PROPULSION SYSTEM AND METHOD

Abstract: The present invention relates to a propulsion system for vessels and a method for moving a vessel in ice conditions. The system comprises a drive shaft (3), a propeller (4) attached to the drive shaft, a nozzle (5) surrounding the propeller, the nozzle having a water inlet (10) and a water outlet, and rotatable blades or vanes (6) attached to a portion of the drive shaft which projects outside the water inlet for breaking and/or crushing ice before the ice enters into the nozzle. The point of maximum diameter of the blades or vanes is positioned at an axial distance from the plane of the water inlet which is 0.02 to 0.25 times the diameter of the propeller. The rotatable blades or vanes are also having a diameter which is 0.6 to 0.8 times the diameter of the propeller.

Description: PROPULSION SYSTEM AND METHOD Fi ld of he inventinn This invention relates to vessel propulsion arrangements, and, in particular but not exclusively, to propulsion systems intended for operation in ice-covered waters and/or in ice conditions . Background of the invention Conventionally the movement of a vessel, such as a ship or a ferry, has been provided by a propeller attached to a drive shaft. The drive shaft is rotated by a drive apparatus positioned within the hull of the vessel, and the drive shaft is then lead through the hull such that the propeller extends to the water. The vessels are maneuvered by separate steering gears, such as by rudder gears. At present much attention is being paid to the application of so called azimuth thruster units or azimuthing propulsion units which provide both the vessel propulsion and also the maneuvering. These atzimuthing propulsion units are gaining increasing popularity, and they are applied for many type of vessels, as they have proven to provide many benefits when compared to conventional solutions. They have proven to be especially advantageous when using the vessels in ice conditions . One widely known azimuth thruster unit for ship propulsion and maneuvering in ice is offered by ABB Azipod Oy, the tradename for these being Azipod. These azimuthing units operate in a pulling mode and consist of a streamlined strut and a torpedo-shaped pod containing drive elements and a propeller shaft with a screw propeller mounted on the overhanging part of the shaft (for more details, see e.g. Azipod, Project Guide, Sept. 1995 or FI patent No. 76977 in the name of ABB Azipod Oy) . A shortcoming of the azimuthing unit of the above type is that the screw propeller is not protected against possible damages caused by the ice while the propulsive efficiency of the fixed-pitch propeller is not sufficient in all conditions . A Norwegian company Ugland Offshore provides azimuth thruster unit which operate in a pulling mode and consist of a streamlined strut and a torpedo-shaped pod containing drive elements and a propeller shaft with a controllable- pitch ducted propeller mounted on the overhanging part of the shaft (for more details, see e.g. Brochures on the Fennica and Nordica Icebreakers published by Ugland Offshore, Norway) . The drawback of the above unit is also that the propeller blades are unprotected against the destructive effect of ice. The performance of a vessel operating in heavy ice is also unsatisfactory as it is not advantageous to use a nozzle arrangement surrounding the propeller owing to the tendency of the nozzle inlet to clog with ice blocks which are drawn in to the nozzle by the propeller. This results in a sharp reduction of propeller thrust and an increase in hull vibration. In case of clogging the ship often comes to a standstill state which, among other disadvantages affects of stopping the ship, increases the danger of collision with the following ship moving in the convoy. If the ice is seized between the blades and the nozzle when the ship is moving through hammocky ice, the removal of this by reversing the propeller has proven to be difficult and in many instances impossible. One known improvement is an azimuth thruster for ship propulsion and maneuvering in ice conditions, which has a streamlined strut and a torpedo-shaped pod containing drive elements and propeller shaft with the ducted propeller and particular ice-breaking elements mounted on the overhanging part of the shaft, thus making it possible to break and crush the ice before entering into the nozzle (see Finnish patent No. 91513 A, int. class B63H 5/16) . The drawback of such a unit is that the nozzle inlet is still unprotected against clogging with ice fragments. It is also impossible to throw ice fragments away from the nozzle owing to the relatively small size of the ice-breaking elements when compared to the propeller, and thus to the nozzle, diameter. The unit disclosed by the FI patent 91513 is intended for breaking (crushing) of ice and admitting it through the nozzle, but this operation can be accomplished only for a substantially thin ice in conditions in which comparatively small propellers are used, for instance in propulsive systems used in harbor icebreakers . In heavy ice conditions, such as in the Arctic, this unit is ineffective and unable to throw the larger size ice fragments away from the nozzle, while the smaller size fragments entrained into the nozzle deteriorate the propeller performance. Summary of the invention The general problem lies on the fact that the prior art proposals have not been able to satisfactorily to solve the problem caused by iced conditions. What is needed is a solution for propulsion units which improves the characteristics of a vessel moving in iced conditions. An object of the invention is to provide an improvement to a performance and characteristics of a vessel used in ice conditions by providing a reliable protection of nozzle inlet against clogging of the same with ice fragments and by raising the effectiveness of propulsion in general in ice conditions. A further object is to provide a corresponding improvement for vessels using azimuthing propulsion units or thrusters in heavy ice conditions. This object is attained by specially designed propulsion system comprising ice-breaking elements which are in form of rotatable blades or vanes and attached to a portion of the drive shaft projecting outside the water inlet of a nozzle for breaking and/or crushing ice before the ice enters into the nozzle are designed. The design is such that the point of maximum diameter of the blades or vanes is having an axial distance from the plane of the water inlet which is 0.02 to 0.25 times the diameter of the propeller and the rotatable blades or vanes are having a diameter which is 0.6 to 0.8 times the diameter of the propeller. The inventive method utilizes the above design. According to a preferred solution the blades or vanes are uniformly placed in a circle on the plane perpendicular to the propeller shaft . According to a further embodiment the propulsion unit is formed by an azimuthing propulsion unit. In the following the present invention and the objects and advantages thereof will be described by way of an example with reference to the annexed drawings . Brief description of the, drawings For a better understanding of the present invention and in order to show how the same may be carried into effect reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows, partially in section, an azimuth thruster with ice-breaking elements, Figure 2 shows the results obtained from model tests of the propulsive unit fitted with ice-breaking elements. Detail d description of the drawings The azimuth thruster disclosed by Fig. 1 comprises a streamlined strut or support 1 rotatably mounted relative to the hull of the vessel. A torpedo-shaped pod 2 is attached to the strut 1 and contains drive elements (not shown in the figure) . A propeller drive shaft 3 is connected to the drive elements, and project outside from the pod 2. A screw propeller 4 is mounted on the overhanging part of the shaft 3 and inside a nozzle 5. The nozzle 5 is a hollow, tube like element (the nozzle is sectioned in figure 1) attached to the pod 2 by means of support arms or mounting brackets 7 and has an inlet 10 for the inflowing water and correspondingly an outlet for the outflowing water. The azimuth thruster as a whole is usually fitted in the rear end 8 of a vessel, but the thruster may also be fitted otherwise, such as in the forward end of the vessel. The skilled person is familiar with the above described basic members of an azimuthing propulsion system provided with a nozzle and the possible modifications and variations thereof as well, and these are thus not explained in more detail herein. According to the present invention the ice-breaking elements 6 are in the form of blades or vanes which are fitted on the propeller shaft 3 fore of the screw propeller and the nozzle inlet 10 at a distance of Δ = 0.02-0.25 D p , where D p is the diameter of the propeller 4. The blades or vanes 6 are robustly constructed, i.e. they are made more solid than it is actually necessary for guiding the flow of water, so that they can effectively fulfill also the other basic functions thereof, namely breaking and/or throwing away the ice in front of the nozzle inlet. The inventors discovered that the diameter of the ice- breaking blades and vanes has to be chosen so that they can effectively perform their basic functions: throwing away and breaking/crushing of ice and formation of flow before the nozzle. For this purpose the blade diameter must be 1.5-2 times larger than that of the propeller hub 9. The upper limit of the blade (vane) diameter is, in turn, dictated by the need to avoid much heavier ice loads on the propeller shaft than what is the case when using an open screw propeller (i.e. no nozzle) . In the course of thruster operation the blades (vanes) will have to frequently mill the ice. In this case, ice anti-torque moment will be proportional to the blade diameter to the power 2-2.5 (see e.g. 5 th Lips Propeller Symposium, Drunen, the Netherlands, 19-20 May, 1983) . Therefore, the selection of the size of the ice-breaking elements was considered to be a subject for study which should be conducted by taking into account both characteristics of the propulsion unit and the ship aft lines, and, further, ice navigation conditions. The inventors found that by selecting a blade (vane) diameter (at the maximum diameter point) which is within the range of 0.6-0.8 times the propeller diameter optimal properties can achieved in this sense. Accomplished model test confirmed this discovery. It was found that the ice-breaking blades or vanes 6 must be mounted fore of the nozzle inlet 10 and spaced from the fore edge i.e. the inlet 10 of the nozzle 5. However, with the blades positioned in too close proximity to the nozzle inlet opening 10, ice casting away by the blades will be hindered by drawing in forces of the nozzle. In this case, all ice pieces in way of the nozzle inlet opening will be destroyed by milling which will, in turn, result to an undesired wasting of the shaft rotation energy and excessive loading of the shaft line. However, the blades cannot be mounted at a too great distance in front of the nozzle either since they will then loose their screw/nozzle protection capability. What was discovered in this sense is that the optimum spacing Δ between the blades (vanes) at the point of their maximum diameter and the plane of the nozzle opening is 0.02-0.25 times the diameter of the screw propeller in the shroud. This was also confirmed by the model test. The inventors also found that in most cases it is preferred to position the ice-breaking blades or vanes uniformly in the plane perpendicular to that of the propeller shaft in order to eliminate inertial loads on the shaft line. The final diameter of the ice-breaking blades (vanes) , their number and spacing from the nozzle fore edge for each particular vessel and navigation conditions should be selected on the basis of data obtained from tests in hydrodynamic and ice model basins. Mounting of ice-breaking blades fore of the nozzle leads to a reduction of hydrodynamic efficiency of the propulsion unit. Hence, it was necessary to estimate the degree of the blades (vanes) effect on the hydrodynamic efficiency of the propulsion unit proposed herein. The inventors carried out special comparative hydrodynamic tests of the proposed propeller and of an isolated ""screw-nozzle 1 ' combination. In both cases, the same "screw-nozzle ' ' set was used, and the blades (vanes) were modelled by mounting, at various distances fore of the nozzle of an additional four-blade propeller model having a diameter equal to 0.7 times the diameter of the screw propeller in the nozzle. Using dynamometers, hydrodynamic thrust T B on the shaft, torque Q Σ , nozzle thrust T H were measured, as well as shaft rotation η and propeller speed V. Values of the following dimensionless coefficients were calculated: T + T - total thrust K j _ = R 2 " pn D propeller torque K 0Σ = Qτ pn l D" where p is water density, and D is ducted propeller diameter V relative advance is λ - — , and nD K λ propeller efficiency is η - —— K Q _ 2π Results of the accomplished model tests are shown in Fig. 2. The values of λ are presented on the x-axis and values of κ τ ∑ / K Q∑ an d Ηp on tne y-axis. The curves (1) , (2) , (3) in this plot correspond to the values of K τ∑ , K Q∑ and η p for the standard v scre -nozzle" propulsion unit. The curves (4), (5) and (6) show the values of K τ∑ , K Q∑ and η p , respectively, for the proposed propulsive unit. Thus, it can be seen that the rotating blades/vanes mounted fore of the nozzle do not impair significantly the hydrodynamic efficiency of the propeller as defined in the appended claims when compared to the traditional ""screw- nozzle 11 combination. The operation of an azimuth thruster can be described shortly in the following manner. A rotating screw propeller develops a thrust that drives the vessel . Owing to the nozzle the thrust is additionally increased by 20-25%. Blades and/or vanes dimensioned as stated above and which rotate together with the screw propeller cast away and/or destroy ice and prevent blocking of the nozzle inlet opening . Thus, the invention provides apparatus and a method by which a significant improvement is achieved in the area of propulsion systems. It should, however, be understood that the above description of an example of the invention is not meant to restrict the invention to the specific forms presented in this connection but rather the present invention is meant to cover all modifications, similarities and alternatives which are included in the spirit and scope of the present invention, as defined by the appended claims. For instance, upon reading the above description together with the annexed drawing it will be obvious to the skilled person to use this invention in connection with conventional propulsion units.

Claims: Claims 1. A propulsion system comprising: a drive shaft ; a propeller attached to the drive shaft; a nozzle surrounding the propeller, the nozzle having a water inlet and a water outlet; and rotatable blades or vanes attached to a portion of the drive shaft which projects outside the water inlet for breaking and/or crushing ice before the ice enters into the nozzle, the point of maximum diameter of the blades or vanes having an axial distance from the plane of the water inlet which is 0.02 to 0.25 times the diameter of the propeller, and the rotatable blades or vanes having a diameter which is 0.6 to 0.8 times the diameter of the propeller. 2. A propulsion system in accordance with claim 1, wherein it comprises an azimuthing propulsion unit used both for moving and manoeuvring a vessel, said propulsion unit comprising a support for rotatably connecting the propulsion unit to a vessel and a pod connected to the support and enclosing drive elements for rotating the drive shaft, and the nozzle is fixedly attached to said pod. 3. A propulsion system in accordance with claim 1 or 2 , wherein the blades or vanes are uniformly spaced over the circumference in a plane normal to the propeller shaft. 4. An azimuth thruster for vessel propulsion and manoeuvring under ice conditions, comprising: a support for connecting the thruster to a vessel, a pod enclosing drive elements, a propeller shaft having a propeller and ice-breaking elements mounted on an overhanging part of the shaft protruding out from a nozzle surrounding the propeller, the ice-breaking elements being in the form of blades or vanes secured fore of the propeller and capable of ice breaking and/or crushing, characterized in that the blades or vanes, at their maximum diameter points, are positioned at a distance of 0.02 - 0.25 times the propeller diameter from the inlet of the nozzle, and the diameters of the blades or vanes are selected to be equal to 0.6 - 0.8 times the propeller diameter. 5. A vessel azimuth thruster according to claim 4, characterized in that the blades or vanes of the ice-breaking elements are uniformly spaced over the circumference in a plane normal to the propeller shaft. 6. A method of moving a vessel in ice conditions by means of a propulsion system comprising a drive shaft, a propeller attached to the drive shaft and a nozzle surrounding the propeller, the nozzle having a water inlet and a water outlet, comprising breaking and/or crushing the ice before the ice enters into the nozzle by means of rotatable blades or vanes attached to a portion of the drive shaft which projects outside the water inlet, the blades or vanes being designed such that the point of maximum diameter of the blades or vanes is positioned in an axial distance from the plane of the water inlet which is 0.02 to 0.25 times the diameter of the propeller and that the rotatable blades or vanes have a diameter which is 0.6 to 0.8 times the diameter of the propeller. 7. A method according to claim 6, wherein the vessel is moved and manoeuvred by means of an azimuthing propulsion unit.

Inventors: ALEKSEYEV YURIY NIKOLAEVICH, ANDRYUSHIN ALEXANDRE VLADISLAV, BELYASHOV VALERIY ADAMOVICH, BEZZUBIK OLEG NIKOLAEVICH, PASHIN VALENTIN MIKHAILOVICH, PONOMAREV ANDREI VASILIEVICH, SPIRO VADIM EVGENIEVICH, ALEKSEYEV, YURIY NIKOLAEVICH, ANDRYUSHIN, ALEXANDRE VLADISLAVOVICH, BELYASHOV, VALERIY ADAMOVICH, BEZZUBIK, OLEG NIKOLAEVICH, PASHIN, VALENTIN MIKHAILOVICH, PONOMAREV, ANDREI VASILIEVICH, SPIRO, VADIM EVGENIEVICH
Applicants: ABB AZIPOD OY, ALEKSEYEV YURIY NIKOLAEVICH, ANDRYUSHIN ALEXANDRE VLADISLAV, BELYASHOV VALERIY ADAMOVICH, BEZZUBIK OLEG NIKOLAEVICH, KRYLOV SHIPBUILDING RESEARCH I, PASHIN VALENTIN MIKHAILOVICH, PONOMAREV ANDREI VASILIEVICH, SPIRO VADIM EVGENIEVICH, ALEKSEYEV, YURIY NIKOLAEVICH, ANDRYUSHIN, ALEXANDRE VLADISLAVOVICH, BELYASHOV, VALERIY ADAMOVICH, BEZZUBIK, OLEG NIKOLAEVICH, KRYLOV SHIPBUILDING RESEARCH INSTITUTE, PASHIN, VALENTIN MIKHAILOVICH, PONOMAREV, ANDREI VASILIEVICH, SPIRO, VADIM EVGENIEVICH
Application year: 1998
Main classification: B63H5
IPC codes: B63H5