Resistance Improvements on a Lagoon Boat by Air Lubrication

Resistance Improvements on a Lagoon Boat by Air Lubrication Igor ZOTT I Department of Naval Architecture, Ocean and Environmental Engineering (DINMA) University of Trieste - Via A. Valerio, 10 34127, TRIESTE ITALY Received : May 14. 2005 ABSTRACT Air cavity ships (ACS) are used in calm water areas both on high speed craft and on full form river ships. De air lubrication reduces the frictional resistance and, consequently, the total resistance of the ship. Since 1960' systematic research has been carried out in Russia, at the Krylov Centre Research Institute and, more recently, in Korea, USA, Holland, Denmark, etc.. From these studies it was found that in a ACS the resistance is reduced by 15 30%. A boat widely used in the Lagoon of Venice, for garbage collection (more than hundred boats navigate in the lagoon canals), was investigated to evaluate the resistance after the modification of the lines and the generation of the bottom cavities. A scale model was built at the laboratories of D1NMA Department and a series of experiments was made with the original hull and with the modified hull. The hull lines were modified to generate continuous or separated air cavities. Different solutions with two or four air injection points were used. The extension of the cavity was obtained through the analysis of video recordings taken by a submerged camcorder. The air pressure and flow rate were measured and modified to define the most appropriate solution to obtain a wide cavity extension and resistance reduction. The most interesting results obtained from the tests will be presented and discussed. Keywords Air cavity Ships, Drag Resistance, Air Lubrication. 1. INTRODUCTION One of the main aims of a naval architect is to design new ships having hull lines of minimum resistance. A recently developed technique to reduce the resistance consists in creating an air film along the hull bottom, to reduce the friction drag component. The method to generate and to maintain the air film on the bottom depends very much on the hull lines and on the hull speed; in general the air pressure (pa) is lightly higher than the atmospheric pressure (pa) (Pa/Po = 1.05 1.15), so as to fill the bottom space, without generating lifting pressure forces. The ships having hull lines in which stable air cavities are generated on the bottom are called Air Cavity Ships (ACS). The purpose of the cavity is to reduce the drag (or friction) resistance although, as demonstrated in many experiments, also the residual and other components are reduced. The air injection is used both on high speed planning craft and in displacement hulls; in this second case the drag resistance is the component which is reduced at most. The first systematic researches on air cavity ships have been carried out at Krylov Central Research Institute, at St. Petersburg, since 1960' [1, 2]. Barges and displacement hulls having a flat bottom were investigated first. In 1965 a series of air nozzles have been installed on the bottom of a 3270 tonnes barge; later a river selfpropelled cargo-ship, called Volga-Don, having a 5300 tonnes deadweight was tested at full scale. From these preliminary experiences it was ascertained that the engine power could be reduced by (16 17) % to maintain a speed of 20 km/h. In general, in a displacement ship, the resistance reduction is estimated to range between (15 +18)% In By taking into account the power required to generate and

maintain the air cavity, a net decrease of propulsion power is estimated to be (10 H2) A or slightly more. More recently improvements in resistance performance of slow and high speed ACS craft have been obtained in Korea and other countries [4, 5, 6]. 2. AIR LUBRICATION PRINCIPLES The purpose of reducing the viscous drag by supplying air or gas under the hull bottom was proposed about one hundred years ago. The presence of an air boundary layer on the bottom surface reduces the hull wetted surface which comes into contact with the flow and consequently the frictional resistance is reduced. In high-speed flows a cavity is generated behind an obstacle. If a wedge is located on a flat plate, then a cavity is generated and it separates a part of the flat plate from the water contact. In a displacement ship, in which the corresponding velocities are moderate, a natural cavity does not appear and consequently an artificial cavity must be created, by supplying air or gas behind the wedge (figure 1). Figure 1: The air cavity on an ASC craft. During the motion the air escapes both from the stern and from the sides, whereas to maintain the drag reduction it is necessary to have a stable cavity. For this reason it is necessary to supply the air, to retain the cavity extent. The cavity size depends both on the hull bottom geometry and also on the craft speed. It has been proved that the maximum cavity length depends on the square of the craft speed by: Lc = K V2 [V: rn/s]; [Lc: im] ( 1) The K value suggested in Ill for displacement vessels is 0.34. The air cavity is artificially generated by injecting the air downstream or backward from one or more nozzles placed in the fore part of the bottom cavity. The minimum resistance is obtained by optimizing the cavity extent and reducing the air consumption. Many cavities can be generated to cover the bottom surface, by defining an appropriate geometry and the side edges to contain the bubble. Attention must be paid to avoid the generation of viscous pressure drag, by placing cross barriers which reduce the air supply, but increase the total drag. 2.1 Geometrical properties of an ACS cavity A typical ACS bottom is shown in figure 2 [1, 7]. From this figure some geometrical features can be put in evidence.

Figure 2 : Typical ACS bottom The bow lines of the hull have the same features of the traditional craft. In the bow sections the flow pressure impacts the hull; going abaft the pressure is decreasing and the area is suitable for a cavity creation. The traditional bow lines are maintained also to allow a better seakeeping behaviour at sea. The height of the side containment edges are decreasing abaft Indeed the air pressure is higher near the outflow nozzles and is decreasing going aft. The stern lines closing the cavity promote the air outflow, without generating viscous pressure drag components. The lateral air outflow does not interfere with the central screw propeller flow in water. The geometry of the artificial cavity is affected by: dl) The edge height h (figure 3A). When an artificial air cavity is not generated, the presence of a step increases the resistance of a boat; the resistance increases with deeper steps. When the artificial cavity is generated, faster boats require deeper steps, which generate longer cavities at high speed and reduce the resistance. d2) The bottom steepness a (figure 3B) The bottom steepness a interacts with the cavity length extension. In general positive a values increase the cavity length. At moderate speeds, whenever the cavity length is unable to extend, more cavities can be generated (figure 3C). By so doing, better resistance results can be obtained. 13 A Figures 3 A, B, C : Geometry variations on an artificial cavity bottom.

2.2 Physical properties of an ACS cavity The physical characteristics of an ACS cavity interest the inflated air. They are: The air flow QA In general, when increasing the hull speed, the air flow QA must be augmented, because the same does the cavity length and, consequently. the side and stern air escape. At a fixed speed a maximum value of the air flow QA for which the resistance is minimum and the cavity length is maximum can be found. When increasing this flow rate, neither the resistance, nor the cavity length increase. The air pressure po At QA = constant, an optimum pressure can be found to have the maximum cavity length and the minimum resistance. When increasing this pressure no extra gains are found. The craft speed V As already mentioned, the speed increase extends the cavity; to obtain this result an air flow and a slight pressure increase are generally required. Attention must be paid also to the observation of stability or instability cavity conditions. In some cases the instability phenomena induce the generation of large bubbles, which separate from the cavity, escape from the stern or side walls and produce oscillatory phenomena; the craft resistance consequently increases. 3.1 The tested model 3. THE PRELIMINARY TESTS The model used for the resistance tests was that of a boat used for the garbage transportation in Venice. More than 100 of these units navigate in the lagoon of Venice to carry out this service managed by VESTA Co.. The main characteristics of these ships are given in table 1. Table Principal characteristics of the ship at full load condition. Ship Model Length over all (LOA, m) 11.50 1.725 Length between the perpendiculars (Lpp, m) 10.60 1.590 Length at the waterline (1-wt, m) 11.25 1.688 Breadth (B, m) 2.30 0.345 Depth (D, m) 1.20 0.180 Draught (T, m) 0.765 0.115 Wetted Surface (WS, m2) 37.808 0.8507 Displacement (A, t or N) 15.65 503.34 The model was built in wood, in 1: 6.667 scale and was tested in two loading conditions, that is in full load and in ballast conditions in a speed range between Sand 10 km/h at

full scale. The body plan of the investigated hull is shown in figure 4. The tests have been made at the towing tank (50 x 3.10 x 1.60 m) of the University of Trieste Figure 4: Body plan of The tested model. tn..* bottom (figure 5). in 7- The model was fitted with turbulence stimulators and was tested in resistance, sinkage and trim. Preliminary, the model flow lines were observed by using wool strings placed on the model Figure 5: Bottom wool strings for flow lines visualization... s ;.1.4 The flow lines permit to define the adequate, ` Ct v ( 21..,t 1 1 location of the artificial t t 1 I cavities and the air supply t ( t i J C J I( I'LI? nozzles. It is suggested that the limiting --/, / I.1 r ) ) streamlines to be almost 1 I (' k? parallel to the advancing direction of the model and the flow nearly uniform. The flow visualization has been made by using a submerged camera and lamps. The flow lines displayed by the bow hull in full load condition at the speeds of 9 and 10 km/h are shown in figure 5. 11. LT -,fskes, Figure 5 :, Flow lines at 9 and 10 km/h (full] scale) for the fun load condition.

3.2 Testing methodology and data processing The experiments with ACS models require additional instrumentation for the air flow and pressure measurements. In the preliminary tests an airflow meter, allowing a maximum flow of 8.1 l/min was used. Later the instrumentation and the air accumulator were changed, because larger quantities of air were needed to reduce the resistance. The new airflow meter has a maximum airflow of 100 Umin and is coupled with a pressure gauge with a measuring range 0 ± 2.5 bar. All tests have been made at constant airflow and air pressure. The observations of the bubble extent have been made with a submerged camera and lights; some experiments for airflow visualization have been made using a Plexiglas sheet transparent bottom, but did not give satisfactory results. The bubble visualization is very important to define a correlation between the bubble extent and the model resistance variation. This extent depends very much on the model speed and on the bottom geometry. The resistance calculation at full scale is made according to Froude scaling law. The total resistance is accounted as: RT = RR + RR (2) RR : Residuary component; RI. : Frictional component; In the presence of an air bubble, the frictional component RF changes to RF'. Starting from the assumption that the residuary resistance does not change, RI: can be obtained from the new total resistance subtracting the residuary component. But a very recent research [7] showed that also the wave resistance Rw is reduced by the airflow and Rw is a component of RR. In some circumstances also the air outflow can produce a positive reaction ARa,,, which reduces the boat resistance. Then, the total resistance of an ACS hull can be expressed as: RT = RF' RR - AR w - ARair (3) ARw: Variation in wave resistance; ARa,r: Air outflow impulse; RF' is affected by the extent of the wetted surface in contact with the air or water and consequently depends on the bubble extent. But the bubble extension changes with the speed and other navigation conditions (for example with the vertical or transverse motions of the boat). The presence of such large number of variables, to which it is very difficult to assign accurate values, suggests to compare directly the total model resistance measured in the different hull geometries. For a better analysis of data, dimensionless results will be compared, by dividing the resistance by the model displacement. Similar data presentation has been used also in other comparisons with ACS hulls. 3.3 Hull modifications No experience in designing an ACS hull was previously gained by the Author; similar experiences do not exist at the Venice lagoon shipyards, nor at local designers. For this reason it was decided to define the new ASC hull by trial and error method, by defining and testing a series of geometrical variations, to recognize the physical aspects which characterize the geometrical changes. The research program has been developed according to the following procedure.

3.3.1 Original hull testing The resistance tests have been made in two loading conditions, that is in the full load and in ballast. The resistance, the trim and sinkage have been measured. Then the flow lines were visualized (figure 5) to define the positioning of the artificial cavity. 3.3.2 One cavity on the hull The first cavity was defined along the whole hull, from the section 17.5, by placing two edges along the hull sides and by fairing the bottom elevation with the original bottom lines. The high BIT ratios (BIT = 3.875 in full load; Brr = 9.63 in ballast condition) suggested to place two airflow nozzles, divided by a separator faired to the bottom (figure 6), to avoid the vortex generation due to the airflow impact. Figure 6: Airflow nozzles. At the stern a double faired termination was placed to favour the air escape, without create vortex shedding phenomena The edge height at the bow was 8 mm and 5 mm at the stern. This solution gave partly satisfactory results, but the maximum cavity extension arrived to the section 11 (from the section 17.5) (figure 7), but only at low speeds (5 km/h at full scale) and in ballast condition. In full load, the cavity extension was smaller and more irregular. The airflow was 8.1 Um in and the pressure was 1.04 bar. Figure 7: Maximum cavity extension. Both in the full load and in ballast air flow leakage from the bow sides has been noticed. At higher speeds the bow is risen and the airflow escape reduces the cavity extension; for this reason the cavity extension does not grow by increasing the hull velocity, contrarily to the expression (1).

3.3.3 One cavity and an aft barrier To make stable the cavity generation it was decided to increase to 10 mm the bow artificial cavity height and to 8 mm the stern height. Then the aft cavity termination was closed by a plane barrier, to avoid the airflow escape. Hie bow bubble extension increased, but lateral flow leakage was noticed, also from the stern sides. The bubble extension never arrived to the end of the cavity but, at higher speeds, decreased in its length similarly as in the previous testing conditions. The boat model resistance increased, certainly for the presence of the aft barrier. 3.3.4 Two cavities on the hull To increase the bubble extension on the artificial cavity, two extra nozzles were placed at the section 11, which was the section to which the maximum bubble expansion was noticed in the previous tests. The new nozzles had only a short cover, faired to the bottom. The aft barrier was eliminated and the previous faired terminator was restored. During the tests it was noticed that the new bubbles did not create a wide cavity but the air flowed from the nozzles created a turbulent vortex, without expanding on the bottom cavity. The hull resistance decreased slightly, when compared to the condition 3.3.3. 3.3.5 Increased airflow To extend the bubble on the hull bottom, the airflow was increased from 8.1 I/min to 30 1/min, and the pressure to 1.1 bar. The new rate of air flow was established after having measured the resistance at different speeds with the airflow ranging between 10 l/min and 40 Umin. At the new rate the resistance decreased slightly at the intermediate speeds, but the air vortex concentration coming out from the stern nozzles did not change. 3.3.6 Stern cavity modification To expand the bubble on the bottom a new artificial cavity was created astern, by placing a new faired barrier (figure 8). The resistance results did not show significant improvements, because the air escaped upstream the section 11 from the side walls. This was caused by air flowing downstream from the fore cavity (figure 9a) and the presence of the new barrier for the new cavity generation diverted the air flow outwards. The aft bubble was extended on the aft hull bottom (figure 9b), but the lateral air leakage increased the hull resistance. A new modification is now required, to converge the fore airflow into the new aft cavity, to extend both bubbles on the hull bottom. at TEST RESULTS The results obtained from the tests are shown in figures 10 and 11, respectively for the ballast and the full load conditions. The resistance is represented as a dimensionless parameter, dividing the resistance by the model displacement; this result is then multiplied by WOOL

Figure 8: The boat bottom with two artificial cavities. The results show that the testing conditions 3.3.4, 3.3.5 and 3.3.6 present the more suitable geometrical conditions for the resistance reduction. These conditions are defined by two cavities on the hull, supplied by four air nozzles. The condition 3.3.3 is the worst one and represents a bad choice of the geometrical cavity, characterised by an aft barrier for the air bubble. In tables 2 and 3 the percent resistance variation, referred to the original hull geometry (3.3.1), respectively for the ballast and the full load conditions and for the full scale speeds of 5, 8 and 11 km/h are shown. Figures 9a and 9b: The fore and aft bubbles generated into the artificial cavities at low speed. Table 2: Per cent model resistance variations Ballast condition. V (km/h) Testing Condition 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 5-0.6-13.5-9.02-15.5-12.59 8-7.79 +5.93-21.18-22.6-19.32 11-5.34 +18.32-3.16-3.16-2.19 - Table 3: Per cent model resistance variations Full load condition V (km/h) Testing condition 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 5-29.63-17.6-31.48-24.07-44.4 8 +4.75 +4.15-16.32-13.35-11.28 11-9.82 +3.99-8.08-8.08-8.59

1 hull These tables highlight the general resistance variations obtained by increasing the boat speed. Excluding the condition 3.3.3, a general resistance reduction decrease is noticed at the highest speed. The explanation can be found by looking at the trim variations, which increase with the speed and reduce the artificial cavity extension. In general the cavity generation reduces the trim variations, but does not remove their consequences. Some trim variations have been measured, but not in a systematic way and a general trim reduction with the cavity generation has been noticed. "ti 91.4604Cf COMIAntS0.1i - II Oa. Loot, -.ova a NENE...novo hai a. TWO 4./11 (01 0.1 20-2 taw. toodalsatan 10 4 0.00 040 060 1.20 I 60 000,OACI 0./30/20 V(111/3) V (MIDI Figures 11:0 and 11: Resistance results in ballast and full load conditions I 60 In figures 12 and 13 the resistance variations measured for the ballast and the full load, conditions for the tested cases 3.3.2 and 3.3.4 with and without the air are shown. The geometrical bottom variations increase the resistance of the tested hull without air, when compared with the base condition 3.3.1. The airflow intervenes to reduce the resistance generated by the bottom appendages; this reduction is very efficient so as to decrease the resistance below 'the values measured for the originaloriginal: condition 3.3.1. APR (AFFECT OM THE HULL Footing Condition 1.52 + ballat without al. APR EFFECT OK THE PlUla rating Condition 1.1.4 4. Full toad, without air X *In WWI toad,...ilk Air A Full load flout SW + 10.41.e. without a V - - - Full End..400 X loadel,..1011" 20 10 0.00 0.40 0.00 ii'-"- 11- - V (rn(s) Figures 12 and 13: Air effects on the boat resistance. 1.20!Cy 0.00 V (110/S) 120 5 'CONCLUSIONS The tests made on a scale model of a boat navigating in the lagoon of Venice confirm the positive results previously obtained by other researchers [2, 3, 41. The low pressure airflow generated under the bottom of a displacement hull reduces its resistance. This

- reduction is affected by many parameters, that is the geometrical bottom lines, the trim variation, the hull speed, etc... When the navigation is made in calm water, as it happens in a lagoon, in a river or in a lake, in suitable trim conditions, the resistance reduction can be optimized and the maximum speed gain can be obtained. The energy loss for the air generation is small if compared to the resistance reduction obtained. The tests carried out at the University of Trieste on two boat models (the examined model and a fast boat model) permitted to verify the positive effects of the air lubrication on the boat resistance and to know which are the technical features to respect, to obtain satisfactory resistance reductions. These results will be improved in progressive geometrical variations of the bottom and future tests. 6. REFERENCES Matveev, K.I.: Modeling of vertical plane motion of an air cavity ship in waves. Proc. Fifth international Conference on Fast Sea Transportation (FAST '99), Seattle, Washington, USA, 31 August 1 September 1999, (p. 463-474). Butuzov, A. Sverchkov, A. Poustoshny, A. Chalov, S. : High speed ships on the cavity: scientific base, design peculiarities and perspectives for the Mediterranean Sea. Proc. 5t1 High Speed Marine Vehicles International Symposium, Capri (Italy), 24 26 March 1999. (p. 111.6.1 111.6.12). I3I Latorre, R. : Ship hull drag reduction using bottom air injection. Ocean Engineering, vol. 24, No. 2, 1997, (p. 161 175). Jinho Jang Hyochul Kim Seung-Hee Lee.: Improvement in resistance [41 performance of a barge by air lubrication. Proc. Practical Design of Ships and Mobile Units (PRADS 1999), The Hague, September 1998, The Netherlands, Elsevier Science By., (p. 655 661). [5] Jinho Jang II Jun Ahn Jaesung Kim Jung-Chun Suh Hyochul Kim Seung-Hee Lee Museok Song : A practical application of air lubrication on a small high speed boat. Proc. Practical Design of Ships and Mobile Units (PRADS 2001), Shanghai, China, 16 21 September 2001, (p. 113 118). 1[6] Jinho Jang Hyochul Kim Seung-Hee Lee.: Improvement in resistance performance of a high speed boat with air lubrication. Proc. WEMT 2002 Conference, Castello di Baia, Italy, 18 20 September 2002, (p. 111.1 111.7). Gokcay, S. Inset, M. Odabasi, A.Y.: Revisiting artificial air cavity concept for high speed craft. Ocean Engineering, vol. 31, 2004, (p. 253 267). 1[7]