Abstract. 1 Introduction

Transcription

1 An experimental research of control rudders on tug manoeuvring E. Cueto, J.J. Achutegui, S. Mendiola, G. Gutierrez University ofcantabria, Santander, Spain Abstract We have accomplished, 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. 1 Introduction The discovery of the Kort nozzle in 1933, 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 manoeuvring 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,flaprudders, 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 and to fit a steering nozzle.

2 504 Marine Technology and Transportation In the last years the manoeuvring problem has worsened due to the reduced space left by crowded docks. In Spain, several in-port tugs have 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 report 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 strain gauges. 2 Experimental methodology Within this conceptual framework we investigated two different types of parameters: a) Those related with the shape and size of the turning circle. b) The strain deformations in the rudder stock. The results obtained have been arranged according to both types of steering devices above described: 1. 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 and the second in Santander. Each series of tests was accomplished in the order below specified: I.- Engine rpm 360, rudder hard-a-port. 2.- Engine rpm 360, rudder hard-a-starboard. 3.- Engine rpm 600, rudder hard-a-starboard. 4.- Engine rpm 600, rudder hard-a-port. 5.- Engine rpm 760, rudder hard-a-starboard. 6.- Engine rpm 760, rudder hard-a-port. 3 Equipment The equipment used in the trials was as follows: - A real 24 m tugboat. - A steering system with rotating nozzle,fixedfin. - A steering system with rotating nozzle, two coordinated rudder fins. - A topographic total station. - Experimental strain analysis gauges.

3 Marine Technology and Transportation 505 Main characteristics of the tugboat Length over all 26,80 m. Length between perpendiculars 24,00 m. Beam at design waterline 7,90 m. 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 13,00 kt. Steering system Rudder torque 9000 m.kn Time for the rudder to go from one side to the other 12 s Nozzle diameter 2136 mnr Area of the fin fixed to the nozzle 0,93 nr Attack angle of the turning nozzle 35 Coordinated rudders system (CT) Area of the first rudder fin 0,42 nr Area of the second rudder fin 0,92 nr Attack angle of the nozzle andfirstrudder fin 35 Attack angle of the second rudder fin 35 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 extensometric bands: Type FLA-6-11 Nominal resistance 120 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 1 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, afterfittinga 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 Engine speed Slow ahead Side Mean diameter 50 metres Half ahead Full ahead 65 metres 80 metres 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 thefirstby 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 Dl = Distance between stock and second fin axis d = Distance regulating second fin turning angle MI = Rotating torque Marine Technology and Transportation 507 During the second set of trials, meteorological conditions were excellent, as was the case during thefirstone in Pasajes. Therefore we can assume that they had no influence on the results. Engme speed Slow ahead Half ahead Full ahead Table 2. Average diameters with steering nozzle and CT 1 Side Mean diameter \ 20 metres 25 metres 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.

6 508 Marine Technology and Transportation Table 3: Miscellaneous data of a turning manoeuvre Engine RPM 360 Steering side Port Attack angle of the nozzle 35 Attack angle of the flap 3 5 Propeller RPM 120 Tug speed Length 2,28 kt 24,00 m Time to change heading 90 18,10s Time to change heading 180 Tug speed during steady turn 39,26 s 0,89 kt Speed loss 70,13% Advance Tactical diameter D< 30,96 m 40,30 m Nondimensional tactical diameter D, /L 1,67 Steady turning diameter D^ 17,80 m Nondimensional steady turning diameter Dg/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 diameter 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 =finaldiameter P = Number of revolutions in the propeller shaft D = P

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.p.m. and the torque 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(m-KTV) = f Relationship between propeller r.p.m. and shearing forces in the CT Using the same parameters, the resulting formula is: F = Shear force on the stock F = P 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 turning diameter, the stock torque and the shear forces in the steering system of a tugboat as a function of the propeller revolutions. 1 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 with 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 Remolques Unidos, S.A. (RUSA) for their support during the trials. References u^j\ji\.. 1. Gutierrez, G. y Pantaleon, M. Apuntes de extensometria 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 journals: 1. Cueto, E. Coordinacion de Timones en Remolcadores, Rotacion, 288, p.35, 104, Cueto, E. Coordinacion 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 la maniobrabilidad 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: 1. Cueto, E. Accion de la Tecnologia del Coordinador de Timones en los Remolcadores, VI Congreso de la Marina Civil, Santa Cruz de Tenerife, Spain, noviembre de 1994: "For la Reactivacion del Sector Maritimo". 2. Cueto, E. Mendiola, S. Exigencias de la Tecnologia del Remolcador Escort. VI Congreso de la Marina Civil, Santa Cruz de Tenerife, Spain, noviembre de 1994: "For la Reactivacion del Sector Maritimo".