Transverse propellers or"thrusters"as they are also called, are mounted in vessels to provide them with better manoeuvring characteristics, e. g. when the vessels have to come alongside quay or be dynamically positioned.

Arranging the casing with clearance in the channel then helps to ensure that pressure waves from the water that is set in motion by the propeller have as little influence as possible on the hull and cause a minimum of troublesome noise and vibrations, since an influence of this kind can have an undesirable effect on safety in general.

A propeller device of this kind is disclosed, e. g., in US 4 629 432. According to this publication, in the annulus between the casing and the channel, with spacing both in the axial direction and in the tangential direction, there are mounted a number of separate elastic buffer elements, which transmit to the hull the forces and moments that the water exerts on the propeller. These buffer elements create a vibration damping or insulation between the casing and the channel, and may be dampers of an elastomer material or elastic gas containers or cushions. These provide a damping effect, e. g. when the frequency of the vibrations from the propeller device passes the resonance frequency during the start or stop phase of the propeller device.

With this propeller device, moreover, between the casing and the channel there are loosely mounted a number of separate, elastic air containers, the object of which is to displace water in the annulus, replacing it with air, i. e. a highly compressible medium, with the result that the pressure waves from the propeller are transmitted to a lesser extent to the channel and the hull. These air containers have thin walls and are loosely mounted between the casing and the channel. The container walls shall have as little stiffness as possible

and may be replaced by air bubbles in the water in the annulus. The object thereof is not to support the propeller device.

This known device can therefore comprise both robust, elastic vibration dampers that are arranged to transmit forces and moments from the heavy propeller device to the channel and thin-walled cushions and holding devices for these, which makes this mounting arrangement complicated and expensive.

A further design of the Norwegian shipyard"Liaaen Verft as"is also known, where a number of separate, elastic mounting elements are provided to support the propeller device. A central portion of the annulus between the casing and the channel is sealed against the surrounding seawater by means of elastic membranes and filled only with air. On the other hand, those portions of the annulus that are located at its axial ends, however, are filled with water, with the result that this design is not acoustically optimal, since pressure waves from the propeller can easily be propagated to the hull sides via the water.

In the known propeller devices the spring stiffness of the elements supporting the propeller device in the hull are not adjustable. The resonance frequency of the propeller device is therefore constant. If this resonance frequency has a value, which is in a range where the propeller's angular acceleration is low, i. e. the time required to pass the resonance frequency is long, the hull may be exposed to an unnecessarily large vibration load and an unnecessarily large amount of noise may be propagated to the hull from the propeller device before the propeller's rotational frequency has passed the resonance frequency.

Furthermore, with the known propeller devices the propeller's rotational frequency may be varied during operation, e. g. if the propeller does not have variable pitch blades and the propeller is required to exert a variable thrust.

Even though the set rotational frequency of the propeller is different to the resonance frequency, in a case like this the rotational frequency may coincide with, e. g. a harmonic frequency, which is undesirable.

The object of the invention is to provide a mounting arrangement of the above-mentioned type that is not encumbered with the above-mentioned disadvantages.

The invention will now be described in greater detail with reference to the drawing, whose single figure schematically illustrates an embodiment of a propeller device according to the invention.

The figure illustrates a section through a sea-going vessel and along a vertical plane, which comprises the longitudinal axis of a channel extending horizontally and transversally through the vessel.

As illustrated in the figure, in a seagoing vessel hull 2 there is provided a channel 4. The channel 4 preferably has a circular cross section and extends horizontally and transversally through the hull 2. The channel's longitudinal axis is indicated by reference numeral 6.

In the channel 4 there is mounted a propeller device 8 comprising a tubular casing 10, which is preferably circular in cross section and in which is mounted a propeller 12. The propeller's drive shaft 14 is rotatably connected to a bracket 16 and is coaxial with the channel 4. In the bracket is mounted a sensor 20 for measuring the propeller shaft's rotational frequency f.

The bracket 16 is securely connected to the casing 10 and to some extent movably connected to the hull via a flexible sealing device 18. Between the casing 10 and the channel 4 an annulus 22 is formed. The sealing device 18 thus creates a seal between the annulus 22 and the cavity in the hull 2 located near the channel.

In the annulus 22, at each of the ends of the channel there is provided a seal or air container 30 and 32 respectively, which extend round the casing 10.

The air containers 30,32 have a hollow cross section and thin walls of a robust, elastic material. They are therefore arranged to be filled with compressed air and to abut against the casing 10 and the channel 4 simultaneously, thereby sealing the annulus 22, and thus preventing seawater from flowing into it.

Furthermore, at other axial positions of the casing 4, e. g. radially outside the propeller 12, an additional air container of the same type may be provided, which can be arranged to abut against the casing 10 and the channel 4, but which does not need to create a seal between them. In the figure two additional air containers 34,36 are illustrated.

In order to hold the air containers 30,32,34,36 in place between the casing 10 and the channel 4, they may be provided with portions, which project into the annulus 22 and between which the air containers 30,32 can be secured in such a manner that they are not moved axially. Thus a centring of the propeller device 8 in the channel 4 is achieved by means of the air containers 30-36.

In order to remove any water from the annulus 22 after the air containers 30, 32 located at the ends of the channel have been filled with air, thus sealing the annulus from the water surrounding the vessel, at the lower portion of the annulus 22, e. g., there may be provided a pump device 38, which is arranged to pump water from the annulus 22 out into the surrounding water, as indicated by the arrows A. To enable air hereby to flow into the annulus 22, at the upper portion of the annulus there may be provided an inlet pipe with a check valve 40 that permits flow in the direction of the arrow B, but not in the opposite direction. Water can thereby be prevented from flowing into the cavity in the hull when the air containers 30,32 are not filled with compressed air and do not create a seal. The pump device 38 may be provided with a sensor (not shown) for detecting water in the annulus 22 and which on detection thereof can start the pump device.

In order to supply the air containers 30-36 with air, a compressed air source or supply device is provided, comprising an air pump 50 driven by a motor 52. This air pump 50 is connected via pipe 54 with a compressed air supply opening P of electrically controlled valves 60,62,64,66, which are connected to the respective air containers by means of additional pipe 56. For the sake of clarity, in the figure only pipe 54 is shown, connecting the air pump 50 with the valves 62,64, and the additional pipe 56 connecting the valve 62 with the air container 34. It should be understood that the inlet openings P of the remaining air containers 30 and 32 are also connected to the pipe 54. By means of appropriate control of the valves 60-66, air from the air containers 30-36 can flow from the air containers 30-36 to the surrounding, open air via outlet openings R of the valves. The pressure of the air in each of the air containers 30-36 can thus be controlled separately by means of separate control of the valves 60-66.

The air containers 30-36 are connected to respective sensors 70-76 for measuring the pressure of the air therein.

For measuring the level of any vibration of the channel 4 and possibly the casing 10, i. e. the amplitude and the frequency of the vibration thereof, suitable sensors 80 and 82 respectively are connected to the channel 4 and the casing 10, these sensors preferably being accelerometers, which are arranged to emit electrical signals corresponding to values for the measured parameters.

For controlling the pressure of the air in the air containers, the valves may be provided with manual operating bodies. The valves are advantageously magnetic valves.

A computer 90 is advantageously provided, which can receive signals via electric cables from the rotational frequency sensor 20, each of the pressure sensors 70-76 and the vibration sensor 80, which is mounted on the channel 4. The computer is further arranged to calculate the values of suitable pressures of the air in the individual air containers and transmit corresponding signals to each of the valves 60-66, based on the received signals, in order to obtain a minimal transmission of noise and vibration from the propeller device 8 to the hull 2.

The computer 90 may also be arranged to receive signals from the vibration sensor 82 on the casing 10 for monitoring the vibration of the propeller device.

The function of the device according to the invention will be described below.

Before starting up the propeller tdevice 8, e. g. the pressure of the air in the air containers 30-36 and thereby the stiffness thereof may be adjusted in order to obtain a minimal transmission ratio, i. e. a ratio substantially less than 1 between the amplitude of the vibration of the channel 4 and the amplitude of the vibration of the propeller device 8 when the propeller 12 rotates at its normal operational rotational frequency f. In this case it must be ensured that the time interval during which the frequency of the propeller device passes the resonance frequency fn, i. e. where the transmission ratio may be equal to or greater than 1 during a slow passing of this kind, is so small that there is no risk of the occurrence of large resonance amplitudes of the channel.

In order to minimise this risk, however, it is advantageous for the time interval during which there is approximate coincidence between the natural

frequency fand the propeller frequency (i. e. the frequency of the propeller device) to be short. In order to achieve this, the pressure of the air in the containers and thereby the stiffness thereof during a start-up of the propeller device must be adjusted in such a manner that the natural frequency fn of the propeller device and the air containers for elastic mounting thereof has a value corresponding to the propeller device's frequency f in a frequency range where the time-derivative of the propeller frequency, i. e. dfDdt, has as great a positive value as possible. The propeller frequency f is then increased to a value that is greater than the value of the natural frequency En, whereupon the propeller frequency is monitored, possibly continuously, and the pressure of the air in the containers is adjusted so as to keep the natural frequency fn of the propeller device less than the propeller frequency f during operation. It will be appreciated that in the event of a stopping procedure a value may be similarly be selected for the resonance frequency in a frequency range where the time derivative of the propeller frequency has as great a negative value as possible.

In order to avoid harmonic resonance frequencies of the propeller device coinciding with a desired rotational frequency particularly for propellers without variable pitch blades during variation of the propeller's thrust, an advantageous feature of the invention is that the air pressure in the air containers can be altered during the operation of the propeller device. The stiffness of the air containers and the value of the harmonic resonance frequencies can thereby be altered.

Even though the invention has been described in the above-mentioned embodiment in connection with a transverse propeller, it will be understood that it may also be employed with other propeller devices, e. g. with water jet devices. Furthermore, it will be appreciated that the invention may be employed in connection with shrouded propellers that are surrounded by fluids in general. The invention can therefore be employed in connection with, e. g. air cushion vehicles, aircraft, etc.