Rosmani, Suandar Baso, Andi Dian Eka Anggriani, Lukman Bochary, Wahyuddin, Muhammad Akbar Asis, Andi Ardianti

Corresponding author: s.baso@eng.unhas.ac.id

Abstract

The stern flap and stepped hull are widely applied for high speed vessels to reduce resistance by adjusting the performance attitude. In this present study, a form design of stern flap has been improved on the high speed ferry as a planing hull, namely I and 2V. The stern flap was attached on the transom stern of the planing hull installed at several angles of 10, 20, 30 degrees. The free running model test and resistance analysis of the planing hull were carried out to determine the effects of the improved stern flaps on the parameters of trim by stern, resistance, and speed. The planing attitude of planing hull captured in the model test were analyzed using the application of Maxsurf Resistance. The study results revealed that both the stern flaps I and 2V on the planing hull afford to reduce the trim by stern, resistance, and to improve speed. The average value of the difference of the increase of trim by stern between the stern flap 2V and stern flap I is 1.79%. With the constant FnV, the planing’s resistance can be reduced by the increase of the stern flap angles. The average value of the reduction of planing’s resistance due to the increase of the stern flap angles is 6.27% for the stern flap I, and 5.05% for the stern flap 2V. Regarding the effect of the stern flap form with its angle on the planing’s resistance, the planing’s resistance due to the stern flap 2V is lower compared with the stern flap I wherein overall the average difference is 4.73%. The reductions of the trim by stern and planing’s resistance due to the stern flap form and angle are caused by the vertical lift force and pressure distribution acting on the transom of the aft planing hull.

 

Keyword

Planing hull; Porpoising phenomenon; Resistance reduction; Stern flap; Trim by stern

 

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Introduction

Energy efficiency plays an essential role in accelerating the clean energy, decarbonizing the economies, securing the energy supplies, and increasing the productivity. This has drawn global attention including the marine engineering field as well. Therefore, the International Maritime Organization (IMO) has urged the Marine Environment Protection Committee (MEPC) to identify and develop mechanisms to achieve a reduction of ships’ greenhouse gas emissions [1]. Also, the MEPC has introduced regulations focused on the limitation of CO2 production of newly built ships [2]. To scale up action on energy efficiency and GHG emissions reduction, a ship’s geometry and an attachment device (appendage) should be optimized appropriately due to its significant relation to ship performance.
For the optimization of the ship’s hull, the effect of hull form parameters on the hydrodynamic performance of a bulk carrier was studied by analyzing the joint optimization process, fast principal-dimension optimization of the origin parent ship considering the integrated performance of ship resistance, seakeeping, and maneuverability [3]. Beside the optimization of overall hull geometry, the bow shape or stern shape has been studied widely to obtain the reduction of ship resistance. The bow and stern hull of KRISO Container Ship was optimized to reduce its resistance through an innovative methodology of synchronous local optimization with the consideration of the whole ship speed range [4]. Numerical method for optimizing the stern shape of a container ship based on the Sequential Quadratic Programming (SQP) method and Reynold Averaged Navier-Stokes (RANS) equation to minimize the pressure resistance was proposed [5]. The improvements of stern part and additional part on the transom affected an increase on the heave and pitch amplitudes, and the effects were dependent on the improvement of the form [6].
Most of high speed ferries have transom stern to consider an improvement of flow around the stern and hydrodynamic performance. Besides the consideration of the optimization of stern shape, the appendages fitted on the stern also have been designed such as stern flap, interceptor, and wedge stern in order to improve the hydrodynamic performance. One of stern appendages is applied to a high speed ship is a stern flap. Then, this application is to be an important concern on a hydrodynamic design in improving a performance of high speed ship. Related to the hydrodynamic performance due to the effect of a stern flap, the followings are some studies that have been carried out. The reductions of resistance due to stern flap were compared based on the variations of chord length and angle as investigated by using the computational fluid dynamics (CFD) wherein the stern flap 1%Lpp with angle 4 degrees is most optimal resistance reduction [7]. Two stern flaps active ride control were designed for the wave piercing catamarans (WPC) to reduce the heave, roll and pitch motions in beam waves wherein the ride control system based on linear quadratic regulator and genetic algorithm [8]. Experimental and numerical simulation methods were carried out to explore the influence of the flap mounting angle coupled with the steps wherein the low speed resistance performance was improved in increasing the mounting angle [9]. The influence of stern flaps was investigated by numerical simulations and model towing test wherein the proper length and optimal angle of stern flaps could be greatly reduced the resistance [10]. The parameter optimization of the stern flaps of the series displacement ships was studied through the model test wherein the result revealed that the ship with stern flaps has lower resistance and could save 3% and 5% of energy [11], the stern flap reduces the ship resistance with the energy savings between 30% and 50%, however it affects the ship model flow field to a certain extent with a consequence in affecting the propulsion performance of the ship [12]. CFD simulations and experimental model testing of a typical high speed displacement vessel equipped a stern flap design in 12 different configurations were conducted for determining most favorable flap design in for reducing the energy efficiency design index [13]. The model experiment and CFD analysis of the stern flap were conducted to examine the effect of stern flap on the running attitudes and wave making at the after portion of the hull on resistance reduction [14].
By referring to numerous studies above, the effects of stern flap on hydrodynamic performance of a ship remain to be investigated extensively. The form and placement of the stern flap also have been the considerations for the improvement of hydrodynamic performance. Regarding the use of methods, the accuracy of the research results has showed the satisfaction. Regardless, the improvement of design of a stern flap remains challenging to be conducted and proposed in order to extend the studies of the hydrodynamic performance due to the effect of the stern flap through using the numerical method, experimental method or computational fluid dynamics (CFD). By the miscellaneous of the studies of the hydrodynamics design regarding the stern flap will enhance the complex considerations to obtain the favorable design.
This paper presents the experimental study of hydrodynamic performance of a planing hull caused by improving the form of stern flap in calm water condition. The aims of this study are to propose the stern flap design and to investigate the effect of the proposed design of stern flap on the resistance reduction and improvement speed. The free running test of the planing hull model with the proposed stern flap was conducted, and then the attitude (trim by stern) of the planing hull model with the angle of stern flap at the constant speed was captured. In addition, various stern flap angles and Froude numbers were considered. The attitude of the planing hull model is used for the resistance prediction using the Maxsurf application.

Conclusions

The free running model test in calm water and the analysis of the planing’s resistance using Maxsurf Resistance were carried out successfully. The effects of the stern flap form with its angle on the planing attitude and planing’s resistance have been discussed. In this present study, the notable contributions are concluded accordingly.
The increase of the angles of stern flap at constant speed affects on the reduction of the trim by stern. For each angle of the stern flap, the trim by stern increases significantly in the FnV range of 1.0 to 2.0 for both stern flaps I and 2V, and it remains to increase gradually in the FnV > 2.0. Regarding the form of stern flap, the increase of the trim by stern due to the stern flap 2V is higher than the stern flap I. The average value of the difference of the increase of trim by stern between the stern flap 2V and stern flap I is 1.79%. The reduction of the trim by stern due to the stern flap form and angle is caused by the vertical lift force and pressure distribution acting on the transom of the aft planing hull.
With the constant FnV, the planing’s resistance can be reduced by the increase of the stern flap angles. The average value of the reduction of planing’s resistance due to the increase of the stern flap angles is 6.27% for the stern flap I, and 5.05% for the stern flap 2V. Regarding the effect of the stern flap form with its angle on the planing’s resistance, even though the trim by stern due to the stern flap 2V at the same FnV is higher than stern flap I, the planing’s resistance due to the stern flap 2V is lower compared with the stern flap I. Overall the average difference is 4.73%. The stern flap form with its angle affects on the wave resistance and pressure distribution on the planing hull’s transom and appendage (stern flap).
Even though the differences show small, these can be a consideration on the improvement of the stern flap form as well as the angle. In addition, the stern flap can reduce the resistance on the planing hull, therefore, it can reduce the propulsion power required to achieve a given speed. Therefore, this study of the improvement of the stern flap form will be extended in our future work.

Acknowledgments

The authors would like to thank Muh.Taslim, Muhamad Toraray Delo, Sunardi Samuel Rinding, Mila Karmila, Nurul Awaliyah Mustari, Ainun Chandra Puspa Ningrum, Muhamad Fachreza Rahman, Hasrul for their kind help in conducting the experiment.

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