The invention concerns a method for maintenance of a water jet provided in a ship, wherein the water jet comprises a water inlet defined by walls, which inlet is in the form of a closed channel, leading from the sea to a pump housing which contains mechanical components.

The invention also concerns a blocking body for blocking a water inlet or outlet for a water jet provided in a ship, wherein the water inlet is in the form of a closed channel defined by walls, leading from the sea to a pump housing, and the water outlet is in the form of a closed channel defined by walls, leading from the pump housing to the sea.

Water jets are used to a certain extent in the propulsion of ships. A water jet comprises a water inlet which passes water from the ship's underside to a pump which is located in a pump housing and driven by a motor. A water outlet directed backwards from the pump housing produces a backwardly directed water jet which propels the ship forwards.

In known water jets the pump housing is normally located in the area of or immediately below the waterline, inside or aft of the ship's stern. The pump housing is connected to a guide apparatus to give the water jet the most favourable velocity and direction with a view to the propulsion, and a steering apparatus located behind the guide apparatus, to steer the ship by altering the direction of the water jet. The water inlet and the motor are provided inside the ship.

In known water jets the pump housing is designed in one piece, with a forwardly directed axial opening connected to the water inlet, and a backwardly directed axial opening connected to the guide apparatus. During maintenance of these known water jets the ship has to be placed in dry dock or a slip, and the connection between the pump housing and the water inlet is loosened, thus enabling the pump housing with the guide apparatus and the steering apparatus to be lifted away from the ship. This is an expensive and time-consuming operation, which restricts the desire to use water jets.

In known water jets the power transfer from the motor to the water jet is performed with an approximately horizontal shaft, possibly via a gear. In order to limit the space occupied by the shaft, and to limit oscillation and

mounting problems which may arise with a long shaft, it is desirable to place the motor near the impeller, i.e. far back in the ship. The weight of the motor and the water jet strongly influence the position of the ship's centre of gravity, and in many cases will be critical for the design of the ship's supporting structures in the stern area.

The above-mentioned factors associated with the weight of the water jet and the motor become more important as the size of the ship increases, and are consequently another reason for the restrictions on the desire to use water jets.

US 3 030 909 describes a water jet where the pump housing with the impeller are located inside the ship, immediately in front of the ship's sternpost. The steering apparatus is located outside the ship, while the guide apparatus is located in an intermediate position. The ship's centre of gravity is thereby located further forward than with those water jets in which the pump housing is located aft of the ship's stern, but the maintenance of this water jet is cumbersome, and will require the use of a slip or dry dock to dismantle the pump housing.

An object of the invention is to provide a method for maintenance of a water jet and a water jet, which method is not encumbered by the above-mentioned disadvantages.

A further object of the invention is to provide a blocking body for blocking a water inlet or a water outlet for use in the method.

This object is achieved according to the invention with a method and a blocking body of the type mentioned in the introduction, characterized by the features which are specified in the claims.

In a water jet according to the invention the pump housing with an impeller for transfer of kinetic energy to the water are located inside the actual ship. The steering apparatus is located outside the ship as in known water jets, but may be located nearer the ship, since the pump housing with the guide apparatus do not occupy space between the steering apparatus and the ship's stern. As in known water jets the guide apparatus is located between the pump housing and the steering apparatus, and on the basis of practical

adaptations to the individual ship design may be located inside the ship, outside the ship or in an intermediate position.

During maintenance of the water jet according to the invention a blocking body is placed in the water inlet, thus blocking off water from the pump housing. The pump housing is then opened for maintenance.

Preferred embodiments of the invention are indicated in the sub-claims. A particularly preferred embodiment is obtained when the invention is employed together with the invention according to the applicant's simultaneously submitted Norwegian patent application, which is referred to here in its entirety. The invention according to Norwegian patent application no. 963452, where the pump housing is divided into an upper and a lower part, is described briefly in connection with the description of the specific embodiment.

The invention will now be explained in more detail in association with a description of a specific embodiment, and with reference to the drawing, in which: fig. 1 is a side view of a water jet with a blocking body according to the invention, fig. 2 illustrates the water jet with the blocking body viewed from above, fig. 3 is a section of the water jet with a second blocking body according to the invention.

The same reference numerals are used for corresponding parts in all the figures.

Figs. 1 and 2 illustrate a water jet 1 with a pump housing 3. The water jet 1 is envisaged located in the rear part of a ship, near the ship's stern. A channel- shaped water inlet 2, defined by walls 17, passes water from the sea 18 under the ship to the pump housing 3. The pump housing 3 contains not shown mechanical components in the form of an impeller, which supplies kinetic energy to the water and forces the water out in a water outlet 13, and a bearing arrangement 5 which forms the impeller's mounting, and which consists of a bearing housing, a rotating stern tube, a mechanical seal, bearings and other mechanical components. A not shown motor drives the

bearing arrangement 5 and the impeller 6 via a substantially horizontal drive shaft 12.

A guide apparatus 21 is located in the water outlet 13, behind the pump housing 3, to guide the water into a backwardly directed water jet, and a steering apparatus 22 is located behind the guide apparatus 21 to steer the water jet and thus steer the ship. The guide apparatus 21 and the steering apparatus 22 do not form part of the actual invention, and will not be described in more detail.

The pump housing's 3 mechanical components are exposed to wear, thus necessitating the opening of the pump housing in order to gain access to these components. The opening of the pump housing may preferably be performed by the method indicated in the applicant's simultaneously submitted Norwegian patent application no. 963452, where the pump housing 3 is divided into an upper and a lower part. Here the pump housing is opened by lifting away the upper part, thus exposing the mechanical components inside the pump housing 3 for access and maintenance, and possible replacement.

In the ship's normal position in the water the pump housing 3 will typically be located below or aligned with the surface of the water, with the result that there is water in the pump housing when the water jet is out of operation. This hampers access to the pump housing's mechanical components, thus complicating maintenance.

When performing maintenance according to the method according to the invention, a blocking body 30 is placed in the water inlet 2, thus preventing sea water from gaining access to the pump housing 3, whereupon the pump housing is opened for maintenance.

The blocking body is preferably placed in the water inlet 2 via a hatch 19 in one of the water inlet's walls 17.

The method may comprise the additional feature that the part of the ship where the pump housing 3 is located is raised, preferably by ballasting/deballasting of the ship, thus causing the water level in the pump housing 3 to sink. Water remaining in the pump housing can be drained away by known means, such as a drain plug 7 in the bottom of the pump housing.

The part of the ship where the pump housing is located can be raised after th blocking body 30 has been placed in the water inlet 2, or prior to placing the blocking body in the water inlet, which is preferred since it simplifies the placement of the blocking body and reduces the need for draining of the pump housing.

In some designs the water outlet 13 will be closed for the inflow of water from the sea outside the ship by having a section of the water outlet located higher than the surface of the water, and in this case a closing of the water outlet is unnecessary, especially in those cases where the part of the ship where the pump housing is located is raised. In a further preferred embodiment of the invention, however, the water outlet is also closed, thus preventing sea water from gaining access to the pump housing by this route too.

Fig. 3 illustrates the blocking of the water outlet 13 by a blocking body 31. The blocking body 31 is placed in a section 32 of the water outlet with an annular cross section, defined by a centrally placed orifice pin 33 and an orifice ring 34 provided along the circumference.

According to the invention the blocking bodies 30, 31 can be expanded, thus enabling them to press sections of the blocking bodies 30, 31 against walls 17 of the water inlet and walls of the water outlet respectively in the form of the orifice pin 33 and the orifice ring 34. A sealing and holding force is thereby produced which blocks the water inlet 2 and the water outlet 13 respectively.

In a second embodiment of the water outlet, where the orifice pin is withdrawn with the result that it does not form a centrally located section in the water outlet, the water outlet can be blocked by a ball-shaped blocking body.

The design of the blocking bodies and the principle for the expansion are not restricted to any specific design, and a number of designs may be employed.

In a preferred embodiment the blocking bodies 30, 31 are designed with one or more internal, expandable cavities which under the influence of a fluid pressure are expanded to press sections of the blocking body against the walls of the water inlet or the water outlet. The cavities may be located

against the outer edges of the blocking body, with two or more placed across the width, thus providing security in the event of a break in one of the cavities. The fluid may, for example, be hydraulic oil, water or air.

In a preferred embodiment, see figs. 1 and 2, the blocking body 30 for blocking the water inlet 2 is in the form of an elastic bladder which is expanded radially outwards under the influence of an internal fluid pressure. The internal fluid pressure is supplied through a hose 37, which is inserted in the water inlet through the hatch 19.

In a second preferred embodiment, see fig. 3, the blocking body 31 for blocking the annular section 32 of the water outlet 13 is in the form of an elastic ring which under the influence of an internal fluid pressure expands radially inwards and/or outwards, pressing against the orifice pin 33 and the orifice ring 34.

If a section of the water outlet in a second embodiment does not have a centrally located orifice pin, a blocking body 31 for this section of the water outlet can be designed analogously with the blocking body 30 for the water inlet.

Compared with the known water jets mentioned at the beginning, a much simpler maintenance is obtained with the invention, since the maintenance can be carried out within the ship. The maintenance is not as in the known water jets dependent on the ship being laid up in a slip or a dry dock, and consequently it can be performed independently of where the ship is located. In addition to the fact that the maintenance in itself thereby becomes simpler and more reasonably priced, a reduction is also obtained in maintenance times, thus increasing the ship's availability for use and its earning power.

Since the expenses of laying up a ship in a slip or a dry dock are greater, the larger the size of the ship, the invention results in a greater area of application for the water jet.

The location of the water jet's housing inside the ship, which is made a practical possibility by the invention, is favourable for the dimensioning of the ship's supporting structures, and is another reason why the water jet by means of the invention can have a greater area of application.

In the above the invention has been explained with reference to a specific embodiment. It is obvious, however, that a number of variants, for example connected with the design of the blocking elements for the water inlet and th water outlet and the shape of their cross sections, since the shape of the cross section must be adapted to the shape of the water inlet and the water outlet respectively, will be able to be implemented by a person skilled in the art within the framework of the claims.