Computation of the Speed Loss in Seaway by Different Approaches

Lu, Ling (National Taiwan Ocean University) | Mao, Yao-Tang (National Taiwan Ocean University) | Hsin, Ching-Yeh (National Taiwan Ocean University)

OnePetro 

ABSTRACT

In this paper, the speed loss is computed by the simulations of self-propulsion tests in calm water and in waves, and different approaches of the self-propulsion test are used. In addition to using the viscous flow RANS method, the potential flow methods are also applied in the computations of wave-making resistance and added resistance. The body force method is used to represent the propeller effects. The KCS and KVLCC2 ships are used to demonstrate the speed loss computations, and results from different approaches are investigated by the comparison of self-propulsion results and power curves.

INTRODUCTION

The fuel efficiency in seaway becomes more important due to environmental regulations, and the speed loss in seaway is quantitatively evaluated by weather factor in EEDI (Energy Efficiency Design Index). To compute the speed loss, the ship hull resistances in both calm water and waves have to be computed, and the ship speeds in different sea states can be evaluated based on the propeller performance in waves. In this paper, instead of using the viscous flow RANS method to directly simulate the whole physical phenomena, we will use both the potential flow method and the viscous flow method. Since the viscous effects are less important in ship motion problems, to use the potential flow method is appropriate. On the other hand, the interactions between the ship wake and the propeller are dominated by the viscous effect, thus the viscous flow RANS method is used.

The speed loss problem has been studied by many researchers. Journée (1976) applied an approximate method to study the ship motion effects to the propeller performance, and he has also carried out experiments to make the comparison. Faltinsen (1980) has investigated the resistance and propulsion in seaway, and he claims that since the encounter frequency of the incoming wave is far smaller than the propeller rotational frequency, only the vertical velocities due to motions are critical to the propeller performance in waves. The variation of the propulsion thrust in wave can be computed by quasi-steady flow method, that is, to solve the thrusts at different time step, and the thrust in waves will be the mean value of these. Nakatake et al. (1986) later developed a panel method using source and sink distributions to simulate the ship hull, propeller and rudder, and studied the interactions of hull/propeller/rudder by computations. Ando et al. (1989, 1990) have verified the above computations by experiments. Recently, Kashiwagi et al. (2004) investigated the propeller performance in waves by using the Enhanced Unified Theory (EUT), which is derived from ship motion theory. Chuang and Steen (2011) have studied the power and speed loss in waves by experiments. The body force method (Kerwin et al., 1994; Hsin et al., 2000, 2008; Wei, 2012) is used to represent the propeller effects, and the reason is not only because the simplicity, but also because we can separate the flow field into “propeller inflow” and “propeller induced velocity” by using this method. The inflow due to the propeller-hull interaction can be added to the inflow due to the ship motions and wave induced velocities, and become the “total inflow” of a propeller in waves.