The Effect of Rudder Existence on Propeller Eccentric Force

Journal of Marine Science and Engineering

Article The Eﬀect of Rudder Existence on Propeller Eccentric Force

Gisu Song, Hyounggil Park and Taegoo Lee * Samsung Heavy Industries Co., Ltd., 217 Munji-ro, Yuseong-gu, Daejeon 34051, Korea; [email protected] (G.S.); [email protected] (H.P.) * Correspondence: [email protected]; Tel.: +82-42-865-4377

Received: 3 November 2019; Accepted: 4 December 2019; Published: 12 December 2019

Abstract: In order to design a safe shafting system in a ship, it is vital to precisely predict load on stern tube bearing. It is well known that load on stern tube bearing is directly influenced by the eccentric force of a propeller. In this paper, the effect of rudder existence on propeller eccentric force was studied based on numerical analysis with a 10,000 TEU class container vessel. To obtain propeller eccentric force, numerical simulations including propeller rotation motion using a sliding mesh technique were carried out. When a ship is turning, propeller eccentric force significantly changes compared to those of straight run. For starboard turning especially, the propeller vertical moment was decreased by about 50% due to the existence of a rudder compared to that without a rudder. In contrast, as for port turning, the results of simulations with and without a rudder were similar to each other. This difference is fundamentally due to the interaction between the direction of propeller rotation and the inflow direction to a propeller. Based on this study, it is inferred that the influence of appendages around a propeller need to be considered to ensure the reliable prediction of propeller eccentric force.

Keywords: propeller eccentric force; shaft force; maneuvering; rudder

1. Introduction The safe design of a shafting system on a ship has been a critical issue for a long time. If unexpected problems on a shafting system occur on the voyage under harsh sea conditions, the safety of people on the ship would be at risk. With this reason in mind, a shafting system has been designed conservatively according to the strict guidance of classification societies. A shafting system consists of a propeller, a shaft, bearings and so on. The load on stern tube bearing located in front of a propeller is directly related to the eccentric force of a propeller [1]. Since a propeller rotates in a non-uniform wake field, eccentric force on a propeller is continuously provided and they are changed dramatically during ship turning. Although a number of processes to calculate or check for the design of a shafting system exist, unintended problems, such as stern tube bearing damage, are rarely but still addressed. Moreover, classification societies such as ABS [2] or DNV-GL [3] have published a revised guideline for the safe design of a shafting system. The predicted eccentric force of a propeller from numerical simulations is used to calculate the stern tube bearing load to ultimately secure the safety of a shafting system. For this purpose, detail conditions, including draft and ship speed for reliable numerical simulations, are strictly clarified in the guidelines of most of the classification societies. Besides, some researchers have investigated a safe shafting system for a long time in line with academic interests or engineering needs. Kuroiwa et al. [4] proposed a quasi-steady method to estimate the wake into the propeller during ship turning and showed that the propeller shaft force evaluated by the proposed method was similar to the measured value from the model test. Vartdal et al. [5] analyzed the lateral force on the propeller during ship turning motion by full-scale measurement in various ship types. They calculated

J. Mar. Sci. Eng. 2019, 7, 455; doi:10.3390/jmse7120455 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2019, 7, 455 2 of 20 the contact pressure in the tail shaft bearing using the FEM simulation. Moreover, they asserted that the CFD simulation can be an effective method of predicting these forces with reasonable accuracy. Ui [1] investigated the influence of propeller force on shaft alignment based on the data from 35 vessels of 10 ship-yards participating in the JIME research committee. He examined two representative trouble cases; (1) A hard turn to starboard side, (2) An extremely shallow draught. Coraddu et al. [6] showed asymmetric propeller behavior obtained by thrust and torque measurement on a twin-screw vessel during a free running test. Different wake distribution into two propellers (leeward and windward) during the ship-turning motion was simulated by numerical method. Based on this wake, the propeller loads were evaluated using BEMT model. Shin [7] studied the effect of propeller force on propeller shaft bearing during straight run and turning. He simulated the nominal wake using commercial RANS solver and evaluated the eccentric force of a propeller using a potential code, MPUF3A. In particular, he considered the hull deflection in the shaft alignment. Ortolani et al. [8] investigated the radial bearing force exerted by the propeller during model ship operation. In particular, a free running test was carried out and the realistic radial bearing forces were measured. Dubbioso et al. [9] analyzed the turning capability of a navy supply vessel with different rudder arrangements; twin rudders with a centerline skeg and single rudder. Due to the centerline skeg in the case of the twin rudders, the inflow to the propeller was fairly different to that of the single rudder during ship turning. Lee et al. [10] investigated the propeller eccentric force and the load of the stern tube bearing on a CRP system, compared to those of conventional single propellers. They implemented the sliding mesh technique to directly obtain the eccentric force on the propeller. They showed the shaft behavior inside the Aft-stern bush bearing in the CRP system. Dubbioso et al. [11] investigated the propeller bearing load of a twin-screw frigate vessel in a straight ahead and steady turning motion. They evaluated the wake distributions using a URANS simulation with an overlapping grid approach. The bearing loads evaluated by the BEMT model and the measurements were compared, and they tended to be similar to each other. Muscari et al. [12] examined the effect of the centerline skeg and the propeller rotation direction on the propeller bearing load in a twin-screw vessel. From their study, it can be inferred that the wake is significantly affected by the appendages around the propeller during ship turning. Ortolani and Dubbioso [13,14] measured the single blade loads on propellers during a free-running operation; a straight ahead and a steady turning motion. Their experimental data provided the insight to understand the interaction between the wake and the propeller. From many previous studies, it can be inferred that several factors are influencing propeller eccentric force. Among them, this paper focused on the effect of a rudder. To analyze the effect of rudder existence, numerical simulations involving two cases with and without a rudder were conducted under the same conditions for straight run or turning motion, and then the propeller eccentric force in these simulations was compared to each other.

2. Numerical Details In this study, the target vessel was a 10,000 TEU class container ship, constructed in SHI and delivered in 2014. The main particulars of the ship and propeller are brieﬂy summarized in Table1, and Figure1 shows the target vessel.

Table 1. Main particulars of target vessel and propeller.

Items Values L 286.0 B 48.2 Td 12.5 D 9.4 Z 5 J. Mar. Sci. Eng. 2019, 7, 455 3 of 20 J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 3 of 21

FigureFigure 1.1. The target vessel.vessel

2.1. Numerical Methodology

2.1.1. N Numericalumerical Setup This study study was was mainly mainly performed performed by bya numerical a numerical simulation simulation with the with commercial the commercial code, STAR code,- STAR-CCMCCM+. The +fundamental. The fundamental information information used in the used numerical in the simulations numerical simulations is presented isin presentedTable 2. in Table2. Table 2. Numerical Setup Table 2. Numerical Setup. Items Description Items Description Code STAR-CCM+ Ver. 10.06 Governing equationCode STAR-CCM+ Ver.RANS 10.06 Governing equation RANS st TemporalTemporal discretization discretization 1st order1 order ConvectionConvection term term 2nd order2nd upwind order upwind GradientGradient method method Green-GaussGreen-Gauss Pressure–Velocity coupling SIMPLE Pressure–Velocity coupling SIMPLE Turbulence model Reynolds stress model TurbulenceWall model treatment WallReynolds function stress model Wall treatment Wall function 2.1.2. Case I: Resistance Test 2.1.2. Case I: Resistance Test At first, the difference of resistance for the target ship between the model test and the numerical simulationAt first, was the compareddifference inof resistance the model for scale. the target As shown ship inbetween Table3 the, the model ship speedtest and was the defined numerical as fromsimulation 19 to 23 was Kn. compared For this simulation, in the model the computationalscale. As shown domain, in Table boundary 3, the ship condition speed andwas grid defined system as arefrom presented 19 to 23 Kn in. For Figures this 2simulation,–4, respectively. the computational To reduce the domain, computational boundary time condition and cost, and grid half system of the hullare presented was modeled in Fig andures the 2–4 symmetry, respectively. condition To reduce was definedthe computational in the centerline time and plane. cost, The half hexahedral of the hull unstructuredwas modeled gridand system, the symmetry called trimmer condition mesh was in defined STAR-CCM in the+ , centerline was applied plane. and The the hexahedral grids were clusteredunstructured for the grid realistic system, elevation called andtrimmer propagation mesh in of STAR waves-CCM+, around was a ship. applied The total and number the grids of gridswere wasclustered about for 3.2M. theThe realistic free surface elevation was and captured propagation by the VOFof wave methods around with HRICa ship scheme.. The total The number time step of wasgrids defined was about as 0.001 3.2M. s and The total freecomputational surface was captured time was by 80 the s. TheVOF number method of with inner HRIC iterations scheme per. timeThe steptime wasstep definedwas defined as 5. as The 0.001 value s and of ytotal+ on computational thew hull surface time was was defined80 s. The as number 40, which of inner was locatediteration ins theper recommendedtime step was rangedefined to as apply 5. The the value wall functionof y+ on forthew the hull high surface y+ wall was treatment defined model as 40, [which15,16]. was located in the recommended range to apply the wall function for the high y+ wall treatment model [15,16].

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Table 3. Ship speed and the Froude number for the resistance test in the numerical simulation. J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 4 of 21 Vs Fr Table 3. Ship speed 19and the Froude number for the resistance test 0.185in the numerical simulation. J. Mar. Sci. Eng. 2019, 7, x FOR PEER21 REVIEW 0.205 4 of 21 J. Mar. Sci. Eng. 2019, 7, 455 23 Vs 0.224Fr 4 of 20 Table 3. Ship speed and24 the 19 Fr oude number for the resistance test in the0.234 numerical0.185 simulation. 25 21 0.2430.205 Table 3. Ship speedV ands the Froude number for the resistance test inFr the numerical simulation. 23 0.224 19 0.185 24 V Fr 0.234 21 s 0.205 25 0.243 23 19 0.185 0.224 21 0.205 24 23 0.224 0.234 25 24 0.234 0.243 25 0.243

Figure 2. Computational domain (side view and front view).

Figure 2. Computational domain (side view and front view). Figure 2. Computational domain (side view and front view).

Figure 2. Computational domain (side view and front view).

Figure 3. Boundary conditions. Figure 3. Boundary conditions.

Figure 3. Boundary conditions.

Figure 4. Grid system for the resistance simulation. Figure 4. Grid system for the resistance simulation. Figure5 shows the time history of the resistance in the model scale (Rtm) for the 23Kn simulation and we judged that the value was converged at 80 s. As presented in Figure6, the di ﬀerence in the Figure 5 shows the time history of the resistance in the model scale (Rtm) for the 23Kn simulation ande wﬀectivee judged horsepower that the value (EHP) wasFigure calculated converged 4. Grid from system at the 80 for Rtms. theAs value resistancepresented between simulation in F theigure model. 6, the test difference and the numericalin the simulation in all ship speed range was within 2%. Figure7 shows that the wave elevation and propagationFigure 5around shows the the time Aft-hull history was of similar the resistance to that observed in the model in the scale model (Rtm test.) for Figure the 238Kn presents simulation the yand+ range we judged simulated that atthe 23Kn Figurevalue ship was4. speed;Grid converged system most for of at ythe +80 valueresistance s. As on presented thesimulation hull wasin. Fi distributedgure 6, the indifference the range in of the 30 to 50. Figure 5 shows the time history of the resistance in the model scale (Rtm) for the 23Kn simulation and we judged that the value was converged at 80 s. As presented in Figure 6, the difference in the

J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 5 of 21 J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 5 of 21 effective horsepower (EHP) calculated from the Rtm value between the model test and the numerical effective horsepower (EHP) calculated from the Rtm value between the model test and the numerical J.simulation Mar. Sci. Eng. in2019 all, 7 , shipx FOR speedPEER REVIEW range was within 2%. Figure 7 shows that the wave elevation5 of and 21 simulationpropagation in around all ship the speed Aft-hull range was was similar within to th 2%.at observed Figure 7in shows the model that test. the Figure wave elevation8 presents and the effectivepropagationy+ range horsepower simulated around at the(EHP 23 AftKn) calculated- shiphull wasspeed similar from; most the to of Rtmth y+at valuevalueobserved onbetween the in thehull the model wa models distributed test. test Figure and inthe 8 presentsthe numerical range the of simulationy+30 torange 50. simulated in all ship at 23 speedKn ship range speed was; mostwithin of y+2%. value Figure on the 7 shows hull wa thats distributed the wave in elevation the range and of propagation30 to 50. around the Aft-hull was similar to that observed in the model test. Figure 8 presents the y+ range simulated at 23Kn ship speed; most of y+ value on the hull was distributed in the range of 30 to 50. J. Mar. Sci. Eng. 2019, 7, 455 5 of 20

Figure 5. Time history of the resistance at the model scale (Rtm) at 23Kn. Figure 5. Time history of the resistance at the model scale (Rtm) at 23Kn. Figure 5. Time history of the resistance at the model scale (Rtm) at 23Kn. Figure 5. Time history of the resistance at the model scale (Rtm) at 23Kn.

FigureFigure 6. The 6. di Theﬀerence difference of eﬀective of effective horsepower horsepower (EHP) (EHP between) between the model the model test andtest theand CFD the CFD simulation at variousFigure ship 6. speeds.The difference of effectivesimulation horsepower at various (EHP ship) betweenspeeds. the model test and the CFD simulation at various ship speeds. Figure 6. The difference of effective horsepower (EHP) between the model test and the CFD simulation at various ship speeds.

(a) (b) J. Mar. Sci. Eng. 2019, 7, x FOR PEER(a) REVIEW (b) 6 of 21 FigureFigure 7. 7.Wave Wave pattern pattern around around Aft-hullAft-hull at 24Kn24Kn in the the ( (aa)) model model test test;; (b (b) CF) CFDD simulation simulation.. Figure 7. Wave pattern around Aft-hull at 24Kn in the (a) model test; (b) CFD simulation. (a) (b)

Figure 7. Wave pattern around Aft-hull at 24Kn in the (a) model test; (b) CFD simulation.

(a)

(b)

Figure 8. RangesRanges of of wall wall y+ y+ valuesvalues at at 23 23KnKn ship ship speed speed in in resistance resistance test test (a) (a) side side view; view; (b) (b) bottom bottom view view..

2.1.3. Case II: Propeller Open Water (POW) Test Since the propeller’s eccentric force is the main topic in this study, the accurate prediction of the propeller’s thrust or torque is essential. Thus, the POW test was carried out to check the reliability of the numerical simulation for the target propeller. The computational domain, boundary condition and grid system for the POW simulation are presented in Figures 9–11, respectively. In this simulation, the hybrid grid system was implemented. As shown in Figure 9, two regions in the computational domain were defined; the inner part and the outer part. In the inner part, the propeller was modeled and the diameter and width were 1.1 D and 0.25 D, respectively, and the polyhedral unstructured grid system was applied for better presenting the propeller geometry [17]. On the other hand, the trimmer mesh was used likewise for the resistance simulation in the outer part. The total number of grids was about 2.2M and the grids were clustered to describe the leading curvature. The value of y+ on the propeller surface was defined as 40, which was same value on the hull in resistance test in this study. Since the computation domain was modeled as an axis-symmetric shape, likewise a cylinder, the MRF method is a reasonable choice for the POW simulation.

Figure 9. Computational domain (side view and front view).

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(a)

(b) J. Mar. Sci. Eng. 2019, 7, 455 6 of 20 Figure 8. Ranges of wall y+ values at 23Kn ship speed in resistance test (a) side view; (b) bottom view.

2.1.3.2.1.3. Case II: Propeller Open Open Water Water ( (POW)POW) T Testest SinceSince the propeller propeller’s’s eccentric eccentric force force is is the the main main topic topic in in this this study, study, the the accurate accurate prediction prediction of ofthe the propeller’spropeller’s thrust or torque is is essential essential.. Thus Thus,, the the POW POW test test was was carried carried out out to tocheck check the the reliability reliability of of thethe numericalnumerical simulationsimulation forfor the the target target propeller. propeller The. The computational computational domain, domain, boundary boundary condition condition and gridand systemgrid system for the for POW the simulationPOW simulation are presented are presented in Figures in 9 Figure–11, respectively.s 9–11, respectively. In this simulation, In this thesimulation, hybrid grid the systemhybrid grid was implemented.system was implemented. As shown in As Figure shown9, twoin Figure regions 9, in two the regions computational in the domaincomputational were deﬁned; domain thewere inner defined; part andthe inner the outer part and part. the In o theuter inner part. part,In the the inner propeller part, the was propeller modeled was modeled and the diameter and width were 1.1 D and 0.25 D, respectively, and the polyhedral and the diameter and width were 1.1 D and 0.25 D, respectively, and the polyhedral unstructured grid unstructured grid system was applied for better presenting the propeller geometry [17]. On the other system was applied for better presenting the propeller geometry [17]. On the other hand, the trimmer hand, the trimmer mesh was used likewise for the resistance simulation in the outer part. The total mesh was used likewise for the resistance simulation in the outer part. The total number of grids was number of grids was about 2.2M and the grids were clustered to describe the leading curvature. The about 2.2M and the grids were clustered to describe the leading curvature. The value of y+ on the value of y+ on the propeller surface was defined as 40, which was same value on the hull in resistance propeller surface was deﬁned as 40, which was same value on the hull in resistance test in this study. test in this study. Since the computation domain was modeled as an axis-symmetric shape, likewise Since the computation domain was modeled as an axis-symmetric shape, likewise a cylinder, the MRF a cylinder, the MRF method is a reasonable choice for the POW simulation. method is a reasonable choice for the POW simulation.

J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 7 of 21 J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 7 of 21 Figure 9. Computational domain (side view and front view). Figure 9. Computational domain (side view and front view).

Figure 10. Boundary conditions for POW simulation. Figure 10. Boundary conditions for POW simulation. Figure 10. Boundary conditions for POW simulation.

FigureFigure 11. 11. GridGrid system system for forPOW POW simulation simulation..

Figure 11. Grid system for POW simulation. The advance ratio, generally denoted by J, was defined from 0.1 to 1.0. The KT, KQ and EtaO from the numerical simulation were compared to those from the model test in Figure 12 and Table 4. The The advance ratio, generally denoted by J, was defined from 0.1 to 1.0. The KT, KQ and EtaO from results from the numerical simulation agreed well with experimental data. The differences of KT, KQ the numerical simulation were compared to those from the model test in Figure 12 and Table 4. The and EtaO were less than 2% in most of the J range (0.1–0.8), respectively. results from the numerical simulation agreed well with experimental data. The differences of KT, KQ and EtaO were less than 2% in most of the J range (0.1–0.8), respectively.

Figure 12. Comparison of KT, 10 KQ and EtaO from the model test and the CFD simulation at various advance ratios (J).

Figure 12. Comparison of KT, 10 KQ and EtaO from the model test and the CFD simulation at various advanceTable ratio 4. Thes (J). accuracy of the POW test results from the numerical simulation and model test. J KT (CFD/EXP.) 10KQ (CFD/EXP.) EtaO (CFD/EXP.) Table 4. The accuracy of the POW test results from the numerical simulation and model test. 0.10 98.4% 99.8% 98.6% J KT (CFD/EXP.) 10KQ (CFD/EXP.) EtaO (CFD/EXP.)

0.10 98.4% 99.8% 98.6%

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Figure 10. Boundary conditions for POW simulation.

J. Mar. Sci. Eng. 2019, 7, 455 7 of 20 Figure 11. Grid system for POW simulation.

The advanceThe advance ratio, ratio,generally generally denoted denoted by J, was by defined J, was deﬁned from 0.1 from to 1.0. 0.1 The to K 1.0.T, K TheQ and K TEtaO,KQ fromand EtaO the numericalfrom the numericalsimulation simulationwere compared were comparedto those from to those the model from thetest modelin Figure test 1 in2 and Figure Table 12 and4. The Table 4. resultsThe from results the numerical from the numerical simulation simulation agreed well agreed with well experimental with experimental data. The data. difference The disﬀ oferences KT, KQ of KT, and EtaOKQ and were EtaO less were than less2% in than most 2% of in the most J range of the (0.1 J range–0.8), (0.1–0.8), respectively respectively..

Figure 12. Comparison of KT, 10 KQ and EtaO from the model test and the CFD simulation at various Figureadvance 12. Comparison ratios (J). of KT, 10 KQ and EtaO from the model test and the CFD simulation at various advance ratios (J). Table 4. The accuracy of the POW test results from the numerical simulation and model test. Table 4. The accuracy of the POW test results from the numerical simulation and model test. JKT (CFD/EXP.) 10KQ (CFD/EXP.) EtaO (CFD/EXP.) J KT0.10 (CFD/EXP.)98.4% 10KQ (CFD/EXP.) 99.8% EtaO (CFD/EXP.) 98.6% 0.20 99.0% 100.8% 98.2% 0.10 98.4% 99.8% 98.6% 0.30 99.7% 101.4% 98.3% 0.40 99.8% 101.0% 98.8% 0.50 99.8% 100.4% 99.4% 0.55 100.0% 100.4% 99.6% 0.60 100.2% 100.5% 99.7% 0.65 100.3% 100.7% 99.6% 0.70 100.2% 100.8% 99.4% 0.75 99.9% 100.9% 99.0% 0.80 99.2% 100.9% 98.3% 0.85 98.3% 100.8% 97.6% 0.90 97.4% 100.9% 96.5% 1.00 96.3% 103.5% 93.1%

Based on the previous two simulations; CASE I: resistance test and CASE II: POW test, it can be concluded that the numerical setup applied in this study is reliable to predict the eccentric force on the propeller.

2.2. Definition for Maneuvering Condition As mentioned above, a propeller during ship turning experiences continuous change in inflow. To simply describe this circumstance, the quasi-steady approach suggested by Kuroiwa et al. [4] was applied. It is a method to define a flow angle against a propeller plane at a specific instant during ship turning by calculating an actual drift angle. The actual drift angle is calculated by Equation (1). The definition of variables in Equation (1) is schematically presented in Figure 13. ! 1 Xp Xp α = β + tan− r β + r (1) Vs · ≈ Vs · J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 8 of 21

0.20 99.0% 100.8% 98.2% 0.30 99.7% 101.4% 98.3% 0.40 99.8% 101.0% 98.8% 0.50 99.8% 100.4% 99.4% 0.55 100.0% 100.4% 99.6% 0.60 100.2% 100.5% 99.7% 0.65 100.3% 100.7% 99.6% 0.70 100.2% 100.8% 99.4% 0.75 99.9% 100.9% 99.0% 0.80 99.2% 100.9% 98.3% 0.85 98.3% 100.8% 97.6% 0.90 97.4% 100.9% 96.5% 1.00 96.3% 103.5% 93.1%

2.2. Definition for Maneuvering Condition As mentioned above, a propeller during ship turning experiences continuous change in inflow. To simply describe this circumstance, the quasi-steady approach suggested by Kuroiwa et al. [4] was applied. It is a method to define a flow angle against a propeller plane at a specific instant during ship turning by calculating an actual drift angle. The actual drift angle is calculated by Equation (1). The definition of variables in Equation (1) is schematically presented in Figure 13.

1 x p  x p     tan  r    r (1)  Vs  Vs J. Mar. Sci. Eng. 2019, 7, 455 8 of 20

J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 9 of 21 Figure 13. Reproduced from [4], with permission from Mitsubishi Heavy Industries, Ltd., 2007. FigureFigures 1314. Reproducedand 15 show from measured [4], with values, permission including from Mitsubishi ship speed, Heavy yaw Industries, rate, and Ltd.,2007. propeller rpm of aFigures full-scale 14 shipand 15during show the measured turning values, circle test including in an official ship speed, sea trial, yaw respectively. rate, and propeller The rotation rpm of angle a full-scaleof the shiprudder during was the+35° turning or −35° circle. From test these in anfigures, official it can sea trial,be recognized respectively. that Thea ship rotation in turning angle motion of the rudder was +35 or 35 . From these figures, it can be recognized that a ship in turning motion reaches to the point◦ −of maximu◦ m yaw rate status, called “yaw rate max.”, in a short period of time reachesjust toafter the a point turnin ofg maximumtest starts. yawAbout rate 300 status, s later, called the yaw “yaw rate rate converges, max.”, in aand short ship period speed of also time becomes just afterstable. a turning This test status starts. is called About “ 300steady”. s later, It the is yawknown rate that converges, inflow andconditions ship speed to a alsopropeller becomes of these stable. two Thissituations status is called are quite “steady”. different It is from known that that of inflowa straight conditions run, also to amaking propeller eccentric of these force two situationsinduced by a are quitepropeller different be different. from that Table of a 5 straight represents run, the also inflow making conditions. eccentric force induced by a propeller be different. Table5 represents the inflow conditions.

Figure 14. Ship motion during the turning circle test (starboard turning) in the sea trial. Figure 14. Ship motion during the turning circle test (starboard turning) in the sea trial.

Figure 15. Ship motion during the turning circle test (port turning) in the sea trial.

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Figures 14 and 15 show measured values, including ship speed, yaw rate, and propeller rpm of a full-scale ship during the turning circle test in an official sea trial, respectively. The rotation angle of the rudder was +35° or −35°. From these figures, it can be recognized that a ship in turning motion reaches to the point of maximum yaw rate status, called “yaw rate max.”, in a short period of time just after a turning test starts. About 300 s later, the yaw rate converges, and ship speed also becomes stable. This status is called “steady”. It is known that inflow conditions to a propeller of these two situations are quite different from that of a straight run, also making eccentric force induced by a propeller be different. Table 5 represents the inflow conditions.

Figure 14. Ship motion during the turning circle test (starboard turning) in the sea trial.

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Figure 15. Ship motion during the turning circle test (port turning) in the sea trial.

TableFigure 5. Information 15. Ship motion of the turningduring the rate turning at three circle representative test (port turning) inﬂow conditions. in the sea trial.

Turning Status Turning Direction r α Straight run - 0.00 0.00 Port turn 1.01 22.45 Yaw rate max. − Starboard turn 0.99 22.15 − Port turn 0.80 27.04 Steady − Starboard turn 0.79 26.20 −

Numerical simulations for the prediction of propeller eccentric force in these conditions were carried out based on methodologies presented in the previous Section 2.1. Approach ship speed and the propeller’s rotation speed were determined by referring to official sea trial data in Figures 14 and 15. Although the wave around the ship is naturally generated in the straight run and turning of a ship, the free surface effect on the propeller eccentric force didn’t consider several previous studies [7,10] on reducing the computational time. In this regard, the boundary condition on the plane representing a design draught was defined as the symmetry condition. The computational domain was expanded by reflecting the symmetric plane used in the previous resistance test to consider a propeller behind the hull, therefore the total number of grids became about 7.2M. The rotation of a propeller is realized by implementing a sliding technique, and the propeller eccentric force could be directly calculated during the simulation. Unlike previous studies with high-speed navy vessels [6,8,9,11–14], the target vessel had a relatively low speed and was operated conservatively meaning that, ship motion such as a heeling during the turning was not considered. The rudder angle for each turning direction in the numerical simulation was defined as being the same as the value of official turning circle test.

2.3. Rudder Conﬁguration The schematic diagram of an asymmetric spade rudder installed on a target vessel is presented in Figure 16. The section shape diﬀers on the rudder’s upper part and lower part in order to prevent cavitation at the leading edge of the rudder. Energy-saving devices such as rudder bulb or post-swirl stator were not considered. J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 10 of 21

the propeller’s rotation speed were determined by referring to official sea trial data in Figures 14 and 15. Although the wave around the ship is naturally generated in the straight run and turning of a ship, the free surface effect on the propeller eccentric force didn’t consider several previous studies [7,10] on reducing the computational time. In this regard, the boundary condition on the plane representing a design draught was defined as the symmetry condition. The computational domain was expanded by reflecting the symmetric plane used in the previous resistance test to consider a propeller behind the hull, therefore the total number of grids became about 7.2M. The rotation of a propeller is realized by implementing a sliding technique, and the propeller eccentric force could be directly calculated during the simulation. Unlike previous studies with high-speed navy vessels [6,8,9,11–14], the target vessel had a relatively low speed and was operated conservatively meaning that, ship motion such as a heeling during the turning was not considered. The rudder angle for each turning direction in the numerical simulation was defined as being the same as the value of official turning circle test.

2.3. Rudder Configuration The schematic diagram of an asymmetric spade rudder installed on a target vessel is presented in Figure 16. The section shape differs on the rudder’s upper part and lower part in order to prevent

J. Mar.cavitation Sci. Eng. at2019 the, 7 ,lea 455ding edge of the rudder. Energy-saving devices such as rudder bulb or post10 of-swirl 20 stator were not considered.

(a) (b)

FigureFigure 16. 16.Asymmetric Asymmetric rudder rudder conﬁgurations configurations (a )(a) Side Side view; view (b; )(b) three-dimensional three-dimensional (3D) (3D view.) view.

2.4. Coordinate System 2.4. Coordinate System AJ.A coordinate Mar. coordinate Sci. Eng. system 2019 system, 7, x used FOR used PEER for for the REVIEW the analysis analysis of of propeller propeller eccentric eccentric force force was was deﬁned defined as as shown shown in in11 of 21 FigureFigure 17 .17.

Figure 17. The coordinate system for propeller lateral forces and moments.

3. Results Figure 17. The coordinate system for propeller lateral forces and moments. To investigate rudder effect on propeller eccentric force, simulation cases were categorized into 3. Results two conditions: straight run and turning motion, including starboard and port turning. In addition, propeller lateralTo investigate force and rudder moment effect were on propeller evaluated eccentric by averaging force, simulation for the last cases one revolutionwere categorized of a into propeller.twoThe condition propeller’ss: straight rotation run angleand turning per time motion step was, including defined starboard as 1◦ for realistic and port simulation. turning. In addition, propeller lateral force and moment were evaluated by averaging for the last one revolution of a 3.1. Straightpropeller. Run The propeller’s rotation angle per time step was defined as 1° for realistic simulation. Nominal wake distributions evaluated by numerical simulations and measured by model tests on the straight3.1. Straight run are Run compared in Figure 18. In model tests, a rudder does not exist due to measuring equipment consistingNominal wake of a pitot distributions tube and traverseevaluated system. by numerical To compare simulations the rudder and eff measuredect, two numerical by model tests simulationson the were straight carried run out are with compared and without in Fig a rudder.ure 18. In model tests, a rudder does not exist due to measuring equipment consisting of a pitot tube and traverse system. To compare the rudder effect, two numerical simulations were carried out with and without a rudder.

(b) (c) (a)

Figure 18. Nominal wake distribution. (a) Measurement; (b) simulation without rudder; (c) simulation with rudder.

As can be seen in Figure 18, the wake distribution in the upper part of a propeller (from 9 o’clock to 3 o’clock) is quite similar to the numerical simulation without a rudder and model tests. If a rudder exists, axial velocity is decelerated on the vertical center line. In Table 6, the thrust and torque of a propeller with and without a rudder were presented, respectively.

Table 6. Change of thrust and torque by rudder existence on the straight run.

J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 11 of 21

Figure 17. The coordinate system for propeller lateral forces and moments.

3. Results To investigate rudder effect on propeller eccentric force, simulation cases were categorized into two conditions: straight run and turning motion, including starboard and port turning. In addition, propeller lateral force and moment were evaluated by averaging for the last one revolution of a propeller. The propeller’s rotation angle per time step was defined as 1° for realistic simulation.

3.1. Straight Run Nominal wake distributions evaluated by numerical simulations and measured by model tests on the straight run are compared in Figure 18. In model tests, a rudder does not exist due to J.measuring Mar. Sci. Eng. equipment2019, 7, 455 consisting of a pitot tube and traverse system. To compare the rudder11 effect, of 20 two numerical simulations were carried out with and without a rudder.

(b) (c) (a)

Figure 18. Nominal wake distribution. (a) Measurement; (b) simulation without rudder; (c) simulation withFigure rudder. 18. Nominal wake distribution. (a) Measurement; (b) simulation without rudder; (c) simulation with rudder. As can be seen in Figure 18, the wake distribution in the upper part of a propeller (from 9 o’clock to 3 o’clock)As can isbe quite seen similarin Figure to 1 the8, the numerical wake distribution simulation in without the upper a rudder part of and a propeller model tests. (from Ifa 9 ruddero’clock exists,to 3 o’clock) axial velocity is quite similar is decelerated to the numerical on the vertical simulation center without line. In a Tablerudder6, theand thrust model and tests. torque If a rudder of a propellerexists, axial with velocity and without is decelerated a rudder on were the presented, vertical center respectively. line. In Table 6, the thrust and torque of a

J. propellerMar. Sci. Eng. with 2019 ,and 7, x FORwithout PEER aREVIEW rudder were presented, respectively. 12 of 21 Table 6. Change of thrust and torque by rudder existence on the straight run. Table 6. Change of thrust and torque by rudder existence on the straight run. Item ItemWith Rudder With Rudder Without RudderWithout Rudder Fx Fx112% 112% 100% 100% Mx 107% 100% Mx 107% 100%

TheThe thrust thrust and and torque torque of of a a propeller propeller with a rudder were largerlarger thanthan thosethose ofof aa propellerpropeller without without a arudder. rudder. Due Due toto rudderrudder existence,existence, inflowinflow velocity to the propeller propeller was was reduced, reduced, consequently consequently making making thethe angle angle of of attack attack of of a a propeller propeller relatively relatively larger. larger. Since Since the the increment increment of of thrust thrust was was bigger bigger than than that that ofof the the torque torque with with a a rudder, rudder, the the propulsive propulsive efficiency efficiency of of a a propeller propeller would would be be increased. increased. Figure Figure 1 199 shows instantaneous pressure distribution normalized by P ; 1 ρV 2 on a propeller face side as well shows instantaneous pressure distribution normalized by Pdd; ½ρ2 Vss2 on a propeller face side as well asas the the difference. difference. Moreover, Moreover, the the pressure pressure on on the the propeller propeller cap cap-end-end surface surface is is also also increased. increased. This This is is a a wellwell-known-known phenomenon phenomenon,, known known as as the the “wake “wake effect”. effect”.

(a) (b)

FigureFigure 19. InstantaneousInstantaneous normalized normalized pressure pressure distribution distribution on the on propeller the propeller face side face (looking side upstream) (looking upstream)during the during straight the run straight (a) without run (a) a rudder;without (ab )rudder; with a rudder.(b) with a rudder.

3.2. Turning Condition The ratio of propeller lateral force and moment in the turning condition for thrust and torque in the straight run is presented in Figures 20–23, including the sets with and without a rudder. During starboard turning with a rudder, the change of the Mz directly related to load of stern tube bearing is remarkable compared to those without a rudder. Due to the existence of a rudder, the Mz was reduced by about 50%, and this is effective for alleviation of load on stern tube bearing. This tendency is similar regardless of turning status (yaw rate max. or steady). In contrast, during port turning, the Mz with a rudder is almost similar to that without a rudder. As indicated in the results above, it can be summarized that the effect of rudder existence on the Mz is asymmetric in line with turning direction. This is schematically presented in Figure 24 to show the asymmetric effect of rudder on the Mz in the case of the ship changing direction.

J. Mar. Sci. Eng. 2019, 7, 455 12 of 20

J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 13 of 21 3.2. Turning Condition J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 13 of 21 J. Mar.The Sci. Eng. ratio 2019 of,propeller 7, x FOR PEER lateral REVIEW force and moment in the turning condition for thrust and torque13 of in21 the straight run is presented in Figures 20–23, including the sets with and without a rudder.

Figure 20. Propeller lateral force and moment during the starboard turn (without rudder).

Figure 20. Propeller lateral force and moment during the starboard turn (without rudder). Figure 20. Propeller lateral force and moment during the starboard turnturn (without(without rudder).rudder).

Figure 21. Propeller lateral force and moment during the starboard turn (with rudder).

Figure 21. PropellerPropeller lateral lateral force and moment during the starboard turn (with rudder).rudder). Figure 21. Propeller lateral force and moment during the starboard turn (with rudder).

Figure 22. Figure 22. Propeller lateral force and moment during the port turn (without rudder).rudder).

Figure 22. Propeller lateral force and moment during the port turn (without rudder). Figure 22. Propeller lateral force and moment during the port turn (without rudder).

J. Mar. Sci. Eng. 2019, 7, 455 13 of 20 J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 14 of 21

J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 14 of 21

. Figure 23. Propeller lateral force and moment during the port turn (with rudder). Figure 23. Propeller lateral force and moment during the port turn (with rudder). During starboard turning with a rudder, the change of the Mz directly related to load of stern tube bearing is remarkable compared to those without a rudder. Due to the existence of a rudder, the Mz was reduced by about 50%, and this is effective for alleviation of load on stern tube bearing. This tendency is similar regardless of turning status (yaw rate max. or steady). In contrast, during port turning, the Mz with a rudder is almost similar to that without a rudder. As indicated in the results above, it can be summarized that the effect of rudder existence on the Mz is asymmetric in. line with turning direction. This is schematically presented in Figure 24 to show the asymmetric effect of rudder on the Mz in theFigure case 23. of Propeller the ship lateral changing force direction.and moment during the port turn (with rudder).

Figure 24. The rudder effect on the propeller’s lateral moment during the straight run or the turning motion.

This is because of the interaction between the direction of the propeller’s rotation and inflow direction. During the starboard turning motion, the direction of flow into a propeller is from the port side to the starboard side. Since a propeller rotates clockwise, it continuously experiences a count- swirlFigure flow in 24. theThe lower rudder part e ffofect a onpropeller the propeller’s (from 3 lateralo’clock moment to 9 o’clock) during during the straight ship turning run or themotion. Figureturning 25 presents motion. the instantaneous pressure distribution on the face side of a propeller without a rudderFigure in both 24. Theturning rudder status, effect andon the it pisropeller known’s thatlateral the moment thrust duringcenter theof astraight propeller run oris naturallythe turning located in theThismotion lower is. because part of a of propeller. the interaction between the direction of the propeller’s rotation and inflow direction. During the starboard turning motion, the direction of flow into a propeller is from the port sideThis to the is starboard because side.of the Since interaction a propeller between rotates the clockwise, direction it of continuously the propeller’s experiences rotation a and count-swirl inflow direction.flow in the During lower partthe starboard of a propeller turning (from motion, 3 o’clock the to direction 9 o’clock) of duringflow into ship a propeller turning motion. is from Figure the port 25 sidepresents to the the starboard instantaneous side. Since pressure a propeller distribution rotates on clockwise, the face side it continuously of a propeller experiences without a rudder a count in- swirlboth turningflow in the status, lower and part it isof known a propeller that the(from thrust 3 o’clock center to of 9 ao’clock) propeller during is naturally ship turning located motion. in the Figurelower part25 presents of a propeller. the instantaneous pressure distribution on the face side of a propeller without a rudder in both turning status, and it is known that the thrust center of a propeller is naturally located in the lower part of a propeller.

J.J. Mar.Mar.J. Mar. Sci.Sci. Sci. Eng.Eng. Eng. 201920192019,, 77,, x7x , FORFOR 455 PEERPEER REVIEWREVIEW 151514 ofof of2121 20

((b)) ((a)) Figure 25. InstantaneousInstantaneous normalizednormalized pressurepressure distributiondistribution onon thethe propellerpropeller faceface sideside withoutwithout Figure 25. Instantaneous normalized pressure distribution on the propeller face side without rudder rudderrudder duringduring starboardstarboard turningturning (a)(a) At yaw rate max.; (b)(b) at steady. during starboard turning (a) At yaw rate max.; (b) at steady. IfIf aa rudderrudder exists,exists, inin contrast,contrast, thethe pressurepressure onon thethe upperupper partpart ofof a propeller was much increased, as shownIf a in rudder Figure exists, 26.. TheThe in contrast, pressurepressure the onon pressure thethe lowerlower on partpart the of upperof aa propellerpropeller part of awaswas propeller somewhatsomewhat was muchincreaincreased, increased,sed, butbut thetheas effecteffect shown waswas in Figurelimited,limited, 26 asas. The tthe upper pressure regionregion on the of lowerthethe rudderrudder part of isis a relativelyrelatively propeller closeclose was somewhattoto thethe propeller increased, due to but the the sweepsweepeffect angleangle was limited, of thethe rudder asrudder the upper.. This regiontendency of the is ruddersimilar isregardless relatively of close turning to the status, propeller namely, due to yaw the sweeprate maxangle.. or ofsteady. the rudder. Figure This27 showsshows tendency thethe instantaneousinstantaneous is similar regardless pressurepressure of distributiondistribution turning status, onon aa namely,planeplane defineddefined yaw rate atat severalseveral max. or heightssteady. above Figure a baseline27 shows in the yaw instantaneous rate max.. condition pressure.. Notably,Notably, distribution thethe differendifferen on a planece of defined results atsimulated several heights with andabove without a baseline a rudder in yawis clearly rate max.contrasted condition. at the Notably,upper region the di offf erencea rudder. of results simulated with and without a rudder is clearly contrasted at the upper region of a rudder.

((a)) ((b))

FigureFigure 26. 26. InstantaneousInstantaneousInstantaneous normalized normalized pressure pressure distribution distribution on on thethe the propeller propeller face face side side with with rudder rudder duringduring starboard starboard turning turning (a)(a) (a A)t Att yawyaw yaw raterate rate max max.;..;; (b)(b) (b at) at steady steady...

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(a) (b)

Figure 27. 27. InstantaneousInstantaneous normalized normalized pressure pressure distributions distributions on a on plane a plane at three at di threeﬀerent different heights heights during starboardduring starboard turning turning at yaw rateat yaw max. rate status max. ( astatus) without (a) w rudder;ithout rudder (b) with; (b) rudder. with rudder.

During port turning motion, the direction of the ﬂowflow into a propeller is from the starboard side to the port port side. side. Due Due to to propeller propeller rotation rotation direction, direction, a propeller a propeller experiences experiences a count a count-swirl-swirl flow ﬂow in the in theupper upper part part of a of propeller a propeller during during ship ship turning turning motion. motion. Consequently, Consequently, as as shown shown in in Figure Figure 2288,, it it isis inferred thatthat thethe centercenter of of thrust thrust is is located located in in the the upper upper part part of of the the propeller propeller without without a rudder, a rudder, and and the Mzthe Mz essentially essentially works works on reducing on reducing the bearingthe bearing load load of the of sternthe stern tube tube in this in this condition. condition.

J. J.Mar. Mar.J. Mar.Sci. Sci. Eng. Sci. Eng. 2019Eng.2019 ,2019 7,,7 x, 455,FOR 7, x FORPEER PEER REVIEW REVIEW 1716 of of 1721 20 of 21

(a) (a) (b) (b)

FigureFigureFigure 28. 28. Instantaneous 28.Instantaneous Instantaneous normalized normalized normalized pressure pressure pressure distribution distribution distribution on on the on the propeller the propeller propeller face face faceside side sidewithout without without rudder rudder rudder duringduringduring port port portturning turning turning (a) (a )At At(a) yaw yawAt yawrate rate max.;rate max.; max.; (b) (b )at at(b) steady. steady. at steady.

AsAs shownAs shown shown in in Figure in Figure 2929,, the the29 pressure, pressurethe pressure on on the theon upper upperthe partupper part of apart of propeller a ofpropeller a propeller was alsowas increasedwasalso alsoincreased increased by rudder, by by ruddersimilarrudder, similar to, insimilar the to starboardin to the in the starboard turning starboard turning cases. turning Although cases. cases. Although the Although pressure the thepressure on pressure the lower on the on part thelower of lower a part propeller part of a of a propellerwaspropeller also was increased, wasalso alsoincreased, the increased, eﬀect bythe a effectthe rudder effect by was aby rudder relativelya rudder was less wasrelatively than relatively that less of lessthan the starboardthan that thatof the of turning starboardthe starboard cases. turningThisturning can cases. be cases. explained This This can by canbe Figure explained be explained 30. During by Figure by port Figure 30 turning,. During30. During the port pressure port turni turni onng, thetheng, lower pressurethe pressure section on oftheon a lower rudderthe lower sectionis notsection signiﬁcantlyof a ofrudder a rudder is increased, not is significantlynot significantly unlike Figure increased increased 27, at, unlike yaw, unlike rate Figure max.Figure 27 status., at 27 yaw, at yaw rate ratemax. max. status. status.

(b) (a) (a) (b)

FigureFigureFigure 29. 29. Instantaneous 29.Instantaneous Instantaneous normalized normalized normalized pressure pressure pressure distribution distribution distribution on on the on the propeller the propeller propeller face face faceside side side with with with rudder rudder rudder duringduringduring port port portturning turning turning (a) (a )at at(a) yaw yaw at yawrate rate max.;rate max.; max.; (b) (b )at (b) at steady. steady. at steady.

Moreover, the My of a propeller is more sensitive to rudder eﬀect rather than Mz for ship turning motion. If a rudder is operating for the starboard or port turning motion of a ship, a rudder area facing a propeller is larger, and the rudder is respectively weighted to a propeller, as shown in Figures 27 and 29. In this context, the pressure distribution on the part of a propeller blade facing the biased rudder is mainly changed, and the My is consequently aﬀected. During starboard turning, this value is reduced by 20% by the rudder; however, it is increased about two times during port turning, as represented from Figures 20–24.

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(a) (b)

FigureFigure 30. InstantaneousInstantaneous normalizednormalized pressure pressure distributions distributions on aon plane a plane at three at three diﬀerent different heights heights during duringport turning port turning at yaw at rate yaw max. rate status max. (statusa) without (a) w aithout rudder; a rudder (b) with; (b) a rudder.with a rudder.

Moreover,As mentioned the My in Section of a propeller 2.3, the rudderis more used sensitive in this to study rudder was effe composedct rather ofthan diff Mzerent for sections ship turning along motion.the rudder If a height rudder to is prevent operating rudder for cavitation.the starboard This or characteristic port turning is motion also related of a ship, to the a asymmetricalrudder area facingeffect ofa propeller a rudder is on larger, propeller and the eccentric rudder forceis respectively at each turning weighted direction. to a propeller, The comparison as shown in between Figure 2Figures7 and 29 31. In and this 32 context, would explainthe pressure why thedistribution rudder’s on effect the on part propeller of a propeller eccentric blade force facing is asymmetrical the biased rudderas per theis mainly turning changed, direction and of the a ship. My is The consequently instantaneous affected. pressure During distributions starboard aroundturning, the this leading value isedge reduced of the by rudder 20% by were the clearlyrudder; di however,fferent to it the is turningincreased direction about two (port times and during starboard). port turning In contrast,, as representedthe turning from status, Figure representeds 20–24. by yaw rate max. or steady, did not significantly affect the pressure distributionAs mentioned on the rudderin Section in each2.3, the turning rudder direction. used in this study was composed of different sections along the rudder height to prevent rudder cavitation. This characteristic is also related to the asymmetrical effect of a rudder on propeller eccentric force at each turning direction. The comparison between Figures 31 and 32 would explain why the rudder’s effect on propeller eccentric force is asymmetrical as per the turning direction of a ship. The instantaneous pressure distributions around the leading edge of the rudder were clearly different to the turning direction (port and starboard). In contrast, the turning status, represented by yaw rate max. or steady, did not significantly affect the pressure distribution on the rudder in each turning direction.

J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 19 of 21 J. Mar.J. Mar. Sci. Sci. Eng. Eng. 20192019, 7, ,x7 FOR, 455 PEER REVIEW 19 of18 21 of 20

((a)) ((b))

FigureFigure 31.31. InstantaneousInstantaneousInstantaneous normalized normalized pressure pressure distributions distributions on a onrudder a rudder during during starboard starboard turning turning (a)(a) Att ( yawyawa) At raterate yaw maxmax rate.. (b)(b) max. at steady (b) at steady (looking(looking (looking downstream)downstream) downstream)...

((a)) ((b))

FigureFigure 32. 32. InstantaneousInstantaneousInstantaneous normalized normalized pressure pressure distributions distributions on ona rudder a rudder during during port port turning turning... (a)(a) ( Aa)tt At yawyaw rate rate max max.,..,, (b)(b) ( bat) steady at steady turningturning turning (looking(looking (looking downstream)downstream) downstream)... 4. Conclusions 4. Conclusions In this study, numerical simulations were performed to investigate the effect of rudder existence InIn thisthis study,study, numericalnumerical simulationssimulations werewere performedperformed toto investigateinvestigate thethe effecteffect ofof rudderrudder existenceexistence on propeller lateral forces and moments. A target vessel, about a 10,000TEU class container ship, was on propeller lateral forces and moments. A target vessel, about a 10,000TEU class container ship, was chosen. For the straight run, the thrust and torque of a propeller were increased due to rudder existence chosen. For thethe straight run, the thrust and torque of a propeller were increased due to rudder and the propulsive efficiency on a propeller was improved by the wake effect of a rudder. The effect of existence and thethe propulsivepropulsive efficiencyefficiency onon aa propellerpropeller waswas improvedimproved byby thethe wakewake effecteffect ofof aa rudder.rudder. a rudder on propeller eccentric force was different in the turning direction. The Mz directly related to The effect of a rudder on propeller eccentric force was different in thethe turningturning direction.direction. TheThe MzMz the bearing load of a stern tube was diminished by about 50% owing to the existence of a rudder for directly related to the bearing load of a stern tube was diminished by about 50% owing to the starboard turning, while the values were similar to each other regardless of rudder existence for port existence of a rudder for starboard turning,, while the values were similar to each other regardless of turning. This difference basically results from the interaction between the direction of a propeller’s rudder existence for port turning. This difference basically results fromfrom thethe interactioninteraction betweenbetween thethe rotation and the inflow direction to a propeller. In particular, the effect of a rudder on propeller lateral direction of a propeller’s rotation and thethe inflowinflow directiondirection toto aa propeller.propeller. InIn particular,particular, thethe effecteffect ofof aa force and moment depends on where flow straightness behind a propeller is relatively strong or weak. rudder on propeller lateral force and moment depends on where flow straightness behind a propeller Although the center of thrust on a propeller was located in the lower part of a propeller for starboard isis relativelyrelatively strongstrong oor weak. Although the center of thrustthrust onon aa propellerpropeller waswas locatedlocated inin thethe lowerlower turning, the magnitude of the Mz was compensated by rudder effect in the upper part of a propeller. part of a propeller for starboard turning, the magnitude of the Mz was compensated by rudder effect In contrast, the center of thrust on a propeller is already located in the upper part of a propeller for inin thethe upperupper partpart ofof aa propeller.propeller. InIn contrast,contrast, thethe centercenter ofof thrustthrust onon aa propellerpropeller isis alreadyalready locatedlocated inin port turning so the effect of a rudder was relatively limited on propeller eccentric force. Additionally, thethe upper upper part part of of a a propeller propeller for for port port turning turning so so the the effect effect of of a a rudder rudder was was relatively relatively limited limited on on a rudder affects the My of a propeller, consequently, that value with rudder showed similar level propeller eccentric force. Additionally, a rudder affects the My of a propeller,, consequently, consequently, that that regardless of the ship’s turning direction. A rudder’s main configurations, including the leading-edge value with rudder showed similar level regardless of thethe ship’s turning direction. A rudder’s main sweep angle, aspect ratio, section shape or distance between a propeller and a rudder, might have an configurations, including thethe leading leading--edge sweep angle, aspect ratio, section shape or distance impact on propeller eccentric force. Furthermore, appendages located around a propeller, including a between a propeller and a rudder, might have an impactimpact onon propellerpropeller eccentriceccentric force.force. Furthermore,Furthermore, appendages located around a propeller, including a rudder and various energy saving devices, need toto bebe consideredconsidered inin orderorder toto estimateestimate rreliable propeller eccentric force.

J. Mar. Sci. Eng. 2019, 7, 455 19 of 20 rudder and various energy saving devices, need to be considered in order to estimate reliable propeller eccentric force.

Author Contributions: Conceptualization, G.S.; validation, G.S., and T.L.; analysis, G.S., and T.L.; writing—original draft preparation, G.S.; writing—review and editing, T.L. and H.P.; supervision, H.P.; project administration, H.P. Funding: This research received no external funding. Acknowledgments: This research was carried out as part of an internal research project. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

B Breath [m] BEMT Blade Element and Momentum Theory D Propeller diameter [m] CFD Computational Fluid Dynamics Eta O Propeller efficiency CRP Contra-Rotating Propeller Fr Froude number EHP Effective Horse Power KT Thrust coefficient FEM Finite Element Method KQ Torque coefficient Fx Thrust of the propeller Kn Knots Fy Force to the vertical axis L Length Between Perpendiculars [m] Fz Force to the horizontal axis P Dynamic pressure [Kg/(m s2)] HRIC High Resolution Interface Capturing d · r Rate of turn [◦/s] JIME Japan Institute of Marine Engineering s Second MRF Moving Reference Frame Td Design draught [m] Mx Moment based on the propeller-shaft axis Vs Ship speed [Kn] My Moment based on the vertical axis xp Distance from the propeller to the gravitational Mz Moment based on the horizontal axis center of the hull [m] Z Number of blades POW Propeller Open Water α Water flow angle against propeller plane [◦] Rtm Total resistance in model scale β Drift angle [◦] SIMPLE Semi-Implicit Method for Pressure Linked Equations δ Rudder angle [◦] TEU Twenty-foot Equivalent Unit ρ Density of fluid [Kg/m3] URANS Unsteady Reynolds Averaged Navier-Stokes VOF Volume of Fluid

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