Thursday, February 24, 2005

Hybrids: Hybrid Boats, Hybrid Ships and Beyond

The New Scientist reports that for several weeks last summer, a team of German engineers from the company SkySails sailed back and forth across the Baltic Sea testing the potential of high-tech kites to pull a ship across the ocean by hitching a ride on winds high above the waves.

The idea isn't to propel a ship by wind alone - a conventional diesel engine will help it along on days when the wind is blowing from the wrong direction, is too strong or dies away entirely. But since the kite reduces the need to use engines, the team at SkySails believes it can halve the amount of fuel a ship burns.

This is just one of the ways in which sail power is being revived.

For nine years a team of naval architects in Copenhagen, Denmark, has been working on a completely new design: a 50,000-tonne cargo ship whose diesel engine will be augmented by a set of high-tech sails set on six masts. Canvas is definitely out. Aerofoils are in.

Led by the naval architects Knud E. Hansen, the research not only produced a new design of ship but also looked at how the ship could make the most of wind power and the cargoes it would be best suited to carrying. Now the team is about to embark on full-scale trials.

The small crew needed on a modern ship, combined with the low wages they are paid, means that the cost of fuel as a proportion of total running costs rose from 10 per cent in 1900 to between 25 and 60 per cent by 2000.

Modern windships can also take advantage of new technologies and materials that weren't available in the days of sail. Wind tunnel tests on different types of rigging and sails quickly showed the Danish team how poorly traditional sails perform. A sail is more than a simple sheet of fabric. To propel a ship it needs to take up an aerofoil shape, and that only happens when the wind fills it. If the wind is too light, or it keeps changing direction, the canvas flaps uselessly and generates drag rather than propulsion.

So the Danish team came up with an alternative that exploits materials borrowed from the aerospace industry. Using high-performance steel for the masts does away with the need for stays to hold them upright. The sail itself is made of fibreglass, with a profile like an aircraft wing. Flaps on the sail's trailing edge generate extra thrust when extended, but can be retracted to minimise aerodynamic drag - important when using engine power alone.

Wind-tunnel tests showed this design to be twice as efficient as the sails on a traditional windjammer. Even more importantly, the sail generates thrust when the ship is sailing close to the wind. Simulations suggest that the vessel will be able to make progress under sail even when the wind is blowing as little as 40 degrees off the bow, which is an excellent performance for a large sailing vessel. With a fresh breeze of 9 metres per second at 100 degrees - blowing only slightly from behind - the sails alone can propel the ship at 13 knots (25 kilometres per hour.

Unlike traditional sails, these fibreglass wings will not need a large crew to operate them, the designers say. They can be controlled hydraulically from the bridge, and because they never need to be lowered there is no need for storage space that would eat into the cargo capacity. The downside is that in light winds, with the ship under diesel power, they exert aerodynamic drag - even with flaps retracted - which negates some of the fuel savings from having them there in the first place.

The SkySails kite suffers no such handicap. The idea is to harness the winds higher above the ocean with an inflatable aerofoil - a kite designed to fly at a height of 100 to 500 metres, towing the ship on a cable fastened to the hull.

At 500 metres, winds are often stronger and less variable than at sea level, and can differ in direction from those immediately above the waves by 10 to 15 degrees, according to Barry Gromett of the UK's Met Office in Exeter. "Although these differences are not huge they could be really useful," he says.

SkySail's aerofoil is designed to maximise thrust whatever the wind conditions. It uses a computer autopilot and patented wind sensors coupled to the ship's steering system to calculate the kite's optimum position. Then the autopilot manoeuvres the kite using motors in a control unit suspended beneath it to change the trim of the aerofoil by adjusting the tension in its control lines. The kite can move along a rail around the hull to maximise its towing efficiency and a winch on the ship adjusts the length of the kite's main line to fly it where the wind speed and direction are most favourable.

Last year's trials in the Baltic, aboard an 8-metre model of a cargo vessel, were mostly carried out in unfavourable conditions of weak and variable winds. Nevertheless, they showed that the SkySails kite can generate 1 to 1.15 kilowatts for every square metre of aerofoil. "In favourable winds it would generate a lot more thrust," says Stephan Wrage, founder of the company. The kite is designed to be retrofitted to ships of almost any size, but SkySail's largest version, with an area of 2000 to 5000 square metres, will generate propulsive power equivalent to a large ship's engine, he says.

Since the kite is controlled by an autopilot, Wrage says it will not need many extra crew to handle it. Compressed air will be used to blow it up when it is deployed and when not in use it is deflated, so storing it should not be a problem either. But as any kite flyer will tell you, launch and recovery are likely to be a little more complicated. SkySails says it will be an automated process, but won't reveal details until its engineers fit their first system to a ship next year.

Selecting a course that maximises the benefits from the wind is an important part of any successful voyage under sail. Here modern windships have another key advantage over their predecessors: they will have access to far more accurate weather forecasts to help get the best from the wind.

On routes such as Rotterdam to New York the ship would save up to 27 per cent of the normal fuel bill at its design speed of 13 knots, the Danish team calculate.

Just as sales of hybrid cars have been spurred on by environmental concerns and the rising price of gasoline, the price of marine diesel (which closely follows crude oil prices) has soared to nearly three times its 1999 levels and there are increasing concerns about pollution from shipping. These engines release pollutants such as sulphur, nitrogen oxides and PM10 particles. In particular, marine diesel oil contains 2.7 per cent sulphur - more than 500 times what is allowed by the EU for diesel sold for cars and trucks. By 2010 it is estimated that cargo ships will account for three-quarters of all Europe's emissions of sulphur dioxide. In the US, the Environmental Protection Agency is stepping in with new regulations to help improve air quality around large ports.

Just as changing driving habits maximises the mpg returned for a hybrid car reducing the design speed of the windship from 13 to 11 knots, for example, would cut fuel bills by a third on both the North Atlantic and the Indian Ocean routes because the engine wouldn't have to work so hard.

In Australia SolarSailor have developed a passenger boat, showcased at the 2000 Olympic Games in Sydney, that can be powered by solar panels, and whose movable wing-like solar panels can also act as sails. They are being considered for use as urban ferries and would use up to 50% less fuel. According to WorldChanging the inability of the Solar Sailor to run 100% off the sun and wind when operating as a ferry is due to the application's speed requirements--the craft can only do 12-14 knots maximum estimate without also using battery/diesel hybrid assist; but cutting fuel usage in half is still a huge and admirable achievement. It's better than today's best hybrid cars do.

SkySails has its eye on a rather different market. "One surprising result from the trials was the vessel's stability in heavy seas," Wrage says. Unlike conventional sails, the kite tends to stabilise the ship instead of making it heel over. This is partly because it is tethered to a rail close to the vessel's centre of gravity, and partly because the horizontal tug of the kite is counterbalanced by the vertical pull it generates, which tends to hold the vessel upright. "The sail acts like a damper so the ship moves smoothly, which will prevent passengers being sick." This is significant because Wrage sees cruise liners, and the growing number of cargo ships that carry passengers, as important markets for the technology. The next step for SkySails is to move from a model to a full-size craft.

So could there be a sea change for sail? "It will now be profitable both environmentally and economically to build the windship," says Anders Carlberg of Knud E. Hansen. Other new sailing ship projects are already in the works, one in Germany and one in Japan. Carlberg and his team estimate that full-scale trials of their design will start within three years.

The point to be made is hybrid engine technology has encouraged a new way of thinking beyond the garage which has many applications from buses, trucks and tractors to even planes. While GreenCarCongress brings us news of an Extreme Flywheel Plug-in Hybrid concept theoretically capable of 250mpg. The overall trend spells an end to head-in-the-sand consumption of oil and the beginning of a transition to a post-oil age with hybrid technology most likely being one of a number of key bridge technologies.

WorldChanging's coverage of the Solar Sailor a Hybrid Solar / Wind Boat

WorldChanging's Original Article and Dicusssion of the SkySails Concept

WorldChanging on an entirely solar powered aircraft concept

Gravity Powered Aircraft Concept

Full Text of New Scientist Article

Wired's 2000 coverage of the Solar Sailor

SkySails Official Website

Naval Architects Knud E Hansen

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Blogger Safety Neal said...

Fascinating post. I'm glad to see that these bridge technologies are being developed. With the price of oil continuing to climb, the economics of these projects will become very persuasive.

4:20 pm, February 27, 2005  
Blogger Engineer-Poet said...

The curious thing is, sulfur emissions may be one of the factors helping to hold global warming in check.

If we do wind up sequestering most of our CO2 emissions, we might find ourselves creating dimethyl sulfide to increase cloud cover and offset some of the greenhouse effect.  See

7:23 pm, February 27, 2005  
Anonymous Anonymous said...

Adrian Akau
Pahala, Hawaii

I think that the use of kites to propel ships is an excellent method. The height of the kite above the boat, the size of the kite and the control of the kite all will help provide the ship with fuel savings. A very large kite could provide a great deal of power to move people and cargo.

It would be like going back to the days of sailing ships but in the case of kites, the forces involved might be much greater. What is also needed is some work on the bottom of the ship to reduce adhesion of the ship to the water. Perhaps some type of vibration could reduce this adhesive force and would permit the ship to move more easily. This has already been accomplished in the design of certain types of torpedos. In this manner, the speed of the ship could be greatly increased without needing too much additional power.

6:51 pm, July 05, 2005  
Anonymous Anonymous said... name is nuno and i am from portugal.
I would like to know where can i find a hybrid maritime engine or another type...but has to be ecologicaly viable.This is to use on a 12m fishing boat (outboard or inboard)...u can reply to


9:34 am, January 25, 2006  
Blogger Analog2Digital said...

One thing bugs me...
I'm assuming the kite is more kite than a solid airfoil. If this is true, then what would happen if the ship is on a Beam Reach(wind is 90deg to midship). Wouldn't it just cause the ship to heal over? Some have tracks where the base point of the kite is mobile, and is allowed to rotate around the ships outer deck. If the ship is caught off guard, with a beam reach and sudden force 4 wind, plus the updraft created by the kite need to raise in a following wind... it could easily heal if not crab walk. which could cause severe damage to the sip.

As far as ones with the fixed point on the ships bow, a 90deg sudden pull would alter it course drastically, and would leave the crew with only 2 choices... 1) pull the kite in, thus making it just extra baggage. 2) fight the Lee-Helm that the kite produces, increases the amount of drag the rudder is producing, thus reducing the speed and also increasing the amount of work the engine and kite have to produce.

if you have any comments or would like to set me straight...
Please email me at

Lewis DE.

4:33 pm, April 12, 2008  

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