Loadstar explains: Wind-assisted ships – smooth sailing or just hot air?

Sails have been retrofitted to bulk carriers and tankers, and with the arrival of a new wind-powered containership concept from Hapag-Lloyd, The Loadstar finally has an opportunity to discuss wind-assisted ship propulsion (WASP).

I heard there are new shipping companies using ‘tall ships’?

There are indeed. Sailcargo is one of them; it has one three-masted schooner, Vega, and another, Ceiba, is currently under construction. When it is finished, it will have a capacity of 250 tonnes of breakbulk, the equivalent of nine teu.

Hammering clippers together from oak planks is something of a lost art, though, meaning that Sailcargo and the handful of companies like it will need a lot of help if they are going to decarbonise shipping. We’re certainly rooting for them.

So, is this a return to the age of sail?

Hardly. In the best possible conditions, the best designs for existing sails can provide about 30% of the propulsion needed to power a ship. It’s certainly nothing to be sniffed at, but it is not a revolution, either – and neither is it a substitute for investment in other areas like hull forms and bubble lubrication.

The benefit of sails is extremely situational. Close to the equator, the ‘trade winds’ blow from east to west, and in the North Atlantic, from west to east. There is a similar and potentially useful gyre in the Pacific: “westerlies” (which, confusingly, blow eastward) across the northern Pacific toward North America, and trade winds further south, which blow westward, toward the (political) east.

There is a slight problem however: the sail is not only thing propelling the ship. If a vessel has to change its route to take full advantage of these drifts and gyres and burn extra fuel, this is a false economy.

But wind is free – surely sails are worth having for any ship?

Wind may be free, but retrofitting them is not. Many more ships stand to benefit from adding wind power than are currently doing so, but shipowners can be quite obstinate when it comes to investing in new technologies, particularly if their vessels are over ten years old.

And shipyards can be rather stuck in their ways, as well. Never mind technologically advanced retracting sails, Foreship chief naval architect Markus Aarnio recently points out that many yards still aren’t even building hulls the correct shape. “Often yards insist on optimising the hull form to contract trial speed only, even if that speed is seldom used in real operation,” he says.

The biggest barrier, though – and one that Hapag-Lloyd is undertaking to solve – is that not every ship can accommodate sails.

Can modern technology help?

Absolutely. Amy Buhl, of WeatherNews (WNI), told The Loadstar her organisation was in dialogue with shipowners about combining advanced weather forecasting – including WNI’s latest ‘probabilistic’ forecasts – with ship sails to create new, unheard-of ship efficiencies. “It’s a really interesting conversation,” she said, “because it requires us to change how we think about voyage optimisation and how to utilise the weather conditions in this respect. I think we’ll see more of it as adoption of the technology increases.”

Could containerships use sails?

Maybe. But it is difficult to do without something getting in the way. Sails are tall, but container stacks will very quickly get in the way of the wind, meaning sails we have seen so far are only used on flat-top vessels. such as bulk carriers, tankers, and ro-ros.

Meanwhile, at the quayside, sails get in the way of rail-travelling gantry cranes, meaning they must perform additional movements to get to cargo. If sails on a containership can reduce fuel consumption, more time spent (usually burning fuel) by the quayside will immediately offset this.

Hapag-Lloyd’s design puts a wall between container stacks and, while this does protect the sails from loading-related mishaps, it does seem that cranes will still have to move more to unload the same amount of cargo.

There is also the matter of leverage. Becker Marine, which made a name for itself developing mewis ducts, twisted fins and other hull-optimisation goodies, recently developed a retractable daggerboard system, akin to that on a sailing boat. The rationale: sails create a force perpendicular to the direction of travel, something engine-powered ships are not necessarily designed to deal with. But if ships are going to have a sail pulling on them from higher up, they need something below the waterline as well, reasoned Becker MD Dirk Lehmann. “When the ship has a certain deviation, it produces lift to the correcting side, like the daggerboard on a sailboat,” he explained at last year’s SMM in Hamburg.

If Mr Lehmann is right, and corresponding underwater accessories are needed to support sails on top, it will be interesting to see how Hapag-Lloyd’s plan of putting them well above the vessel’s centre of mass will work out.

You don’t seem very certain about any of this?

Weather is uncertain – it is the main reason we stopped using wind. And it is only likely to become more so, according to Ms Buhl. “…the effects of global warming are not being observed uniformly… some areas are more affected than others, meaning those temperature differences increase”.

And recently she said: “…we have seen… rapid intensification of tropical cyclones – tropical systems that rapidly increase in wind speed over short intervals of time. We also see more ‘bombing lows’ – a similar type of dynamic for a mid-latitude system, where the low rapidly deepens over a short period of time.”