Sailing merchant ships: utopia or reality?

Is sail­ing propul­sion a ser­i­ous option for redu­cing the car­bon foot­print of mari­time freight?

Since 2011, we have seen the emer­gence of pro­jects for the use of dies­el propul­sion in the mer­chant navy. And the cur­rent situ­ation – in par­tic­u­lar the very high cost of energy – is increas­ingly favour­able to the sec­tor. Wind is a source of energy that is free from the prob­lems of spec­u­la­tion, secur­ity, and infra­struc­ture. In addi­tion, the Inter­na­tion­al Mari­time Organ­isa­tion and the European Com­mis­sion are demand­ing increas­ing decar­bon­isa­tion efforts from shipown­ers, and sea freight is expec­ted to be included in the EU Emis­sions Trad­ing Scheme by 2023. 

The huge advant­age of dies­el propul­sion is its avail­ab­il­ity: it is an ideal solu­tion in the short term, and com­ple­ment­ary to altern­at­ive fuels in the longer term. There are already 15 ships in the world that are using sail propul­sion sys­tems. Some tech­no­lo­gies are mature, such as Flettner rotors, and oth­ers will be tested very soon. For example, the Nantes-based start-up Air­seas has been test­ing its kite since Decem­ber 2021 on a ship chartered by Air­bus between France and the United States. Zéphyr & Borée plans to launch its Can­opée sail­ing cargo ship this year, which will trans­port the future Ariane 6 launch­er from main­land France to French Guiana.

You men­tion sev­er­al tech­no­lo­gies: in con­crete terms, what is the nature of sail propulsion?

There is a real pro­lif­er­a­tion of innov­a­tions and tech­nic­al solu­tions. The most mature solu­tion is the Flettner rotor, dis­covered in 1924. Since 2010, the firm Ener­con has demon­strated that this tech­no­logy is com­pat­ible with the con­straints of mari­time freight. The rotors need to be rotated by means of anoth­er energy source, a motor, or a huge fan in the case of suc­tion pro­files. Cur­rently, oil-fired gen­er­at­ors are used on board to pro­duce this power, and altern­at­ive fuels could be con­sidered in the future. 

There are also oth­er wind propul­sion tech­no­lo­gies that use only wind power. Con­ven­tion­al flex­ible fab­ric wings, such as those on old rigs like the three-masted Belem, age quickly and are not very effi­cient. Today, shipown­ers are turn­ing to rigid pan­els, made of com­pos­ite mater­i­al, or thick pro­files that resemble an air­craft wing. 

All these sys­tems are loc­ated on the deck of the ship, and there­fore require space. One of the latest solu­tions explored is the kite: this time the wing is aer­i­al. This is quite innov­at­ive, and the tech­no­logy is cur­rently being developed.

Can all ships be equipped, and can we achieve 100% wind propulsion?

It is pos­sible to ret­ro­fit exist­ing ships today. A study ¹ estim­ates that 40–45% of the world fleet (37,000–40,000 ships) could be ret­ro­fit­ted by 2050, due to the lower cost of this propul­sion and its wide avail­ab­il­ity. Wind will in most cases be an aux­il­i­ary energy source, partly redu­cing the need for the ship’s com­bus­tion engine.

New ships will have to be built to achieve almost entirely wind propul­sion (they will still carry an engine to secure the port approach). This per­form­ance can only be achieved by ships designed for this pur­pose, and not by ret­ro­fit­ting: their hulls must integ­rate anti-drift sys­tems linked to the use of sails, and the pos­i­tion­ing of the masts must respect the bal­ance of masses and access to cargo spaces by port ser­vices. The Neo­line sail­ing freight­er, cur­rently under devel­op­ment, will be equipped with four masts provid­ing 90% of the energy require­ment for its propulsion.

The heav­ier the ship, the more power is needed to move it. Sail­ing propul­sion makes sense on medi­um-sized ships – up to 200 metres in length – at a speed of about 15 knots. By com­par­is­on, a con­tain­er ship can be up to 400 metres long and travel at 20 knots.

So, deploy­ing vee-propul­sion on a large scale will require a reor­gan­isa­tion of mari­time trade?

Yes, the concept of mas­sific­a­tion using con­tain­er ships and logist­ics hubs is out­dated. Ship propul­sion addresses new tasks, such as the long-dis­tance trans­port of goods on inter­me­di­ate-sized ves­sels. They can call at more ports, espe­cially sec­ond­ary ones: this brings the goods closer to the con­sumer and reduces pre- and post-car­riage, which is usu­ally done by road. Speed is also reduced: the Neo­line cargo ship will sail at an aver­age speed of 11 knots. This does not pre­vent ship­pers such as Man­it­ou and Ben­eteau from show­ing interest. It will be a vir­tu­ous logist­ics chain, and con­sumers must be involved by accept­ing longer deliv­ery times to avoid a rebound effect. 

Anoth­er change con­cerns the mode of nav­ig­a­tion. Wind propul­sion needs wind so some­times sea routes will have to be mod­i­fied to take advant­age of it. To do this, crews can rely on mari­time rout­ing – tak­ing into account the weath­er con­di­tions – which is essen­tial for sail­ing solu­tions. But shipown­ers are not used to this type of nav­ig­a­tion, and there is a psy­cho­lo­gic­al brake. Finally, it should be noted that the propul­sion of sail­ing boats is more favour­able on transat­lantic routes in the North, and less so on the route between Europe and Asia.

What is its decar­bon­isa­tion potential?

We do not know pre­cisely. A study ² shows that 3,700 to 10,700 ships could be equipped with dies­el propul­sion sys­tems by 2030. This could avoid the emis­sion of 3.5 to 7.5 mil­lion tonnes of CO₂ in 2030. We can also rely on the estim­ate of the ret­ro­fit poten­tial of exist­ing ships pro­duced for the UK gov­ern­ment (37,000 to 40,000 ships) ³: if 10% of fuel can be saved by the use of sail propul­sion, an over­all reduc­tion in con­sump­tion of around 3% is achieved. 

But the major uncer­tainty con­cerns the actu­al energy gain of the sys­tems. There are no stand­ards for meas­ur­ing per­form­ance, only the man­u­fac­tur­ers provide an estim­ate of the gain made by their solu­tion. On this basis, it is estim­ated that the energy gain asso­ci­ated with Flettner rotors is around 8%, up to 20% for the kite or even up to 30% for thick pro­files. But these are the­or­et­ic­al meas­ure­ments: today we lack meas­ure­ments in nav­ig­a­tion as well as a stand­ard, such as an ISO norm. This is an import­ant issue, which would enable shipown­ers to accur­ately assess the return on invest­ment of these decar­bon­isa­tion solutions.