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| Fig. 1: Cargo ship at dock. (Source: Wikimedia Commons) |
Cargo ships are big. Extraordinarily humongously big. These giants, as in Fig. 1, serve as the backbone of global trade moving over 13 billion tonnes of goods per year, a mass is equivalent to a cube of concrete a little over 2 km per side, ie, a mountain. [1] Achieving this feat takes a fleet of over 100,000 fuel-burning ships powered at huge monetary and environmental cost, consuming roughly 330 million tonnes of fuel every year. [2] From this we can simply calculate the annual CO2 emissions due to shipping by comparing the atomic weight of raw carbon to CO2:
| emissions | = | 44 14 |
× 330 × 106 tonnes y-1 | = | 1.04 × 109 tonnes y-1 |
While the environmental concerns over these 1 billion tonnes a year of CO2 emissions are significant, the estimated price of retrofitting the global shipping fleet easily runs into the 10's of billions of US dollars, so any such move will have to be driven by economic, not environmental, arguments. As a reference point, we can estimate the fuel costs based on the price of bunker fuel, the cheapest energy-per-dollar option. [3] This value is slightly out of date as finding reliable figures is tricky, yet following the maxim that costs always go up, even in a sector as volatile as oil markets, we can take this as a cost floor:
| US $300 tonne-1 × 1.04 × 109 tonnes y-1 × | = | US $3.12 × 1011 y-1 |
This US $100 billion price tag, in comparison to the US $10 billion retrofit estimation, informs us that upgrades to the shipping fleet which offer > 10% fuel efficiency savings are economically viable, a performance figure which as it turns out, is entirely doable. Even if the ship costs rise higher, the year-on-year savings will quickly add back up in our favor.
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| Fig. 2: Cargo ship with Flettner rotors. (Source: Wikimedia Commons) |
It comes as no surprise that the wind provides a humongous bounty of energy which can be harnessed for sailing. The very name of sailing comes from the centuries of sailors using the titular sails to catch the winds. Despite this rich tradition, only those with questionable taste in hobbies even consider operating such vessels for trans-oceanic travel. Pure sail drives are slow, unpredictable, and all too dangerous. No matter the other concerns, it is quite clear that the mainstay of shipping will remain burning fuel, the wind alone is simply not enough. That said, between modern engineering and meteorology much good can be done, especially over long haul shipping routes with stable prevailing winds. [4,5]
There are three primary technologies being explored for wind assisted propulsion (WASP); sails, Flettner rotors, and wings. [6] The first is conceptually the simplest, a return to historical form, but runs afoul of serious challenges in being deployed onto a modern ship's deck, largely because they take up too much space. I should note that this has lead to certain efforts to turn the sails into vast kites, but these are also impractical. The later two methods are more compatible with modern ship design and are already displaying real-world success. [7,8] While there remains much development to optimize the design and integrate into hull plans, the basic principles are well understood and confirmed by live testing, leading to rapidly growing industry buy-in, now with hundreds of orders for WASP system installations.
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| Fig. 3: Cargo ship with collapsible hard sail wings, concept illustration. (Source: Wikimedia Commons). |
Flettner rotor drives work on the Magnus effect, generating thrust by rotating a tall cylinder in the wind, Fig. 2, and can provide 10-20% fuel savings. This approach has the advantage of simplicity and minimal deck space usage, however suffers from the requirement that the rotors need be driven, leading to general inefficiency and longer term maintenance costs. Ship assist wings (also called hard sails or hard wing sails), Fig. 3, work much the same as on an airplane, generating thrust by redirecting airflow over an airfoil. These have the advantage of higher efficiency, offering fuel savings of up to 20-50%, and require minimal movement. [9] Further, the nature of wings allows not just for the optimization to current weather conditions, but also safer ship handling at either low or extreme wind conditions compared to rotors and can even be designed to fold and retract making the loading/unloading of cargo easier. The optimal aerodynamics design for these large ship wings is an outstanding challenge and upper limit performance has yet to be reliably demonstrated in live testing. In general, publicly available figures give about 10% fuel savings for current commercially deliverable technologies following either approach.
WASP cargo ships are clearly the future. The direct monetarily savings are clear and desirable even before accounting for regulatory pressure to reduce emissions. Ship wings look to be the most likely long term platform as they offer comparable current day performance and have the most room for engineering improvements. Additional integration with modern weather tracking can provide further performance and reliability. All of this however does not escape the reality that modern shipping now and in the foreseeable future rely on burning vast amounts fuel and replacing these engines will require a technological revolution beyond even the wind.
© Andrew Land. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. Borriello et al., "The EU Blue Economy Report 2025," European Commission, 2025.
[2] M. Kass et al., "Understanding the Opportunities of Biofuels For Marine Shipping," Oak Ridge National Laboratory, ORNL/TM-2018/1080, December 2018.
[3] R. Ura-Martnez et al., "Primer on the Cost of Marine Fuels Compliant with IMO 2020 Rule," Oak Ridge National Laboratory, ORNL/SPR-2021/2088, August 2021.
[4] L. Taylor, "Supplementing Fuel with Wind-Power for Long Distance Shipping," Physics 240, Stanford University, Fall 2023.
[5] J-M. Chen et al., "Seasonal Climate Associated with Major Shipping Routes in the North Pacific and North Atlantic", Terr. Atmos. Ocean. Scie. 25, 381 (2014).
[6] M. Kolodziejski and M. Sosnowski, "Review of Wind-Assisted Propulsion Systems in Maritime Transport," Energies 18, 897 (2025).
[7] "A New Age of Sail Begins," The Economist, 21 Jun 24.
[8] S. Neuman, "New Technology Uses Good Old-Fashioned Wind to Power Giant Cargo Vessels", NPR, 8 Oct 23.
[9] Md. D. Hussain and O. Md. Amin, "A Comprehensive Analysis of the Stability and Powering Performances of a Jard sSil-Assisted Bulk Carrier", J Mar. Sci. Appl. 20, 426 (2021).
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