Out on the pull: making wind assistance work

Wind power is worth attention: theoretical ‘castles in the air’ are rapidly gaining foundations, writes Stevie Knight

Wind Assisted Propulsion – WASP – “is not an efficiency measure, it’s a propulsion provider”, underlines Gavin Allwright of the International Windship Association. Sadly, it’s regularly left out of carbon discussions despite wind assistance breaking into the big tonnage: last winter’s WASP installation onboard the 163,000gt New Vitality tanker is now being followed by an even larger VLCC order.

While New Vitality’s results are as yet unconfirmed, validated WASP test results have been climbing. Maersk Pelican’s Norsepower installation returned an average, year-long fuel saving of 8.2%: an important milestone as though others may have returned higher values during peak performance or on selected routes, Pelican is a fairly ordinary long-range tanker – and that class-validated ‘average’ matters.

However, WASP technology comes in many different flavours. Airbus’ spin-off development, Seawing is a lightweight, textile parafoil designed to fly at 150m “where you get above most of the gusts and turbulence to constant and powerful winds”, says Luc Reinhard of Airseas. Despite this, it’s designed to automatically unfurl and lift in a minimal 8kn breeze.

The pull is transferred to a winch on the foredeck, the line incorporating a data cable which runs to the kite’s brain. Hanging just below the parafoil, this control pod directs the wing into a dynamic figure-of-eight flying pattern, increasing traction with speed.

The Seawing has already started picking up orders, doubtless helped by the fairly simple demand of space on the foredeck and a control link to the bridge. As a result, K-Line is fitting it to a capesize bulk carrier – significantly, there’s an option for 50 more so it may well fly in more ways than one.


There’s now a whole range of rigid and hybrid sails including models with single and even double flaps, but few are realised onboard larger commercial vessels.

Still, Eco Marine Power’s (patented) EnergySail is working on a broad remit: not only can it make use of a wide, 270deg arc of wind direction but it’s opening up its operating window by doubling up as a platform for other technologies. While the base version has been tested in high winds the company is now working on a slightly more rugged model that will hold an array of photovoltaic panels. By utilising these and other areas, it’s possible to harvest 1MW of solar power per ship: additional energy storage could allow emission-free running in port.

Further, there are plans for “wind-in-wind” says EMP CEO Greg Atkinson: incorporating small turbines could deliver a useful feed to the onboard grid alongside the push to the hull.


Towers, on the other hand, modify the low pressure area created by the wind’s travel path around a cylinder, amplifying the Magnus effect. However, they achieve this in rather different ways.

The Ventifoil has an elliptical cross-section, adding a flap and ventilator explains eConowind’s Guus Van der Bles. This accelerates the airspeed at the boundary layer and mitigates turbulence: as a result, the low pressure area clings to a broader section, multiplying the generated thrust. It’s effective in incoming wind-angles over 30degs from the bow with maximum thrust in beam winds: given 17m/s wind speeds, four 10m VentiFoils should yield a reduction of up to 420kW of engine power, maintaining a ship speed of 11 kn.

So, what does it take to integrate them? “It’s not that difficult,” says Van der Bles: the first plug-n-play, folding containerised eConowind-unit onboard Lady Christina was fitted to the deck by common twistlocks: a standard electrical connection fed the 7.5kW ventilators.

However, he adds that ‘bigger is better’: the returns significantly stepped up for the two free-standing 10m VentiFoils on Van Dam Shipping’s bulker MV Ankie. Early indications are that 6m extensions (bringing them up to 16m) will provide about the same thrust as four 10m containerised versions. The yield from the next order, two 20m freestanding Ventifoils, should be interesting – but even here heavy deck reinforcement is unlikely to be necessary “although we check for class”, says Van der Bles. It also opens up the potential for swapping units between vessels, depending on route.

This all helps the business case: “You can talk about thrust force per metre, but thrust force for invested Euro is most important,” he remarks.


Instead of sucking, Flettners spin to accelerate the airflow on one side while decelerating it on the other, the pressure difference adding to the pull: rotation is generally adapted to wind speed through a variable electric drive – although some, like ThiiiNKsail’s model, also incorporate a wing flap. This results in a somewhat bigger demand than ventilated towers, Norsepower’s largest 35m-by-5m diameter model draws on average 40kW and up to 143kW, though that’s set against a larger contribution to the total energy budget.

While the output per metre is just a little higher than other towers, Flettners can also make use of over 300deg wind angles and even a 30m/s storm. Norsepower’s 24m tall, 4m diameter Rotor Sail can produce 2,000kW propulsion equivalent from a true wind speed of 22m/s (though tower size, vessel speed, energy conversion and other parameters affect comparison).

Along with tow, rotor spin lends a fluctuating gyroscopic or precession component to the forces acting on the foundations points out Rogier Eggers of MARIN. Accordingly, they’re fairly meaty, adding between 30% and 40% to the flettners’ 20 to 59 tonne weight although these are still fairly straightforward retrofits: an adaptor is welded on during ordinary docking.

Accordingly, the foundations are fairly meaty, adding between 30% and 40% to the Flettners’ 20 to 59 tonne weight although they’re still fairly straightforward retrofits: an adaptor is welded on during ordinary docking.

Interestingly, a study for a DAMEN BTa 19500 tanker carrier carried out by Nico van der Kolk of Blue Wasp noted that positioning three Flettners along the side of the vessel allowed integrating the base with the vessel frame. This could facilitate cargo operations (at least from one side) although different headings made for asymmetric WASP effects.

While the study generally fell in line with eConowind’s discovery that fewer, larger rotors are more effective than smaller multiples, Van der Kolk’s colleague Giovanni Bordogna adds that spacing does matter: according to his modelling, WASP installations can interfere with each other if they’re too close.

However, powered towers do have one advantage over sails: you can hit the ‘off’ switch if things get rough. While it won’t remove all the drag, it’ll rapidly take the edge off the Magnus effect.

Which brings us to automation. Each WASP unit is designed to catch the wind so the forces acting on it total far more than weight alone.

Deployment, recovery and optimisation has to take place with little or no human interaction, but a ‘fold-down’ strategy for critical conditions also means the kit has to be capable of automatic, seamless, and timely retraction, which in turn demands a robust monitoring and control system. That could skyrocket costs but it’s possible to get clever: EMP, for example, bases its system on a reliable, class-approved data logger.

Interestingly, the Seawing mitigates some of these concerns as it can move “from a dynamic power pattern to a holding position above the ship in just a few seconds,” explains Reinhard. Once there, it exerts almost no pull, which allows the kite’s AI to pause and wait for developments, minimising launch and recovery operations.


As to be expected, newbuilds offer greater WASP optimisation potential than retrofits, whether just by avoiding wind-blocking structures, saving weight on the foundations or going for a whole redesign incorporating a streamlined hull, fuel cells, batteries and other innovations. Interestingly, the transit pace for both EMP’s 240m Eco Ship and the 220m Conoship 33000 ZE bulker designs sits between 10 and 13 knots as slower speeds allow for greater wind assistance, and both predict average savings of over 40%. However, Van der Bles adds that with favourable Bft 5 conditions on a North Atlantic route, “it should be possible to meet the ship’s entire propulsion requirement for an 11kn speed” with power to spare.

But despite the usually fairly simple nature of the retrofits, it’s not straightforward.

Given 40% wind assistance “the manoeuvring and sea-keeping characteristics of the ship itself could change” says Eggers.

If the propeller isn’t providing much thrust “there is little flow over the rudder”, reducing the ship’s ability to counter the force of waves he explains: it could be enough to push the vessel off course and potentially increasing roll, although he adds that some WASP systems actually confer a roll-dampening effect.

Still, the forces arising from the most productive crosswind conditions may result “in the vessel having to steer into the wind, reducing the power available”, says Philip Holt of MAN ES. Even if the ship doesn’t change heading, Van der Kolk says the benefits of WASP technology have to be considered against other effects. He adds the main issue is the aerodynamic side force will likely be large enough to create considerable heel and leeway angle, resulting in potentially significantly raised hydrodynamic resistance. Further, Eggers points out the rudders could also become overloaded as they attempt to correct the balance.

Generally, WASP-optimising design means “main particulars and section shapes will need a careful look” he explains, adding that adapting appendages could make a difference. While in principle the solution is to move the weight down or widen certain sections of the vessel, in reality “this may be a challenge”, he says.

However, he points out there are easier, operational strategies that will mitigate course keeping issues such as “temporarily sailing at a higher speed to increase flow over the rudder or by doing the opposite: reducing speed to shorten the encounter period of the waves”. He also adds that given spare capacity, steady heel “could be compensated by asymmetric ballasting”.

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