Blow by blow: a guide to Wind Assisted Propulsion Systems

Wind power is experiencing a renaissance.  

For centuries it was wind that propelled traders, travelers and explorers across the oceans, connecting nations, facilitating development and enabling discovery. Steam power, and then combustion engines, took the wind out of shipping’s sails, but, after being consigned to the sidelines throughout the 20th century this readily available, renewable and zero emission propulsive power is very much back on the industry radar. And with good reason. 

The world, as we all know, is facing a climate crisis. Consumption patterns must change to divert catastrophe, and shipping, this unique driver of world trade, has a key part to play. In short, it’s time to show fossil fuels the door. The IMO has fired the starting gun in the race to decarbonize, with ambitious goals of a 20% emissions reduction by 2030, a 70% cut by 2040, and full-scale decarbonisation by or around 2050. 

However, unlike previous transitions there’s no steam or combustion ‘silver bullet’, leaving owners and operators scratching their heads about how these targets can possibly be reached. Uncertainty about future fuel choices hangs heavy during discussions, while availability of those fuels looks unlikely to meet demand (or budgetary considerations) for some time. 

Which brings us back to the freely available, proven power of wind. And, more precisely, Wind Assisted Propulsion Systems (WAPS). 

What are Wind Assisted Propulsion Systems? 

Wind Assisted Propulsion Systems (WAPS) are the marriage of modern engineering with traditional wind power.  

At their core, WAPS function much like conventional sails, capturing wind energy to propel vessels forward. However, whereas bygone ships depended entirely on sails, WAPS serve as an auxiliary system, providing up to a third of the propulsion needed. This reduces main engine loads, while dramatically cutting fuel consumption, emissions and costs. 

How do WAPS work? 

The working principles of modern WAPS are the same as traditional sails and aircraft wings – combining the basic aerodynamic forces of Lift and Drag. However, when compared to traditional sails, modern WAPS are capable of generating significantly more force due to shape optimisation and advanced aerodynamics.  

In the case of bound4blue’s innovative eSAILs®, they generate up to seven times more thrust than conventional rigid sails of the same size, meaning they can be far more compact and efficient, with less intrusion on a vessel’s design (while opening the door for simple, flexible retrofits).  

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Although arguably not the silver bullet, or magic wand, to instantly herald a zero-emission future, WAPS are increasingly being recognized as an absolutely essential enabler for a cleaner, more cost effective, and brighter maritime future. What’s more the technology is available, and proven, NOW.

What are the different types of WAPS? 

The world of Wind Assisted Propulsion Systems is diverse, offering a menu of technologies for different vessel types and operational needs. However, there are four main types forward-thinking owners and operators can get a taste of.  

They are: 

1. Rigid wings:
   Inspired by airplane wings, these large, rigid structures harness wind efficiently, especially when sailing close to the wind. However, the maximum lift these can generate is limited, meaning they can be less effective in certain wind directions.  
2. Flettner rotors:
   These cylindrical structures spin to generate lift through the Magnus effect, producing significant thrust. While highly thrust, their rotating mechanism can be mechanically complex. 
3. Suction sails:
   Using suction as a method of active aerodynamics, suction sails like bound4blue’s eSAIL® can generate very high thrust. The simplified mechanics of the technology reduces maintenance requirements and costs. 
4. Kites:
   Deployed from the bow of a ship, kites capture wind at higher altitudes where it is often stronger and more consistent. While innovative, their deployment and control can be challenging, especially in certain conditions. 

From an aerodynamics point of view, the different technologies can be split into two main categories: 

• Passive aerodynamics: Rigid wings and kits – this refers to designs which are limited to fixed components and shapes. Simply put, the airflow will follow the paths it wants to follow. 

• Active aerodynamics: Suction sails and Flettner rotors – this refers to designs where parts may be moved constantly or suction/blowing of air is included to manipulate the aerodynamics and therefore force a desired behavior, resulting in larger aerodynamic forces.   

All WAPS varieties offer advantages and disadvantages when compared, but the most critical issue is the fuel savings they deliver for the vessel, and the return on investment. This equates to a combination of the thrust capability, system cost, and maintenance costs, as well as many other factors, including the ship, WAPS configuration, and route. 

Why do you need WAPS? 

As already discussed, shipping must transition to cleaner energy to meet climate targets, as well as the environmental expectations of diverse global stakeholders – from cargo owners to society itself. In an effort to catalyze change, regulators are looking to a new breed of regulations, with stringent penalties for non-compliance, such as the European Union’s Emissions Trading System (EU ETS) and, from 1 January 2025, FuelEU Maritime. 

These regulations demand rapid action and innovative solutions, but the development of alternative fuels and fully carbon-neutral ships takes time. As such, Wind Assisted Propulsion Systems are garnering growing acclaim, and increased adoption, as a practical, cost effective and immediate way to reduce emissions. 

By enabling significant fuel savings – often up to 20% or more – WAPS not only help shipowners meet requirements, but they offer clear economic incentive; both by reducing operating expenditure (OPEX) and delivering powerful regulatory cost benefits. 

What sets bound4blue’s eSAIL® apart? 

An increasing number of world class shipowners and operators are looking to bound4blue to help them embrace the wind revolution, with our DNV Type Approved eSAIL® gaining ground as the industry’s preferred WAPS solution. The fully autonomous units, which require zero crew interaction, stand out as a hybrid system combining the best attributes of both rigid wings and Flettner rotors.  

Here’s what makes then truly unique: 

• Wide wind range operation: Wide wind range operation: Unlike many systems, the eSAIL® can operate effectively even at low apparent wind angles, as narrow as 10°, similar to rigid wings. This versatility allows it to generate high lift or high thrust under a broader range of conditions. 

• High thrust efficiency: The eSAIL® achieves a lift coefficient up to seven times greater than that of conventional rigid sails, providing exceptional performance in terms of thrust generation. Meaning significant thrust can be provided in favorable wind conditions, similar to Flettner rotors. 

• Low maintenance costs: By simplifying its mechanical design, bound4blue has minimized moving parts, making the eSAIL® both reliable and cost-effective to maintain. 

• Adaptability: The eSAIL® is suitable for both retrofits and newbuilds, making it a versatile choice for a variety of vessels, including tankers, bulk carriers, ferries, and vehicle carriers. 

• Cost efficiency: The efficacy of the eSAIL®, providing exceptional propulsive force while delivering strong OPEX and regulatory benefits, makes it the best value WAPS on the market. Payback is typically less than five years. 

Setting sail for tomorrow 

As the maritime industry sets its sights on a more sustainable future, WAPS are limbering up to play a pivotal role – offering a practical, scalable solution to the twin challenges of emissions reduction and cost control. 

bound4blue’s eSAIL® technology exemplifies the potential of this approach, blending innovative engineering with proven aerodynamic principles to create a system that is both efficient and accessible. 

For shipowners and operators, embracing Wind Assisted Propulsion Systems isn’t just an environmental imperative – it’s a sound business decision. With the capability to reduce fuel consumption and comply with stringent regulations, these systems represent a win-win for both the industry and the planet. 

Is it time for your fleet to undergo a clean power renaissance? Get in touch with our expert team to find out more! 

About the author

Alberto Llopis Pascual holds the current position of Head of Aerodynamics at bound4blue. He is responsible for aerodynamic development of bound4blue’s technology, both numerical and experimental. Alberto holds a PhD in Aerospace Engineering in the field of experimental aerodynamics (University of Manchester, 2017). Previously, he spent 6 years working with University of Manchester, BAE Systems and a NATO task group researching and developing fluidic control effectors for aircraft, designing, and building the first aircraft to fully integrate and demonstrate this technology. Later, he also spent 2 years carrying out aerodynamic design of medical delivery drones.