I. Wind-assisted Ship Propulsion
As the energy resource is more and more in short supply all over the word, there is a trend to develop recyclable energy resource. Saving oil and improve the economy has been raised to a new height. Besides, the United Nations and the International Maritime Organization have the strict monitoring of emissions of ships. So, further development of the use of new energy-saving and emission reduction ship is in an urgent need. According to the special conditions at sea, many countries choose to develop the use of wind energy to give impetus to ships.
The earliest use of wind energy is from sailing . Traditional sailboats use wind energy as propulsion, so their reliability are poor. At the end of the century, the emergence of marine steam engines, diesel engines and propellers, causes a strong impact on the previous driving force for ships - sailing. Although the traditional sail is gradually entering the history museum, many countries and regions are still use them in accordance with local conditions. For instance, shipping in some countries’ inland waterway, as well as some small-scale fishing operations, are still use purely sailing. Nevertheless, for large ocean-going vessels, if only controlled by wind, marine weather is easy to affect the safety of the ship, which is more dangerous than diesel-powered ships. Therefore, modern shipping does not use purely sailing, but to consider the wind as an assistant propulsion, thereby reducing the fuel consumption of diesel engines, and ultimately achieve both the effect of energy saving and emission reduction.
Wind-assistant ship propulsion is a technology that transferring wind energy to ship propulsion directly. Wind-assistant propulsion ship can utilize keeping sailing speed, reduce the main engine output power  or main engine output power unchanged, increase the sailing speed and other ways to achieve the purpose of saving fuel consumption.
A. Development of Wind-Assistant Ship Propulsion
In the 1960s, Germany began to study the 10,000-ton large- scale wind-assistant propulsion ship. They designed the six mast of the wind-assistant propulsion ship “DYNA”, and total length of it is about 160 meters.
Since the 1970s, the United States has also noted the good prospects for wind-assistant ship propulsion, set up a special committee, and developed a wind-assistant propulsion sailing program. They conducted a series of experimental research work in the barge, fishing boats and floating crane drilling boats, offshore and ocean-going vessels, and achieved some results. For example, the “Atlantic” cargo ship installed a single mast triangle sail in 120 square meters , which could increase the speed of 1. 5 kn in the 6 ~ 8 wind speed, so that 1,200 kW diesel engine could save about 1,000 L oil every day and night, that means it could save about 20% than the ship without sail.
In 1980, Germany helped Indonesia developed a wind-assistant propulsion ship, the ship carried out trade activities between Indonesia and other islands. In the same period, the French Mediterranean Maritime Service also built a number of sailing ship, ship length was 53 m or 134 m. Since then, they built “La Fayette” sailing in the Caribbean Sea, which had a total length of 187 m, and known as the world\'s largest wind-assistant propulsion ship. Most of these vessels use electronic computers to automatically the sail.
In 1980, Japan built the world\'s first modern wind-assistant propulsion ship “New Love Pill”. The ship was loaded with a capacity of 1,600 t and a displacement of 2 400 t. It was equipped with two-sided crimp-reinforced fiber sails and a low-speed diesel engine as an auxiliary power. Sailing with wind energy as a driving force to make ships sailing, which can save fuel. The ship saved 50% of the fuel compared to the same conventional motorboat, but the transport time was increased by 20%.
On December 15, 2007, the world\'s first ship powered by a rectangular kite, was launched in a port in Hamburg, Germany (\"Beluga Skysail\", see Figure 1). “The advantage of the sky sail is that it can take advantage of the stable and strongest winds in the upper deck,” says the German engineer. This high-tech windsurfing can operate with wind-assisted engines, reducing fuel consumption by 10% to 35%. More importantly, it can also significantly reduce greenhouse gas emissions. But it is also flawed, that is, it cannot use normally when the speed is beyond 16 kn or under the condition of against the wind. And it can only rely on fuel power once encountered hurricanes.
Fig. 1 Beluga Skysail
B. Energy Efficiency and Economic Benefits
More than 90% of the world\'s international trade through the ship to achieve , the development and utilization of wind energy on the route not only reduces the annual operating costs of the ship, but also greatly reduces energy consumption.
The ability of wind-assistant propulsion to help sail is related to a wide range of factors, the most critical of which is the size of the boost on the sail. The size of the sail can be calculated by the following formula:
Where - sail thrust (kg);
- sail thrust coefficient (The value is derived from the wind tunnel test. The magnitude of the value is related to the type of sail and the wind angle acting on the sail. The wind direction depends on the ship speed, ship course and real wind speed, wind direction, and the best value for is 1.6).
- air density();
V - relative wind speed (The wind speed acting on the sail is synthesized by real wind speed and ship speed vector) (m/s);
S – sail area.
From Equation 1, we can see the most basic factor in the effect od wind energy is the wind speed on the sailing line. According to the calculation, if the annual average wind speed is 4.5 to 9.5 m/s, the ship sailing energy can save 9-15 %, and the concrete value depends on other factors, especially the wind direction and ship course. The range of calculated values has been proven for many countries ship operation. For example, Japan is a country that pay a lot attention on studies of wind-assistant propulsion ship, so far has built a number of real ships operating at sea, and it is reported that this kind of sailing can save fuels about 15%.
The economic benefits of wind-assistant ship propulsion are mainly dependent on the energy saving effect of sailing and the investment cost of the sailing device. The investment cost is related to the degree of automation of the sailing system, generally not more than 10% of the cost of the ship. For instance, Japan’s “New Love Pill” ship controlled by microcomputer hydraulic pressure. According to the wind speed and wind direction, it can fold and stretch the sail automatically, adjust the sail corner to achieve the most effective use of wind energy. The price of the investment on it was about 7 million Japanese yen which was about 10% of the cost of the ship.
C. Existing Problems
Wind-assistant ship propulsion is the new type ship for international to solve the energy crisis and environmental pollution problems and develop the use of marine wind energy. It not only to consider the air and fluid power, but also take into account the economy, ship type, manipulation, stability, strength and automation and other aspects.
1) The Use of Sail\' Shape
Contemporary sail material is different from the ancient sail, the current new material has been able to create a huge stretch of sails, it can be controlled easily and conveniently. Therefore, for the cargo ship, we can break through the traditional cumbersome and huge sailing equipment, such as bulky cross girders, girder, wind pole, stretch and static cable, complex mesh cable structure, etc., and thus become lightweight and flexible.
There are many kinds of sail used for the wind-assistant propulsion ship. From the shape of sail, there are square sails, triangular sails; from the relative position of the sail and the mast, there are vertical sails, horizontal sails; from the structure form, there are hard sails, soft sails; from the cross section of the sail to see the line shape, there are laminar flow type, arc type, SY type, and so on. So, the problem is which shape can produce greater thrust and be applied to modern ships. For the study of different shapes of sails, the results show that: The aerodynamic performance of the arc-type sails is excellent, making it easier to manufacture and manipulate, it seems most suitable for modern ships .
2) The Match of Sail, Main engine, Paddle
There are two energy saving effects: One is to keep the sailing speed, change the main engine load, change the main engine output power; the other is to keep the main engine output power unchanged, the sailing speed changes. When use the fixed speed, the main engine load change is random, the main engine power is changed with the wind speed and wind direction, that is, with the value of assistant thrust provided by the sail. Due to the instability of the wind speed, the resulting energy is unstable, and in order to ensure the optimum propulsion efficiency of the propeller, it is preferable to use the variable pitch propeller to achieve the best matching control of the sail-main engine-paddle. So, it is essential to conduct the scientific analysis and demonstration of operation characteristics for the match of sail, main engine, paddle.
3) The choose of sailing direction
In order to obtain the voyage increased the speed by wind, we often need to change direction, get tilting voyage, and this time the ship will deviate from the scheduled route. So the ship will go a little more distance, which will make the diesel engine run longer, burn more fuel, and also increase the ship\'s voyage time. Therefore, we require the study of how to utilize winds and optimizing the deviations without affecting the schedules.
It is more favorable for wind-assistant propulsion ship to sail in a cyclical sea monsoon route. Stretching the sail when it’s downwind, folding the sail when it’s against the wind. Do the scientific analysis of the global marine climate, select the most favorable route for sailing, can make the application of sail gain the ideal effect.
4) Mutual optimization of energy efficiency and safety
Wind-assistant ship propulsion can be divided into two cases: one is use wind as the main thrust, the diesel engine power as the auxiliary propulsion; one is use the wind as an auxiliary propulsion, diesel engine power as the main thrust. Obviously, the former can produce greater energy saving effect, but by the impact of climate factors even greater, and once encountered hurricanes, security is not as good as the latter.
The ship adopts the former or the latter, can be adapted to the local conditions: the former can be applied to relatively safe route of the river or offshore monsoon with less hurricane; ocean-going vessels can use the latter. Therefore, in order to achieve the best energy saving and safety, we should also study on the climatic conditions on the ship’s route.
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