Verticalization maneuver of a 22-m-high bound4blue eSAIL during installation on board Ville de Bordeaux, owned by LDA and chartered by Airbus, February 2024 in Vigo, Spain.

 

By Dr. Alberto Llopis Pascual • David Ferrer Desclaux • Marc Martí Arasa

Wind is where it’s at. Awareness, appreciation, and adoption of wind-assisted propulsion systems (WAPS) are growing exponentially throughout the maritime industry as forward-thinking owners and operators look to unlock significant environmental, regulatory, and commercial advantages. However, there’s one big issue that’s hiding beneath the sails: aerodynamic interference.

Spain’s bound4blue has been tackling this challenge and is on a roll. The pioneering WAPS developer had a breakthrough year in 2024, landing the largest-ever wind propulsion contract (for 20 of its eSAIL suction units with Maersk Tankers) and securing orders with shipping companies such as Klaveness Combination Carriers, Eastern Pacific Shipping, Amasus, and Marflet Marine, to name a few.

While the company is reveling in the wider appreciation of wind power as a key enabler in shipping’s energy transition, it is one of the few players to caution that real care is needed to optimize WAPS and unlock the true propulsive power of nature.

Aerodynamic Interference

The challenge of aerodynamic interference has not received the attention it deserves.

Let’s start at the beginning. If you have one WAPS unit that is capable of generating the power of X, it’s common sense to assume that installing four units on the vessel would generate 4X. But it’s not that simple.

When two aerodynamic bodies, in this case WAPS, are placed in the vicinity of one another, they affect the airflow (the speed and direction of the wind), which modifies the resulting aerodynamic forces. Therefore, any installation of two or more lifting devices on a vessel can create an interference, which will reduce overall, combined performance.

The problem is one of physics. For any shipowner that’s considering adopting wind, it is absolutely essential to work with your suppliers from the very beginning of the process and understand the degree of potential efficiency loss. To do that, you have to consider factors such as the number of units, the size of the WAPS and their positions, and, from that starting point, you can then work to optimize the configuration to reduce the impact of aerodynamic interference and maximize propulsive force.

As an example of the difference this can make, a recent modeling project for a Kamsarmax vessel with four eSAILs showed that an optimized configuration, compared to three slightly different setups, could increase savings by up to 28 percent.

Suction Sails

bound4blue’s eSAIL is a suction sail. In contrast, examples of competing WAPS technologies are kite sails, Flettner rotors (spinning cylindrical structures) and rigid wings. All WAPS offer advantages and disadvantages that may or may not be applicable to individual vessels, but the most critical issues are the fuel savings they can generate (which leads to emissions reductions and easier regulatory compliance) and the return on investment.

We’ve developed our fully autonomous suction sail to deliver a mechanically simple, powerfully effective solution for a broad range of vessel types. Typical return on investment is less than five years.

Lift force is the “holy grail” in WAPS. You want to generate as much propulsive power as possible, while minimizing size and weight. From a purely aerodynamics point of view, mathematics tells us a thick shape, or a circle, is best to maximize lift. However, these shapes have poor aerodynamic properties due to flow separation, where the airflow detaches from the surface and the power “stalls.” That’s where the genius of suction sails comes in.

By using a fan and creating suction along the surface of the WAPS, the flow can be reattached and used to generate very high lift forces. In fact, the thick surface of an eSAIL with suction power produces up to seven times more lift than a similar size, non-suction, slender-bodied WAPS, such as an airfoil, wing, or wingsail.

This exceptional aerodynamic efficiency allows for smaller size and weight, with less impact on ship stability, visibility, lights, deck space, and air draft, and much easier integration. This helps owners maintain operational flexibility while accessing the many benefits of wind power.

Furthermore, unlike other solutions, suction sails excel in sailing upwind, ensuring low power consumption and mechanical simplicity. It’s a combination that, as shown by findings from Clarksons Research’s World Fleet Register, is pushing suction sails into pole position in the orderbook.

 

 

Designing with Aerodynamic Interference in Mind

Aerodynamic interference has to be taken into account from the very first stages of any project. Traditionally, computational fluid dynamics (CFD) studies have been the go-to method for this, but CFD requires high-power computing solutions, and CFD studies are incredibly complex, time consuming and expensive. That puts CFD out of reach for many owners and installations.

To solve that issue, we’ve developed something called POINT (POtential INterference Tool). This is an advanced simulation tool that can be run on any standard laptop, working up to 25 times faster than CFD to assess as many eSAIL configurations as possible on board a selected vessel. It compares the impact of potential interference, enabling informed, optimal decision making and giving owners realistic insights into the savings that can be unlocked with eSAILs.

In short, it tackles the issue, democratizes the whole study process (opening up the benefits of CFD without the price tag), fast-tracks simplified adoption, and creates transparency around the scale of future savings and benefits.

The Right Time for a New Wind Revolution

The International Maritime Organization has set ambitious decarbonization goals, and we all have a part to play in the transition to a more sustainable maritime industry. Wind power is an abundant, renewable, proven and free energy source, making it the natural choice for voyaging toward sustainability. Given the lack of availability of other green fuels at present, it’s also a key enabler, as it reduces the overall energy demand of ships for a more cost-effective move to alternatives, such as hydrogen, ammonia or batteries. It is fully complementary with these technologies.

While the commercial benefits of lower fuel consumption, reduced OPEX, and improved asset value and competitive advantage speak for themselves, it’s worth diving a little deeper into the regulatory perspective. It’s here where wind really comes into its own, with added value realized through easier compliance, reduced penalties and more future-proof assets.

If you consider how the regulatory landscape has evolved in recent years, we’ve seen a flood of new environmental legislation, defined by steadily increasing targets, and ever more stringent penalty costs for non-compliance. Wind delivers benefits across the board.

For example, with the Energy Efficient Design Index (EEDI) and Energy Efficiency Existing Ship Index (EEXI), WAPS reduce the by-design CO2 emissions for vessels, while Carbon Intensity Indicator (CII) ratings improve with less operational emissions. A reduction in the tonnage of CO2 emitted reduces the economic cost of EU Emissions Trading System (ETS) compliance, while a “wind reward factor” in FuelEU Maritime works to reduce the greenhouse gas intensity of vessels.

Let’s take one ship, for example: a 17,000-dwt LPG tanker sailing a route between Antwerp and Houston at 15 kt. Installing eSAILs could reduce fuel costs on that vessel, without deviating from the normal route and speed or buying alternative fuels, by 9.8 percent a year, translating to annual OPEX savings of $283,000.

Now, let’s consider EU ETS and FuelEU penalty costs. There’s a heavy price to pay for non-compliance. eSAIL installation addresses regulatory compliance while enabling savings that could climb to 18.4 percent by 2027, equivalent to $532,000 a year. By 2040, those savings rise to 22.66 percent, or $655,000.

This is really just the tip of the iceberg. There are powerful regulatory advantages, and positive financial rewards, for owners opting for wind. And, when you consider that regulations are only going to grow in complexity and stringency, the incentives for adoption will multiply.

So, seeing the big picture, we believe the stars are really aligning for wind power. The quicker you take advantage of that fact, the more you save, the greater the impact, and the easier compliance will be—today and far into the future.

The Role of Collaboration

Collaboration with shipowners and operators on the installation of WAPS is essential. There’s no standard, one-size-fits-all approach. We’d advise any client to start with a feasibility study to understand the potential saving and suitability for individual vessels, working hand-in-hand with a trusted, specialist supplier to ensure a chosen system fits operational needs, commercial objectives, and any other stakeholder requirements.

From our perspective, selecting the optimal eSAIL configuration is a case-by-case exercise, and close collaboration with the customer allows for fine-tuning of the number, placement, and size of eSAILs for maximum efficiency. Transparency, communication, and mutual understanding are absolutely key to successful implementation and long-term success.

In terms of installations, coordination, timing and flexibility are key factors. As we can all appreciate, vessels are “moving targets,” and it’s not always possible to plan everything down to the second. So, again, communication is important, as is the ability to adapt to unforeseen circumstances. At bound4blue, we do everything we can to ensure smooth, efficient and high-quality installations. We’re lucky our customers are as focused on those objectives as we are.

Looking Ahead

It’s an exciting time for the segment. Wind power is achieving more and more recognition as a viable, market-proven, and cost-effective means to decarbonize shipping, enhance competitiveness, and work toward the most demanding regulatory requirements. With each new order and installation we undertake, we’re edging ever closer to a tipping point of wide-scale adoption.

By 2030, we anticipate that WAPS will be well-established as a preferred decarbonization solution for the maritime industry, working standalone or integrated with hybrid systems/alternative fuels. This will propel us closer to our goal of a more sustainable maritime sector, with less dependency on fossil fuels.

Dr. Alberto Llopis Pascual is the head of aerodynamics at bound4blue.

David Ferrer Desclaux is the CTO and co-founder of bound4blue. 

Marc Martí Arasa is an aerodynamics engineer at bound4blue.