Stability, propulsion system and rudder evaluation of a riverine support vessel to optimize its operational performance
Stability, propulsion system and rudder evaluation of a riverine support vessel to optimize its operational performance Javier Serrano Tamayo, Naval Mechanical Engineer Naval Academy “Almirante Padilla” Colombian Navy, Office of Education Tel: (57) (1) 2742410 email@example.com Abstract—The present article summarizes the study of thestability and the hull integration with the propulsion system of I. INTRODUCTIONa riverine support vessel, in order to optimize the efficiency ofthe propulsion plant and improve its maneuverability in itsoperations area. The relevance of this study originated from thefact that the vessel originally was a Tug Boat converted into a T he consolidation of the Democratic Security Policy in Colombia requires the highest possible performance of naval vessels, for which the Navy exercises sovereigntymother vessel to transport troops. Other vessels of the same over the navigable rivers of the motherland. Within thetype were used in operations to control public order, due to the organization, the Riverine Brigade has twelve riverine tugslack of methodology in the conversion process the end results built in the 80’s to serve as a tugboat and cattle transportis not optimal. vessel , work done until at the end of 90’s when they were stranded and converted in riverine supported vessels. But To initiate the work, it was necessary to forego a Field here was the extent of repairs, without considering that thereview that permitted to measure the vessel in 3D; change in mission required a different conception of themodeling and executing the estimation of weight and other components of the vessel, particularly the integrationcomponents of the vessel using SWBS (Vessel Work of the hull with the propulsion system.Breakdown Structure), as well as determining the locationof its center of gravity (CG), work performed using GHS The increase of weights, was construed by the shieldingsoftware (General Hydrostatics) and Rhinoceros. made out of steel plates and sand interspersed, suggests that there have been significant variations in displacement and An important evaluation for the criteria of stability was in the vertical position of the CG, so it was necessary toapplied using standards such as DDS-079 USN and study the loading conditions and to evaluate its transverse046CFR170 USCG. Once a study was undergone, the stability with acceptable criteria for such boats, like DDSresistance was predicted using systematic series with the 079-1 NAVSEA, U.S. Navy for intact stability. The mainNAVCAD software, as well as the optimal propeller engine, designed to develop an average of Rated Power 180selection and evaluation in terms of fuel consumption and BHP @ 1800 RPM did not exceed 1500 RPM in its bestoperating autonomy. In regards to the controllability case, indicating it was overloaded or the propeller was notsystem, a study was performed utilizing the current rudder properly projected. In the same term, it was a desire toin which recommendations were formulated in achievement overcome the presented cavitations. Regarding the rudder,of the appropriate one, better located and which absorbed manoeuvrability was being affected by a tactical diameterthe propeller turbulence, as well as state of the art that could be reduced.recommendations to improve maneuverability. The purpose of this document is to summarize the The results turned to a stable vessel and the studies undergone to evaluate the stability as well as toimprovement in the propulsion system efficiency, which show the procedures to obtain an optimal and commercialshowed an increased in speed, cavitations reduction and propeller, and calculation to design a rudder thatlonger range. The applied rudder remarkably improved significantly improved manoeuvrability. This study solvedmaneuverability as well as coursekeeping. a problem of poor performance of a vessel in the river due to a partial adaptation of a towing vessel as a personnel Keywords—Fairing, weight estimation, loading carrier with capacity of a mother vessel. This pattern ofconditions, stability criteria, squat effect, propeller study could also be replicated in the same type of vesselsefficiency, rudder selection, Schilling rudder. presenting similar problems.
2 II. 3D MODELLING OF THE VESSEL shape is passed to the program to define the cut-off to optimize the material, since the steels have a high cost in For the hull measurement was created a table of offsets, the construction process. To change the lines and view thewhich were drawn stations from conspicuous points it applied fairing using Rhinoceros software, which worked hinoceroscontains, which are referenced to an initial origin point that the vessels hull as a series of surfaces. First from susually corresponds to one of the ends of the hull. To that checkpoints were "pulling" those imperfections until giving imperfecteffect it was taken as a reference point (0,0,0) point of the hydrodynamic shape of the hull.intersection of the imaginary tip of the bow with the centerline to the height of the baseline. Most of the vessel,specially the parallel section, keeps a depth of 1.2 m andhas a sharp rise at the bow and slight one at stern. Once the measurements were obtained and theinformation of the hull shape collected, the first table of ,offsets was developed in order to organize information andto take the first record of the shape lines of the stations. s Fig. 2. Checkpoints used to the bow fairing After long hours of iterations station by station and iter point by point, was reached an acceptable and reliable a model in 3D. Table I. Table of offsets format for one station ffsets s The information collected and entered in the producedtable offset format, necessarily had to be supplemented andverified with different available information of the vessel. The first introduction of data was attained with highly assatisfactory results in semi-tunnels shape definition of the tunnels shape, Fig. 3. Final 3D model mstern section and parallel. In the bow the results were lessfavourable given the muddy terrain, which led to a long hichprocess for shape lines refinement, known as fairing fairing. III. FORM COEFFICIENTS AND HYDROSTATIC CURVES Form coefficients are used to show the shape of the hull and provide an estimation of power. According to the basic on power features of the studied vessel is a full forms vessel, as reflected in its main coefficients. coefficients 126.2 m3 CB = = 0.7963 ≈ 0.8 (1) 31.15 m ⋅ 7.28 m ⋅ 0.7 m 4.976 m2 CM = = 0.9765 (2) 7.28 m ⋅ 0.7 m Fig. 1. Reference point for the table of offsets and erence ∇ ∇ C imperfections details at the bow CP = = = B = 0.8168 (3) L ⋅ AM L ⋅ B ⋅ T ⋅ CM CM The fairing of a hull is intended to avoid discontinuities,voids or tipping points that result in concentrating s stress, Once the coefficients were obtained, the hydrostaticgreater resistance and lack of aesthetics in design. In curves were calculated which indicated different values thatshipbuilding, is of great importance, since after the faired pbuilding, affected the vessel’s stability at variable water lines. It is
3customary to calculate the curves with the vessel at flat keel IV. WEIGHT ESTIMATING(no trim), which is often shown in auxiliary curves. Thedrafts range is shown from the minimum possible, when the To study the loading conditions were necessary to knowvessel fully shedding (light weight) to the highest possible, the weight of the vessel and all its components, as well aswith the boat fully loaded. the bending moment respect a reference point. Three conditions were considered for study: lightship, minimum Nowadays, these calculations are made by stability operation condition and full load. But, when the projectsoftware, in general the important factor is to select the began, before using SWBS, some methods for structuremost appropriate software and enter the information weight calculation were studied satisfactory results werecarefully and make sure that the program delivers useful not achieved.results, as well as user friendly and consistent with theparticular form to be integrated. In this case was used GHS Studied method Results(General Hydrostatics), but before obtaining the Benford method Little displacementcorresponding values the tanks of the vessel must be edited Danckwardt method Little L/D ratiointo the model for make the calculations properly. Lamb method Little length Mandel method Ilogical value Gilfillan method Just for bulk carriers Murray method Ilogical value Osorio method Could be use as a reference J.L. García G. method Very little value Table II. Weight estimating methods for main features1 Considering that no method satisfied the accuracy required to determine the weight of the vessel, proceeded to weigh it according to each one of its components. Detailed procedure for weighting and CG estimation is determined in SWBS (Vessel Work Breakdown Structure) which is a Fig. 4. Model including the different tanks. detailed summation of weights designed by the U.S. Navy, which considered the vessel as a set of elements condensed Once this was done, a program was developed to obtain in seven structural groups, detailing all the components ofthe coefficients of form and hydrostatic curves as shown. light ship and dead weight. Group Concept 100 Hull structure 200 Propulsion plant 300 Electric plant 400 Command and Surveillance 500 Auxiliary Systems 600 Outfit and Furnishings 700 Armament M Margins F Loads Table III. SWBS structural groups Fig. 5. Curves of form, trim cero, heel cero. There are basically three types of weight according to our knowledge of them. The first is that which we know its CG with certainty, and their properties. The second is a type of weight in which the weight and centre of gravity are likely known. The majority of the vessels weight is collected. While there is some information available, the weight or CG is not defined, which could cause variations of displacement of the CG and more time and delay in detailed design. The third are margins, which are integral part of weight estimating and are expected to reflect the weight of the vessel or KG at the time of delivery. Fig. 6. Hydrostatic curves, trim cero, heel cero. 1 Taken from MIEZOSO Manuel, “Ecuación del desplazamiento, Peso en Rosca y Peso Muerto”, ETSIN, UPM, Madrid, 1990.
4 The CG location of a combined loaded system, as a s C. Full Load.vessel may be regarded, can be calculated multiplying the The vessel is fully loaded, i.e., the total dead weightweight of each component by the distance from the CG to a plus the light vessel in accordance with the characteristicsreference point (0,0,0), to find moments in the three of its design.coordinates, which makes a final sum and are divided overthe total weight. The CG location is determined when the Once the weights, CG’s and their bending moments fordistance from each of the three planes has been established. the three axes were completely defined, and set the loadingThe importance of its determination is that it exerts a conditions to study. The information was organized to .bending moment on the axes x, y, z that affects the trim, understand the behaviour of the vessel according to theirheel, and KG height of the vessel. Once the process of . weight distribution. For this purpose the loading curves orweighing the vessel’s components finished came to define were developed.the loading conditions. V. LOADING CONDITIONS2 Chapter 096 of the Naval Ships’ Technical Manual(NSTM) which deals with weight and stability define theloading conditions for surface vessels. For th case three thisweight conditions were selected: Light Ship, MinimumOperating Condition and Full load. A. Light Ship. Combine elements of the vessel from the group 100 to ombine700, ready for service in every aspect. While excluding thedead weight must take into account some weights as: fixed eightballast (if applicable), basic spare parts, machinery for Fig. 7. Loading curves for min. operating condition minfluids in minimum levels of operation. B. Minimum Operating Condition. VI. STABILITY CRITERIA The vessel has the least possible stability characteristicsto survive in normal operation. The liquid cargo is included rmal The addition of weights due to the "shield" installed,in such a manner that seeks to keep a good stability and consisting of three steel plates of ¼" with 2 cm of sand in ting ¼trim, but otherwise the components of the dead weight , the middle of them throughout the superstructure of the hemdepend on both the type of vessel and its service. Accurate vessel which added a total weight of 17 tons, required of apercentages for the components of deadwe deadweight are defined initial stability criteria for surface ships using the standardin the next table, which is presented below. This is a critical DDS-079 of the USN and 46CFR Part 170 of used by the heoperation condition, because many tanks are empty and USCG.most often high weights remain constant. Standard DDS-079 begins by establishing a requirement 079 esta Crew Same as full load for spacing between transverse bulkheads and the bow collision bulkhead. To be considered effective, the main Ammunition 1/3 of full load transverse bulkheads must be spaced a minimum distance Provisions and stores 1/3 of full load of 10 feet + 0.03 LBP (length between perpendiculars) Lubricants 1/3 of full load separated. Food and drinking water 2/3 of full load Fuel 1/3 of full load The measure for the vessel case would be: Load 1/3 of full load Since 10 ft = 3,048 m and LBP = 31,15 m, Ballast tanks Empty 3,048 m + 0,03 (31,15 m) = 3,98 m ≈ 4 m Passengers Same as full load Table IV. Percentage of variable loading for minimum ge Moreover, one of the transverse bulkheads can be used operating condition3 as collision bulkhead in order to limit the flooding of the compartment closest to the bow, must be located Note: The above table related only values according to approximately 5% to stern, measured from the forwardthe characteristics of the load of the vessel case. perpendicular (FP).2 The measure for the vessel case would be 5% of the NAVAL SEA SYSTEMS COMMAND, Naval Ships’ Technical Manual. total length is (0,05 x 31,5 m = 1,575 m) and the FP is 0,35Chapter 096, pg. 96-4. 1996.3 m behind 00 station, then, 1,575 + 0,35 = 1,925 m. This can NAVAL SEA SYSTEMS COMMAND, Design Data Sheet 079,Stability for surface ships of US Navy, pg II-11. be compared with the 2D tanks distribution. distribution
5 The value of factor P for service in shallow waters4, defined according the standard as those that have no special risk, such as most rivers, harbours, lakes, etc.., is defined by the following equation: P = 0.028 + ( L 1309) 2 (8) Fig. 8. 2D Tank distribution and bulkhead spacing The value A is the lateral section of the projected vessel As it can be seen; a less spacing of bow and stern peaks, above the water line and the value H to the height from theas recommended by the standard, also the location of the center of area A through the center of the submerged area ormachinery room affects the spacing of the cellar and the about the midpoint of draft.water tank aft. On the other hand, the forward collisionbulkhead is 2.1 meters from the station 00, which is a The value of W corresponds to the displacement forsomewhat higher than the minimum requirement as set by each load condition and the value of T at the lower anglethe standard technique. between 14˚ or half of the freeboard. Furthermore, the standard states warships have to bear Finally, the program was edited and were obtainedthreats and external influences, which can affect stability. satisfactory results in terms of stability, which wereThe main threats are: predictable, considering its high B/T ratio = 6, even in the minimum operating condition. 1. Beam wind combined with rolling. 2. Haevy lifting over one side. 3. Towing forces. 4. People crowding over one side. 5. High speed turning. Table V. GHS run to check stability criteria 6. Top icing. In order to not move the centres of gravity of each tank The first and the last two pose no threat to the vessel caused by the free surface effect, the software calculates aconsidering its characteristics and surroundings. The other total value of heeling moment to artificially alter thethree could represent a risk regarding the safety of the unit, position of CG.which is an appropriate stability measure attained bycomparing the curves of upright arm with the curves ofthreats of heel. Factors to be considered are the static angleof heel with its associated arm upright as well as thedynamic stability reserve. To edit these threats sums were made from the threearms of heeling simultaneously, which are described bymathematical formulation to obtain a most critical arm, thatthe multiplied by the displacement for each load condition;calculate the most critical moments that were imposed onthe GHS program to check the set criteria. Fig. 9. Stability curve for min. operating condition• Heavy lifting over one side HA = W × a × cos θ ∆ (4)• Towing forces VII. BEAM VESSEL ANALYSIS 2 HA = 2 × N × (SHP× D) 3 × S × h × cosθ (38× ∆) (5) GHS also gives structural information provident of the• People crowding over one side vessel by a percentage comparison of the maximum stress HA = (W × a / ∆) cos θ (6) that the vessel will entail riding a trochoidal wave against its tensile strength, as well as providing information for On the other hand, standard 46CFR provides a criterion shearing and bending moments of the beam vessel.about minimum permissible metacentric height, which is Determining the design section modulus, Z, which mustimportant for stability analysis which must be equal to or find continuous material in the middle section, angreater than the following for each load condition: acceptable bending stress σad must be introduced into the PAH 4 GM ≥ (7 ) United States Coast Guard. 46CFR. Subdivision & Stability. Part 170. W tan(T ) Subpart E – Weather Criteria, pg. 85
6equation. Vessels below 61m in length the strength Method studied Resultrequirements are based in locally induced stresses than in Basic formula Very wide rangelongitudinal bending ones. Holtrop method Low BWL/T (2.1-4.0) Oortmerssen method Low BWL/T (1.9-3.4) For practical purposes was created a spreadsheet of the U. Denmark method Low LWL/BWL(5-8), 4.4Moment of Inertia and Section Modulus (SM), adding the USNA YPseries method Characteristics matchstructural components that go along at least 40% of thebeam vessel, which are related to the master frame, Series 60 Round bilgeobtaining a value of 206251.36 cm3, equivalent to 2062.51 Series U. Brit. Columbia Unevenly shapescm2-m SM of the studied vessel . Once the modulus section Series Nordstrom y YP 81 High death sliverand the values of the wave applied were calculated, this Series 64, SSPA, y Dutch Planing hullsdata and the values of modulus of elasticity and tensile Table VI. Systematic series analysisstrength of vessel material were introduced to the program. The series chose was the U.S. Naval Academy. After Finally, the program receives information from a beam that was necessary to complete information for usingvessel with some evenly distributed weights and other NAVCAD in three datasheets: Environment, Hull andlocals, which is subject to hogging and sagging, and is Appendices. All data were taken for the full load conditioncompared in terms of percentage with the characteristics of in which resistance is greater. Some values were required tothe material of construction of the vessel indicating if it will calculate independently, but many of them are calculated bybe able to navigate properly in terms of structural the software.resistance. According the results obtained can be concluded thatmeets the structural requirements satisfactorily, comparingpercentage of maximum stress suffered by the vessel withthe material is no more than 3%. (5) Stress, 1 = 0.0001 T.M./cm2 (1) Weight, 1 = 0.1 T.M./m (4) Shear, 1 = 0.04 T.M. (3) Buoyancy, 1 = 0.02 T.M./m (2) Point weight, 1 = 0.7 T.M Fig. 11. Hull Data in NAVCAD The wetted surface required a special report on GHS for Section Modulus = 2062.51 cm2 - m its importance in the frictional resistance: RF CF = (9 ) 1 ρ SV 2 Fig. 10. Beam vessel longitudinal resistance 2 With regard to the environment in shallow waters such some sectors of Caquetá River, the most significant effect VIII. RESISTANCE on resistance is squat effect. First of all, there is a significant variation in the water flow around the hull as the For this case, the resistance curve was made using water passing under and going sideways faster than in opensystematic series of the software NAVCAD. It has series of waters with a reduction in pressure and a sinking of the bowmodels ran in certificated towing tanks. The engineer or the stern (squat), as well as an increase in trim andability is not just to match the series consistent with the therefore the resistance, determining the maximumvessel, but read and obtain useful information of the results allowable velocity without bottoming.5 Taylor and Tuckpresented. define squat as the change of draft and trim of a vessel that is the result of variations in hydrodynamic pressure on the The most appropriate series is selected according form hull, in its movement at any water depth.6curves, main ratios and Froude number. To make the bestselection were studied the displacement hulls similar with 5 LEWIS Edward, “Principles of Naval Architecture”, The Society ofcharacteristics of ARC Sejerí. Naval Architects and Marine Engineers, 2nd Revision, Vol. II, Ch. V, Section 5, Pg. 42, 1988. 6 HERREROS Miguel, ZAMORA Ricardo y PÉREZ Luis, “El fenómeno squat en áreas de profundidad variable y limitada”. XXXVI Sesiones
7 After entering the characteristics of the hull of the The previous graph shows clearly the peak of thevessel that provide resistance in NAVCAD and studied critical region.how they affect the speed of the vessel, resulting in tresistance curve presented below. To understand better how the change in the river deep o affects the squat effect prediction the software NAVCAD prediction, Resistance Curve was used to compare squat effect at various depths, and 24000 increasing speed step by step, determine the minimum allowable depth to be able to navigate the subcritical llowable 21000 16.2 region. On the other hand will be seen, graphically, the 15.84 drastic increase in the total resistance when the depth 18000 15.48 15.12 decreases to critical levels. 14.76 Resistance (Newtons) 15000 14.4 12000 12.6 9000 10.8 6000 7.2 3000 3.6 0 0 0 2 4 6 8 10 12 14 16 18 Speed (kph) Fig. 12. Vessel case resistance curve This graph shows the increase of resistance as well asthe speed. The vessel will respond readily to low speedswithout significant opposition, but from 14 m the slope ion, mphrises sharply and the vessel will require a much largermachine or an optimal propulsion system that allowsmaximizing the actual configuration. IX. SQUAT EFFECT INFLUENCE Fig. 14. Squat vs. Spe curve at four depths Speed The vessel must be designed for the subsubcritical region incases where there is displacement of ships and the The previous graph shows the squat effect variation assupercritical region in the case of planning hulls. 7. increasing the speed of the vessel for different river depths. The curve corresponding to one meter deep, has the three critical regions and a 0.32 m peak in the critical area that added to the full load draft, 0.87 m, causes the vessel bottoming, being impossible the navigation is in such conditions. The problem of the squat effect and difference in the curves for one and three meters, raising the need to know the minimum depth to avoid this undesirable peak for safe navigation, for which some iterations were made with curves between 1 and 3 m deep. The first iteration resulted in the critical region even show up to 2.5 m deep, but the vessel will bottom with only Fig. 13. Design regions values less than 1.7 m, nearly the double the draft for full load vessel, 0.87 m, which shows the true importance of thetécnicas de Ingeniería Naval. ETSIN Universidad Polítécnica de Madrid, squat phenomenon.Pág. 2, 2000.7 HOFMAN M. and KOZARSKI V. Shallow Water Resistance Charts ForPreliminary Vessel Design. International Shipbuilding Progress. Volume47, Number 449. Pg. 63. 2000.
8 The curves have similar characteristics, but between three to six meters deep curves is a difference of 4000N, which affect the performance spe of the vessel with the speed installed power. 40000 PREDICCIÓN MANACACÍAS-3m.nc4 SEJERÍ SQUAT PREDICTION 3.0 m. nc4 PREDICCIÓN MANACACÍAS-6m.nc4 SEJERÍ SQUAT PREDICTION 3.0 m. nc4 SEJERÍ SQUAT PREDICTION 3.0 m. nc4 PREDICCIÓN MANACACÍAS-9m.nc4 SEJERÍ SQUAT PREDICTION 3.0 m. nc4 PREDICCIÓN MANACACÍAS-12m.nc4 30000 Rtotal N 20000 RT difference of 4000N at max. speed: 8.4 kts 10000 Fig. 15. First iteration between 1.5 – 3 m . The second iteration also presented in all curves thepeak values, in other words, up to 2.9 m deep will be aspontaneous effect of trim, affecting safe navigation of the 0 0 1 2 3 4 5 6 7 8 9 10unit. For that reason only for depths equal to or greater than Vel kts3.0 m, the vessel will sail in a subcritical zone. Fig. 17. Resistance curves for 3 – 12 meters deep Moreover, the following graphics shows the squat effectimportance on resistance of the vessel. In the first one, the . X. SELECTION OF OPTIMAL PROPELLERcurve of 1 m deep shows an important differe difference with thefollowing three, with two meters apart, while it is observed One of the initial important requirements was more speed,a minimum difference of total resistance from 3 to 9 m, six but not changing the current configuration by a largerof difference. motor or gearbox due to budget problems. On the other hand the pronounced slope of the resistance curve from 14 kph away, discard the changing idea and the solution is to work out with reference to the existing diesel engine and gearbox in order to determine th most the appropriate propeller with the help of NAVCAD. Propulsion system technical data of ARC Sejerí: • 01 main diesel engine DD671L, 180 BHP@1800 RPM DD • 01 gearbox Twin Disc DD DD-5091V, reduction ratio: 2.45:1. • 01 three blades fix pitch propeller, B Series, 36”diameter, 32”pitch. With the engine performance curve available to work with NAVCAD, we can combine it with the vessel resistance curve and compare the speed of the machine with the Fig. 16. Resistance curves for 1 – 9 meters deep power supplied to the axis and the maximum speed to achieve by the vessel in its current state as can be seen in state, Second graph is a zoom of the first, rejecting the one econd the next graph.meter deep curve in order to see the difference in resistancewhen the vessel is in the subcritical region.
9 The results show an excess of cavitation as the speed increases, obtaining a value of 11.2%, above the 5% acceptable, according to the criterion applied. According the report the current propeller has three problems: First, the engine is underutilized; its current irst, maximum output is less than 150 HP to 180 HP available ata revolutions from 1200 to 1500 average rpm, which causes carbon in combustion chambers of engines. Second, the engines current propeller has a cavitation problem that is wearing the blades and reducing their thrust. Third, the current maximum speed of the vessel is 8.4 kts, which is likely to increase with an optimal propeller. NAVCAD on the menu of propeller data allows a comparison up to three different propellers and have a selection for optimize the pitch. ep The diameter is optimal due to the semi-tunnel he restrictions in geometry. The first iteration was done for 3 and 4 blades propellers. Fig. 18. Resistance and engine curves comparison This curve shows an abnormal performance of the mainengine, which is working below the nominal speed,reaching only up to 1500 rpm, as was evident in the recordsof the engine. These low revolutions affect the enginesperformance promoting carbon in combustion chambers. In addition, the propeller expanded area ratio (EAR)was obtained involving it in a paper and passing thedrawing to AutoCAD for the necessary calculation. Among other parameters to select is the application of acavitation criterion for which was determined to choose theKeller equation.8 AE (1.3 + 0.3Z )T = +k (10) AO ( p O − p v ) D 2 Fig. 19. Optimum pitch selection for 3 and 4 blade propellers with the same diameter This criterion may be implemented by the software andgives an indication to establish if EAR allows an acceptable The next graph has a grid that shows that lowering the gripressure differential. However, the equation leaves aside he pitch and therefore the value of the P/D, the speed curvevariables that affect cavitation such as the influence of the can be moved to the apex of the curve of the enginewake and blade geometry. performance. These variables are absorbed by the software that The current propeller is 32" pitch and 36" diameter, for heinvolves these factors and calculates an overall percentage a P/D of 0.889. If this value is decreased to obtain an idealof cavitation in the current propeller as well as the other P/D, keeping the same diameter, the software recommendsproposals. a new pitch of 0.5545 m and consequently a P/D of 0.607. To study the laws that govern the propeller behavior it In reporting results also identified a reduction ofis tested with no hull in front which is known as "open cavitation due to pitch change from 8.4% to 3.6%, thuswater". meeting the criteria set, but kept the value of the differential teria pressure between the two sides of the blade, which is within acceptable values.8 LEWIS Edward, “Principles of Naval Architecture”, The Society ofNaval Architects and Marine Engineers, 2nd Revision, Vol. II, Ch. VI,Section 7, Pg. 183, 1988.
10 0.50 BS-3: 0.914x0.555x0.450 BS-3: 0.914x0.546x0.800 GA-3: 0.914x0.503x0.800 0.48 0.46 PropEff 0.44 0.42 0.40 1 2 3 4 5 6 7 8 9 Vel kts Figura 22. Comparative curve between B-Series and GAWN geometry propellers Finally, the designed propeller was consistent to aFig. 20. Optimum and current P/D comparison showing the commercial one, Aquapoise 45 of Teignbridge Propellers performance area at max engine RPM Ltd., found to be a better fit, which presented the highest efficiency and acceptable parameters of cavitation From the reports, analysis concluded that the optimum according the criterion established.propeller is one of three blades with optimum pitch, as canbe seen in the graph below: Once selected the optimum propeller, a comparison against the previous showed a moderate increase in speed 0.50 but a significant reduction of cavitation. BS-3: 0.914x0.813x0.450 BS-3: 0.914x0.555x0.450 Moreover, although in the current configuration the BS-4: 0.914x0.530x0.610 engine is only reaching a maximum of 1500 RPM, is consuming more fuel than if it develops its maximum speed. The results of the runs show a difference of nearly half a gallon per hour for the maximum speed of 15 kph, 0.48 finally running over several days are three days of operation, which is a significant additional cost, avoided by installing an appropriate propeller. PropEff XI. RUDDER SELECTION 0.46 Optimizing the rudder includes three basic aspects: governability, which is the ability to maintain the desired course; maneuverability, defined as the controlled change in the direction of movement; and change of speed, which is the controlled change of speed variation including 0.44 2 3 4 5 6 7 8 9 10 stopping and reversing9. Vel ktsFig. 21. Comparative curve of propellers efficiency at open Two criteria were used to meet the rudder area water requirement. The first, Lamb & Cook, establishes a general rule of 2% of the area product of the length at water line by An additional consideration is geometry comparison the average draft, and to this type of vessels particularizesbetween a GAWN and B-Series propeller. Normally, these by 2.5%. The second is a general formula of Det Norskeapplications use a propeller with a larger EAR, and GAWN Veritas, that criterion is based on the following formula:used to be bigger, slower and may become more efficient.However, the results of the run widely favored B-Series.The reason is that this optimization is just of the propeller, 9 LEWIS Edward, “Principles of Naval Architecture”, The Society ofthe motor and the gear box remained the same. Naval Architects and Marine Engineers, 2nd Revision, Vol. III, Ch. IX, Section 1, Pg. 191, 1988.
11 T × LBP B 2 AR = 1 + 25 (11) 100 LBP Replacing with known data of ARC Sejerí plus thebarge that pushes in certain operations: 0.75 m × ( 26 + 26) m 5.5 2 2 AR = 1 + 25 = 0.5 m 100 26 + 26 Therefore, 0.5 m2 is the minimum required rudder area. areaHowever is required a little increasing for stability indirection (concept of governability), particularly for small ), Fig. 24. Increasing of lift coefficient for different aspectvessels, so the minimum required rudder area of the vessel , ratios at different rudder angles Source: PNA, Vol III. angles.is 0.6 m2. The rudder of ARC Sejerí had a height of 65 cm over a chord of 138 cm for an aspect ratio of 0.49, i.e., outside of However, the reality of ARC Sejerí was different Each different. curves of the previous graph, which gave a clear indication revious graphrudder had a total area of 0.68 m2, and with the two rudders of a defective rudder.configuration a total of 1.36 m2, too much area without abenefit for the vessel, so the vessel can operate with just Adjusting the height of the rudder to the diameter of theone rudder without affecting the area requirement requirement. propeller in order to absorb all its turbulent flow, it was determined a height of 88 cm. On the other hand, as the cm rudder area had been determined at 0.6 m2, the chord was deduced to 68 cm, to a final aspect ratio of 1.3, increased significantly the lift force. The position of the rudder was another critical aspect, since there was much separation between the rudder and propeller. Instead of one-design considerations of rudder is its ease of fabrication, installation, i.e., rudders were hung from the transom, the distance between the start of the rudder, and propeller core was 92 cm. T . This distance must not exceed a propeller radius, in this case, no more than 44 cm, so that the rudder blade can absorb as water flow as possible. Fig. 23. Comparison of one of the two old rudders against . the installed Moreover, the trim towards the bow allowed to show a small portion of the upper rudder above waterline, causing Because of a single rudder could be placed at the semi- a vibration, erosion and wear, and undermines the stability oftunnel middle, behind the propeller, the concept of the direction due to external interference. The practical solution Thighest possible rudder improved its aspect ratio, the higher ra was installing the rudder stock, supported by three bushingsthe greater lift to increase the rudder angle involved. The and nut up at the top to easily absorb the propeller flow andnext graph shows the evolution curves of the lift coefficient is completely protected and water covered.depending on the angle involved with rudders for differentaspect ratios, allowing understanding the conceptimportance.
12Fig. 25. Previous position of two rudders layout vs. current position of one rudder layout Furthermore, a system of two rudders behind onepropeller must be avoided10, unless the requirements ofminimum rudder area require it. This layout generates Fig. 26. Tactical diameter improving using the Schilling acticaleffects of interference between the rudders when the ship is rudder. Source: Japan Hamworthy & Co. .turning, especially with compensate rudders The same rudders.way, the propeller flow is lost amid the rudders, which doesnot take advantage of the turbulent flow from it. Another design factor to consider is the balance ratio ordegree of compensation11, which is given in terms of thecoefficient block, equivalent to 0.8 for ARC Sejerí at fullload condition, so its range of balance ratio should be rbetween 0.265 to 0.27012. This concept refers to the r ratiobetween the blade area ahead of the rudderstock over thetotal blade rudder area, to facilitate turning of the vessel(concept of maneuverability). Once defined the dimensions of the rudder, the mostappropriate profile must be selected. In the Colombian .jungle, the available technology just allowed building flatplate rudders, but in meeting and training with the weldingteam, this process improved significantly with the Fig. 27. Lift coefficient comparison in turns going aheadintroduction of a recent technology rudder building the building, and astern. Source: Japan Hamworthy & Co.Schilling rudder. Considering the advantages of the Schilling rudder andan Just try to explain the features of this rudder, would th in order to make a significant innovation in vesseldeserve another paper, but let us summarize the most maneuverability, the rudder was built according to thisimportant advantages. The rudder major innovation is the i profile. In the tests, the tactical diameter decreased fromstall angle is bigger than 35°, as is used in others, rising to , four to just two lengths, as well as an excellent steering, ,70° without loss of water flow, due to its cross section in it maintaining a steady heading. headithe form of “fish-fin”. The manufacture, like the NACA, is . NACAin one piece, so do not require additional maintenance. Ithas great stability in direction, which benefits fuelconsumption. The lift coefficient is also high when thevessel going astern (concept of speed changing). concept changing The following graphs show the significant difference inthe tactical diameter as well as the higher lift coefficient ofa vessel using Schilling rudder, comparing with a ,conventional NACA and a movable flap rudder.10 LEWIS Edward, “Principles of Naval Architecture”, The Society ofNaval Architects and Marine Engineers, 2nd Revision, Vol. III, Ch. IX,Section 17, Pg. 365, 1988.11 PEREIRA Heber, “Teoría del Buque”, Timones: Teoría y sus efectosevolutivos sobre el buque. Pg. 261, 1984.12 Ibid 9.
13 The difference that can have some benefit of this non- standardized procedure is the installation costs. One square meter of certificated steel installed in a riverine support vessel built by Cotecmar requires 37.5 kg of steel at a cost of $ 40.000 pesos kilogram installed, for a total of $ 1500.000 pesos per square meter. On the other hand, the square meter, with the replacement of sand by injected polyurethane, worth $ 700.000 pesos. However, this difference, slightly more than double, offset certified security costs and the remarkable difference benefit the stability of the vessel. On the other hand, the steel with the sand assembly have additional costs associated with lower load capacity and increased fuel consumption which affects directly the operating costs, thus offsetting the installation costs. XIII. CONCLUSIONS The vessel keeps a good initial stability instead of the Fig. 28. 3D view of Schilling rudder and the built one for almost 18 tons added by the superstructure shielding, ARC Sejerí, according corresponding model. revealed in the stability evaluation, according criteria applied to the calculations, as was expected, considering is high B/T ratio of 6. XII. IDENTIFICATION OF NON-STANDARD PRACTICES The engine installed is operating below its rated speed, that cause carbonization of the combustion chambers, speed Current shielding is made of a combination of three ¼” lost and higher maintenance costs.naval steel plates with two layers of sand of 2 cm thicknessbetween the plates, with a composed specific weight of 222 The propeller installed aboard exhibits relatively goodKg/m2. performance characteristics, however the optimal recommended will reduce cavitation, improve efficiency, Comparing this shielding with the ballistic steel used for and such, will reduce fuel consumption, extending thethe construction of riverine support vessels build by vessel range.Cotecmar is nearly six times less than the weight of thecurrent assembly. This certificated steel is just one 3/16” The reduction of cavitation at an optimal level willballistic steel plate with 50 Rockwell C hardness and a prevent blade erosion, loss of thrust and generation of noisespecific weight of 37.5 Kg/m2. This means that if the and vibration in the hull.calculated weight for shielding, as detailed in SWBS wasalmost 18 tons, with the application of ballistic steel would The rudders of the vessel were inefficient due to its lowbe only 3 tons. aspect ratio (0.43) and a low balance ratio (0.12), as well as its wide distance from the propeller (1.5 times propeller Moreover, the combination of three steel plates in diameter), which leads to design a rudder more efficient andaddition to two sand layers estimates that it could give better placed to improve the maneuverability and generalbetter protection than ballistic steel. However, Cotecmar performance of the vessel .test is made by firing a rifle AK-47, 7.62 calibers at adistance of 15 meters, with satisfactory results, while The type of shielding installed does not affected theRiverine Brigade tests, with the same rifle and distance, the initial stability seriously, but has done some damagebullet never reached the third plate, but did damage even in associated with an uncertified armor, less stability, lessthe second one. This would weaken the structure and cargo capacity and higher fuel consumption.allowing for the passage of moisture into the arena, gainingmore weight, and thus less stability. The lack of an appropriate methodology and an investigative process in the project of vessel conversion Today, the procedure have been improved using lead to a non optimal result, thus economic damage ininjected polyurethane instead of sand which has allowed operating costs, instead of at the beginning there is anbetter results in the ballistic tests, as well as weight apparent saving in repairing costs.reduction.
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