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A ship is a highly dynamic system that is simultaneously exposed to a number of different environmental conditions. The result is a complex arrangement of cause and effect, whose factors have a linear correlation in only the most seldom of cases.
Environmental factors can be such things as the wind, waves, water temperature or the salt content of the water. The state of a ship will also change depending on its loading conditions or hull fouling. Ships vary among themselves in terms of their cruising speed, the efficiency of their propellers or main engine, as well as in the shape of their hull. All these factors have an impact on ship resistance and in turn on the propelling performance of a SkySails-System. To be able to predict savings, real ships must be examined under real operating conditions. In only the rarest of cases will all the needed information be available, which is why all forecatings are only approximations.
Put another way: The fuel savings achieved with the use of SkySails propulsion depend for the most part on the efficiency of the ship's propeller, on the ship resistance, the ship's speed, the wind conditions (wind speed and direction), on the routes traveled and the manner in which the crew and shipping company employ the system. This is the reason why no across-the-board statement can be made regarding fuel savings.
In calculating the performance of the SkySails-System, a differentiation must be made between the effective tractive force under standard conditions and the average performance.
* The effective load of the SkySails propulsion indicates the maximum tractive force of the system in the direction of the ship’s course under optimal conditions. The effective load is substantial when dimensioning the SkySails-System as it can be directly compared to the thrust required by the ship to reach its cruising speed.
Since ships differ in construction and operate on very diverse routes, it is difficult to make a general statement regarding the propulsion power of the SkySails-System. However, and in order to give an indication of the system’s propulsion power, SkySails states its effective load under standard conditions. The system’s power is determined for a clearly defined system status at a specified point in time. This system status is defined as follows:
SkySails standard conditions
System variable Values
Wind speed: 12.8 m/s (25 knots)
True wind direction: 130°
Ship speed: 5.1 m/s (10 knots)
Sea State: 2
Towing kite flight mode: dynamic
The SkySails’ effective load under standard conditions is specified for an exemplary vessel which cruises at a speed of 10 knots at a true wind course of 130°. The wind speed is 25 knots, waves are up to 60 cm high, and the towing kite is maneuvered dynamically.
The determination of the system’s propulsion power under standard conditions is based on measurements tested in practise of the SkySails technology and theoretical calculations. Currently, SkySails is offering towing kite propulsion systems for cargo vessels with an effective load* of between 8 and 32 tons. The planned product program comprises towing kite propulsion systems with an effective load* of up to 130 tons.
An effective tractive force of 8 tons by a SkySail corresponds to approx. 600 to 1,000 kW installed main engine power - depending on the ship‘s properties (propeller efficiency degree, resistance, etc.)The following chart gives an overview of the dimensions of the SkySails propulsion on different ships:Ideally, the effective load* specified for the towing kite should range between 70 and 150% of the thrust at NCR generated by the ship's main propulsion.
The SkySails average performance is the power that a SkySails-System generates on a given ship within the period of one year averaged over all the voyages made. This average performance depends greatly on the ship and route factors outlined above and must therefore be determined individually for each vessel.
SkySails does separate savings calculations for each of a shipping company's vessels. The savings potential can be determined with the help of the ship's technical data and detailed information about the routes traveled.
The first step involves meteorologists determining the wind conditions that were prevailing on the routes traveled. To do this they need an extract from the logbook showing the coordinates where the ship was located at intervals of 6 hours. They then use weather databases to determine the wind strengths and directions that prevailed at altitudes of approximately 200 meters at these various routing points.
With the help of weather data, the performance is calculated to reflect what a SkySails-System would have achieved under the given conditions. By combining the SkySails-System's performance data with the respective ship's propulsion and resistance characteristics, it is then determined to what extent the output of the ship's engine could have been reduced. Fuel savings can then be determined on this basis in the final step. These fuel savings are computed for all the points along the route and result in the total fuel savings that could have been realized through the employment of a SkySails-System on the route examined.
One of the most important economic metrics is the amortization or payback period of a SkySails-System on a specific ship. To determine this, a calculation of the potential fuel savings is made for numerous routes that the ship navigates. As a result the potential annual fuel savings and, by taking into account the procurement, maintenance and servicing costs for the SkySails-System, the payback period itself can be determined.
SkySails has conducted numerous studies on the savings potential of SkySails-Systems based on logbook extracts, historical weather data and field testing results. Actual ocean voyages were also recomputed. The result of these computations is a savings potential of between 10% and 35% depending on the ship.
The amortization period is subject to a number of other influencing factors besides the performance generated by the SkySails-System.
Heavy oil, also known as bunker fuel, is the fuel used by oceangoing ships. Depending on the type of fuel used, about 200 grams of oil must be burned to generate one kWh of power. The higher the price of oil, the more expensive it is to propel the ship using the main engine, and the greater the savings per kWh generated through SkySails propulsion. Consequently, a rising oil price results in a shortening of the payback period.
The price of bunker fuel is also influenced by its quality. The various fuel grades (IFO 380, IFO 180, MDO, MGO) available to shipping are also priced differently. There is a price difference of about 50% between the low-sulfur, expensive grades (MDO, MGO) and the sulfur-rich, inexpensive grades (IFO 180, IFO 380). Therefore the SkySails-System's payback period gets shorter the higher the grade of fuel that is used. This factor will gain in importance in light of the latest developments in marine and environmental protection (IMO MARPOL Annex VI) that prescribe a reduction in the sulfur content of bunker fuel.
The payback period also depends on the costs that arise for the purchase and operation (maintenance, servicing, routing) of the SkySails-System. The lower the costs of SkySails propulsion, the shorter the payback period.
For the sake of simplicity, capital commitment costs were not taken into account in the analyses presented.
The operating days at sea are those days when the ship is actually sailing, i.e. not in port or at a shipyard. The SkySails-System can only be used when the vessel is underway. This means that the number of operating days at sea influences the payback period. A ship normally sails 210 - 240 days a year. Conservatively SkySails adopt 210 days for calculations.
The effective employment time of the SkySails-System during the operating days at sea depends on the navigated area, the weather, the wind conditions, how the crew utilizes the system and the shipping company's strategy. Extensive routing analyses that SkySails made for shipping companies show that the SkySails-System depends on shipping routes to see use in 30 - 50% of the days.
For technical reasons, the overall efficiency factor (Pe) of a ship's propulsion including losses from the propeller is a maximum of 0.6. This means that in the best-case scenario, 60% of the energy generated with the help of the main powertrain can be turned into propulsive energy for the ship. The lower the main powertrain's efficiency factor, the greater the fuel savings from using the SkySails-System and the shorter its payback period. In its calculations SkySails uses 0.6 as the overall efficiency factor of the ship's powertrain. This is a conservative approach, since the overall efficiency level is often below this figure in practice.
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