Analysis: Flying high - another kind of wind power

Why the idea of using wind to propel ships is enjoying a comeback.

Is it a bird? Is it a plane? No, it's a ship being towed by a kite, which is one of the latest technologies that ship owners are looking at to reduce their fuel bills.

German firm Beluga Shipping has fitted its newest cargo vessel with a high-tech kite made by Hamburg-based SkySails. The multi-purpose heavy lift ship 'Beluga SkySails' is testing the system on a voyage from Bremen to Venezuela, with the expectation that it should reduce annual fuel costs by between 10 and 35 per cent, depending on prevailing wind conditions, with the added benefit of reducing emissions. The first results are to be expected in the next few months.

SkySails founder and managing director Stephan Wrage says there has been an upsurge in interest from shipping companies all over the world during the last year in the light of rising oil prices.

This is not the first time that sails have been used to reduce fuel consumption on cargo ships. In the late 1970s and early 1980s there was considerable interest in wind-assistance for merchant ships. Several vessels were built or converted, including the Japanese tanker 'Shin Aitoku Maru' and a bulk/log carrier 'Usuki Pioneeer'.

In 1985 the sailing-enthusiast captain of the German-flagged 'Bold Eagle' container ship used the cranes on the ship as masts for a pair of spinnakers – large lightweight sails used when travelling in roughly the same direction as the wind. It was claimed that, with a good tail wind, the ship could turn off its engines and sail at a comfortable five knots, saving about 2,000 litres of oil per day.

A more sophisticated approach was adopted by the board of Stephenson Clarke Shipping Ltd, when they fitted a Walker Wingsail system (basically an aircraft wing turned on end) to their ship Ashington and evaluated it between 1986 and 1988. However, in the ship's trading area, "usable wind" (when the wind was from a suitable direction) was restricted during the trial to approximately 30 per cent of the total passage time, and average wind speeds were relatively low. There was also significant downtime due to necessary wingsail maintenance, and the vessel's fuel consumption results were not consistent.

The problem with most of these systems is that, to gain a significant advantage from wind assistance, the vessel route has to take advantage of the prevailing winds. None of these systems offer any advantage when the vessel's course is into the wind, and some that use fixed equipment (although not the SkySail) can cause extra wind resistance, making them detrimental to fuel economy.

With ships recently sailing into the headlines over problems encountered in gales resulting in evacuation, one has to wonder what will happen if a ship using sail technology encounters such strong winds.

After all, the critical decision would be over when to winch the kite back onto the deck, not to mention how it should be protected when it is safely down.

SkySails technical manager Stephan Brabeck acknowledges that only practical experience will provide answers to these questions. "Certainly, the daily routine at sea will still pose many challenges, and in the process we will learn many very valuable lessons," he says. "It is now particularly important to raise the manageability and robustness of the system to the level demanded by our customers."

Questions arising

There are still many questions that need answering. If a small kite gives you a small improvement in fuel consumption, will a big kite give you a large improvement in fuel consumption and if so, where does it end? How big a kite can you fit to a ship before it becomes completely unmanageable by the small number of crew used to man ships nowadays?

Wind speed and direction is also dependent on height. Many will remember the images of the Virgin Atlantic balloon, in which Richard Branson crossed the Pacific in 1991. This travelled at speeds in excess of 245mph (394km/h), breaking all existing records. 

If this technology 'takes off', the temptation will be to fly these kites higher and higher and further away from the ship. At what point will they become a hazard to aircraft and other ships and how long will it take to winch them back in?

With much depending on the price of oil, only time will tell if harnessing the wind is a success or if it will be just be another idea that costs more than it saves.

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