ABSTRACT

Due to the effect of the strong oblique flow, the traditional Maneuvering Modeling Group (MMG) model cannot provide accurate prediction of the maneuvering motion for Azimuth-Stern-Drive (ASD) tugs. This study establishes a new propulsion model of azimuth thruster, which is obtained by interpolation of the thrust calculated under a series of different speed and drift angles and steering angles. Compared with the traditional MMG model, the improved MMG model improves the prediction accuracy of turning performance for ASD tugs by more than 10 %. It has been verified that this model can be used for subsequent parametric research.

INTRODUCTION

The azimuth thruster combines the propulsion performance of traditional propeller and the operation performance of rudder, and can rotate 360 degrees around its vertical axis, which greatly enhances the flexibility of ship maneuvering. This makes the application of the azimuth thruster more and more widely in practical engineering. Therefore, it is very significant to accurately predict the maneuverability of the ship equipped with the azimuth thruster. The MMG model is generally used to predict the turning performance of ships. However, the traditional MMG model is not suitable for ASD tugs equipped with the azimuth thrusters. In the case of different drift angles and steering angles, the strong oblique flow makes the predicted value of the traditional MMG model deviate greatly from the experimental value. So, it is not enough to only consider the wake fraction and thrust reduction factor. It is necessary to add the consideration of hull-propeller interaction under different drift angles and steering angles to the original MMG model.

MMG model is a mathematical model for ship maneuvering motion solution, proposed by a research group called Maneuvering Modeling Group (MMG) in Japanese Towing Tank Conference (JTTC) in 1970s. MMG method is widely used due to its specific physical meaning of the hydrodynamic derivatives, and accessibility to optimization of partial design and shallow water maneuvering problem, etc. However, the MMG model cannot be applied to all situations, and it is necessary to improve the MMG model accordingly for different research objects. And the forces on the hull, the rudder, and the propeller are separated in the MMG model. This separation allows for more accurate and convenient evaluation of each force. It should be noted that this separation also brings errors due to the coupling effects, such as the hull-ruder-propeller interaction. Therefore, corrections should be considered in such situations (Du. et al, 2022).

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