The present invention relates to a propulsion device for a surface going water craft and specially the design of rotor blades and guide vanes of a pumpjet propulsor attached to an electric motor immersed in water in a so-called pod arrangement.

Pumpjet propulsors is in itself known from other marine applications, such as torpedoes and, to some extent, submarines (nuclear submarines). A pumpjet propulsor is able to create the required pressure force needed for the craft to travel at the relatively high speed as it is designed for, with relatively good efficiency and low radiated noise. A pumpjet propulsor is in essence an axial turbine pump with a nozzle or tunnel which surrounds a fixed stator that puts spin on the water and a rotor with usually more blades than a conventional propeller.

The starting point of the invention is a pod arrangement that can propel a water craft at high speed with high efficiency. A rapid rotation of the rotor can give rise to a highly fluctuating load on the rotor blades as well as the guide vanes. The interaction between the vanes and rotor blades is critical in trying to create a load case as even and static as possible, which in turn implies that the components can be made thinner. This provides several benefits from a noise, efficiency, weight and cost perspective.

A pumpjet with a nozzle has the advantage of leveling out the variations in cavitation in the disturbed wake field when the rotor is rotating, and thereby reducing the vibration and noise, particularly in the low frequency range. A nozzle generally allows you to use a larger blade tip load on the rotor blade by minimising the gap between the rotating blade tip and the inside of the nozzle thereby reducing blade tip cavitation and pressure equalizing overflow from the pressure to suction side of the rotor blades .

For an electric powered boat with a pod with a direct driven pumpjet it is essential to increase the rpm as much as the pumpjet propulsor allows without any significant loss in efficiency. The reason for this is that the hydrodynamic resistance decreases significantly with a higher rpm because a proportionately smaller electric motor can deliver the required power. There are also cost-related reasons for such a development. A propeller blade that cuts through the water has the maximum speed at its blade tip. At a given constant rpm the velocity increases with radius, ie. a propeller with a larger diameter has a higher blade tip speed than a smaller diameter. Turbulent phenomena such as separation and cavitation tends to increase with increasing Reynolds number. Reynolds number increases with increasing speed and increasing characteristic length. For a hydrodynamic profile, like the cross section of a propeller blade, the profile chord normally constitutes the characteristic length. The chord is the length of a given profile from its front edge to its trailing edge. It is normal to have different chord lenghts at different radii, usually denoted by rote chord length at blade base, top chord by the tip and the mean chord which is the average over all radii from its base to its tip. The thickness of the profile along the chord then defines the blade shape.

One purpose of the invention is to solve the technical problem of noise and vibration when the rotor blades and vanes are hydrodynamically on- and off-loaded as the rotor blades rotates around its drive shaft. Decreasing vibration can be used to optimize the strength of the components. The invention solves this problem, as set out by the following independent patent claim. Other claims relates to advantageous embodiments of the invention, which provides further improved efficiency of the pumpjet as the rotor blades and vanes can be further optimized.

In what follows, the invention is to be described further with reference to the attached drawings, in which

fig. 1 shows an electrical pod motor with a stern mounted pumpjet and its fastenings in the form of a fin; fig. 2 shows a pumpjet with a nozzle, stator and rotor seen from the rear and fig. 3 shows a principle sketch seen from the rear of a tilted attachment of a guide vane.

Fig 1 shows a pod 1 , containing an electric motor. The pod can be attached to the bottom of a water craft with a fin 2. In other cases, the pod can be attached to the transom of a boat with a slightly different fastener, more like a stern drive. Even in such cases, the fastener attach at the top or the front upper part of the pod. The figure below shows how a pumpjet 3 is attached to the outgoing drive shaft directly after the pod. The figure also shows the horseshoe-shaped vortex 4 that occurs around the fin where the boundary layer upstream of the fin separates by forming a vortex. This horseshoe vortex bends around the fin and follows the flow downstream.

Figure 2 shows the pumpjet composition viewed from the rear. A pumpjet includes a rotor with rotor blades 5 mounted on the electric motor outgoing drive shaft 6, a stator with guide vanes 7 and a nozzle 8 which surrounds the rotor and stator. To reduce problems with interference between the stator and rotor, which induces self- oscillations, both are fitted with an odd number of guide vanes 7 and rotor blades 5. The guide vanes and rotor blades are mounted with an even distribution around a symmetry axis along the drive shaft. The number of rotor blades is an odd number less than the number of vanes.

As mentioned, the main objective of the invention is to solve the technical problem of noise and vibration when the rotor blades are hydrodynamically on- and off-loaded when they pass through the disturbed wake field. When an individual rotor blade 5 rotates around the drive shaft, the blade will experience locally different angles of attack of the incoming flow due to the speed variations that occur when the flow is affected by disturbances in the flow induced by the guide vanes 7 and the horseshoe vortex 4 created by the fin 2 or other fastening joining the pod 1 with the craft's hull, see below for the latter.

By designing the rotor blades 5 chord length so that the projected blade width 9, in a plane perpendicular to the drive shaft, matches the distance between the guide vanes 7, the blades will be on- and off-loaded in a more smooth and balanced way. The leading edge of a rotor blade 5 enters the flow behind a guide vane 7 before or just as the trailing edge of the rotor blade passes by the closest previous guide vane seen in the direction of rotation. With such a design there will be a more continuous load cycling on the blade instead of the discontinuous on- and off-loading, which would otherwise have been the case. The latter would have resulted in significant vibration and noise problems. The vibration suppression means that you can design the rotor blades 5 thinner than otherwise would have been the case.

The rotor blades 5 have substantially radially directed leading and trailing edges and a maximum projected width 9, which is equal to the angular distance between the guide vanes 7 or up to 8% larger than this. For best effect, the maximum projected width 9 should be 5% larger than the angular distance.

With the electric motor attached to a fin 2 or other fastener the boundary layer upstream of the fin will separate and create a so-called horseshoe vortex 4. If a guide vane 7 is positioned so that the horseshoe vortex 4 passes over the vane, a very unfavorable load case is created for this individual vane. The vortex creates a disturbed flow with large variations in pressure and velocity. Disturbances like this are called variations in the wake field. This means that this guide vane must be dimensioned significantly thicker compared with the other unaffected vanes. A thicker blade will lead to higher wet surface, and a larger cross-sectional area and thus higher resistance and lower efficiency. Also the rotor blades 5 dimensional design is influenced by these variations in pressure and velocity in the wake field with the same negative effect on efficiency. Thus, it is very unfavorable for a guide vane to be directly positioned in the path of the horseshoe vortex 4.

In a preferred embodiment of the invention the odd number of guide vanes 7 are fitted so one of the vanes is directed substantially downwards, which therefore means that no vane is directed upwards in the position to which would be most influenced by the horseshoe vortex.

The guide vanes 7 initates a rotation of the flow and generates a lifting force. This lifting force tends to bend out the vanes in the angular direction in a plane perpendicular to the drive shaft 6. The tip of the guide vanes 7 are fastened to the nozzle 8, which in itself generate a net force in direction of the drive shaft 6. This leads to a very complex load case which gives a complex bending mode of the vane with large local stress concentrations as a result. The nozzle 8 will as a result of this tend to rotate slightly. In order to reduce the bending stress you can attach the vanes 7 at the hub and the nozzle so that the vane is loaded more in a pure state of compressive or tensile forces. In this way, you can create either main compressive or tensile stress in the vanes, which means that they can be made thinner as compared to if they would have to withstand the complex load case.

You can create the desired more pure compressive or tensile load case by installing the vanes 7 not straight out of the radial point but tilted, as seen in a plane perpendicular to the rotor drive shaft 6, so that the attachment in the nozzle 8 is located before or after mounting in the center seen in the rotor rotation (Ω in Figure 3). If the attachment in the nozzle attachment is located before the attachment at the center (α in Figure 3) the vane 7 will essentially experience compressional stresses and in the opposite case (β in Figure 3) the vane will essentially experience tensile stresses.