An expenmental research of control nidders on tug manoeuvring

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1 TCHNISCHE WVR&TET Laboratorlum voor $choepshydromechanjca kchlef eko1weg CD Deift,a eieae* -, oie-iaia An expenmental research of control nidders on tug manoeuvring E. Cueto, J.J. Achutegui, S. Mendiola, G. Gutierrez University of Cantabria, Santander, Spain Abstract We haveaccomplished. in an operating tugboat, a series of'manoeuvres in order to determine new relationships between the turning circle diameter, the revolutions of the propeller the torque in the rudder stock and the most significant stresses The tests have been accomplished with two configurations, the first one with a steering system composed by rotating nozzle and a fixed rudder blade, and the second with a fixed nozzle with two coordinated helm blades. I Introduction The discovery of the Kort nozzle in for the propulsion of tugs where traction is'the main task, was consolidated in the 1970s, after tests showed an increase in the bollard pull of 20% to 35%, depending on their design. Such an increase led to the gradual but wide installation of fixed nozzles. There were two practical results: The first one was positive because the pull increased but not so the second since the manoeuvring capacity was clearly limited. This limitation of the manoeuvring capacity produced by this system and the scarce effects of a conventional helm with a fixed nozzle led to different systems seeking to improve the rnanoeuvring capacity. Several solutions were tried to gain manoeuvring power. One of them was to install a transverse propeller in the bow, or several other solutions in the stern. such as active rudders flap rudders,.rotary cylinders, nozzle helms, Shilling helms. Tow Master, Becker and Rudder Coordinators. The most widely spread solution to the manoeuvring problems with, a fixed nozzle in new tugs was to remove it arid to fit a steering nozzle.

2 In the last years the manoeuvring problem has worsened due to the reduced space left by crowded docks. In Spain. several in-port tugs haie. been fitted with either fixed or steering- nozzles together with a couple Of coordinated rudder blades. Such solution provides a greatly increased manoeuvring power at a low cost. This paper contains a short epon of the tests of the manoeuvres of a real tugboat. 24 meters long between perpendiculars and with a power of 2050 bhp. The tests have been-carried out with two types of steering arrangements. One composed by a turning nozzle with a fixed fin. The other consists of the same turning nozzle with two coordinated rudder fins. During the trials, turning circles were measured with a Global Positioning System.. GPS. and the strains in the rudder and flap stock by means of strajn gauges. - 2 Experimental methodology I Within this conceptual framework we investigated two different types of parameters: ThOse related with the shape and size.of the turning circle. - The strain deformations in the rudder stock. The results obtained have been arranged according to both types of steering devices above described: I. Turning nozzle with fixed rudder fin.. 2. Turning nozzle with two coordinated rudder fins. The tugboat manoeuvring capacity was tested in several series of six turning circles-at sea. The first series of the tests were accomplished in Pasajes a.nd. the second in Santander. Each series of tests was accomplished in the order below specified:. Engine rpm 360. rudder hard-a-port Engine rpm 360, rudder hard-a-starboard.. Engine rpm 600. rudder hard-a-starboard. Engine rpm rudder hard-a-port. Engine rpm 760, rudder hard-a-starboard. Engine rpm 76Q, rudder hard-a-port. 3 Equipment The equipment used in the trials was as follows: - A real 24 rn tugboat. -. A steering system with rotating nozzle, fixed fin. - A steering system with rotating nozzle, two coordinated rudder fins. - A topographic total station. - Experimental strain analysis gauges.

3 Marine Technology..- and Transportation 505 Maui characteristics of the tugboat Lengthoverall 26,SOm. Length between perpendiculars 24,00 m. Beam at design waterline 7,90 In. Maximum draught.::: 4,30 m. Normal service draught 4,00 m. Displacement at normal service draught 305 Mt. Bollard poll 32,00 Mt. Main engine 160 bhp. Speed kt. Steering system Rudder torque. 9000m.kN Time for the rudder to go from one side to the other 12 S Nozzle diameter 2136 mm2 Area of the fin fixed to the nozzle 0,93. rn2 Attack angle of the turning nozzle.35 Coordinated rudders system (CT) Area of the first rudder.fin 0,42 rn2 Area of the second rudder fin 0.92 m2 Attack angle of the nozzle and first rudder fin 35 Attack angle of the second rudder fin 350 With this steering system, when the nozzle rotates 35. the fin rotates the same angle. triggered and synchronized by the rudder coordinator (CT). as shown in Figure 1. Topographic equipment Global Positioning System GPS Compact Station system Geometer. 400 Time between measurements... 0,4 Maximum length 3100 m Standard recording book RS - 232C communication with a compatible PC Strain measurement instrumentation Technical data of extensothetric bands: Type FLA-6-11 Nominal resistance Ohms Gauge length 6 mm Gauge thickness.. 0,0125 mm Gauge factor 2,132

4 506 Marine Technology and Transportation Trial results The two tables below show the results of the trials. Table I refers to the conventional system of a rotating nozzle with a fixed rudder blade. These trials were carried out in the approaches to Pasajes with calmed sea and no wind. We show the average values of the turning circle diameter, together with the engine speed. Table 2 shows the results of the second series of trials, after fitting a second coordinated fin to the rudder. as shown in Figure 1. These trials were carried out off Santander, on the 23 of November of Table 1. Average diameters with steering nozzle and fixed fin Engme speed Side Mean diameter Slow ahead Starboard-port 50 metres Half ahead Starboard-port 65 metres Full ahead Starboard-port 80 metres A Figure 1: Coordinated rudders (CT). The coordinated rudder consists of two rudder fins. The first is joined to the rotating nozzle, and the second one joined to the first by means of a hinge. The main parts of the system are: T = Nozzle M = Stock P = Pintle C = Liner

5 B = Arm A = Second blade DI Distance between stock and second fin axis d = Distance regulating second fin turning angle M = Rotating torque Marine Technology and Transportation 507 During the second set of trials. meteorological conditions were excellent, as was the case during the first one in Pasajes. Therefore we can assume that they had no influence on the results. Table 2. Average diameters with steering nozzle and CT Engine speed Side MEdiameter:. Slow ahead Starboard-port 20 metres Half ahead Starboard-port 25 metres Full ahead Starboard-port 35 metres Figure 2 shows the shape and dimensions of the turning circle. The reader can find in Table 3 some additional details about the manoeuvring data and conditions during the trials Figure 2: Turning circle.

6 Table 3: Miscellaneous data of a turning.manoeuvre Engine RPM 360 Steering side Port Attack angle of the nozzle 350 Attack angle of the flap 350 Propeller RPM 120 Tug speed Length Time to change heading 90 Time to change heading lo Tug speed during steady turn 2,28 kt 24,00 rn 18,10 s 39,26 S 0,89 kt Speed loss 70.13% Advance 30,96 m Tactical diameter D1. 40,30 m Nondimensiorial tactical diameter D1/L 1,67 Steady turning diameter Dg 17,80 rn Nondimensional steady turning diameter D/L 0,74 4 Results As can be observed, the manoeuvring capability of the tug has increased two and a half times We should outline that the relationship length / final diafneter is less than one (when originally was of 2). This means that the ship practically turns on the spot. 5 Mathematical processing of data obtained in the trials Final turning diameter It is obtained with the following expression D = final diameter P = Number of revolutions in the propeller shaft D = )'

7 Marine Technology and Transportation 509 The equation above gives acceptable results with propeller speeds of 100 to 300 rpm since, with the engine stopped.. we'd get a turning diameter of 2.88 m which simply can't be true. Relationship between propeller r.pm and the tor4ue in the stock Many equations have been found to calculate the forces on the rudder stock, but we. propose the. following: M = Torque on the stock P = Shaft revolutions M (mkn) = -l p Relationship between propeller r.p.m. and shearing forces in the CT Using the same parameters. the restilting formula is: F = Shear force on the stock F = -37.5l.6P The above expressions are only valid for similar ships with a CT system, in fine weather, with appropriate draught and a clean hull. 6 Conclusions This paper presents some equations to calculate the final ti.ining diameter, the stock torque and the shear forces in the steering system of a tugboat as a function of the propeller revolution& I The turning diameter varies with the propeller rpm. 2 The torque and the shear forces increase with the revolutions. 3 We appreciated a moderate increase in the final diameter with an increase in the shaft revolutions. These equations for a tugboat of 24 meters fitted wjth a coordinated rudder show a considerable improvement of the manoeuvring capacity at low speeds. Acknowledgements.. The authors wish. to show their gratitude to the general manager and staff of Remoiques Unidos, S.A. (RUSA) for their support during the trials. References 1. Book: I. Gutierrez, G. y Pantaleon, M. Apüntes deextensometria y fotoelasticidad. Escuela Tecnica Superior de Ingenieros de Caminos, Canales y Puertos. Universidad de Cantabria. Santander, 1983;

8 510 Marine Technology and Transportation 2. Papers in,joumals: Cueto, E. Coordinacion de Timones en Remolcadores, Rotacion, 288, p.35, 104, Cueto, E. Coordinación de Timones en buques de pesca. Rotacion, 292, p.96, Cueto, E. Coordination of Rudders, n 72, pag. 32, Oct Cueto, E. Las estaciones topograficas totales contribuyen al estudio experimental sobre Ia mariiobrabilidad de los remolcadores, Topografia y Cartografia, ed C.O. de Ingenieros Tecnicos en Topografia, Vol. 11, pp 9-15, Madrid, Octubre de Papers in conference proceedings: Cueto, E. Accion de La Tecnologia del Coordinador de Timones en los Remolcadores. VI Congreso de Ia Marina Civil. Santa Cruz de Tenerife, Spain. noviembre de 1994: "Por Ia Reactivación del Sector Maritimo". Cueto, E. Mendiola, S. Exigencias de Ia Tecnologia del Remolcador Escort. VI Congreso de Ia Marina Civil, Santa Cruz de Tenerife, Spain, noviembre de 1994: "Por Ia Reactivación del Sector Maritimo".