Maneuverability of Wind-Assisted Ships

A Time Domain Simulation Tool

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Abstract

Wind-assisted ships are installed with wind propulsors capable of generating thrust for aiding the propulsion of the ship, with the purpose of saving fuel by reducing the required propeller speed and thus engine power. Wind-assisted ships are becoming more common due to high fuel prices and rising environmental concerns. The subject of this thesis is wind-assisted ship maneuverability, which is a relatively new and unexplored topic. There are many factors which this study can involve, and so the goal of this thesis is to study the general behavior of wind-assisted ships during the Turning Circle and Zig-Zag standard maneuvers, and investigate in depth the effect of different control optimization schemes for the wind propulsors. This study requires building a 4 degree-of-freedom (DOF) time domain maneuvering model in which the equations of motions for a wind-assisted ship are solved and the required parameters are input in order to execute the standard maneuvers. The model includes calculation of all the major forces on the ship, including hydrodynamic forces common among maneuvering models, and with the addition of aerodynamic forces due to the wind propulsors. Once the model is built and shows sufficient accuracy, the maneuvers are simulated for a variety of wind conditions, sail configurations, and wind propulsor trim optimization schemes. The behavior of the ship due to these different conditions and the compliance with maneuvering standards are discussed.

The results of the simulations show that the maneuvering behavior of the ship is indeed significantly dependant on the wind, and also how the wind propulsors are controlled. Based on the results, in general, the ship shows the behavior to turn quickly into the wind and slowly while turning against the wind. The effect this has on the maneuvers is that the Turning Circle is generally tighter when the initial turn is turning toward the wind, and larger when the ship is initially turning away from the wind. For the Zig-Zag maneuvers, the ship is alternating in turning toward and away from the wind and so the behavior of turning faster toward the wind is evident in the measured quantities of the Zig-Zag maneuver. The standardized requirements on maneuvering do not yet have considerations for wind-assisted ships and thus it is unknown what will be allowed in the future in terms of wind propulsor control during the standardized tests. Additionally, the abilities of the ships and wind propulsors when it comes to active optimization and wind propulsor trimming are not well known. Thus in this thesis a variety of options for wind propulsor control are studied in order to provide some insight on what is possible, covering some more conservative and more aggressive approaches. The wind propulsor optimization shows to have a big effect on the ship maneuverability, and with thoughtfully constructed optimization schemes there is a general improvement in the measured quantities that result from the maneuvers.