30 May 2023
SKYTUG: Primary Wind Propulsion for All Ships
Full power wind propulsion will soon be available to existing ships. A bold and surprising statement, perhaps, but one which I believe to be true. SKYTUG, which is being developed by my company, Bluewater Engineering, is a wind propulsion concept with a novel twist – the sails aren’t attached to the ship.
As you might have guessed from the name, SKYTUG is a tug – but we’re not talking about harbour operations here. This is ocean towing, on a scale designed to pull deep-sea cargo ships for tens of thousands of miles across the world’s oceans. The tug is designed to tow ships from A to B within their usual timeframe, using only the power of the wind to do so.
In doing this, it will reduce carbon emissions by substituting wind propulsion for fossil fuel usage, reduce costs for alternatively-fuelled vessels by lowering their consumption of expensive fuels, and help to roll out zero carbon propulsion systems across the world fleet by filling a huge gap in the available green fuel supply with the freely accessible, and highly efficient, direct power of the wind.
The obvious question is why? Why build a whole new vessel rather than install a system aboard the ship, or even build a new ship? There are several answers to that.
The first is space. There is limited space aboard existing ships for wind propulsion installations. Container ships have least space of all, but even aboard bulkers and tankers there is still a practical limit set by the deck length and cargo handling constraints, which prevents wind from providing anything like the majority of their propulsion. For newbuilds, of course more deck space could be added, but at the cost of higher port charges, restricted access to canals and harbours, and a drastic change in the shape and dynamics of the ship, since increasing length without offsetting the
benefits with increased water resistance, means reducing draft.
The second, closely related answer is power. If one accepts that the available space aboard ship is what it is, then to achieve full power wind propulsion, space needs to be found elsewhere – that’s where SKYTUG comes in. There is quite literally an ocean of space available away from the ship, in which to place a wind-propelled tug which can provide ample power to tow the ship at full speed. Of course, there are some common-sense constraints on that too – it clearly isn’t sensible to build an additional vessel as big or bigger than the ship in order to tow it across the ocean, so SKYTUG relies on kites as a highly compact, power-dense wind propulsion solution, which can expand to vast size in flight, yet pack away tightly aboard the tug when not in use. Of course, kites can be used on ships too, but a sufficiently large kite array to provide full wind propulsion would still require increased length or the sacrifice of cargo space.
The third answer is the one which should get shipowners and their charterers really excited. As an entirely separate vessel, SKYTUG can provide wind propulsion as a service, providing towing for a fee, so there is no CAPEX outlay, no risk of stranded assets, and no under-utilisation arising from the ship’s inevitable time spent in sheltered waters or in port, loading and unloading. To cap it all, there is no need for modifications aboard the ship. That means no expense or downtime for intrusive work, no maintenance burden, and no additional structures getting in the way of cargo operations. And neither is there a training burden for the ship operator, since the tug will have its own specialist crew. An additional financial benefit is flexibility – the tug can be deployed to different ships as required, to deliver fuel savings and CII compliance, while operating continuously on the most favourable routes, thus maximising its impact across fleets rather than being fixed to a particular ship indefinitely.
So that’s why, but what about how? How does it work, and how can it be economical to build and operate such a system? Let’s deal with the latter question first.
The tug, in its initial design, which will be the first and smallest size to hit the water, is about the size of a typical SOV, at 70m long. Without going into details, that should give knowledgeable readers an idea of the construction cost, which is
similar, albeit for different reasons. Adding in kites and their control systems and the various other SKYTUG machinery adds about 15% to that. When considering OPEX, including a highly trained crew, kite repairs and replacement, kite handling system maintenance and other factors, and considering revenue, from displacing fuel as the source of energy, it quickly becomes apparent that SKYTUG could offer propulsion at a fee not only competitive with future green fuels but competitive with today’s fossil-based bunker fuels, and generate a competitive return for shareholders too.
Put in context, this economic potential is not that surprising. Future e-fuels are expected to have less than 10% efficiency from renewable energy input to ship’s wake, as opposed to 100% for direct wind propulsion, and yet cost only 3-5x more than today’s fossil fuels, despite the massive infrastructure investment required to bring them to the market.
Most of that renewable energy may come from wind turbines, which have seen a major reduction in the levelised cost of energy over recent years, due largely to increased scale. SKYTUG deploys wind power right at the point of capture, short-cutting all those conversion losses, and its grand scale makes for a high level of propulsive power, and therefore revenue, which overcomes the costs of operation to make it a profitable venture. Economy of scale has long been a feature of successful industries, and that paradigm applies no less here.
Economic to Technical
I know readers will want to know how SKYTUG works, so I
will explain the basics here. For this description I will refer to the parameters of the first-to-be-launched SKYTUG design, which is sized to tow ships of 35,000dwt and above, saving around 26 tonnes of fuel per day, and can in fact tow far larger ships on some windier routes, such as those routes rounding the Cape of Good Hope, saving even more fuel and emissions in the process.
Design & Working Principle
The tug is a little over 70m long, and has a catamaran hull form. This was an obvious choice because the kites themselves are large but light, so the vessel to carry them should be sufficiently large without displacing too much water and creating too much resistance; a multi-hull form makes sense, and a catamaran specifically is best at countering heeling moments. The tug form and general arrangement has been designed by our excellent naval architecture partners, ICE Marine Design. The design is laid out with the towing apparatus, accommodation and machinery spaces in the aft portion, the kite connection and airborne handling systems area up forward, and the kite stowage bay in the middle.
“SKYTUG is a very exciting concept – the most cost-effective way to decarbonise transoceanic shipping. I’m pleased to be supporting the project and keen to see it progress to full demonstration.”Steinar Draegebo OBE, Chairman & CEO, ICE Marine Design
Each kite is around 5,000m2, meaning it has a wingspan similar to the length of a Handymax bulk carrier, and the tug carries ten of these. The kites work as a series array in flight, which means they fly in a ladder formation, exerting their wind forces through common tethers down to the ‘flight deck’ on the foredeck of the tug. Much work has been done by BGD, our kite development partners, on these kites and their flight. More or fewer kites can be flown, depending on the conditions and towing force required, and they can be independently launched and recovered as needed, by systems being developed both in-house and by our remote-handling experts at RED Engineering. Being so large, the kites are not unfurled on deck, but first hoisted up onto the array as a folded package, before expanding to fill with air and start creating wind force; later, they will be retracted by the reverse procedure.
The kites are of the ‘foil kite’ type – lightweight, high performance, neatly packable and self-inflated by the incident wind, after launching. They have one-way valve structures in the fabric to retain inflation pressure until deflation is carried out during recovery, which means they maintain a good flying shape, are resistant to water ingress, and are more easily recoverable from adverse incidents. Along with BGD, sailcloth manufacturers and sailmaking experts at Bainbridge International have been working with us to design the full-scale kites and plan the process for their manufacture.
SKYTUG uses daggerboards to keep it sailing forward even in upwind conditions – it can sail very close to the wind, which is a necessity because the forward speed of ships on passage generally creates sufficient headwind to swing the angle of the apparent wind well forward of the beam. Seafarers will know this intuitively from standing out on the bridge wings or on deck, facing forward and feeling the breeze on their faces more often than over their shoulders. The daggerboards act like a yacht’s keel to counteract the sideways part of the kite array’s forces, leaving only the forward thrust which pulls the tug and ship through the water. They are deep, but retractable, so their draft will not cause a problem when entering harbours or navigating shallow straits.
Towing is managed by means of an adaptive towing winch, to smooth out the variations in towing force arising from sea state and the vessels’ differing dynamics. The tug will be fitted with a system for passing a towline from a safe distance, rather than the usual heaving line technique, to allow a safe approach under sail. This allows the SKYTUG kite array to be launched, checked and ready to go before towing commences.
This ambitious project has some way to go – so far we have carried out proof-of-concept trials of key features, operations and sub-system prototypes, independently, showing that the largest of the challenges for SKYTUG are indeed surmountable. CFD and tank testing of a model tug, carried out in conjunction with ICE, Lab Sea Systems and Ifremer, has successfully proven the performance of the hydrodynamic aspects. Kite flying trials carried out by BGD have shown stable kite array flight and the ability to launch and recover kites independently to and from the array. Kite handling system prototyping by Bluewater Engineering and RED have shown us how the kites can be handled mechanically to furl, unfurl, reel and control them. Meanwhile, studies by Bainbridge and ICE have provided preliminary designs, plans and cost estimates backing up the all-important economic viability of this great decarbonising concept.
The potential of SKYTUG is vast. As a solution for all ship types, flexible in its operation, the eligible market is huge. The emissions reduction potential is almost absolute for the ships served, and the cost savings that can be achieved are on a scale that exceeds any other proposal for decarbonisation of the shipping industry. So we must press ahead, and quickly.
“We consider the SKYTUG concept very interesting for marine transportation and one of the methods which may contribute to significant reduction of fuel consumption and GHG emissions without the need for capital investment on the towed ships.”Zenon Kleopas, Executive Vice President, Green Energy & Technology, Star Bulk
Conceptual unknowns and feasibility risks have been largely addressed in the first phase of the project. Now we must set about the engineering challenge of building a full working system – we plan to begin that work by fitting out a test and development vessel, at workboat scale, and getting out on the water next year. This is to be followed by construction of a demonstrator vessel, and finally commercialisation and growth of the SKYTUG fleet to serve the world’s merchant ships in a new, clean, green maritime industry.