Numerical Analysis of Patrol Boat Performance with a Stern Flap

⎯ ship resistance signifies the amount of fluid force acting opposes the ship movement. Past researches revealed that installation of a stern flap reduces the value by 5% to 10%. The current research investigates resistance reduction on a patrol boat due to variation of the flap span width and angle of installation. The flap chord length was varied by 50% and 100% of the ship transom width (BT) whilst the installation angle was set to 0 and 5. Numerical tests were carried out using CFD method employing NUMECA Fine Marine software. Simulation result disclosed that the stern flap length of 50% BT installed at 0 indicates the most optimum arrangement. Reduction of the resistance value increases correspondingly to the ship resistance as the ship speed increases. The wave form developed conforms with the flap width such that the wider the flap results in wider wave form. A reduction at the most of 1.87% was yielded at 15 knot ship speed. No significant reduction was observed due to installation angle at 0 and 5. Keywords⎯ NUMECA fine marine, resistance reduction, stern flap installation, waveform.


I. INTRODUCTION 1
Pat rol boat is a ship that serves as an operational support in the field of defense and security. For that the ship must have a high speed, to support the operational of the ship. The ship moves at a certain speed through the fluid water, this causes the ship to experience resistance. Reducing the value of the resistance on the ship can be done by adding stern flap or the addition of the stern of the ship. From journal, it was explained that with the addition of stern flap, it was able to reduce the resistance of the ship by 5-10%, compared to the rudder bulbows which only reduce the resistance value by 2-3% [1].
In this study numerical test was conducted to determine the effect of adding stern flaps on patrol boats by looking at the shape of the flow produced. The variation used in the stern flap are variations in the flap width of 100%BT and 50%BT, for variations in the angle used at 0 0 and 5 0 . In this study the test were carried out using 3 speeds, speeds of 10 knots, 13 knots, and 15 knots Test was done by using NUMECA Fine Marine software with the Computational Fluid Dynamic. In this study it is expected that the addition of angular variations can reduce the value of the greater resistance on the ship.

A. Patrol Boat
Patrol boats are operational support in the defense and security sector [2]. Patrol boats are naval vessels designed for defense. They are commonly found in border protection, smuggling, piracy [3]. In the discussion of the final task used is the type of planning hull. Planning hull is type of hull V that have a double at  [4]. High speed planning hull is used to reduce negative pressure on the hull when the ship reaches high speeds.

B. Flow Characteristics
Fluid flow is characterised into two groups [5]. 1) Laminara fluid flow that moves and has a layer of layer that forms the flow line and does not intersect with each other 2) Turbulenta flow that has a relatively large flow velocity and the motion of particles is irregular with each other

C. Froude Number
Froude Number is a number used to measure resistance from object that move through water and compare object of different size [6].

D. Ship Stern
The stern of the ship is the back of the ship, and there are several controlling components including rudder, propeller, and other. Ship resistance is the same as force and because it is produced by water, this is called a hysdrodynamic force [8]. The shape and type of stern deopends on the type of steering installed [9]. In the past the stern which was widely used on ships was counter or eliptic stern, but along with the many developed stern forms that are widely used now on ships are cruiser or transoms. The cruiser stern type is considered to have a good shape, and also provides hydrodynamic usability that can minimize the value of the resistance on the part of the ship below the water line [10]. The shape of the stern flap and stern wedge on the ship can reduce viscous pressure resistance and can also reduce the resistance that occurs on the ship [11].

E. Ship Resistance
Ship resistance is the study of fluid reactions due to the movement of ships through the fluid. The resistance is the same as the force component which works parallel to the axis of the ship speed of movement [12]. The total value of the total resistance on the ship is the total number of resistance working on the ship as follow [13].
1) Friction Resistancea resistance that is received by the ship when driving which result from friction between the skin of the hull of the ship and water. 2) Wave Resistancethe loss of energy caused by waves when the bow of the ship hits the water 3) Additional Resistancethe pressure caused by addition resistance to wave resistance and friction resistance

F. Advantages of Stern Flap
Stern flap is the addition of the hull length of the ship after transom in the base plate form [14]. Important parameters of stern flap are flap length, flap width, and flap angle along the transom. Stern flap has the effect of the performance of a ship like a stern wedge. All stern flaps depend on the type and size of the vessel used [15]. On the planning hull stern flap the effect of the trim angle is 4-5 0 . The basic advantage of adding a stern flap is the change in flow around the propeller. This flow provides reduce drag on the stern and changes the wave resistance on the ship [16]. Stern flap result in a decrease in the flow of the hull in the location of the length of the addition. The decrease in flow velocity will increase the pressure on the hull, this causes drag reduction. The advantages of adding a stern flap are as follows.
1) Reducing powering resistance. Research shows a reduction in resistance due to the addition of a stern flap of 5-10% 2) Increase maximum speed 3) Change the transom wave system

G. Hydrodynamic Effect on Stern Flaps
Installation of the stern flap on the ship creates a hydrodynamic effect on the ship. The hydrodynamic effect that are added include changes in the wave system, changes in stern flow, and lift as shown by Figure 1. The following will be explained about the hydrodynamic effect caused by the installation of a stern flap [17].

1) Wave System Changes
At the same speed the stern flap result in reduced flow areas: height, slop, and waves. With the presence of a stern flap it can reduce wave height in the wave system near the stern area and far from the stern 2) Change of Stern Flow Stern Flap reduced flow speed and increases dynamic pressure under the hull of the ship. Increasing the pressure area causes a greater lift which results in a positive effect on the direction of the ship. Stern flap increases the speed of outflow on the trailing edge compared to transom without flap. This increase in speed will reduce the flow rate and make the separation flow in a reduction in the viscous pressure in a reduction in the viscous pressure resistance.

3) Lift and Drag
Stern flap can produce lift and drag at all speed and conditions. The advantages is the interaction with the hull and propeller so that it can reduce vessel resistance. Lifting forces and drag increase more as the chord, span, and angle of the flap increase

H. Computational Fluid Dynamic
It is a calculation method with a control of dimensions, area, and volume by utilizing computer assistance to perform calculations on each element of the division [18]. The initial of the equation is boundary condition. Boundary condition is a condition where calculation controls are defined as the initial definition which will be involved in the calculation controls and adjacent to it through equations [19]. In general, the calculation process in the CFD method consist of 3 main part, preprocessor, processor/solver, postprocessor.

I. NUMECA Fine Marine
NUMECA is a trademark of an international company providing NUMECA international CFD processing software. NUMECA has product for design processors and meshing and solvers for CFD processing software [20].

A. Ship Modeling using Maxsurf and NUMECA Fine
Marine The patrol ship model was made based on the existing lines plan of the real ship. Modeling was done in two International Journal of Marine Engineering Innovation and Research, Vol. 5 (2), Jun. 2020. 122-129 (pISSN: 2541-5972, eISSN: 2548-1479 124 steps. Firstly, the surface model was made using Maxsurf software then solidified using NUMECA Fine Marine. Figure 2 displayed the patrol boat model obtained.

B. Model Verification
Verification was performed to check the correctness of the initial model made prior to any further modification. Ship resistance was chosen as variable for verification and two resistance values originated from numerical simulations using Maxsurf and NUMECA Fine Marine software was used for comparison.

C. Ship Modeling with Stern Flap
The research focuses on the investigation of resistance reduction due to stern flap installation vary with span width and angle of installation. Based on existing research on stern flaps, the recommended chord length is 1% to 2.5% of LPP. The recommended span width according to the transom width (BT, Breadth Transom) may use full width i.e. 100% BT, 75% BT, up to 50% BT. Previous research suggested that the longer and wider the size of the stern flap, the greater reduction in the value of the resistance [6]. Based on the finding, the research designed the stern flap to possess a chord length of 2.5% LPP whilst vary with span width and angle of installation respectively by 50% BT and 100% BT and 0 0 and 5 0 . Table 2 outlines the stern flap design variation corresponds to Figure 3.

D. Analysis of Total Resistance and Flow Forms at
Various Ship Speeds Various simulations at different ship speeds were carried out employing the model with stern flap using NUMECA Fine Marine software. Three speed variations were chosen viz. 10 knots, 13 knots and 20 knots. Finally, the total resistance and flow form resulted from the simulation were analysed and compared with the initial value and condition prior to installation of the stern flap.    122-129  (pISSN: 2541-5972, eISSN: 2548-1479)

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A. Resistance Profiles Table 3 provides resistance value of the patrol boat obtained using Maxsurf and NUMECA Fine Marine. Correspondingly, Table 4 summarises the simulated resistance values of the model fitted with the four stern flaps designed. Figure 4 profiles the resistance of both models i.e. without and with stern flaps. Resistance profile of each design have been analysed against the original patrol boat model for evaluation of resistance reduction behaviour.

1) Resistance of Patrol Boat Model (Barehull)
As summarised in Table 3 Percentage decrease in resistance value on flap 1, the higher speed value, the smaller percentage reduction in resistance. This means that due to the addition of the stern flap on the stern of the vessel the resistance value becomes smaller, and the performance of the stern flaps can reduce the greatest resistance at a speed of 10 knot. In previous studies (Abriansya, 2018) the greater speed, the greater the percentage of resistance reduction. However, resistance from simulation model stern flap 1, the higher speed, the smaller the value of the reduction in resistance. This can be due to the lack of fineness of the model when meshing the software.

3) Resistance of Patrol Boat Model without and with
Stern Flap 2 (50 % BT and 0 0 installation angle) Resistance of the model with stern flap 2 also decreases in the value. The resistance decreases at a speed of 10 knot at0.031, at a speed of 13 knots at 0.04, and at a speed of 15 knots the resistance value decrease by 0.071. The percentage decrease in the value of resistance on flap 2, the higher the speed value, the greater percentage reduction in the value of the resistance. This means that due to the addition of the stern flap on the stern of the vessel the resistance value becomes smaller, and the performance of the stern flap can reduce the greatest resistance at a speed of 15 knots. In the previous study (Abriansyah, 2018) the greater speed, the greater percentage reduction in resistance value. This decrease in resistance is very good, because from the speed of 10 knot the ship has experienced a decrease in resistance and an increase in the percentage reduction in resistance to a speed of 15 knots. This means that due to the stern flap on the stern of the ship the resistance value will be reduced.   Table 5 that difference between both resistance values at all running speeds is less than 8 %. Adopting the statistical criteria for judging significance of important levels, the value implies that the research is  International Journal of Marine Engineering Innovation and Research, Vol. 5 (2), Jun. 2020. 122-129 (pISSN: 2541-5972, eISSN: 2548-1479 127 92 % confidence that the model produced is correct.

C. The Most Optimum Stern Flap Design
The difference in the value of the resistance defines the most optimal stern flap. The value of the resistance that occurs due to the addition of a stern flap on several stern flap models made with different BT width, and with a variation of the angle 0 0 and also the angle 5 0 can be compares with the difference in the value of the resistance that occurs in the model that has not been given a stern flap, and with the model given the stern flap, and with the model given the stern flap is shown in a graph. From the graph above it can be concluded that the most optimum stern flap model, which can reduce resistance is the stern flap 2 model with a width of 50%BT angle 0 0 . In the stern flap model 2 resistance can be reduce at speeds of 10 knots to 15 knots. In addition, the reduction in the resistance value is greater with the increase in the speed of the ship, so that if the ship goes at a faster speed, then the reduction in the value of the resistance will be even greater. So the resistance on the ship will be smaller. This means that the resistance can be reduced by the addition of length on the stern of the ship at an angle 0 0 . If the angle is too deep, then the angle will add the resistance on the ship, such as the stern flap model 3, and the stern flap model 4.

D. Stern Wave Profiles
It is evident that installation of stern flap changes the transom wave system. The research further investigates the phenomena using NUMECA software. Simulations to observe the flow patterns and waveforms of the original patrol boat model and the modified ones due to the speeding vessel and installation of stern flap have been performed. All simulations were executed at 15 knot ship speed to ease the observation of the stern wave formed. Figure 5 displays the stern wave pattern of the patrol boat without installation of stern flap. As seen from the figure, the flow pattern of the patrol boat model that passes through the stern creates a whirlpool or turbulent flow. It is expected that the installation of a stern flap changes the flow patterns hence reducing the ship resistance.

1) Stern Wave of Patrol Boat Model
2) Stern Wave of Patrol Boat Model with Stern Flap Installation Figure 6 to Figure 9 respectively show the stern wave pattern of the patrol boat fitted with stern flap 1 to stern flap 4. Generally, the flow pattern behind the ship is elongated after the installation of stern flap. The waveform is changed from previous wave density of the initial model without the flap such that the waveform becomes wider. This may occur due to the flow pattern which is changed in direction by stern flap installed. Certainly, different design creates different profile. As comparison, the wave formed by stern flap 1 at a speed of 15 knots is wider that the wave formed by the stern flap 2 at the same speed. This may occur due to stern flap 1 having a  at the speed of 15 knots is wider than the stern flap 4 for the same speed. As a common rule, the wider the size of the stern flap will create wider wave density.

IV. CONCLUSION
Based on the simulation results on patrol boat performance without and with stern flaps, highlights are given as follows.
1) The influence of the addition of the stern flap on   International Journal of Marine Engineering Innovation and Research, Vol. 5 (2), Jun. 2020. 122-129 (pISSN: 2541-5972, eISSN: 2548-1479 129 the resistance value, that with the addition of the stern flap the value of the resistance becomes reduced with increasing speed of the ship, and the difference in the reduction in the value of prisoners increases with increasing speed. This happens on ship with a stern flap 2 model with a size of 50% BT at 0 0 installation angle. 2) Simulation results show that with the addition of a stern flap on the stern of the ship it can change the pattern of water flow around the stern of the ship.