBA from INSEAD. Earlier he worked as a research scientist at Thinking Machines Corp. Alex enjoys hiking and astronomy.

Duncan Stringfellow Founder

Duncan is an accomplished engineer with varied experience in delivering complex engineering projects. He studied General Engineering at Cambridge University before spending 14 years working for a major international Engineering Consultancy where he took on roles of increasing responsibility and technical complexity in buildings, masts, towers, bridges and complex moving structures. Later, Duncan worked as the technical lead on Anemoi’s pilot projects; the m/v Afros and m/v Axios. Duncan is passionate about innovation and the adoption of green technologies to address today’s environmental challenges.

Nick Contopoulos Founder

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Rotor Sails: The Future of Sustainable Shipping

Imagine reducing the shipping industry’s carbon footprint by a significant margin while also saving on fuel costs.

How, you ask? Welcome to the future of sustainable shipping – Rotor Sails.

These engineering marvels employ the Magnus effect to generate propulsion, offering a green alternative in an industry traditionally dominated by fossil fuels .

Our in-depth exploration of this innovative wind propulsion technology will cover everything from its scientific principles to real-world applications, impact on sustainability, and future projections. Join us as we sail into a greener tomorrow!

What are Rotor Sails?

Rotor sails are innovative wind propulsion devices installed on ships, utilizing the Magnus effect to enhance their energy efficiency and significantly reduce fuel consumption and emissions.

They are tall, spinning cylinders designed to capture wind energy and convert it into a forward thrust. The process begins when wind meets the spinning rotor sail, which accelerates the airflow on one side and decelerates it on the other, creating a pressure difference. This pressure disparity then generates a lift force perpendicular to the wind direction, providing an additional forward thrust to the ship.

Rotor sails stand out for their ability to deliver greater thrust force per square meter of sail area compared to other wind propulsion technologies, making them a promising solution in the pursuit of sustainable shipping . By implementing rotor sails, ships can minimize their reliance on fossil fuels, leading to lower operating costs and reduced environmental impact.

Their practical design includes an end plate for optimizing aerodynamic efficiency and a control panel for manual operation. Additionally, many rotor sail systems come with automated features that can monitor wind speed and direction, adjusting the rotational direction and RPM for optimal fuel savings.

The introduction of rotor sails to the maritime industry marks a significant step towards greener, more sustainable shipping practices. As the world grapples with the pressing need to reduce carbon emissions, rotor sails offer a viable path to an eco-friendly future in the shipping sector.

Named after their inventor, Anton Flettner, a German aviation engineer, rotor sails are ground-breaking wind propulsion systems that have been revolutionizing the maritime industry since their conception in 1920.

Rotor sails are designed to augment vessel efficiency by considerably reducing fuel consumption, bunker costs, and harmful emissions. They stand out from other wind propulsion technologies due to their unique ability to generate a significantly higher thrust force per square meter of sail area.

What Is The Magnus Effect?

Ever wondered why a tennis or golf ball curves during its flight path? The scientific reason behind this is the Magnus effect.

Magnus effect is when wind meets the spinning rotor sail, the airflow accelerates on one side of the rotor sail and decelerates on the opposite side creating a pressure difference.

Thrust is produced by the rotation of the sails in an airflow, which results in a pressure differential on the rotor’s aft and front surfaces. This thrust force is determined by wind angle and speed; the greatest thrust is produced when the wind direction is directly from the beam.

In operation when the rotor is switched on and starts rotating in presence of wind, the change in the speed of airflow results in a pressure difference which creates a lift force that is perpendicular to the direction of wind flow. This provides the ship with an additional forward force which reduces the engine’s fuel consumption .

How Does The Rotor Function?

There is an end plate fitted on the rotor which is used to optimize its aerodynamic efficiency. The crew has control of the rotor sails from the control panel on the bridge, also some systems can automatically monitor the wind and speed direction to determine the optimal rotational direction and RPM values which thereby maximizes the fuel savings for the vessel.

The rotor sails are mounted on vessels’ tailored foundations which also house a variable drive motor that can control the speed and direction of the rotor.

Flettner rotors are vertical cylinders that spin and generate lift forces due to the Magnus effect. These lift forces support the overall thrust forces acting on the hull for propulsion thereby reducing the loads on the main engines .

The total number, size, and location of rotor sails depend on the following factors,

• Available deck space
• Location of the structural members on the deck
• Vessel operation parameters and functions

What Are The Benefits Of Rotor Sails?

Most rotor sail manufacturers make sure that the rotors are designed and certified in line with the norms of the world’s main classification societies , assuring compliance with the highest maritime quality requirements.

The main benefits of rotor sails are ship performance and environment improvement, lightweight and flexibility, endurance, safety and compatibility, and versatility.

• Lightweight – Rotor Sails are made of the most modern composite structures and the most advanced manufacturing process to come up with lightweight yet strong materials to ensure exceptional performance while adding as little weight as possible. A sturdy steel tower at the center of the Rotor Sail securely transfers all loads into the ship structure.
• Endurance – By Class Requirements, rotors are intended to endure the most severe weather conditions as they stand very high above the main deck. Rotors can be designed for high seas and winds to withstand up to 35m/s (70kts). Some automated rotor sails automatically shut down in harsh situations that exceed their working limits. Reefing is not necessary, as it is with other wind technologies.
• Compliant with regulatory bodies – A 5-30% reduction in hazardous emissions such as CO2, SOx, and NOx benefits the environment while also assisting vessels in meeting regulatory requirements and objectives. Rotor Sails have a favorable influence on a newbuild vessel’s EEDI score and can give a path to compliance with the forthcoming 2025 Phase 3 standards, as well as improve possible operational indicators for retrofit installations (e.g. EEXI and CII).
• Performance benefits – Annual economic savings potential of 100 000 – 200 000 EUR per Rotor Sail under global average wind conditions.  Typical average fuel savings of 5-20% per ship.
• Environment Benefits – The biggest Rotor Sail type has an annual average savings potential of roughly 300-350 kW. Annual fuel savings of around 300-400 tons. A 900-1200 ton decrease in yearly CO2 emissions .
• Movable – Sails may be redeployed between vessels based on operational needs, giving owners confidence in the lifetime of their investment and ensuring return.
• Safety and compatibility – Rotor Sails are appropriate for a wide range of vessel types, including newbuild and retrofit, Bulk Carriers (geared and gearless), Tankers, LPG and LNG Carriers , Ferries, and RoRo ships .

Rotor Sails Technical and Materials

The Rotor Sail System is made up of the Rotor Sail, the Foundation, the Deployment System (if needed), wind sensors, and the Electrical and Control System. The following diagram depicts the position of the equipment.

The “Rotor” (the cylindrical, revolving section), the Tower, the Upper, and Lower bearings, and the electrical motor are the four basic components of our Rotor Sails. The “Rotor” is composed of lightweight composite material, while the Tower is made of steel columns.

The material selection is based on guaranteeing that the Rotor Sails operate well in all-weather situations. with the use of advanced composite like carbon fibers , the rotor sail systems are often less than 0.1% of the vessel’s deadweight.

Rotor Sails Controls And Electronics

A Control Station for the Rotor Sails is positioned on the bridge. This automatically regulates the speed and direction of the Rotor Sails while also monitoring the system’s performance and status. With automatic speed and direction setting, equipment monitoring, and safety shutdowns, the rotor sail control system is designed to maximize performance while minimizing crew involvement.

The electrical input power needs of the Rotor Sails vary with Rotor speed, which is affected by wind speed. When operational, the average power input of one Rotor Sail is approximately 40% of the rated power of the motor. A Rotor Sail with a 75kW motor, for example, would utilize 30kW on average.

Leading Rotor Sail Technology Providers

The following are a few of the major industry-leading companies that provide rotor sail technology and have successfully installed them on different kinds of vessels.

A company named NORSEPOWER which is a leading sail technology provider has already installed rotor sails on different types of cargo ships such as tankers, dry cargo, and passenger vessels. Norsepower’s Rotor Sail system is the first complete commercial mechanical sail that has been third-party confirmed. The completely automated control system detects favorable wind conditions and operates the Rotor Sails with no intervention necessary from the crew.

The company has recently installed two 35m high world-first tiltable rotor sails, which allow the vessel plying in height-resisted routes to benefit from this technology to save fuel. The range of cost for a Flettner rotor varies from $1 000 000-$4 000 000 depending on the size and utility.

This company was started in 2007 to combat the global emissions from the shipping industry and to come up with a design solution that could revolutionize the industry. From the period 2008-2014, they started working on the Flettner rotor technology and even developed a full-scale model for testing and trials.

At present, they deliver clients with outstanding design solutions which include rotor sails that can be retractable and even removable which gives the owner flexibility in different operational conditions.

Hyundai, based in South Korea, has been working on a new rotor sail design to help in environmentally friendly shipping throughout the world. This is gaining popularity in the maritime sector as a feasible method for reducing carbon emissions. When in use, this new system is said to reduce carbon emissions and fuel consumption by 6% to 8%.

Hyundai stated that its design “improves the stability of the drive system compared to the belt method of existing commercial products by applying the reduction gear method to the driving part connecting the electric motor and the rotor.”

The system has been named the Hi-Rotor, and Hyundai expects to undertake land demonstrations with it by the end of 2022, after having their Rotor Sail system Approved in Principle by the Korean Register of Shipping .

Rotor Sails Summary

The rotor sails are efficient solutions in terms of reducing fuel consumption and emissions and can have a huge role in decarbonization in the maritime industry . Shortly, we can find more vessels that implement the rotor sail technology making them more efficient.

What are Rotor Sails and how do they work?

Rotor sails are wind propulsion devices on ships. They utilize the Magnus effect to enhance energy efficiency, converting wind energy into forward thrust and reducing fuel consumption and emissions.

What is the Magnus Effect and how does it apply to Rotor Sails?

The Magnus effect explains the curve of spinning objects in flight. In Rotor Sails, it creates a pressure difference when wind meets the spinning sail, generating a lift force and additional forward thrust.

What are the benefits of using Rotor Sails?

Rotor Sails enhance ship performance, are lightweight and flexible, endure severe weather conditions, and comply with maritime regulatory bodies. They reduce fuel consumption and harmful emissions like CO2 , SOx, and NOx.

Which companies provide Rotor Sail technology?

Norsepower, Anemoi, and Hyundai are industry leaders in providing Rotor Sail technology. They’ve installed rotor sails on various types of vessels.

What materials are used to construct Rotor Sails and how are they controlled?

Rotor Sails are made from lightweight composite materials and steel. Control stations regulate the sails’ speed and direction, with some systems offering automatic monitoring for optimal operation.

About the author

I worked as an officer in the deck department on various types of vessels, including oil and chemical tankers, LPG carriers, and even reefer and TSHD in the early years. Currently employed as Marine Surveyor carrying cargo, draft, bunker, and warranty survey.

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Flettner and Magnus

I add this section just for completeness. Flettner rotors and the Magnus effect have no direct bearing on sail lift. However, they are a part of fluid dynamics, and a couple of ships have been powered by the force of the wind in this way, so read on. Suppose, instead of considering the circulation of air about the airfoil, we consider airfoils that are circulating in the air. If this rather flippant way of introducing the bizarre Flettner rotor suggests that I am talking about two faces of the same coin, well, I suppose there is a sense in which this may be true. You decide.

The Flettner rotor* was developed in the 1920s as a radically different way to propel boats and ships via wind power. A vessel propelled by this strange mechanism is shown in figure A.14. It looks more like a giant floating candlestick than a boat. The idea is that the vertical column, which has a roughened surface, rotates and carries air with it as a result of the Coanda effect. Here, we have another example of Galilean relativity: it doesn't matter if the air moves and the airfoil surface is stationary, or if the air is stationary and the airfoil moves. All that the Coanda effect requires is relative movement between air and airfoil. In fact, in our frame of reference both air and airfoil are

*Named after Anton Flettner, a German aviation engineer.

moving: the air moves at the wind speed, and the rotor moves at a controllable rotation rate.

The effect of the rotating cylinder on the airflow is illustrated in figure A.15. The streamlines become curved as the airflow is deflected by the cylinder. Why does rotation cause airflow deflection? There would be no net deflection without rotation, after all. The explanation is as follows. The thin boundary layer of air that is carried along the cylinder surface becomes detached when the airflow speed relative to the cylinder becomes too great, just as we expect from the Coanda effect (fig. A.7b). When the cylinder is not rotating, the separation points are symmetric, one on either side of the cylinder. When the cylinder is rotating, however, the separation points migrate to the surface region where relative air speed is highest—in the case of figure A.15, this is the aft portion of the cylinder. In detail, the air separates from the cylinder as vortices, which create circulation as we saw in figure A.4. Calculating the lift that arises from this circulation field yields a force in the direction shown in figure A.15. The ship heading can be adjusted by altering cylinder rotation speed as well as by steering the rudder.

I have given this brief explanation of how a Flettner rotor generates lift force—with the boundary layer peeling off asymmetrically—from the Euler/ Bernoulli viewpoint. The same mechanism, given the name of the Magnus effect, after the German engineer who proposed it, can be explained via the naive momentum flux approach as follows: ''Airflow is deflected by the rotating cylinder and so, from Newton's Third Law, the ship experiences a reaction oppos-

Figure A.15. The Flettner rotor. A rotating drum replaces the sail. The drum drags air around its surface (Coanda effect), but this air detaches where the relative speed of airflow and drum surface is high (here, the aft section of the rotor), creating an asymmetric flow (Magnus effect). From the momentum flux perspective, the airflow is deflected, and lift is provided in the direction shown.

ing this deflection, which is the lift force." The more sophisticated version of momentum flux can fill in some details: "The rotating cylinder causes streamlines to curve because of the Coanda effect; the curvature gives rise to lift in the observed direction by generating pressure gradients." Whichever explanation you prefer, the effect is real enough. Flettner was able to propel a ship across the Atlantic in 1926 using two 50-ft-diameter rotors. He found that the vessel worked well to windward and in bad weather. Unfortunately, it was less efficient than conventional steamships or sailing vessels, and so Flettner rotors were never commercially viable.

But the physics is interesting. A number of popular-science articles explain the Magnus effect (which is responsible for the curve of a baseball or soccer ball in flight,* as well as for Flettner rotor motion), and several websites present video demonstrations of it (see the bibliography for details of these).

*''Bend it like Magnus.'' I doubt if David Beckham, the English soccer star famous for his ability to send a soccer ball on a curved trajectory past helpless goalkeepers, does circulation calculations prior to each shot. He applies ''English''—spin—to the ball, and Magnus does the rest.

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Readers' Questions

How to build a flettner rotor?

Gather the necessary materials and tools: marine grade stainless steel, welding equipment, drill, reciprocating saw, heavy duty scissors, measuring tape, and stainless steel screws. Measure and cut the stainless steel pieces to the appropriate dimensions for your Flettner rotor; usually a cylindrical tube with a diameter of 2.4-2.5 meters and a length of 2.5-3 meters. Assemble the pieces together, welding them at the seams. Drill mounting holes in the top and bottom of the cylinder. Attach the top and bottom plates to the rotor by bolting them through the mounting holes. Cut slots in the rotor blade to create an airfoil shape. Drill holes in the center of the rotor blade and attach a motor to the rotor blade. Connect the motor to a power source and test the rotor. Adjust the motor and rotor blade to achieve optimal performance and maximum lift.

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Spinning up a solution to rolling

R olling underway? Ugh! It’s the bane of power boating. It turns some folks green.  

Over the years, various methods of dampening this sickening side-to-side motion have been developed —paravanes, stabilizer fins and gyro stabilization. Now comes the new, new thing: rotor stabilization.  

We were recently given the opportunity to sea trial the very first American installation of one of these systems. As life-long delivery skippers who’ve run hundreds of passage-making motor vessels, sometimes by necessity in heavy seas, we were very curious to check out this rotor stabilization system.  

According to their Dutch maker Dynamic Marine Systems, rotor stabilization is an all-electric method suited for low-speed displacement and semi-displacement motor yachts of up to 30 meters LOA. Called the MagnusMaster, it relies on the “Magnus Effect,” first discovered by Heinrich Gustav Magnus in the mid-nineteenth century.  

Magnus wanted to find out why spinning artillery shells had a trajectory deviation. By definition the effect works this way: “If you pass a rapidly rotating smooth object through a fluid medium, a resulting force is created on the object causing its path to be deflected.”  

In other words the rotor acts like an airplane wing, causing lift. On a boat, it’s the movement through the water that produces flow over the spinning rotors, which in turn produces either an upward or downward force — depending on the direction the rotors spin.

First installation These first Magnus rotors in the US were installed in April, 2022, in San Diego on a Cape Horn 58 trawler Lahaina Sailor , owned by Dave Abrams. He’d purchased the boat at the Seattle Boats Afloat show in 2016, but she was built in 2000 in eastern Canada. Lahaina Sailor has an ice-rated steel double hull with bilge keels. Carrying 4,300 gallons of fuel Lahaina Sailor has a range of 5,000 miles at 8 knots. Turned out, that’s well within the 12-knot maximum speed allowable for the Magnus to work.  

“She will take us anywhere in safety,” Abrams said of Lahaina Sailor , “and can handle a lot more than the crew can. But her only short coming was she was not stabilized. She has a rather flat stern section and would roll like a pig in quartering seas.”

After making trips up into British Columbia, then down to Mexico’s Sea of Cortez and then back to her home base in San Diego, Abrams realized that his beloved Lahaina Sailor ’s one negative characteristic was almost enough to put her up for sale and look for a boat that didn’t roll so badly.  

At the same time, he started to investigate stabilization. He determined that gyros wouldn’t work because of her 86-ton displacement; the necessary multiple systems would take up too much space. Fins wouldn’t work either, because of her double hull with fuel in between and bilge keels.

Abrams also investigated DMS Magnus Master and eventually decided to go with it. Seeing that a retrofit stabilization would work for Lahaina Sailor , he decided to stick with the boat.  

He said he determined DMS was the best fit for several reasons. Their system could be installed in his engine room and give easy access to the actuator motors. Also having it back aft was best placement for access and had ample space. A big plus was that this aft placement would counter Lahaina Sailor ’s roll where it was occurring, in the aft section of the vessel where the hull was getting lifted in following and quartering seas.  

He liked the idea of an all-electric systems; he could run it off either his generator or his inverters and would not have to install additional hydraulic components as fin stabilization would have required. He liked being able to put the rotors in a park position whenever stabilization was not needed, thus eliminating any speed reduction penalty from when they were in use. And for safety, internal shear bolts prevent hull damage in case of impact to the external rotors.

Due to Lahaina Sailor ’s double-hulled steel hull, Abrams’ two main considerations were 1) where to place the actuators and 2) how to mount them to maintain structural integrity. DMS came up with a unique mounting-box system and also sent him a technician from Holland to help with the installation during Lahaina Sailor ’s haul out in San Diego.  

Hooking up the actuators was pretty straightforward because the electrical control box is right near them, and he just had to run wiring to the actuators and up to the control head in the pilot house.

Poking around on board When we came on board Lahaina Sailor at her slip at a Shelter Island marina, Dave and Amanda Abrams gave us the tour of the boat, and we studied lots of photos and videos taken before and during their recent haul out when the DMS was installed.

Next was seeing the DMS installation in the engine room, all the way aft. We could see that, although this is a double hulled boat, the floor of the engine room is not. If it had been doubled, then the installation would have been considerably more complicated.  

We remarked about the ample open space. Abrams showed us where, in order to open more room around where the port and starboard DMS motors and thru-hulls were planned to go, they decided to first remove an old wing engine that had been installed closer to the center line by the previous owner. Visible there now was a large flat welded steel patch covering the 12-inch round hole where that wing-engine shaft had previously passed through the hull.

All the DMS stuff is installed pretty compactly right where the engine-room floor meets the hull walls, on both the port and starboard sides. What you see is the top of the steel mounting boxes that Abrams had designed and approved by DMS specifically for Lahaina Sailor ’s specs. The mounting boxes were welded securely between the boat’s steel frames and stringers, beefing up the area. But the mounting boxes mainly provide new stable platforms about inches above the actual floor where the DMS actuator motors and thru-hulls were installed.  

Atop each mounting box is the DMS actuator motor. Mounted on a bulkhead nearby is that motor’s electrical control box, and from it four color coded wires lead forward, to the electronic operational panel up on the bridge.

What you can’t see is the actuator’s five-inch diameter steel shaft that houses the minimal wiring bundle as it goes straight down through the mounting box and through the vessel’s single 5/8-inch steel hull. If we were diving today, we’d see that just below the hull on each side, the actuator motor shaft goes into a stubby 90 degree elbow from which protrudes the actual DMS cylindrical rotors. The cylindrical rotors are smooth, black carbon fiber, no blades or fins.  

With Lahaina Sailor parked in her slip, the rotors are pivoted aft in “park mode.” But when underway and deployed the rotor cylinders swing forward until they’re perpendicular to the center line of the hull.  

Abrams installed a new nine-kilowatt Northern Lights generator to provide power solely to the DMS motors. Lahaina Sailor also has six-kw worth of inverter power, and each rotor requires only 1.5 kilowatts to operate at max load.  

Sea trials off San Diego Up in the pilot house. Abrams lit off this new generator and the main engine (John Deere 6068 turbo), and he flipped a breaker to energize the DMS electronic control panel. Then we headed out to sea.

Still in the harbor making a steady 7.6 knots, Dave demonstrated how to operate DMS’s touch-screen control panel, which he had mounted conveniently at about four o’clock from the wheel. At first, the rotors were locked in “park” position, indicated on the touch-screen panel like two stubby wings swept aft. So to deploy the stabilization, we first unlock them (red light touched off) and then touch deploy.  

The panel’s graphic display showed the rotors gradually swinging outboard or 90° from their swept aft in park position. It took exactly 12.5 seconds for the DMS rotors to swing outboard and begin stabilizing the boat.  

Did we feel it? Yes, as the rotors worked (rotating up to 1,000 RPM), we could immediately feel the boat’s slight rolling motion steady up. We were still in San Diego’s harbor channel and making less than eight knots.  

Abrams demonstrated how, by moving the main engine control lever into neutral, the DMS rotors automatically swung back into their park position. This could be considered a safety feature, so the pilot can’t forget and accidentally damage the rotors while coming alongside a dock or into a slip.  

It also demonstrates why DMS stabilization is not available when you’re having to pop the main propulsion in and out of gear, such as during close maneuvering. More significantly, it shows why it’s not available while the boat is at anchor.

Again, we deployed the rotors as we began to feel long ocean swells at the seaward end of Zuniga Jetty. This time we watched two different inclinometers. Lahaina Sailor went from 1.5 degrees of roll down to zero as the rotors swung into action.  

Before we reached open ocean conditions, Abrams instructed us to grab on to something, because he was going to demonstrate “forced roll” mode. We complied, and when he activated it, the rotors spun in the opposite directions from their normal stabilization mode. It was astonishing how quickly this 85-ton vessel doing just a little less than eight knots heeled over to 10 degrees by the inclinometers. It became obvious what a significant amount of torque the Magnus effect exerts on a vessel.  

A mile outside the shelter of Point Loma, we had six- to eight-knot WNW wind with initially only about a one- to two-foot swell right on the beam. Without the stabilizers engaged we had a five-degree max roll. With the MagnusMaster units engaged that roll was reduced by about 80%, based on the inclinometers.

Discouraged by such light seas, we decided to induce our own roll, so we performed several helm-hard-over maneuvers in different directions relative to the swell, which was finally building. Without the DMS stabilizers engaged we noted a 12-degree roll on both the inclinometers, and on the same maneuver with the DMS rotors engaged we got a six degree roll. In this case, a roughly 50 percent reduction during hard maneuvers of the kind that you rarely need to perform.  

Out past the San Diego sea buoy, we finally had white caps and bigger swell. We continued testing the stabilizers’ effect by turning down swell with the swell over our quarter, a couple times over both port and starboard quarters.  

Quartering: this was the exact situation where Lahaina Sailor had always rolled her worst. This was her only negative characteristic, but it was the one that almost got her sold off.  

During our tests, while quartering she consistently experienced six-degree rolls without stabilizers, yet amazingly registered less than one degree of roll in nearly identical down-swell runs with the DMS stabilizers engaged. We think she passed her exam.

On the way back into San Diego Bay, we checked our speed with and without stabilizers engaged. We calculated that the stabilization had cost Lahaina Sailor 0.4 knots of speed. Not bad.

Admittedly our number of data points was quite low, but even in light seas we could see that the Magnus effect is a real thing.

Lahaina Sailor ’s owners loved her enough, despite her roll tendency, to invest in a rotor stabilization retrofit. n

Pat and John Rains are both licensed masters, power voyaging delivery captains and authors based in San Diego.  

Researchers are looking to a surprisingly old idea for the next generation of ships: wind power

Lecturer in energy and transport, UCL

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Disclosure statement

Tristan Smith consults to a number of organisations on the subject of wind assistance technologies. He receives funding from both the UK government, and a number of industry and NGO parties, to undertake work to understand the mix of technologies (including wind assistance) and policy solutions which might enable shipping to transition to lower carbon emissions.

Nishatabbas Rehmatulla works for UCL Energy Institute and receives funding from the UK government (EPSRC) and a number of industry parties and NGO's to undertake research on barriers to implementation of technologies that enable shipping to reduce its CO2 emissions.

University College London provides funding as a founding partner of The Conversation UK.

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In many ways, it’s an obvious solution. For many centuries, world trade over the oceans was propelled by wind power alone. Now that we’re seeking an alternative to the fossil fuel-burning vehicles that enable our modern standard of living, some people are turning again to renewable solutions such as wind to power our tankers, bulk carriers and container ships. Globalisation and economic growth might mean a direct reversion to the wooden sailing boats of yore makes no sense, but there are several 21st-century ideas that could make wind-powered shipping commonplace again.

Ship design certainly has a way to go to return to its heritage and take advantage of the wind’s free, renewable resource in the same way we have reinvented the windmill to produce electricity. However, it’s worth remembering wind turbines took a long time to evolve into the structures optimised and deployed at scale we have today. In fact, they’re still developing. Scientists and engineers have debated for years about the relative merits of two, three or more blades, of horizontal versus vertical configurations, and of onshore versus offshore generation.

For ships, the design process for wind technologies is potentially even more complicated and multi-dimensional. There are soft sails, rigid “wing” sails, flettner rotors (a spinning cylindrical vertical column that creates lift using the Magnus effect , originally conceived by Flettner in the 1920s) and kites all vying for a share of this market. Soft sails are fabric sails, most reminiscent of existing sailing ship designs, examples include the Dynarig and Fastrig . Rigid wing sails replace the fabric with a rigid lifting surface like a vertically mounted aircraft wing - for example the oceanfoil design .

A flettner rotor is a vertical cylinder rotated by a motor. The rotation modifies the air flowing around the cylinder to generate lift much like the lift generated by an aircraft wing (it’s referred to as the Magnus effect). While there are many examples of all four, so far it’s the kites and the flettners that have seen the most significant implementation on large merchant ship designs.

Notable examples include the work that Cargill and Wessels have done trialing kite systems , and the experience of two separate operators, Enercon and Norsepower with installations of different flettner designs on different ships. These trials have produced important full-scale experience, lessons about costs, performance data, and evidence for investment cases. All of which are undoubtedly taking us closer to the tipping point when wind once again becomes a ‘no brainer’.

Trials of these new technologies, in combination with the history of wind turbines, can help us understand why any transition to modern wind-powered ships won’t happen overnight. For one thing, no one yet knows which of the many candidate designs will be the most successful.

Modern wind-powered shipping technology also carries a significant engineering challenge that wind turbines don’t: it needs to be mobile. It’s not as simple as bolting a rig to the deck. The highest safety standards have to be maintained and the rig must pose no constraints to loading and unloading cargoes in an uncertain and wide range of different ports (many of which might be obstructed by bridges).

Resolving these issues will take time, money and investors with the appetite for risk and stamina to see an emerging technology from a prototype to a fully developed new product. But I believe the change will happen because of the price of fossil fuels and environmental regulation. Wind power is free so the technology will become a worthwhile investment once it can be clearly evidenced that the saving from moving away from fossil fuels outweighs the costs of installing and operating a wind-powered ship.

Many think that threshold oil price has already been achieved and exceeded, as evidenced by the large and growing number of projects proposing wind propulsion solutions, even allowing for the recent fall in oil prices.

While there is currently only weak regulation on shipping’s greenhouse gas emissons, the sector – like all those producing carbon dioxide – is likely to face more stringent controls as its emissions continue to grow . Exactly what form such controls will take remains the subject of further ongoing work. But any meaningful regulation would reinforce the case for wind-powered shipping as a favourable investment.

Shipping is a vital, if somewhat hidden, part of modern economies. Decarbonising those economies is the only way to avoid destroying them (and the environment). Wind power presents an astoundingly obvious and elegant solution to these combined challenges. But it will languish in the sidelines until we see rapid change from investors, politicians, or ideally both.

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Flettner Rotor For Ships – Uses, History And Problems

When one thinks of a ship’s propulsion system , the conventional propeller and rudder setup is what comes to mind. While there are many other forms of powering a craft (such as waterjet propulsion , sail power etc.), a rather uncommon type of propulsion system is the Flettner rotors.

Rarely used on commercial vessels, it is an experimental concept in wind-based power generation that has been in development since the 1920s.

You can spot a Flettner rotor ship easily by the large stacks or rotors that project from the deck of the ship.

In this article, we will take a look at the rotors, the driving physical concepts, and ships that use this form of propulsion and stabilization.

Table of Contents

What Is A Flettner Rotor?

Flettner rotors are an unconventional means of vessel propulsion and stabilization. First developed by German engineer Anton Flettner in the early 1900s, it uses a phenomenon of fluid dynamics known as the Magnus effect to propel the ship.

The thrust developed by the system and the direction is dependent on several factors and features, which are broadly categorized into:

1. Wind speed (kinetic), 2. Wind direction (directional), 3. Vessel heading (directional), 4. Rotor height and diameter (geometric), and 5. Surface properties of the rotors (dynamic and kinetic)

The driving principle is that when a cylinder is rotated about an axis, and a medium (air or water) flows past it perpendicular to the axis, then a force is generated in a direction orthogonal to both the axis and flow stream.

This force is the result of a pressure difference across the 2 halves of the rotor and is known as the “Kutta-Joukowski” force .

The concept of this force was first quantified by scientists Martin Kutta and Nikolai Joukowski who studied the case of a rotating cylinder.

Flettner rotors are used in both planes and ships, although it is still an experimental concept that hasn’t been commercially mass-produced.

The Flettner rotor is also known as the Magnus rotor. Another name for the large cylinders mounted on the deck is rotor sails or Flettner sails.

When used in the airplane industry, such aircraft are known as Flettner planes.

While the main purpose of this article is to look into the marine applications of the Flettner system, it suffices to say that planes use a rotor at the fore of the craft and ahead of the wings to generate thrust perpendicular to the ground. This thrust creates lift while the ensuing wake provides the required downwash for liftoff.

What is Magnus Effect?

The Magnus effect, which is the physical concept behind the Flettner rotor, was first studied by the German physicist Gustav Magnus.

He observed that a spinning body would deflect off a straight path. This deflection directly depended on the manner in which it spun.

A pressure difference between the 2 halves of the body would create a force that altered the course of the body. This pressure gradient was directly related to the geometry of the object and its kinetic properties (roughness coefficient, form factor, speed of approach, angular velocity).

This effect is most commonly studied in 3 industries- the sports field as a part of many ball sports, the defence industry where the effect could throw a guided missile off target, and in engineering applications (the Flettner rotor).

The force that is generated is known as the Kutta-Joukowski lift and plays an important role in marine hydrodynamics and naval architecture.

This is the guiding force that is utilized by Flettner rotors and certain high-speed crafts to generate lift over an airfoil (or hydrofoil).

The force generated per unit length of the body is given as: F/L=ρvg Here, the length L is the characteristic length of the body.

It is the diameter in case of spherical bodies, and the tip-to-tip length along with the incoming flow in case of anomalous shapes.

The main reasons that the body deviates off its original intended path are because of a phenomenon known as flow separation.

This is when the flow around a body is no longer able to stick to the surface due to certain physical alterations. In this situation, a wake (warp in the flow downstream of the body) is created.

Due to the spinning nature, this wake is formed in certain regions that create a pressure difference between opposite ends of the plane in which the ball is spinning.

The generation of the deviant force is perpendicular to both the axis of the spinning body and the direction of linear motion.

Another dynamic component that comes into play is vorticity. It is the formation of vortices behind a bluff body or spinning object.

These vortices are disturbances or turbulences created in the medium due to the spin or flow separation imparted by the body.

Vorticity is measured in terms of the strength of the vortex generation. An easy way to visualize vorticity is the whirlpool patterns created when water rushes down a sink drain.

Higher the strength, more turbulence generated and faster the water spins. The same principle is applied to the Flettner rotors.

The vorticity strength is calculated as: G=2πωr^2

Where, r is the radius of the body (sphere or cylinder) and ω is the angular velocity (tangential velocity x radius of the body).

The most common application in engineering is the Flettner propulsion system on both airplanes and marine crafts. It is used for both propulsion and craft stabilization.

Where Are Flettner Rotors Used?

The Flettner system serves 2 main purposes on board a ship:

1. For propulsion, and 2. For stabilization

Propulsion is an integral component in marine vessels, and the more thrust that can be generated with minimum input work is always an ideal scenario. While conventional propellers are powered by marine diesel engines, the environmental damage is to be considered.

With the large number of vessels that ply the Earth’s oceans, a move to cleaner energy alternatives is preferred. One such energy source is Flettner rotors that make use of wind powers to generate thrust behind the ship.

These rotor sails are powered by small motors that are located within the hull, while the rotors themselves project vertically upwards for propulsion. As they rotate, the Magnus effect comes into play, and a horizontal thrust is generated to the aft of the vessel. The main source of energy is the motors that power the rotors, while the output is provided by the relative motion of the surrounding air.

Note, the motors alone are not sufficient due to the lower output power in a no-wind situation. For maximum efficiency from Flettner rotors, the wind must flow perpendicular to the ship’s length.

Faster the incoming wind, larger the generated thrust. For this reason, ships fitted with Flettner rotors can sail even when the wind is not in the direction of sailing. However, if the wind changes direction and approaches from the other side (port or starboard), the ship will move in reverse since the thrust is now generated towards the fore. Thus, careful analysis of the incoming wind direction must be undertaken to ensure correct heading for the vessel.

Stabilization is another function performed by Flettner rotors, although the structure orientation varies. While it projects vertically in the case of propulsion, the rotors extend laterally from the hull of the vessel for roll stabilization. Located below the waterline, they measure a few meters on each side and can be of 2 main types- active or passive stabilization.

In passive stabilization, as the vessel rolls from port to starboard, the rotors are activated, and they begin to spin. Based on the speed and direction, they can impart either a lift or downward force on the vessel.

By judging the type of force required, the roll can be stabilized by providing a righting motion using the Magnus effect. The medium in this case that provides the turbulent wake is the water flowing over the rotors.

In passive rotor stabilization, the Flettner systems on both sides of the craft rotate at the same speed in the same direction.

In the case of active stabilization, instead of having the same characteristics, the 2 halves provide different lift and down forces depending on the situation that arises.

It is incredibly precise, and if properly designed, can stabilize the vessel to a near standstill in even harsh weather conditions. However, engineering this design is complicated, and requires extensive experience.

Due to the still-experimental nature of using stabilization rotors, there are not many vessels in operation that use this system for roll stability.

History And Innovations Of The Flettner Rotor

The first ship that was used to develop the concept of the Flettner rotors was the Buckau developed in Germany by the Germaniawerft. Alongside being one of the largest builders of the German U-boat submarines, it was involved in the development and testing of scientific ideas.

The team of designers that worked on the theory and oversaw the construction of the rotors were experts in the fields, and are well-known names in the aerodynamics and fluid mechanics industry.

Albert Betz (pioneered wind turbine technology), Anton Flettner (an expert aerospace engineer), Jakob Ackeret (Swiss aeronautic expert and fellow of the Royal Society), and Ludwig Prandtl (developed several mathematical and analytical models of fluid mechanics) designed the rotor vessel which was built in 1924.

The Buckau employed a twin-rotor system that measured 15 meters in height and 3 meters in diameter. The motors used to rotate the rotor sails drew 37 kilowatts of power.

Its first voyage was from Danzig (Gdansk in Poland) to Scotland in 1925, and then, later on, sailed to the Americas in 1926. However, the ship was inefficient and had difficulty in mustering sufficient power to propel the craft at an adequate speed. As a result, the project was eventually scrapped and the proposal was shelved.

Another German ship, the Barbara, was built in 1926 that attempted to use a 3-rotor system to propel the craft. Built out of Bremen by the Weser shipyard, it was also scrapped due to project delays and difficulties.

Eventually, interest renewed in the Flettner systems during the 1980s, and studies were taken up to analyze the feasibility of such a propulsion system.

Current Models And Prototype Tests

For reasons outlined in the next section, there are very few vessels with Flettner rotor systems. However, on an experimental and trial basis, there are some ships that are currently operational.

There was a proposal in 2007 to construct a fleet of 1,500 ships that would operate using Flettner rotors. Their aim was to improve cloud reflectivity by spraying seawater and creating an aerosol atmosphere above the ocean.

It was considered a novel solution to global warming and was developed jointly by Stephen Salter and John Latham. The hull design was a trimaran that could achieve a cruise speed of up to 6 knots.

Enecron which deals in wind energy generation and technology commissioned the E-Ship-1 in 2008 that acts as an equipment and turbine transport vessel. According to the official statement from the company, it helped reduce fuel consumption by 25% compared to conventional systems.

The Viking Line company that runs ferry services in Finland and the surrounding regions commissioned STX Europe to construct a Flettner rotor powered ship in 2011. While the project was completed in 2012, the rotors were a later addition in 2018. Christened MS Viking Grace, the design was conceived by Wärtsilä in 2009.

The Norsepower company began prototyping a practical Flettner rotor system that could be installed on ocean-going vessels. The model was installed on the Maersk Pelican tanker in 2018 and is awaiting trial tests.

It utilizes a series of twin Norsepower rotors to generate cruising speeds that are said to match conventional propulsion systems.

Similarly, the MV Afros (which is a bulk carrier), has conducted tests with a 4-rotor system that has helped advance research in the design of a commercial Flettner rotor system.

For stabilization purposes, the largest vessel in use is the Eclipse, owned by Russian-Israeli billionaire Roman Abramovich, which is a private superyacht. Operational since 2011, it uses active roll stabilization and has state of the art facilities including a mini-submarine for underwater exploration.

Issues With The Flettner Rotor System

Unfortunately, Flettner rotors are not known for their efficiency, as it suffers from many types of transmission losses. A transmission loss is when total generated power is reduced due to dynamic issues that exist between where the source of propulsion is produced, and the actual propelling mechanisms.

In conventional systems, transmission losses are in the shaft vibrations and resistance faced by the propeller blades. To overcome these, streamlined blade shapes and dynamically isolated shafts are used (dynamic isolation refers to removing any connections between the shaft and surrounding media to prevent loss of energy).

However, for the Flettner rotors, the cylindrical rotors are a source of loss. Despite highly attuned designs to provide thrust in the right direction, the biggest drawback is that wind can blow from any direction, while the ship is only supposed to move at a certain heading.

For this reason, the direction of thrust varies erratically, while the propulsive power is also not constant. These are sources of inefficiency that make the Flettner system inconsistent in power generation.

Moreover, to generate sufficient power to move a large ship with the same speed as a conventional system, rotor sails exceeding 20 meters in height will need to be used. As this can also create hydrodynamic instability in the ship, it is not preferred and the height is restricted to below 15 meters.

Projections above a certain height can place excess stress on the bearings and joints that connect the rotor sails to the deck, leading to an increased chance of an accident.

So, by restricting the height of the rotors, the generated thrust is lesser than an equivalent marine diesel engine. Due to these 2 main reasons, Flettner systems are not used commonly for propulsion.

Regarding stabilization, the biggest issue is the increased beam of the vessel. Ships have a very tight range of values within which the beam can be so that it can safely dock at ports. In order to stabilize the vessel, the rotors need to extend horizontally, which creates an issue while attempting to dock.

Accidental damage to the rotor plus to the wharf are primary concerns, while there is also the risk of accidentally grazing or damaging other vessels nearby. To compound this problem, since the stabilization rotors are located below the waterline, it is near impossible to detect it from another vessel or the shore.

For this reason, Flettner rotors are rarely used for stabilization, and underwater fins are more preferred (passive and active stabilization).

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• Understanding Water Jet Propulsion: Working Principle, Design And Advantages
• Propeller, Types of Propellers and Construction of Propellers
• Marine Propeller Shaft: Design And Construction
• Controllable Pitch Propeller (CPP) Vs Fixed Pitch Propeller (FPP)
• Understanding Design Of Ship Propeller

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Ajay Menon is a graduate of the Indian Institute of Technology, Kharagpur, with an integrated major in Ocean Engineering and Naval Architecture. Besides writing, he balances chess and works out tunes on his keyboard during his free time.

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Your Physicist

I will answer anything from the world of physics.

Magnus effect

What is the magnus effect.

The Magnus Effect is a phenomenon in fluid dynamics wherein a spinning object experiences an aerodynamic lift or drag force perpendicular to the direction of the flow. This effect was first observed by Heinrich Gustav Magnus, a German physicist, in 1852, while studying the flight of spinning projectiles.

The Magnus Effect is caused by the difference in air pressure between the sides of a spinning object. When an object spins, it creates a region of high-pressure air on one side and a region of low-pressure air on the other, due to the Bernoulli’s principle, which states that the pressure of a fluid decreases as its velocity increases. This pressure difference causes the object to move in the direction perpendicular to the flow of air, resulting in a lift or a drag force.

How does the Magnus Effect work?

The Magnus Effect can be explained through the concept of circulation, which is a measure of the flow of fluid around an object. When an object spins, it creates a circulation of air around it, which generates a lift or a drag force, depending on the direction of the spin. This effect is more pronounced at high speeds and with objects that have a smooth surface.

The direction and magnitude of the Magnus Effect depend on several factors, such as the speed of the spinning object, the shape of the object, the viscosity of the fluid, and the angle of attack. The Magnus Effect is also affected by the orientation of the spin axis relative to the flow of air.

Examples of the Magnus Effect in action

The Magnus Effect can be observed in several real-world examples, such as the flight of a spinning ball in sports like soccer, golf, and baseball. In soccer, when a player kicks a ball with a spin, the ball curves in the air due to the Magnus Effect, making it difficult for the goalkeeper to catch it. Similarly, in golf and baseball, the spin of the ball affects its trajectory and distance.

The Magnus Effect is also utilized in several engineering applications, such as helicopter rotors, wind turbines, and sailboats. In the case of wind turbines, the blades are designed to spin, creating a circulation of air that generates a lift force, which is then converted into electricity. In sailboats, the shape and orientation of the sail determine the direction and magnitude of the Magnus Effect, which helps the boat to move forward.

Applications of the Magnus Effect

The Magnus Effect has several practical applications in various fields of science and engineering, such as aerodynamics, ballistics, and fluid mechanics. The understanding of the Magnus Effect is crucial in designing efficient wind turbines, propellers, and jet engines, which rely on the lift and drag forces to generate thrust.

The Magnus Effect also has potential applications in the field of transportation, such as designing more efficient cars and airplanes. Researchers are exploring the possibility of using the Magnus Effect to reduce the drag force and improve the fuel efficiency of vehicles. Moreover, the Magnus Effect can also be used in the development of new sports equipment, such as balls and racquets, to enhance the performance of athletes.

Cargo Ships Are Turning Back to Wind Power—But Don't Expect Big Triangular Sails

Sailboats are back, but this time the sails are spinning.

The next time you see a hulking cargo ship plying the ocean, you could be looking at a sailboat. The shipping industry has begun to experiment with rotating towers that can be installed on ships to harness wind, but in a much more high-tech way than rigging up canvas.

Rotor ships , sometimes called Flettner ships, have been around a while. The big development now is that the major players in shipping are taking the idea seriously. Yesterday the shipping colossus Maersk installed 100-foot-tall rotors on one of its tankers, the Pelican , according to the Wall Street Journal .

Maersk thinks the advanced sails could cut its fuel costs by 10 percent. That may not sound like much, but consider that the Danish mega-corporation spends $3 billion annually on fuel to move the world's cargo, so we're talking about $300 million. If the Pelican tests succeed, the Maersk could try out rotors on other vessels and turn more cargo ships into hybrid sailboats.

Why now? According to the WSJ:

Shipping executives said previous efforts didn’t catch on with operators because either the costs of such technologies were too high or tests didn’t yield the expected fuel savings. But modern, lightweight and relatively cheap rotating sails show more promise, they said.

Andrew's from Nebraska. His work has also appeared in Discover, The Awl, Scientific American, Mental Floss, Playboy, and elsewhere. He lives in Brooklyn with two cats and a snake.

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MAGLift™ Rotor Stabilizer

Retractable rotor - zero & slow speed.

The MAGlift™ Rotor Stabilizer Advantage – Applying the Magnus Effect to Boat Stabilizers

Quantum’s team of marine engineers, naval architects and hydraulic system designers understood that a motion control solution for low speed stabilization required a paradigm shift from the traditional fin stabilizer working principle for design and active stabilizer technology. As world leaders in motion control, Quantum takes pride in finding innovative solutions to hydraulic stabilizer challenges. Based on the Magnus Effect, Quantum’s patented MAGLift™ design uses hydraulic power to deploy and rotate composite cylinders directed by the SMC 4000 stabilizer control unit. This advanced technology provides roll reduction at anchor, and at lower speeds.

The MAGLift rotor stabilizer is fully retractable to eliminate drag for higher speeds making it superior to traditional fin stabilizers. The combined swinging and spinning of the rotor result in hydrodynamic forces that stabilize the vessel while it is at Zero Speed™ as well. During underway operations, the rotor is deployed to a set position, and the righting force direction is altered by simply changing the direction of rotor spin. In addition, the MAGLift™ rotor stabilizer has the highest lift to weight characteristics and the smallest interior footprint.

After several years of numerical analysis and tank testing, Quantum installed the first MAGLift™ Rotor System in 2004, applying the Magnus Effect to generate lift proportional to the speed and direction of rotation. To date, several systems have been installed on a variety of military vessels and large yachts ranging in length from 24.3 meters (80 feet) to over 162 meters (525 feet). The use of this technology increases the performance envelope of the vessel and the crew when considerable time in station-keeping or slow patrol speed modes are required.

How the Maglift Rotor stabilizer system works:

• High damping (also at low speed)
• System redundancy (each rotor can work independently)
• Ability to deliver roll stabilization at relatively low vessel speeds (3–18 knots) based upon vessel specifications
• Compact and easy installation
• Increased safety and comfort for crew
• Increased fuel efficiency
• Underway “Fully Retractable” system
• Reduced “Haul-Outs” for vessels when using the underway “Fully Retractable” system
• Reduction in appendage drag at high-speeds utilizing semi- retractable or fully-retractable system
• High lift to footprint capacity compared to fin stabilization

Simple Installation

• ML200/300/400SR Series

The ML200/300/400SR Series is designed to provide maximum energy transfer in terms of lift force based on the “Magnus Effect.” The unit is a compact rugged design built to withstand the most demanding military and commercial applications.

For maximum roll reduction, the ML Semi- Retractable System is mounted in the turn of the bilge with the rotor oriented perpendicular to the vessel hull during underway operations. The rotor spin direction and speed are altered during operation, as needed, to obtain optimal performance. To minimize appendage drag, the rotor stows neatly, parallel to the vessel hull, when the stabilizers are not in use. The rotor orientation and spin functions are precisely managed by Quantum’s hydraulic and electrical control systems.

Designed using the latest technology and high-quality materials, the ML Semi-Retractable Series system offers the best performance of any system where significant roll reduction is required at loitering speeds.

Smooth Operation: The unit provides smooth power transfer from the ship’s hull to the rotor via precision roller bearings used in both the spin drive shaft and the main shaft.

Precision Control:  By varying the RPM and direction of rotation of the tube, the lift force can be controlled with razor sharp precision, resulting in instantaneous response.

Safety: The rotor’s hydraulic system is equipped with an automatic retract mechanism to allow the rotor to swiftly stow in case of impact during operation. While not in operation, the rotor is safely stowed along the hull, where it presents minimal possibility of impact. In case of severe impact, half of the rotor tube will break away, designed intentionally to preserve the integrity of the hull.

MAGLift™ Rotor

• Spin Control Manifold
• Swing Hydraulic Cylinders
• Swing Drive Housing
• Hydraulic Spin Motor
• Servo Control Valve
• Main Hydraulic Control Indicator

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Magnus Effect Propels This Flettner Rotor Boat

The Magnus effect is a interesting and useful phenomena. [James Whomsley] from [Project Air] decided to put it to work on a small radio-controlled boat, successfully harnessing the effect. (Video, embedded after the break.)

The Magnus effect is an interesting thing, where fluid flowing over a rotating object generates an aerodynamic force at a right angle to the direction of the flow and the axis of rotation. (It’s why curveballs curve.) This can be used for propulsion on a boat, by spinning a tall cylinder called a Flettner rotor. This takes advantage of Magnus effect to generate thrust.

The boat uses a 3D-printed hull, sealed up with a leak sealer spray and lots of spray paint to avoid leaks.  In the center of the catamaran design, there’s a spinning rotor belt-driven by a brushless motor. Outside of the rotor for thrust, a simple rudder is used for steering.

With the rotor turning, the boat was able to successfully sail along with the benefit of the thrust generated from the wind. However, there were teething issues, with heavy winds quickly capsizing the boat. [James] realized that adding some proper keels would help avoid the boat tipping over.

We’ve seen [James] around these parts before, namely with the Magnus-effect aircraft that preceded this build.

Continue reading “Magnus Effect Propels This Flettner Rotor Boat” →

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