Energy Saving Technology of PBCF (Propeller Boss Cap Fins) and its Evolution * Takeo Nojiri ** Norio Ishii *** Hisashi Kai **** ABSTRACT

Energy Saving Technology of PBCF (Propeller Boss Cap Fins) and its Evolution * Takeo Nojiri ** Norio Ishii *** Hisashi Kai **** ABSTRACT Currently, there are more than 1,800 vessels that are equipped with PBCF (Propeller Boss Cap Fins), a trustworthy energy saving device which contributes not only to fuel oil consumption savings, but also to the reduction of greenhouse gas emissions. The PBCF mainly recovers the energy loss at the propeller hub vortex downstream of the propeller, and lowers fuel consumption by 5% at constant speed operation, or boost speed by 2% with the same fuel consumption. In the earlier section of this paper, the basic hydrodynamic mechanism of the PBCF as an energy saving device is described. Then, outline of the recent R&D efforts carried out in the past few years to improve the PBCF s fuel saving effect are described. Although it is still ongoing, the geometric shape of the existing PBCF is being improved by taking into account knowledge of the hydrodynamic mechanism obtained through Computational Fluid Dynamics (CFD) analysis. The effectiveness of improvements made was confirmed by the model tests. propeller disc area. As a result from those influence, especially the down flow from the root of blade trailing edge accelerates the vortex flow around the propeller boss. Those vortex flows from each propeller blade are summed up and converged, resulted in very strong vortex flow at the boss end. The strong down flow from propeller blade trailing edge is rectified by the PBCF fins and the hub vortex is broken up, so that the fins will produce the force reducing the propeller shaft torque by 3% and more, and increasing thrust by over 1%. As a result of such hydrodynamic mechanism, the PBCF will improve propeller efficiency itself and its effect will not essentially depend on the type or size of the vessel, horsepower, RPM, vessel speed, etc. Finally, in the later section, the possible direction of PBCF improvements necessary for it to be a more effective energy saving device is explained. 1. INTRODUCTION The PBCF was proposed as a novel energy-saving device in 1987. In PBCF, small fins are attached on the boss cap (hub cone). The number of the fins is the same as that of the propeller blades, and they rotate together with the propeller. Figure 1 shows a PBCF installed on a ship. The water flow is accelerated and twisted when it passes the * Received February 16, 2011 ** MOL Techno-Trade, Ltd., Tokyo, Japan *** Akishima Laboratories(Mitsui Zosen) Inc., Akishima, Tokyo, Japan **** Yokohama National University, Yokohama, Japan This paper describes the latest knowledge about principle and effect of the PBCF, recent results of reverse Propeller Open Test (POT) and CFD analysis, and also reports actual results of significant reduction of fuel oil consumption in full scale ships. 2. CONTRIBUTION OF THE EXISTING PBCF A large number of PBCFs have been installed on ships and they are contributing to reduction of the fuel oil Journal of the JIME Vol. 46,No.3(2011) 63

333 Energy Saving Technology of PBCF (Propeller Boss Cap Fins) and its Evolution consumption and greenhouse gas emission. PBCF was born in 1987, and spreading its presence since then. In this section, track number and the contribution of the PBCFs will be described. 2.1 The R/D works of PBCF at the stage of its birth PBCF was born from heavy and hard R/D works by Ouchi et al.1) 2) Reverse Propeller Open Test (Rev. POT) has been thought up and utilized for the development, and this experimental methodology would be the key factor of the PBCF birth. Figure 2 shows the test arrangement of normal POT and Reverse POT. 2.2 The track record of PBCF The PBCF have been adopted on 1,800 vessels around the world. A large number of PBCFs are installed on all types of ship such as Tanker including VLCC, Container, PCC, Ferry etc., and including the vessels with CPP. Figure 4 left shows track record in horsepower-wise, 45% is less than 10,000ps, 30% is 10,000~20,000ps, 15% is 20,000~40,000ps and 10% is more than 40,000ps. Figure 4 right shows track record in ship s type wise, 20% is Container, 12% is Tanker (Inc. Product), 20% is Bulker, 10% is PCC and RORO, 10% is General cargo and Reefer, 8% is Gas and Chemical, and 20% is passenger ferry and others. We have also experiences of the PBCF installation on more than 50 vessels of CPP, and many of those CPP vessels have twin propellers. Figure 3 shows the improvement of propeller performance by PBCF installation, in which torque decreases, thrust increases and the propeller efficiency improves. 2.3 Performance on real ships The PBCF performance on Real Ship have been analyzed and evaluated on their trial data and/or on their daily voyage data. Table 1 shows the analyzed results of the PBCF FOC saving effect on actual vessels in recent few years. Δηs denotes the analyzed fuel saving value in percentage. The vessels including in this table are from inter-coastal small size to large size container carrier and VLCC. The M/E output is also widely spread, from 1000 to 76,000 PS. From 2% to 10% of efficiency improvement is shown on this table. Those figures show rather large scattering of the analyzed results and approximately 5% will be the average value of the improvement. Those scattering would be mainly coming from the difficulty on the analysis of actual voyage data. In actual voyage, the fuel consumption is influenced by many factors like speed, draft, trim, sea condition of wave and wind, tide or current etc. Through the plenty of and deep experimental works at its birth stage, PBCF was born with good DNA. From this table, it is indicated that efficiency improvement of PBCF will have nothing to do with vessel type and/or M/E output level etc. Journal of the JIME Vol. 46,No.3(2011) 64

333 Three new PBCFs with different fin contour shapes have been designed in considering lots of model test results and CFD simulation until then. PBCF models newly made are shown in Figure 6. 3. 3 EXPERIMENTAL STUDY ON PBCF IMPROVEMENT PBCF has kept evolving through the continuous R & D works since it was born. Especially, deep experimental studies have been carried out to improve its performance as a energy saving device in these few years. In this paper, some recent R & D results are described focused in the geometric shape of the PBCF fin, from lots of experimental studies on the variation of the shapes of constitutional elements of PBCF. In this section, some results are shown from many experiments which have been carried out and investigated on their performance for various contour of PBCF fin. All the experiments have been carried out in the Cavitation Tunnel at Akishima Laboratories (Mitsui Zosen) Inc. by reverse POT arrangement as described in previous section. 3.1 Experiment on 5 bladed propeller Figure 5 shows the test arrangement for 5 bladed propeller in the cavitation tunnel. Table 2 shows the principal particulars of the propeller used for the experiment. The basic shape of the boss cap (W/O PBCF) is trapezoid, and the experiments have been carried out on the PBCF attached with various shape of fins on the basic boss cap. Fin contour shapes of the model PBCF are as follows; Original shape PBCF5-0 Leading edge side cut shape PBCF5-1 Trailing edge side cut shape PBCF5-2 Trailing edge side cut & increased height shape PBCF5-3 Experiments have been carried out on two advance ratios (J=0.6 & 0.8) and the results are shown in Figure 7. This figure shows the difference in percentage of the experiment results of KT, KQ and η0 for with PBCF combination compared with those for without PBCF. KT increases and KQ decreases, and the efficiency of original PBCF 5-0 is larger than that for W/O PBCF by about 1.5%. Journal of the JIME Vol. 46,No.3(2011) 65

333 Energy Saving Technology of PBCF (Propeller Boss Cap Fins) and its Evolution In case of PBCF5-2, it has been found that KT is about the same level and KQ decreases a little, and the efficiency η0 increases compared with the original PBCF 5-0. In the next section, the possibility of increasing PBCF effect is described that the efficiency would be improved by cutting off some PBCF fin area which seems to be acting not so much for the PBCF effect from the CFD simulation. 3.2 Experiment on 4&6 bladed propeller In order to investigate whether the same tendency will be obtained to improve the PBCF effect found from the 5 bladed propeller tests, experimental study have been carried out on 4 and 6 bladed propellers with various shapes of PBCF. Table 3 shows the principal particulars of 4 and 6 bladed propellers and Figure 8 shows the model PBCF photos used for the experiments. The basic boss cap (conventional cap) shape is trapezoid. Fins have been attached on the basic boss cap surface and their performance has been tested in the cavitation tunnel to compare the effectiveness of the conventional cap with that of PBCFs by reverse POT. Tested models are conventional cap (W/O PBCF), original PBCF (PBCF4-0, PBCF6-0) and trailing edge side cut type (PBCF4-1, PBCF6-1). Test results for 4-bladed and 6-bladed propellers are shown in Figure 9 and Figure 10 respectively. KT increases and KQ decreases, and efficiency increases by approximately 1.5% for the original PBCF (PBCF4-0) compared with the conventional cap (W/O PBCF) in case of 4 bladed propeller. KT of PBCF4-1 is almost the same as that of PBCF4-0, while KQ is slightly decreases, and the efficiency is improved moreover. Journal of the JIME Vol. 46,No.3(2011) 66

333 In the case of 6-bladed propeller experiment, KT increases and KQ decreases, and efficiency is improved for the PBCF6-0 compared with the conventional cap. The efficiency of PBCF6-1 indicates larger value than that of PBCF6-0, as the 4 and 5 bladed propellers results. From the test results shown in Figure 7, 9 and 10, the possibility to improve the PBCF performance is indicated by the modification of the fin form from the original shape. The detail study for various propellers and PBCF models will be proceeded to confirm that the direction of the modification of PBCF fin shape described above will be the general solution for the wide range of propellers. And this direction of the modification will also be confirmed by the evaluation on actual vessel performance. It should be noted that the tested 5-bladed propeller and the PBCF is the actual model of the 5500TEU Container Vessel listed in Table 1. As shown in the table, the PBCF FOC saving effect of this vessel has been analyzed from her voyage data and about 4.2% of fuel saving have been indicated from the analysis. to solve flow field, finite volume method including SIMPLE method is included. SST k-ω model in low Reynolds number type is selected as one of some turbulence models. This model is widely used to calculate for the prediction of performance about wing with separation. Convection term is discretized by MARS scheme. As nature of the scheme, it is well known that the same solutions are obtained without dependency on cell type and stability, and high accuracy is shown. 4.1.1 CFD Simulation on 5 bladed propeller and PBCF CFD simulation has proceeded on the 5 bladed propeller for a Pure Car Carrier in model scale. The principal particulars of the model propeller are shown in Table 4. 4. 4 NUMERICAL SIMULATION OF FLOW AROUND PBCF Ouchi et al. 1) reported the PBCF effect as follows; Propeller thrust increases by the higher pressure distribution at propeller boss end due to breaking up the hub vortex and higher pressure on the face surface of the propeller blade due to ground effect phenomena from PBCF fin. Total propeller torque decreases because the force acting on the fin reacts as inverse torque by the rectification of down flow from the propeller blade trailing edge. In order to study the detail of such phenomena, it is very important to analyze and investigate properly the character of the vortex flow from propeller blades. 4.1.2 Results of CFD Simulation Figure 11 shows the grid mesh on the propeller with and without PBCF, of which the number of the grid is 400,000, to show the image of the generated grids. Actually much more minute grids are applied for the calculation. The grids are generated on every blades and fins considering the future simulation in non-uniform flow as shown in this figure. Further more, the shape of the root of the propeller blades and edge part of the fins are simplified by using box shape to prevent from becoming enormous number of grids. One 3) of the authors proceeded the CFD analysis on the flow around propeller and PBCF, and explain qualitatively its effect and fluid dynamic mechanism. Finite volume method has been applied for the analysis to solve the Navier-Stokes Equations directly, to obtain precise flow field around the PBCF. 4.1 Calculation on 5 bladed propeller The STAR-CD for multipurpose problems in computational thermo-fluid dynamics is used in the present calculations. In the software, Navier-Stokes equations is discretized by dividing a calculation volume to small elements and in order Journal of the JIME Vol. 46,No.3(2011) 67

333 Energy Saving Technology of PBCF (Propeller Boss Cap Fins) and its Evolution propeller blade and PBCF fin, the cause of such phenomena would be considered to be damming effect from PBCF. The slower velocity region only appears just after the propeller blade trailing edge for the case without PBCF. On the other hand, slower velocity region mainly exist at the propeller blade trailing edge and widely spread to the back side region of PBCF fin for the case with PBCF. Figure 13(b) shows the velocity distribution of the section at a little behind the fin leading edge around boss cap. Stream lines of the down flow from a random point at boss cap end are shown in Figure 12 for the case with and without PBCF. These figures show the stream lines on the fixed coordinate system to the propeller. In the case of without PBCF, vortexes of the same rotating direction to the propeller are generated from each blade root and converged to strong vortex at boss cap end. This strong vortex flows out from boss cap end to aft ward, and grows up to hub vortex. In case of with PBCF, vortexes are not formed at all at boss cap end. This means the vortexes flowing out from each blade root are suppressed and vanished by some effect in the area from the blade root to the boss cap end. Or the vortex generated from the blade root does not exist in this area. It is obvious from these results that hub vortex is not formed in the down stream area from the aft end of the boss cap for the case with PBCF. In order to make clear the mechanism of these phenomena, flow field around boss cap is investigated. The velocity distribution (m/s) at the section just before the leading edge of PBCF fin perpendicular to the propeller axis is shown in Figure 13(a). In this figure, left side figure shows the velocity distribution without PBCF and right side shows that with PBCF. The velocity distribution shows some difference between without and with PBCF even at the forward position of PBCF leading edge. As slower velocity region appears even between the Comparing with Figure 13(a) and 13(b), slower velocity region only extend slightly for the case without PBCF. But slower velocity region pretty largely expands for the case with PBCF compared with the section shown in Figure 13(a). Velocity reduction is remarkable for the back side region of PBCF fin and also this appears even for the face side region. Figure 13(c) shows the velocity distribution at the section of aft end of PBCF. Slow velocity appears at the whole range where PBCF fins exist for the case with PBCF. From those Journal of the JIME Vol. 46,No.3(2011) 68

333 effects, velocity near the boss cap surface remarkably reduced and the velocity becomes actually almost down to 0. Little slow down velocity is observed and remarkable reduction does not appear for the case without PBCF. The velocity reduction part is the stagnation point at the aft end of the boss cap, and hub vortex would grow up aft ward from this point. That is to say from the difference of the velocity distribution described above, flow between the fins is dammed, and vortex generated from the trailing edge of propeller blade root cannot converge at the propeller center axis. As a result, hub vortex generation would be prevented. Vortex flow from the trailing edge of the blade roots would avoid the slow velocity region indicated in green color in the figure and diverged out from the trailing edge of upper part of fins. Therefore the mechanism of preventing hub vortex generation would come from damming the flow around the boss cap by PBCF fins. with PBCF, aft end part of the boss cap rather than the fin trailing edge part will act major role for the PBCF thrust increase. This thrust increase will be more significant at the region from the aft end to the trailing edge than the part between the leading edge to the trailing edge of the fins. Such output like pressure distribution described above cannot easily be obtained from experiments, and application of CFD simulation is very useful method to obtain the valuable information. Pressure (Pa) distribution on fin surface is shown in Figure 15. The face side in this figure denotes the fin surface being opposite to the back surface of the propeller blade, and back side denotes the reverse side of the face surface. This figure shows the whole face area of the fin surface is covered with negative pressure, while the almost whole back area is covered with positive pressure. The pressure distribution on the boss cap surface looked from aft is shown in Figure 14. High negative pressure appears on the center region of the boss cap for the case without PBCF. This negative pressure is caused from the strong swirl and hub vortex around the propeller center axis. These pressure distributions on PBCF fin surface are inverse relation to the pressure distribution on propeller blade. Generation of negative torque by PBCF fins is clearly explained from such quantitative physical information of pressure distribution. For the case with PBCF, high negative pressure at the center region of the boss cap disappeared as the strong swirl flow and hub vortex does not exist. Comparing the pressure distribution for the case without and It is found that negative pressure is acting on the whole area on the face side of the fin and especially high negative pressure exists near the area at the leading edge of fins, while positive pressure is acting on the back side and bigger negative one also exists at the area near the leading edge. The difference of pressure distribution between back and face surface is found to be small except some area around the leading edge of the fin. From the pressure distribution as a result of the CFD Journal of the JIME Vol. 46,No.3(2011) 69

333 Energy Saving Technology of PBCF (Propeller Boss Cap Fins) and its Evolution simulation shown in Figure 15, the fore half part of the fin is mainly acting for the reduction of torque, while the aft half and upper end part is not so contributing to the torque reduction. 4.1.3 Possible suggestion to improve PBCF performance from CFD simulation The study is still on going and improvement of the PBCF by the modification of its geographic shape is not concluded yet. From qualitative consideration of the CFD simulation results described here, however, the possible improvement of PBCF performance would be removal of the aft and outer edge part of fin from the view point of reduction of propeller torque. The reasons are; Damming effect is one of the major roles of PBCF to prevent the vortex flow from the root of blade trailing edge converging to strong hub vortex. This damming effect would mainly depend on the fore half part of the fin and this would not be largely affected by the removal of the aft part of the fin. From the pressure distribution on the fin surface, aft half and outer edge part would play little for the reduction of propeller torque. Moreover it would be effective to improve the torque reduction by decreasing the friction resistance by removal of some area of PBCF fin. From those considerations, the possible direction of the modification of PBCF fin shape to improve its effect is tentatively indicated from the CFD simulation. Besides, this indication is matched with the model test results described in previous section. We have to mention that the propeller and PBCF used for the experimental study are different from those for the CFD simulation (different vessel). Pressure distribution on the fin surface is very useful to study the physical phenomena and improve performance of PBCF, such pressure distribution is, however, almost impossible to obtain from the experiment. We will carry out the CFD simulation on the same propeller used for the experiment, and proceed the further study on PBCF to improve its performance. 5. 5 CONCLUSION In this paper, the present situation of PBCF is described in the beginning section, and PBCFs have already propagated for more than 1800 vessels and contributing to not only energy saving but also the reduction of green house gas emission. And then, the possible direction to improve the energy saving effect of PBCF is indicated from the experimental study on various shapes of PBCF models. Moreover, the detailed phenomena of PBCF hydrodynamic mechanism are becoming to be clear by CFD simulation. The possibility is indicated that CFD simulation will qualitatively explain the modification of fin shape studied from the model tests to improve its energy saving performance. More efficient PBCF will possibly be developed from making the most of utilize the CFD simulation to optimize its detailed shape and dimensions. Through the R & D works already explained here, PBCF was originally born with good features and survived from the struggle for existence up to now, and the possibility of its evolution in the future has been presented. 6. 6 ACKNOWLEDGEMENT The authors would like to express their gratitude to Mr. Yoichi Shimizu and the staffs at Ship Planning and Development Group, Technical Division of Mitsui O. S. K. Lines, Ltd for their cooperation and support to the study. The authors would like to express their gratitude to the all staffs at Akishima Laboratories (Mitsui Zosen) Inc. for their helps and supports to provide us the experimental results to this paper. The authors would also like to express their gratitude to the all members of Yokohama National University to provide us the valuable results of the CFD simulation. REFERENCES 1) K. Ouchi, M. Ogura, Y. Kono, H.Orito, T. Shiotsu, M. Tamashima, H. Koizuka : A Research and Development of PBCF (Propeller Boss Cap Fins) - Improvement of Flow from Propeller Boss -, Journal of the Society of Naval Architects of Japan, 163, 1988, pp.66-78. 2) K. Ouchi, M. Tamashima : Research and Development on PBCF (Propeller Boss Cap Fins), Technical Paper for PBCF presented at PRADS 89, October, 1989 VARNA, BULGARIA Journal of the JIME Vol. 46,No.3(2011) 70

333 3) H. Kai, S. Bito, Y. Miura : A Study on Fluid Dynamic Mechanism of PBCF, Journal of the Japan Society of Naval Architects and Ocean Engineers, Volume 10, December 2009, pp37-47 (In Japanese) 4) Y. Tanaka, T. Nojiri, T. Koh : Effect and Application of PBCF (Propeller Boss Cap Fins), International Symposium on Ship Design & Construction Environmentally Friendly Ships - on September 1st and 2nd 2009, organized by JASNAOE, JIME, JIN & RINA. 5) Masatoshi Yokoo, Takeo Nojiri : Fuel Saving Effect of Propeller Boss Cap Fins evaluated from Actual Vessel Data, JASNAOE spring meeting, 2007 Journal of the JIME Vol. 46,No.3(2011) 71