WO2015096841A1

WO2015096841A1 - Marine propulsion unit

Info

Publication number: WO2015096841A1
Application number: PCT/DK2014/050447
Authority: WO
Inventors: Jens-Hagen Brandt
Original assignee: Hydro Blaster Impeller Aps
Priority date: 2013-12-23
Filing date: 2014-12-23
Publication date: 2015-07-02
Also published as: KR20160102044A; US20160325811A1; EP3086999A1; CN105873818A; JP2017501083A

Abstract

Marine propulsion unit, where said unit comprises a streamlined body, where said body has a front end and a rear end, where in use the front end will be facing upstream and the rear end downstream, where the front end has a larger cross section perpendicular to the intended travelling direction than the rear end, and where the front end is provided with a water inlet opening, where said opening is in communication with an impeller, which impeller rotates around an axis parallel to the intended travelling direction, where said impeller has one or more vanes whereby the water entering the front end is expelled radially away from the impeller's rotating axis, where said expelled water is forced through one or more rectifier nozzles directing said expelled water along the outer surface towards the rear end of the streamlined body.

Description

Marine propulsion unit

Field of the Invention

The present invention is directed to a marine propulsion unit, where said unit comprises a streamlined body, and where the propulsion unit has significantly less energy consumption as compared to traditional/conventional marine propulsion means.

Background of the Invention

In a time where energy resources and environmental issues are on the agenda worldwide, new ideas arise from the refining processes of known marine technologies. In this process the sources of losses originating from parts of marine propulsion systems have been under constant focus and analysis for many years. Improvements in compo- site materials are influencing the development of more efficient marine propulsion technologies.

It is the aim of this proposed new propulsion system to obtain a higher overall propul- sive efficiency by using a new and different active energy transferring unit to transfer mechanical energy to water, combined with a unique passive energy absorbing unit.

Basically there are two major propulsion systems or concepts used commercially. Propulsion by means of a propeller arranged in the aft end of the vessel and water jets, issuing jets of water from the vessels aft end and having water inlets somewhere else on the submerged part of the hull.

The key areas to losses in today's marine propulsion systems are the major, still existing losses of the propeller swirl losses or angular momentum losses which can be up to 30% and the losses in water jet installations stemming from the flush ducted inlet (angled inlet relative to hull side) up to 30% and the skin friction or shear wall stress (boundary layer effects) for both systems.

In this connection swirl losses shall be understood as the energy which is imparted to the water as a propeller churns water, creating an angular rotating volume of water (angular momentum losses) around and behind the propeller. This, volume of agitated water has a high energy loss, energy which could otherwise be used for forward propulsion. The skin friction or shear wall stress is especially pronounced at the inlet of water jet systems. On a wet surface there will be a boundary layer of water close to the surface where the velocity of water is zero, and at the same time the frictional relationship with the surface will require extra energy in order to overcome the skin friction or shear wall stress resulting in an increased energy use.

Object of the Invention

It is therefore an object of the present invention to provide a propulsion system which eliminates the losses of conventional propulsion systems, as well as provides further advantages.

Description of the Invention

The invention addresses this object by providing a marine propulsion unit, where said system comprises a streamlined body, where said body has a front end and a rear end, where in use the front end will be facing upstream and the rear end downstream, where the front end has a larger cross section perpendicular to the intended travelling direction than the rear end, and where the front end or adjacent the front end, is provided a water inlet opening, where said opening is in communication with an impeller, which impeller rotates around an axis parallel to the intended travelling direction, where said impeller has one or more vanes whereby the water entering the inlet open- ing is expelled radially away from the impeller's rotating axis, where said expelled water is forced through one or more nozzles directing said expelled water along the outer surface of the streamlined body.

When the expelled water is forced through one or more nozzles, preferably rectifying nozzles directing the tangential component of said expelled water preferably into a radial direction relative to the axis around which the impeller rotates, and further on along the outer surface of the streamlined body where the curvature of the streamlined body turn the flow in axial direction towards the rear end of the streamlined body this effect is pronounced.

The term rectifying nozzles in this context is to be understood as nozzles which directs, and controls the water being forced through the rectifying nozzles in a predetermined and well defined direction flow. The background for the concept of the present invention's new marine propulsion unit is the fact that modern impeller's from pump technologies when tested isolated can achieve efficiencies well above 90% which when combined with the low total drag coefficients of streamlined bodies provides very efficient propulsion, also when compared with prior art devices.

The components of the new propulsion system each have very low individual friction- al losses which in combination creates less loss, thus reaching a higher overall propul- sive efficiency.

The new propulsion system preferably consists of a streamlined body of revolution at ideally zero incidence, and will in the text below also be referred to as a POD, meaning the entire system.

The thrust from the propulsion system stems from the water reacting on the system. Due to the relative momentum change of the water passing the system, forced by the impellers dynamic mechanism accelerating the water and the change of flow direction of the tangential component into radial direction (again relative to the impellers rota- tion axis) caused by the nozzles as the water flow passes by and along the streamlined body where the flow direction is changed to axial direction, towards the rear end of the streamlined body, caused by the curvature of the surface of the POD.

At least in the present context "a streamlined body of revolution at ideally zero inci- dence" shall be understood as a symmetrical body being designed as close to the theoretical definition, however allowing for minor deviations in order to be able to utilize the body for practical applications. For example, the body is supplied with one or more struts/beams in order to attach the body to a vessel for transferring the propulsive force to said vessel.

The impeller therefore acts as a pump drawing water in from an area in front of the pod, accelerating the water and. expelling it in a controlled manner due to the provision of one or more nozzles along the curved outer surface of the pod. As the impeller rotates, it will create an under-pressure and thereby suck water into the impeller which water due to the rotation force of the impeller will be accelerated and expelled along the impeller's periphery. In this position one or more rectifying nozzles will be arranged substantially circumscribing the impeller. The rectifying noz- zles serve to change the direction of the tangential component of expelled water flow into radial direction and then direct the accelerated water along the body of the pod where the surface curvature of the pod changes the flow direction into an axial direction and an aft flow while the body is being thrusted in the opposite direction. In the embodiment of the invention where only one rectifying nozzle is provided, this nozzle may cover part of the periphery of the impeller or the entire periphery and in this manner create a substantially homogenous water flow substantially evenly distributed over the surface of the pod. Where more nozzles are provided it is foreseen that the nozzles may be individually adjusted such that the accelerated water flowing over the surface of the pod may have different layer thickness, speed and kinetic energy con- tent such that there may be a variation in the momentum change of the water flow along the body of the pod whereby the pod becomes steerable.

It is foreseen that in other embodiments the exterior part of the rectifier nozzles can be diminished or removed when the curve at the beginning of the surface of the body has an even, smooth curvature that alone will change the flow direction from radial towards axial direction.

It is also foreseen that the nozzles may be directed to provide a reverse thrust thereby breaking the pod's advancement and thereby the vessel's advancement on which the pod is mounted.

It is also foreseen that the nozzles may be directed to fine tune the tangential flow component, optimizing the flow direction of the water which is directed into the nozzles into a direction towards the aft of the streamlined body.

In order to create a substantially homogenous thrust from the pod for smooth operation, it is advantageous to have a symmetrical pod. construction such that the thrust created by the change of direction of the water flow across the surface of the pod will be substantially even over the entire pod's surface. In a still further advantageous embodiment of the invention the general cross section through a longitudinal plane of the streamlined body generally has a drop shape, where the rear end tapers towards a pointed end.

This configuration provides a pod with important aspects in that the water flowing across the body of the pod will be provided with the optimum energy dissipation resulting in maximum thrust. Especially having a rear end that tapers will minimize the tendency to create turbulence and a turbulent flow and as such the kinetic energy stored in the flow will be effectively transferred from the rotation of the impeller along the surface of the pod and into the thrust from the tapered (rear) end of the pod.

The pod is naturally also provided with means for attaching the unit to a vessel, where said means are suitable to transfer the thrust (propulsion force) to said vessel. The attachment means shall be of a nature that enables it to transfer the forces generated by the pod and at the same time it will be advantageous to design the attachments with a minimum of flow resistance such that the energy loss is minimized.

For these purposes it may in some embodiments be necessary to deviate from the symmetrical construction of the pod in order to compensate for the influence of the means for attaching the unit to the vessel. A complete combined design of attachment means (i.e. struts and/or beams) and the pod itself will naturally be carried out, in order to minimize the generation of turbulence around the pod or between the pod and the vessel.

In a further advantageous embodiment of the invention the water inlet opening, the impeller and the one or more nozzles are arranged on the surface of the streamlined body, such that expelled water exiting through the nozzles will be guided (directly) along the outer surface of the streamlined body.

By arranging the inlet opening and the impeller on the surface of the streamlined body it is achieved that upstream water is not disturbed before it hits the impeller. At the same time water ejected from the impeller through the nozzles is directed directly onto the surface of the streamlined body whereas in other embodiments where the impeller is arranged in a more protected position the protective arrangements may cause disturbance of the water flow both into the impeller and when exiting the impeller.

When the impeller is not arranged such that the accelerated water is expelled directly along the pod's surface, the surface from the nozzle to the pod's surface, is designed hydraulically correct, i.e. such that a laminar flow is assured.

In a further advantageous embodiment of the invention the nozzle has a circular shape arranged around the impeller on the surface of the streamlined body and the nozzle is separated into sections.

The nozzle serves to control and direct the ater exiting the impeller and as such directs the accelerated water along the streamlined body. By dividing the nozzle into separated sections and being able to control the geometry of each section it is possible to control the water flow along the streamlined body and thereby the propulsion force of the water along different sections of the pod and thereby improve the control of the vessel to which a pod is attached.

In a further, very important embodiment the vanes arranged on the impeller all are spaced at a radial distance from the rotation center of the impeller, leaving a central area of the impeller as a vane-free surface, said surface having a plane substantially perpendicular to the impeller's rotation axis. The vane-free surface may be conically shaped pointing in the direction of travel or be provided with another suitable contour. By providing an open section in the central part of the impeller the impeller will not cause any swivels or turbulence and as such there will be no energy loss in the water entering the impeller providing the impeller with a much better ratio between energy used to power the impeller and energy transferred to the water leaving the impeller. In comparison to a traditional ship screw there is a substantial energy loss in a screw propeller due to the fact that the screw is normally arranged on an axle forward of the screw and that typically half of the angular momentum loss (loss close to the axle, loss at the tip of the blades and loss due to a water volume being dragged around by the propeller) will be created before the water arrives at the screw blades being imparted with propulsion energy by the blades. By arranging an open surface in the center of the impeller this angular momentum loss is also eliminated or at least greatly reduced. The invention is also directed at a method of providing propulsion wherein a propulsion unit as discussed above is arranged on a submerged section of a marine vessel where water is channeled into the propulsion unit's inlet opening, and the water is accelerated when coming into contact with the vanes arranged on a rotating impeller, such that the water is expelled through nozzles from the impeller along the surface of the propulsion unit, such that the accelerated water on a front section of the propulsion : unit creates an under-pressure and along the rear part of the unit provides a thrust, providing the propulsion.

The advantages achieved by implementing the propulsion unit are corresponding to the advantages discussed above.

Because the risk of damaging cavitation in the propulsion system is minimized as the velocity of the vessel and propulsion unit is increasing, this embodiment is favored as a high speed propulsion system, where the inception of cavitation is controllable and higher vessel velocities will be obtainable, provided more power is added.

In this context also another advantageous embodiment of the invention wherein the vanes arranged on the impeller can be oriented or adjusted relative to a radial direction is important. By being able to adjust the vanes on the impeller, the optimal angle of attack between the water-flow and the vanes can be achieved at substantially any combination of speed, i.e. rotation speed of impeller, vessel speed through water etc.

Description of the Drawing

The invention will now be explained with reference to the accompanying drawing wherein figure 1 illustrates a marine propulsion unit according to an embodiment of the invention figure 2 illustrates a water inlet opening 13 in which opening an impeller is arranged

figure 3 illustrates an embodiment where the nozzles are arranged in a nozzle member

figure 4 illustrates a computer simulation of a traditional screw attached to an axle which is brought to rotate

figure 5 A unit travelling through the water

figure 6 illustrates an embodiment where the body's surface has dimples and a drive shaft extends in the travelling direction. Detailed Description of the Invention

In figure 1 is illustrated a marine propulsion unit 1 according to an embodiment of the invention. The propulsion unit 1 comprises a streamlined body 10 which body 10 has a front end 11 and a rear end 12. In use the front end 11 will be facing upstream, i.e. will be arranged in the travelling direction of the vessel to which the unit is attached. The front end 11 has a larger cross section perpendicular to the travelling direction than the rear end 12.

Furthermore, in the front end 11 is provided a water inlet opening 13 in which opening an impeller, see figure 2, is arranged. The impeller rotates around the axis 14 thereby accelerating the water entering through the inlet opening 13 and at the same time changing the direction of the water flow to a direction substantially radial to the rotation axis. The water flow along the axis 14 in front of the pod, as the marine propulsion unit 1 travels through the water attached to a vessel will be the same as the vessels speed through the water. Due to the rotation of the impeller the water will be ac- celerated and expelled from the impeller at a substantially higher speed. The accelerated water will be forced through one or more rectifier nozzles 15 and thereby be directed along the outer surface of the streamlined body 10.

In the embodiment illustrated with reference to figure 1 the nozzles 15 are arranged in a nozzle member 16, see also figure 3. In the embodiments illustrated in figures 1 and 3 rectifier nozzles 15 are arranged along the entire periphery of the impeller, but any design and combination of nozzles may be utilized depending on the desired flow of water along the body 10 of the marine propulsion unit 1. In figure 2 is illustrated a schematic impeller 17. In this embodiment the impeller comprises a disc 18 arranged for rotation around the center 19 such that the impeller 17 will receive water from a direction indicated by the arrow 20 and expel the water in substantially equal amounts in a direction indicated by the arrows 21, i.e. having a combination of a radial and a tangential component preferably radial and evenly distributed from the disc 18 (even though the arrows 21 only indicate two directions, it is clear that accelerated water will be expelled radially along the entire periphery of the disc 18). Due to the impeller's rotation the water will be accelerated such that water leaving the impeller in the direction 21 will have a higher kinetic energy level than the water 20 entering the impeller 17.

The impeller 17 is provided with vanes 22 arranged at desired intervals along the impeller surface substantially in a radial orientation. The design of the vanes is in this embodiment curved but may also be designed as straight and flat vanes. Typically the vanes will be angled slightly relative to the radial direction 23. It is also foreseen that the orientation of the vanes may be adjusted/fine-tuned depending on the desired thrust, number of impeller revolutions, vessel speed etc. The vanes angle of attack on the water (i.e. how much the vane is turned relative to the radial direction) can therefore be adjusted, simply by turning the whole vane or part of the vane.

In this embodiment the impeller has vanes which are positioned at a certain distance from the center 19 of the disc 18 leaving a central area substantially open and undisturbed. This is important in that the water entering through the inlet opening 13, see figure 1, will be substantially undisturbed until it comes into engagement with the disc 18 and the rotation of the vanes propelling the water in the direction 21. From prior art as will be discussed with reference to figure 4 a swivel loss occurs if the vanes had engaged the water immediately, for example by starting directly from the center of the disc. This would have created a certain amount of inlet swirling flow which would have diminished the effectiveness of the impeller.

In figure 4 is illustrated a computer simulation of a traditional screw 30 attached to an axle 31 which is brought to rotate, thereby creating propulsion in a traditional manner for a marine vessel. Due to the rotation of the screw a number of swivels indicating turbulent water flow arise. Centrally in an axial direction a hub swivel loss occurs due to the creation of a rotational swivel 32 which is caused by rotation of the axle 31 and the innermost part of the blades of the propeller 30 rotating around the axle such that the axle does not contribute to the propulsive force.

Furthermore, the single blades 33 of the propeller 30 has distal ends, where tip losses occurs 34, which does not contribute to the propulsion either, but only disturbs the water, thereby creating a turbulence 34 at the distal end of each propeller blade 33. These turbulences 34 do not contribute to the thrust of the propeller arrangement.

Furthermore as the water is accelerated through the screw it is accelerated mostl in axial direction, but a part is accelerated in angular direction adding further to the swirl losses and in this way contributing to the above mentioned losses, which totally amounts for up to 30% of the kinetic energy imparted to the water due to the torque.

A naval designer will naturally find the most optimum relationship between the number of blades, the area of the blades and the loss described above such that for traditional propeller solutions the most optimum energy use and the highest propulsion per unit energy is achieved. With the present invention, however, none of these swirl and turbulent losses 32, 34 occur.

As the unit 1 travels through the water, see figure 5, with a given velocity, water will flow into and out of the impeller and rectifier nozzles, and along the propulsion unit's body 10 as illustrated in figure 5.

Due to the rotation of the impeller 17 (see fig. 2) water will be sucked in at the front end 11 of the unit 1. This suction is indicated by the flowlines 41. Due to the rotation of the impeller and the pump action of the vanes 22 (see fig 2), water will be expelled substantially perpendicular to the inflow direction 41 such that water will be expelled through the rectifier nozzles 15 along the surface 10 of the unit 1. Due to the Coanda- effect the accelerated water 42 will follow a curved surface. The Coanda-effect is a long established phenomenon where instead of flowing in a straight line and departing from a curved surface a jet of fluidum, for example air or water, will remain attached and follow the surface along a curved path. When the friction between the fluidum, in this case water, and the surface along which it flows, in this case the outer surface 10 of the unit 1 is neglected, the only forces acting on the fluid particle are due to pressure which will therefore cause pressure differences between the inside and outside of the layer 42 of accelerated water such that the outside pressure is larger than the inside pressure.

There will be a pressure gradient across the flow 42 such that under-pressure will be present close to the surface 10 keeping the flow along the surface of the unit. Therefore, as the accelerated water 42 flows along the surface 10 of the marine propulsion unit 1, thrust, will be the result as the pressurized water leaves the rear (tapered) end 12 of the marine propulsion unit 1. In this manner a thrust is generated such that the impeller accelerates the water, i.e. imparts kinetic energy to the water creating a momentum change in the water flow 42 along the surface 10 of the marine propulsion unit 1 creating a thrust 43 which provides the propulsion of the marine unit 1 and the vessel to which it is attached,

Depending on the speed, i.e. velocity, of the vessel and thereby the marine propulsion unit through the water, the curvature of the unit 1 , the density of the water (which will vary with temperature, salt content etc.) the rectifier nozzles openings 15 may be designed accordingly or may be adjustable in order to vary the flow pattern of the pressurized water 42 over the surface 10 of the unit. By varying the thickness of the kinetic energized water 42 flow or the speed of it, it may be possible to urge the propulsion unit 1 into a different direction due to the variation of resulting thrust, and in this manner steer the marine propulsion unit and thereby the vessel onto a desired direction.

In the embodiments illustrated with reference to the figures the impeller is arranged substantially on the surface 10 of the marine propulsion unit 1, but other embodiments include the impeller and optionally the rectifier nozzles being arranged below the surface in a cavity in the front of the unit 1 such that the inlet opening 13 is in communication with the impeller and that the impeller expels water through the rectifier nozzles along a surface being in smooth communication with the outer surface 10 of the propulsion unit 1. Depending of the speed of the vessel and thereby the marine pro- pulsion unit 1, the ambient pressure rise, in the water in contact with the submerged impeller and the rectifier nozzles, will delay the inception of cavitation in the thrust generating parts of the total propulsion system, stemming from the relative flow velocity in relation to the local pressure. Because the risk of damaging cavitation in the propulsion system is minimized as the velocity of the vessel and propulsion unit is increasing, this embodiment is favored as a high speed propulsion system, where the inception of cavitation is controllable and higher vessel velocities will be obtainable, provided more power is added. In fig. 6 the surface 10 is provided with dimples 48 whereby the boundary layer forming on the outside of the unit _: during travel through water as described above, is prevented from growing whereby the pressure gradient from the flow 42 is sustained and the under-pressure will be further pronounced. Instead of dimples, small indentations (i.e. bulges into the body of the unit) or bulges (bulges coming ou of the surface (plane) of the unit) or "shark' s skin"( well-known wet surface finish especially for high speed vessels) may be provided in order to achieve the same purpose, i.e. restrain the boundary layer from growing.

Also illustrated in fig 6, is a drive axle 44 extending in the pod's travelling direction: This drive axle is comparable to the traditional propeller axle, such that the conventional ship motor installations and arrangements inside the vessel's hull, does not have to be altered when fitting pods according to the invention.

The pod is furthermore provided with an attachment means in the shape of an attach- ment beam or strut 46. This beam 46 is used to attach the pod to the vessel, and carry the pod and also transfer at least a part of the thrust from the pod to the vessel.

In order to be able to compare the present invention to traditional ships propellers and water jets, the well-established and recognized calculation models disclosed in "Prin- ciples of Naval Architecture, Second revision Vol. II, 1988" see in particular pages 132-135 and 225-227, which hereby are incorporated by reference. In the calculations only minor assumptions and adaptations are necessary. The theory provides for a significant better energy use using the present invention as compared to traditional methods, as summed up in the table below. For calculative purposes, the present invention is compared to the propellers used on the latest series Triple E Container carriers as delivered to Maersk line. These propellers are believed to be the most (energy) efficient propellers ever designed and used. "HBI" refers to the present invention.

The above table illustrates comparative calculations between the present invention and a propeller use in Maersk lines Triple E series container carrier. Each column represents different inlet areas.

The results in the second last line indicate that an inlet area of 31.42 m² provides more thrust with less energy consumption. The larger the inlet area, the better the results.

Claims

CLAIMS 1. Marine propulsion unit, where said propulsion unit comprises a streamlined body, where said body has a front end and a rear end, where in use the front end will be fac- ing upstream and the rear end downstream, where the front end has a larger cross section perpendicular to the intended travelling direction than the rear end, and where the front end or adjacent the front end, is provided a water inlet opening, where said opening is in communication with an impeller, which impeller rotates around an axis parallel to the intended travelling direction, where said impeller has one or more vanes whereby the water entering the inlet opening is expelled radially away from the impeller's rotating axis, where said expelled water is forced through one or more nozzles directing said expelled water along the outer surface of the streamlined body.

2. Marine propulsion unit according to claim 1 wherein the streamlined body or body of revolution at zero incidence is symmetric around the impeller's rotation axis.

3. Marine propulsion unit according to claim 1 or 2 wherein the general cross section through a longitudinal plane of the streamlined body generally has a drop shape, where the rear end tapers.

4. Marine propulsion unit according to claim 1 or 2 or 3 wherein the streamlined body is provided with means for attaching the unit to a vessel, where said means are suitable to transfer the thrust to said vessel.

5. Marine propulsion unit according to claim 4, wherein the means is the vessels propeller axle, where the unit is installed instead of the propeller, and where the axle rotates the units' impeller.

6. Marine propulsion unit according to claim 1 or 2 or 3 or 4 wherein inside said stream-lined body motor means are provided for rotating the axle of the impeller.

7. Marine propulsion unit according to any preceding claim wherein the water inlet opening, the impeller and the one or more nozzles are arranged on the surface of the streamlined body, such that expelled water exiting through the nozzles will be directed along the outer surface of the streamlined body.

8. Marine propulsion unit according to any preceding claim wherein the nozzle has a circular shape, arranged around the impeller on the surface of the streamlined body, and where the nozzle is separated into sections.

9. Marine propulsion unit according to any preceding claim wherein the vanes arranged on the impeller all are spaced at a radial distance from the rotation centre of the impeller, leaving a central area of the impeller as an open vane free surface, said surface having a. plane substantially perpendicular to the impeller's rotation axis.

10. Marine propulsion unit according to claim 1 or 9 wherein the one or more vanes are arranged substantially radial on the impeller, and where the one or more vanes are curved relative to a radial going through said vane, whereby the vane will expel water substantially radial from said impeller.

11. Marine propulsion unit according to claim 1 or 10 wherein the vanes arranged on the impeller can be oriented or adjusted relative to a radial direction.

12. Marine propulsion unit according to claim 1 where the nozzles are provided with rectifying means in the shape of walls and a lid, where said walls and lid may by controlled relative to the water flow, said rectifying means directing the expelled water into a radial direction relative to the impellors axis, whereby the expelled water is led along the surface of the body.

13. Marine propulsion unit according to any preceding claim wherein at least part of the surface area of the streamlined body is provided with a plurality of dimples and/or indentations and/or bulges.

14. Method of providing propulsion, wherein a marine propulsion unit according to any of claims 1 to 13 is arranged. on a submerged section of a marine vessel where water is channeled into the propulsion unit's inlet opening, and the water is accelerated when coming into contact with the vanes arranged on a rotating impeller, such that the water is expelled through nozzles from the impeller along the surface of the propulsion unit, such that the accelerated water on a front section of the propulsion unit creates an uhder-pressure and along the rear part of the unit provides a thrust, providing the propulsion.

15. Method of providing propulsion, wherein a marine propulsion unit according to any of claims 1 to 13is arranged below on a submerged section or a submerged cavity in the front end of the pod whereas the speed of the vessel and thereby the marine propulsion unit 1, the ambient pressure will rise, in the water in contact with the submerged impeller and the rectifier nozzles and will delay the inception of cavitation in the thrust generating parts of the total propulsion system, stemming from the relative flow velocit in relation to the local pressure.