The Study On Various Methods Of Reducing The Resistance Of Patrol Craft Have Been Carried Out By Many Researchers. These Methods Included The Application Of Stern Flaps, Stern

Wedges, And Microbubble Injection. However, Due To Its Simplicity And Practicality, Stern Flap Is The Most Promising And Cost Effective Method. The Effect Of A Stern Flap On The Resistance Performance Of The Planing Hull Crew Boat Is Presented. Model Tests Were Conducted To Prove The Effectiveness Of The Stern Flap On Reducing Planing Hull Craft Resistance. Five Different Stern Flap Designs Were

Tested As Part Of Systematic Investigation To Determine The Optimum Geometrical Characteristics Of The Stern Flap. Results Of Model Resistance Experiments Showed That Four Of The Flaps Tested Showed An Increase In Resistance While The Flap At Zero Degree Angle Reduced The Total Resistance By 7.2 Percent At 23 Knots, And An Average Reduction Rate Of 4.5 Percent. At 23 Knots, An 8.2 Percent Reduction In Effective Power Was Predicted

Koh, Kho King

Shamsuddin, Suhaili

Yaakob, Omar

English

Universiti Teknologi Malaysia Institutional Repository

STERN FLAP FOR RESISTANCE REDUCTION OF PLANING HULL CRAFT 43Jurnal Teknologi, 41(A) Dis. 2004: 43–52© Universiti Teknologi MalaysiaSTERN FLAP FOR RESISTANCE REDUCTION OF PLANINGHULL CRAFT: A CASE STUDY WITH A FAST CREW BOATMODELOMAR YAAKOB1, SUHAILI SHAMSUDDIN2 & KOH KHO KING3Abstract. The study on various methods of reducing the resistance of patrol craft have beencarried out by many researchers. These methods included the application of stern flaps, sternwedges, and microbubble injection. However, due to its simplicity and practicality, stern flap is themost promising and cost effective method. The effect of a stern flap on the resistance performanceof the planing hull crew boat is presented. Model tests were conducted to prove the effectivenessof the stern flap on reducing planing hull craft resistance. Five different stern flap designs weretested as part of systematic investigation to determine the optimum geometrical characteristics ofthe stern flap. Results of model resistance experiments showed that four of the flaps tested showedan increase in resistance while the flap at zero degree angle reduced the total resistance by 7.2percent at 23 knots, and an average reduction rate of 4.5 percent. At 23 knots, an 8.2 percentreduction in effective power was predicted.Keywords: Patrol boats design, model testing, fast craftAbstrak. Kajian terhadap pelbagai cara mengurangkan rintangan bot peronda telah dilakukanoleh ramai penyelidik. Kaedah yang dikaji termasuk penggunaan kepak buritan, baji buritan dansuntikan gelembung mikro. Walau bagaimanapun, disebabkan oleh sifatnya yang mudah danpraktikal, kepak buritan didapati amat berpotensi. Kertas kerja ini membentangkan kajian terhadapkesan kepak buritan terhadap prestasi rintangan bot kelasi berbentuk planing. Ujian model dijalankanbagi membuktikan keberkesanan kepak bagi mengurangkan rintangan. Lima kepak berlainantelah digunakan dalam rangka kajian sistematik bagi menentukan ciri geometrik optimum kepakburitan. Hasil ujian model menunjukkan empat kepak menambah rintangan kapal manakala kepakkelima pada sudut sifar mengurangkan rintangan sehingga 7.2 peratus pada 23 knots danpengurangan purata sekitar 4.5 peratus. Pada 23 knots, pengurangan 8.2 peratus kuasa berkesandiperolehi.Kata kunci: Reka bentuk bot peronda, ujian model, pesawat laju1.0 INTRODUCTIONShip resistance will not only affect the choice of propulsion machinery but also theattainable speed, and economics of operation of the vessel. If resistance can bereduced, less power will be required to propel the ship at a given speed, or for agiven power, the ship can travel faster. Bulbous bow is commonly used to reduce1,2&3 Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor.E-mail: omar@fkm.utm.myJTDIS41A[04].pmd 02/16/2007, 22:5743OMAR YAAKOB, SUHAILI SHAMSUDDIN & KOH KHO KING44resistance of displacement hull vessels like tankers [1,2]. However, planing hulls aredifferent from displacement hull in terms of their shape, range of speed, hydrodynamiccharacteristics, etc. Thus a different approach is needed in reducing the frictionaldrag.There are several methods or devices that are being used or still underexperimentation for reducing resistance of planing hulls. Amongst them are thestern flap [3-6], wedges [3,7], microbubble injection [8-10], riblets [11], and boundarylayer additives [12]. Most of these methods can be applied to both hull types, but theeffectiveness may be different, depending on their respective resistance reductionmechanisms.A brief comparison was made among the various methods for installation on aplaning hull craft. After careful consideration of the merits and demerits of variousmethods, stern flap was selected. Table 1 summarises the reasons for not selectingother methods.2.0 THE STERN FLAP AS RESISTANCE REDUCTIONMECHANISMStern flaps have been used on many high-speed small craft, such as workboats,patrol craft, and pleasure craft [3]. A stern flap represents an extension of the hull aftof the transom in the form of a flat plate. The flap is mounted to the transom at anangle relative to the centreline buttock of the ship [4], as in Figure 1.Previous works [3-6] have shown that a stern flap has a similar effect on shippowering performance as a stern wedge. All stern flaps, independent of what vesselssize or type they are used on, create a vertical lift force at the transom, and modifythe pressure distribution on the after portion of the hull. The modification of theafterbody flow field causes the principal performance enhancement on a displacementhull. A stern flap causes the flow to slow down under the hull at a location extendingTable 1 Summary of demerits of various methodsMethods DemeritsBulbous bow Not suitable for planing hullRiblets Marine growth causes loss in drag reduction performanceBoundary layer additives Complex additive requirement high costStern wedge Poorer performance compared with stern flap. Slots weld mightbe neededMicrobubble injection Problems of gas flow generation system high costJTDIS41A[04].pmd 02/16/2007, 22:5744STERN FLAP FOR RESISTANCE REDUCTION OF PLANING HULL CRAFT 45from its position to a point generally forward of the propellers. This decreased flowvelocity will cause an increase in pressure under the hull, which in turn, causesreduced resistance due to the reduced afterbody suction force (reduce form drag).Wave heights in the near field stern wave system, and far field wave energy, areboth reduced by these devices. Localised flow around the transom, which representslost energy through eddy making, wave breaking, and turbulence, is significantlymodified by the stern flap. The flow exit velocity from the trailing edge of the flap isincreased in comparison with the baseline transom, leading to a lower speed forclean transom flow separation, and again, reduced resistance [5].Secondary effects of the stern flap include the lengthening of the hull, improvedpropeller-hull interactions, and improved propeller performance due to reducedloading, and reduced cavitations tendencies.Figure 1 Stern flap locationSide viewStern flapAngleChord lengthSpanStern viewStern flapJTDIS41A[04].pmd 02/16/2007, 22:5745OMAR YAAKOB, SUHAILI SHAMSUDDIN & KOH KHO KING463.0 SHIP CHARACTERISTICSIn this study, a stern flap was designed, and tested on a fast crew boat model. Thebody plan of the fast crew boat is shown in Figure 2, whilst the main particulars aregiven in Table 2. A scaled model designated MTL 006 was constructed, and testedat the Marine Technology Laboratory, Universiti Teknologi Malaysia. Testing wascarried out in the 120 m × 4 m × 2.5 m towing tank, whose towing carriage can reacha maximum speed of 5 m/s.Figure 2 Body plan of the fast crew boatTable 2 Ship particularsLOA 34.00 mLWL 31.80 mLBP 30.80 mBreadth moulded 7.60 mDepth moulded 3.30 mVolume displacement 126.05 m3LCB (aft of amidship) 2.02 mBlock coefficient 0.43Prismatic coefficient 0.73LCG (from AP) 14.20 mKG 2.49 mModel: Ship scale ratio 1:13.6Full load draft 1.26 m4.0 STERN FLAP DESIGN AND SELECTIONThe general practice with regard to stern flap design is to conduct model experimentsthat optimise the selected stern flap geometry with regard to the selected span,JTDIS41A[04].pmd 02/16/2007, 22:5746STERN FLAP FOR RESISTANCE REDUCTION OF PLANING HULL CRAFT 47chord length, and flap angle. Past experience with model testing stern flaps of variousspans has indicated that the maximum span of the flap should be one that avoids theregion of turbulent flow, which typically originates from the transom corner [3].Computational fluid dynamics (CFD) efforts have been invaluable with regard tounderstanding the hydrodynamic mechanisms responsible for the improvedperformance with stern flap. However, these calculations are still costly and difficultto perform. The limited level of experience with these calculations precluded theiruse as the sole design tool without the benefit of model test. Thus, the decision wasmade to conduct model tests and rely on engineering experience to generate severalstern flap designs for testing. Tests were conducted at the full load conditions of129.2 tonnes, and LCG = 14.2 m from AP.Five model stern flaps were designed and manufactured. The geometry of thesestern flaps are presented in Table 3. These stern flaps were designed as severaldifferent series to systematically investigate variations in flap chord length, span, andangle of attack. The first series, comprised of Flaps 1 and 2, were designed to investigatevariations in flap chord length, while holding the angle and span constant at 13degrees, and 250 mm respectively. This span was judged as the maximum reasonablewidth possible, without impinging on the high-speed wake off the transom corners.A second series, comprising of Flaps 3 and 4, was designed to investigate variationsin span, while having chord length at 35.1 and 46.8 mm. The last flap is the samegeometry as Flap 4 but in a different flap angle.5.0 RESULTSSummary of model-scale results for stern flap series are shown in Table 4. Theresistance performance for each flap portrayed as a model total resistance versusship speed are shown in Figure 3.The performance of Flap 1 shows a reduction on model resistance at the first twospeeds tested beyond which resistance is higher than the resistance of the bare hull.Flaps 2 and 3’s performance were poorer than Flap 1, although Flap 3 is slightlybetter than Flap 2. Flap 4’s performance showed a slight reduction on resistance forthe first two speeds tested. However, similar as the other flaps, the resistance increasedfrom the third speed.Table 3 Geometry of model-scale stern flapsFlap Chord length (mm) Span (mm) Angle (°)1 35.1 250 132 46.8 250 133 46.8 200 154 35.1 200 135 35.1 200 0JTDIS41A[04].pmd 02/16/2007, 22:5747OMAR YAAKOB, SUHAILI SHAMSUDDIN & KOH KHO KING48The results from the four tests (Flap 1 to 4) have indicated that, while changingflap geometry does affect the resistance of the boat, the resistance value is still muchhigher than the bare hull (without flap) resistance, especially at high speeds. Therefore,an attempt has been made to investigate the effect of changing the flap angle ofFlap 4 to zero degree. The results obtained from this zero degree angle flap,designated as  Flap 5, showed a significant reduction on resistance at all speedtested. The performance shows a large resistance reduction attained at speeds of 23to 27 knots.Table 4 Summary of model-scale stern flaps resistance performanceShip speed Model total resistance (N)(knots) Bare hull Flap 1 Flap 2 Flap 3 Flap 4 Flap 520 45.5435 44.3679 46.0909 45.8349 44.5364 43.042323 54.3901 53.9700 55.8062 56.5000 53.8460 50.398827 62.3015 69.2977 81.1866 73.3873 67.1354 58.702030 66.9531 84.6019 – 88.4957 77.0137 65.337632 70.5636 – – – – 69.3357Figure 3 Resistance performances for flap selection404550556065707580859019 20 21 22 23 24 25 26 27 28 29 30 31 32 33RTM (N)Flap 1Flap 5Bare HullFlap 3Flap 2Flap 4VS  (knots)RTM (N)Flap Flap 3Flap Bare hulll  Flap 1VS (knots)JTDIS41A[04].pmd 02/16/2007, 22:5748STERN FLAP FOR RESISTANCE REDUCTION OF PLANING HULL CRAFT 496.0 DISCUSSIONS6.1 Performance of Flap 5Flap 5’s results, as shown in Figure 3, indicated a decrease in resistance at all speedstested with the average decrease in resistance of approximately 4.5 percent. Themaximum resistance reduction due to Flap 5 was predicted to be 7.2 percent at aspeed of 23 knots. In terms of full scale effective power and speed, the flap caused amaximum decrease effective power of 8.2 percent at 23 knots, and increased about0.88 knots at design speed (30 knots). The full scale effective power performances ofthe boat are shown in Table 5, and Figure 4. It should be noted here that this resultwas not the optimum reduction that can be achieved by using stern flap. Furtherworks need to be carried out to investigate effects of variations of flap parametersaround Flap 5.6.2 Stern Flap Scale EffectWhile reduction on resistance are indicated by these MTL 006 stern flap experiments,the actual full scale stern flap on the Fast Crew Boat would generally be expected toexceed the performance indicated on the model. Based on past experiences, theactual performances of full-scale prototype stern flaps have been found to exceedthat of their model scale predictions [4-6]. It was determined through comparisons ofpredicted resistance at three different model-scale ratios that stern flap performancedid improved as model size was increased [3]. Ship trial indicated that the experimentalmodel tend to generally under-predict the stern flap performance in the range ofroughly 2 percent to as much as 12 percent, with the greatest performancediscrepancies at the lower end of the speed range [1, 3-6].6.3 LimitationsIt was observed that running trim is very important on evaluating stern flaps effecton planing hull craft. It is desirable to get running trim on the tests. However, it wasvery disappointing when a technical failure occurred during the tests which werecarried on. The data acquisition system on the towing carriage could not read outthe running trim of the model. Based on that, this report did not include the effect ofthe ship’s trim on its resistance performance.Time limitation also means that full calm condition could not be obtained. Thespeed ranges of these tests are very high that is between three to four meter persecond on the towing tank. This range of speeds created big waves on the towingtank, and resulted in a very long time to wait for the water to calm again. Thatsituation may have a small effect on the results attained (added resistance).JTDIS41A[04].pmd 02/16/2007, 22:5749OMAR YAAKOB, SUHAILI SHAMSUDDIN & KOH KHO KING507.0 TRANSOM FLOW OBSERVATIONExcessive wave height, eddy making, and turbulence, represent lost energy in thelocal transom flow of a vessel. A great deal of qualitative information can be obtainedabout the performance of a stern flap by careful observations of its effects on theflow past the transom, and the localised waves generated at the transom. Transomflow can be categorised by three simplified descriptions. At low speeds, the transomand flap are fully wetted, and the flow is said to be “attached”. Resistance is increasedby the “base drag” of the immersed transom, and significant eddy making. As speedincreases, the transom becomes less submerged, and less water tends to flow backover the flap. Over a small speed range, the stern flow becomes “transitional”,periodically breaking free of the transom, and flap, then rolls forward to wet themagain. At a yet higher speed, the flow detaches cleanly or “breaks-away” from theTable 5 Full scale effective power performancesRTS (kN) PE (kW)Vs (knots) Bare hull Flap 5 Bare hull Flap 520 102.01 95.72 1049.50 984.7723 120.56 110.73 1426.42 1310.0827 135.43 126.38 1881.00 1755.2430 142.79 138.72 2203.49 2140.7832 148.78 145.69 2448.97 2398.13Figure 4 Full scale effective power with and without Flap 5’s performance05001000150020002500300019 20 21 22 23 24 25 26 27 28 29 30 31 32Bare HullFlap 5PE (kW)Bar  h lllap 5Vs (knots)JTDIS41A[04].pmd 02/16/2007, 22:5750STERN FLAP FOR RESISTANCE REDUCTION OF PLANING HULL CRAFT 51bottom edge of the transom or flap. The speed at which this detachment occurs isaffected by factors, which include ship displacement, ship trim, transom design anddepth of submergence, and the specific design of the transom and the stern flap.The effect of the stern flap on the localised flow around the transom was observed.Photographs comparing the transom flow, with and without the stern flap installed,taken at full load condition at 23 knots, are shown in Figure 5. Referring to thecomparison photographs at 23 knots, the baseline still exhibits attached flow, whilethe stern Flap 5 exhibits detached flow. This shows that stern Flap 5 caused thetransom flow to detach (break-away) early.8.0 CONCLUDING REMARKSModel testing results have shown that attachment of the stern flaps modifies hullresistance. In one particular stern flap design, at zero degree angle, substantialreduction in resistance was obtained. The model resistance predictions indicate adecrease in resistance due to stern Flap 5 of an average of 4.5 percent. The maximumstern flap resistance reduction attain is 7.2 percent. The stern flap model-scaleparameters to attained those results are; chord length = 35.1 mm, flap angle = 0degree, and flap span = 200 mm. The summary of these results is shown in Table 6.It should to be kept in mind that the results attained is still not the optimumperformance that can be attained by attachment of stern flap to the fast crew boat.Further analysis is still required before an optimum design can be determined.The performance benefits of decreased resistance or improvements on powerrequirements can be expressed directly in terms of reduced fuel consumption, andincreased speed and range. The capability to maintain ship speed with less deliveredpower, and lower shaft speed, may also reduce maintenance costs, and extend theservice life of the propulsion plant machinery. Beneficial ship characteristics such asFigure 5 Model-scale transom flow comparison, bare hull(upper) and with stern Flap 5 (lower), full load, 23 knotsJTDIS41A[04].pmd 02/16/2007, 22:5751OMAR YAAKOB, SUHAILI SHAMSUDDIN & KOH KHO KING52reduced propeller loading, cavitation, vibration, or noise tendencies are alsoachievable, as a result of the decreased power levels.9.0 ACKNOWLEDGEMENTThe authors gratefully acknowledge the assistance provided by the staff of MarineTechnology Laboratory, in carrying out the experiments.REFERENCES[1] Hoyle, J. W., B. H. Cheng, B. Johnson, and B. Nehrling. 1986. A Bulbous Bow Design Methodology forHigh-Speed Ships. SNAME Trans. 94: 31-56.[2] Schneekluth, H., and V. Bertram. 1998. Ship Design for Efficiency and Economy. Second Edition. GreatBritain: Butterworth-Heinemann.[3] Karafiath, G., D. S. Cusanelli, and C. W. Lin. 1999. Stern Wedges and Stern Flaps for Improved Powering– U.S. Navy Experience. SNAME Annual Meeting. Baltimore.[4] Cave, W. L., and D. S. Cusanelli. 1993. Effect of Stern Flaps on Powering Performance of the FFG-7Class. Marine Technology. 30(1): 39-50.[5] Cusanelli, D. S., S. D. Jessup, and S. Gowing. 1999. Exploring Hydrodynamic Enhancements to the USSARLEIGH BURKE (DDG 51). FAST’99, Fifth International Conference on Fast Sea Transportation, Seattle.[6] Karafiath, G., D. S. Cusanelli, S. D. Jessup, and C. D. Barry. 2002. Hydrodynamic Efficiency Improvementsto the USCG 110 Ft. WPB ISLAND Class Patrol Boats. http://www.sname.org/AM2001/paper10.pdf, 12.50a.m., 15.06.2002.[7] Karafiath, G., and S. C. Fisher. 1987. The Effect of Stern Wedges on the Powering Performance. NavalEngineer Journal. 99: 27-38.[8] Lattore, R., A. Miller, and R. Philips. 2002. Microbubble Resistance Reduction for High-Speed Craft. http://www.sname.orgAm2002/papers/paper11/pdf, 9.35 p.m., 03.07.2002.[9] Kato, H. Skin Friction Reduction by Microbubbles. http://www.turbulence-control. gr.jp/PDF/symposium/FY1999/Kato.pdf, 3.45 p.m., 17.06.2002.[10] Sugiyama, K., T. Kawamura, S. Takagi, and Y. Matsumoto.  Numerical Simulations on Drag ReductionMechanism by Microbubbles. http://www.turbulence-control. gr.jp/PDF/symposium/FY2001/Sugiyama.pdf.3.50 p.m. 17.06.2002.[11] Choi, K. S. 1991. Drag Reduction by Riblets for Marine Applications. RINA Trans. 133: 269-282.[12] Canham, H. J. S., J. P. Catchpole, and R. F. Long. 1971. Boundary Layer Additives to Reduce ShipResistance. RINA Trans. 113: 187-213.Table 6 Flap 5’s performance summariesChord length 35.1 mmSpan 200 mmFlap angle 0°Average resistance reduction 4.50%Maximum resistance reduction 7.2% @ 23 knotsAverage delivered power reduction 5.40%Maximum delivered power reduction 8.50%Increase of speed atDesign speed (30 knots) 0.95 knotsJTDIS41A[04].pmd 02/16/2007, 22:5752

Stern Flap for resistance reduction of planing hull craft: a case study with a fast crew boat model

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Stern Flap for resistance reduction of planing hull craft: a case study with a fast crew boat model

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