Wind above: what goes on below?

Could wind’s promised efficiency impact other systems? Stevie Knight asks the questions…

Currently, “Flettner rotors or kites are rather easy to integrate” says Philip Holt of MAN ES. That’s partly because the effective power provided by these systems has historically been quite low: “We have heard of fuel saving of around 3.5% to 5% and up to 10% in special cases on merchant vessels”, he explains. That puts it in the same ballpark “as sailing in small waves with a tail wind…. for the same rpm setting, the propeller simply requires a lower amount of torque”.

However, if the whispers of a near-future 20% and potential 40% vessel efficiency are to be believed, there are further considerations.

“As these mechanical sails push the ship forward through the hull, the amount of additional energy introduced could vary a lot, from zero to half the installed power,” says Tuomas Riski of Norsepower, adding that the onboard plant needs to be flexible enough to compensate for these reduced loads. However, given fairly steady conditions, this still falls within the efficiency range for long-haul ships’ two-stroke engines.

“If you’re operating on 50% engine load with the wind propulsion aid, and 90% without it, the specific fuel oil consumption (SFOC) will only deviate by a few grams,” says Holt, adding that MAN’s slow speed, two-stroke models tend to be efficient down to 10% MCR.

It’s plausible that these engines’ load response might be a little slow, but as Van der Kolk goes on to explain, while most studies look at retaining a fixed speed, in fact there is a wider operating envelope as in reality some wind variation will simply be taken up by a greater ship speed. MARIN’s Rogier Eggers adds that even if a conventional, two-stroke propulsion plant isn’t fully adjusted for WASP, “the savings are likely to still be significant, even if they could be higher”.

Riski remarks that experience bears this out: Maersk Pelican’s impressive 8.2%, savings resulted from installing a pair of large, 30m Rotor Sails without paying the engines any special attention. The 61,000gt tanker “has faced variable wind conditions on continuous basis, but” he stresses “it has not had any challenges in adapting to fluctuating loads”.

However, a greater degree of wind assistance may change the picture: “We will have to see what is really possible with adjusting two-stroke propulsion,” says Eggers, though Holt adds that while “it’s mostly speculative”, during high levels of wind assistance, the engine might benefit from alternative optimisation parameters – although “firm consideration” is still over the horizon, “actual numbers will be required to perform a study of the potential”.

Despite this, Holt underlines that even given sizeable WASP capacity, MAN ES “doesn’t recommend reducing the main engine power based on an installation of some sort of wind assistance… as it cannot be ensured to operate in heavy weather”.

Still, WASP installation could give rise to alternative power configurations. Installing a shaft-generator PTO along with the WASP on deck “could be beneficial” says Holt, as the main engine may be used to supply the electrical power rather than the auxiliaries. It would make for an interesting set up, says Sami Kanerva, ABB Marine and Ports: the gensets could take a break as the two-stroke engine picks up the load during periods of greater wind assistance.


Ship type does play a part says Eggers: higher speed results in “the effective wind direction above deck being mostly from ahead”, so don’t expect long-haul containerships to line up for WASP installation, even if they could arrange it without sacrificing capacity. However, more modest general cargo vessels and coasters “are interesting, because their lower speed translates to a more favourable wind angle”, he remarks.

And in this arena, four-stroke engines reach a broader audience.

As Kanerva explains “in contrast to the larger oceangoing cargo ships, diesel-electric propulsion is typically applied to smaller vessels with a wider operating profile”, given this, it is “usually more effective to have multiple medium-speed engines”. This, of course, applies doubly to those looking at low-carbon or coastal emission-mitigation strategies.

“Wind assist is like another form of hybrid – with its own particular requirements… so you need to tie together and balance the various power inputs”, says MARIN’s Alex Grasman. He underlines, this demands high level power management, not just engine control. But for first movers, it’s a good idea to be able to track what’s really going on so Viking Grace has installed ABB’s Octopus system which can validate the collected data and a machine learning module which can carve up the various effects into their components: drag, wind, wave resistance as well as the push from the WASP.

The recent Flettner trial on Viking Grace uncovered a picture that underlines the subtleties: for example, threading through an archipelago or long fairway “where a vessel needs to move precisely but the wind can be inconsistent, it’s actually difficult for either Flettners or even the power plant to run that optimally”, says Kanerva. However, the picture changes if there’s a longer sea leg.

He remarks: “When we look into ro-ro and merchant vessels with steadier wind conditions and longer ocean passages where they can slightly optimise their heading, then it is really very viable as propulsion support.”

Eggers also believes that some vessels could gain an advantage, he cites ro-ro as presenting “little conflict” with loading or unloading patterns: further, ferries and cruise vessels “may operate in specific areas with favourable winds”.

Moreover, as Kanerva remarks, a four-stroke plant will likely be better able to follow the load curve, which may result in a closer match with the desired speed. Eggers comments that, “even without changing schedules and routes, large savings could be achieved, although it will still not be at its theoretical maximum”. He also points out that for cruise and longer-distance ferries especially “there is the additional incentive of good PR”.

“Of course you want to be confident in selecting the appropriate route and speed distribution” for each voyage, he adds: if the engines have limited response to the load curve, “speed variation may be wider over the duration of an individual trip”.


This brings us to route choice. Usefully, the rapid advance of artificial intelligence means that voyage optimisation services release WASP-installed vessels from traditional 19thC trades, enabling savings on a wide range of less windy routes.

It also allows realistic choices. For example, NAPA’s software uses AI to tailor the output for forecasted weather patterns, specific vessels and type of assistance, yielding a balance between time and fuel saving options.

However, while suitable routes can offer general consistency, that’s spread around a probability curve. So, while the wind conditions don’t usually change that fast, “fast changes can occur”, remarks Riski.

It will take careful handling: inadequate power management could result in “generators cutting in and out several times an hour, eating away at the savings” says Kanerva.

Even with a forgiving operational envelope, a number of four-stroke applications might usefully entrain an energy storage system. Kanerva explains that this would be for “strategic loading, not peak shaving”: the battery potentially supporting the propulsion for longer periods, “maybe even up to half an hour”, he adds.


This yields a synergy with other, potentially interesting power sources. As Riski points out, “whether that’s LNG-electric, hydrogen-electric or some other electric hybrid, everything gets routed through the motor”. He adds that typically, this leads to “a more responsive power plant”, making the integration of WASP systems somewhat easier.

Atkinson explains EMP’s Aquarius Eco Ship design “would most likely incorporate an onboard DC electrical grid and this would allow a range of power sources to plugged in” he says, from batteries and photovoltaics to fuel cells.

Importantly, the idea avoids penalising first movers. He adds: “As these technologies develop further, the existing equipment on the ship could be upgraded or even perhaps replaced.”

In fact, the sail rests on the company’s Aquarius MRE (Marine Renewable Energy) base, a control solution that’s developed to be adaptable, taking on new forms of energy as they reach fruition.

One more point to add from Holt: there might be a challenge in the resistance created by the WASP during adverse weathers and designers will have to take this into account when designing for compliance with the rule on the minimum propulsion power which is currently being set.


However, when it comes to the business case, the cargo industry simply has the “wrong model” concludes Gavin Allwright. He points out there’s still a disconnect between who pays for the investment and who gains, but adds the system could do with a complete overhaul. “If you are tweaking the existing paradigm you’ll get significant gains, but what’s really needed is a move from a time-charter to a slot-charter model with a fuel or carbon-based clause,” he explains. “If you are prioritizing decarbonisation, it could benefit the environment and bottom line.”

By Stevie Knight


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