WO2008020796A1

WO2008020796A1 - Propulsion system for a surface water vehicle

Info

Publication number: WO2008020796A1
Application number: PCT/SE2007/000708
Authority: WO
Inventors: Magnus Eriksson
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-08-15
Filing date: 2007-08-07
Publication date: 2008-02-21
Anticipated expiration: 2009-02-15
Also published as: SE0601682L

Abstract

The present invention relates to a propulsion system for a surface water vehicle, comprising an electric motor (10) of permanent magnet type in an azimuthing pod (2) underneath the hull (1) of the water vehicle. A pump jet (3) comprising a rotor (6) and a stator (5) surrounded by a duct (7), is placed aft of the electric motor (10) and mounted together with the azimuthing pod (2), whereby the pump jet is connected to the outgoing shaft (9) of the electric motor.

Description

Propulsion system for a surface water vehicle

The present invention relates to a propulsion system for a surface water vehicle. The starting point for the invention was to design such a propulsion system making it possible to drive the water vehicle at high speeds and which is silent. The propulsion arrangement in question can however also be used for water vehicles that is not driven at a high speed. The invention uses a motor in a so-called pod arrangement.

An azimuthing pod is in essence an electric motor housed inside a pod which encloses the electric motor and protects it from the outside sea water. The azimuthing pod is mounted underneath the hull of a water vehicle and can normally rotate around the axis attaching it to the underside of the hull of the water vehicle, which makes the manoeuvrability substantially better than for a traditional propeller/rudder arrangement. With a rotating azimuthing pod the need to install a separate rudder is eliminated. The propulsor itself does normally consist of a propeller in a pushing or pulling configuration or a combination of these. Such azimuthing pod units are in operation in commercial ships and predominantly in cruise ships which are well suited for installing a pod since the ship, due to its very high electric hotel load, already has a high capacity to generate electric power.

Electric power for the electric motor and control signals to the azimuthing pod is passed through the connection between the hull and the azimuthing pod.

A few displacement pleasure boats (sail boats and slow displacement motorboats) are installed with azimuthing pod units. Ail known azimuthing pod installations use as propulsor either a single conventional propeller or some sort of combined design.

These existing azimuthing pod arrangements are not able to drive a boat at high speed. On the one hand there are limitations that are related to the use of a propeller and on the other hand a rotationally symmetric body, e.g. an azimuthing pod should have a length/width ratio of about 7, i.e. the body should be at least 7 times longer than its diameter, in order to reduce the hydrodynamic resistance. Known azimuthing pod designs fall far short of this, and if an attempt is made, based on their general design, to achieve a high propulsion power, the hydrodynamic resistance will at the same time be disadvantageousiy high. The invention combines the basic design with an azimuthing pod having a pump jet located in direct connection with the electric motor. This gives rise to advantages in several respects. It becomes a positive spiral in which one advantage leads to several other advantages.

Pump jets themselves are known in completely different marine applications, such as torpedoes and, to some extent, (nuclear) submarines. A pump jet manages to deliver the required thrust to drive the vehicle at the relatively high speed, for which it is designed, with comparatively high efficiency and low radiated noise A pump jet rs in essence an axial turbine pump with a duct or a tunnel surrounding a fixed stator, which rotates the water, and a rotor with more blades than a conventional propeller,

increasing the propeller efficiency by adding a duct is an old idea. If the duct is designed to accelerate the water into the propeller disc some of the force is trans- ferred to the duct and the efficiency increases. The drawback with an accelerating duct is the increased risk of cavitation due to the decreased effective cavitation number in the position of th© propeller than for a conventional propeller (lower pressure and higher speed).

If one is interested in increasing the cavitation performance a decelerating duct is employed instead, Decelerating ducts of course have the drawback that the efficiency is lower than for a conventional propeller in a conventional application at moderate rpm. A high rpm can, however, not be achieved with a conventional propeller i.a. due to problem with cavitation.

Correctly designed both types of ducts have the advantage of averaging out the variations in cavitation in a wake field during a revolution, Which reduces the low frequency vibration- and noise-generating forces. Generally a duct renders it possible to increase rotor blade tip-loading by reducing tip vortex cavitation and pressure overflow from the high pressure side to the low pressure side of rotor blades because the distance between the tip and the inside of the duct is small.

A circumstance that has a favourably effect on the possibility to carry out the present invention is that electric motors of permanent magnet type has of late achieved an increasing importance for a large number of applications, as required electronic control equipment has gained performance in line with the development of the digital technology. Electric motors of permanent magnet type has generally significantly lower volume and weight in relation to the possible power output (m³/kW, kg/kW) in comparison with traditional AC or DC motors.

In the light of what has been presented, it is the purpose of the invention to create a propulsion system which can drive a surface vehicle at high speeds.

Another purpose is to create a propulsion system which is significantly quieter than existing combinations of propulsion systems for driving boats, especially fast-going boats.

Yet another purpose is to create a propulsion system which minimises the append- age resistance (the hydrodynamic resistance arising from components not belonging to the main hull such as rudders, propeller shaft bossing etc) and simultaneously has a high efficiency at higher rpm than do conventional non-aired propeller arrangements.

Another purpose is to offer the possibility to install alternative energy storage and energy conversion technologies, to reduce the environmental impact in the form of for instance carbon dioxide and particle pollution,

Yet another purpose of the invention is to create a shallower propulsion system, which is created by the fact that the diameter of the pump jet can be allowed to be smaller than in a conventional propeller arrangement, but still deliver the same propulsion thrust.

Another purpose of the invention is to protect the rotating parts from the surround- ings, so as to reduce the risk for damages in near by objects and the rotating parts.

Yet another purpose of the invention is to create a vectorised thrust which is parallel with the speed vector of the boat, which leads to a higher efficiency.

The present invention satisfies the described purposes by being designed in the way that is evident from the following independent claim. The remaining claims define advantageous embodiments of the invention. The invention will be described in more detail in the following with reference to the accompanying drawing, in which

Fig. 1 shows a principal drawing of a boat with an azimuthing pod equipped with a pump jet,

Fig. 2 shows the azimuthing pod arrangement of fig. 1 in more detail and FIg, 3 shows a concrete design, according to the invention, of the aft part of an azimuthing pod wiih a pump jet

The basis for the invention and that what solves the described problems in a way that no previous drive systems have done, In spite of a long-required need, is that the invention combines technology from totally different fields of technology. The invention employs an electric motor in an azimuthing pod arrangement, which directly in the azimuthing pod drives a pump jet. In the constructive design of the invention concrete solutions from the different fields can be used- The azimuthing pod itself can be designed based on what is known for azimuthing pod units. The pump jet can also be designed based on what is known regarding those.

In Figure 1 a principal drawing is shown of a boat 1 with an azimuthing pod 2 mounted together with a pump jet 3. Figure 2 shows in more detail how an azimuthing pod according to the invention is mounted underneath a boat hull. The azimuthing pod 2 is mounted on a rotating axis which in the example in the figure is surrounded by a fin shaped hydrodynamic fairing 4. Power cables and signal cables are led through the connection between the hull and the azimuthing pod. Suitably this is done so that the cables are not exposed to water.

The duct of the pump jet surrounding both the stator and the rotor is in this embodiment for a high-sped boat a decelerating duct. !f high speed is not important an accelerating duct can be employed instead of a decelerating one, At least a slightly accelerating duct can be used,

It is usually preferable that the inlet area of the pump jet duct is larger than the outlet area,

A pump jet generally consists of a stator 5 and a rotor 6 surrounded by a duct 7. The stator may be placed either upstream or downstream of the rotor, If the stator is mounted upstream of the rotor its function is to create a pre-swirl of the flow into the rotor disk. If it is instead mounted downstream of the rotor its function is to straighten up and decrease the swirl of the flow that is created aft of the rotor, fn both cases the purpose is to reduce the rotational losses in the pump jet as a whole. In the embodiment of tha invention shown in Figure 3 the rotor 6 is placed upstream of the stator 5.

The stator 5 consists of blades which by its angle of attack relative to incoming flow or through the angular curvature of their cross section initiates a rotation of the water passing through the stator. The stator can be attached to the rest of construction in different ways. In the example in Figure 3 the stator 5 is attached to and acts as a supporting structure to the surrounding duct 7, which in its turn transfers the torque to the azimuthing pod by being attached to the pod via fins 8.

The task of the rotor is to transfers the torque delivered by the motor shaft 9 into a thrust force driving the boat 1 forward through the water. The stator S and rotor 6 is characterised by having considerably more blades than a conventional boat propeller.

When a fast boat is running through the water the added resistance from appendages is a considerable addition to the totaf resistance for the boat as a whole. With a pump jet of the above type the rpm can be allowed to be much higher, meaning that the electric motor driving the rotor rotates faster, which in its turn means that the electric motor delivers more power than the same electric motor at a lower rpm. For a. certain required thrust an electric motor with a smaller diameter can be used as compared with if the same propulsive thrust should be developed by means of a conventional propeller, This leads to a reduction in azimuthing pod diameter which makes it easier to reach a better length/breadth ratio for the azimuthing pod with the pump jet, which is very advantageous from a resistance perspective, especially at higher speeds. The boat can then reach higher speeds and noise disturbances and performance reducing perturbations in the wake field decreases as a result of the ratio between the pump jet diameter and the azimuthing pod diameter can be allowed to increase.

As initially pointed out electric motors of permanent magnet type have significantly lower volume ratio and weight ratio relative possible power output (m³/kW, kg/kW) compared to traditional DC and AC motors, which means that is normally very suitable to use such an engine when putting the invention into practice. An electric motor of permanent magnet type can substantially contribute to a favourable azirnuthing pod length/breadth ratio.

Figure 3 shows a concrete embodiment of a pump jet in use according to the invention, The electric motor of the azirnuthing pod 1 is designated 10, Its drive shaft 9 transfers the motor rotation to the rotor 6. Aft of the rotor the stator 5 is mounted which in its outer edge is attached to trie surrounding duct 7. The transmission module 11 houses, amongst other things, thrust bearings and radial bearings. The seals 12 force the water from leaking into the transmission module.

The proposed propulsion unit is very suitable as the main propulsion unit for a boat, even a fast boat, However nothing prevents it from being used as an auxiliary unit for propulsion.

it is common for an azimuthing pod to be rigidly mounted underneath a boat hull. It is however possible to envisage the azimuthing pod to be able to be vertically raised and lowered in a tunnel inside the hull so that the azimuthing pod, especially in the case of an auxiliary unit, can be raised into the hull when not in use or being in the way for other reasons.

Claims

Claims;

1. Propulsion system for a surface water vehicle, comprising an electric motor (10) in an azimuthing pod (2) underneath the hull (1) of the water vehicle c h a ra c- t e r i s e d in that the electric motor is of permanent magnet type, and in that a pump jet (3) comprising a rotor (6) and a stator (5) surrounded by a duct (7), is placed aft of the electric motor (10) and mounted together with the azimuthiπg pod (2), whereby the pump jet is connected to the outgoing shaft (9) of the electric motor.

2. Propulsion system according to claim 1, ch a ract e ri s e d in that the duct (7) is a decelerating duct.

3, Propulsion system according to claim 1 , ch a r a ct e ri s ed in that the duct (7) is a slightly accelerating duct.

4, Propulsion system according to any one of claims 1-3, c h a ra cte r- i s e d in that the duct (7) has a larger inlet area than its outlet area.

5, Propulsion system according to any one of claims 1-4, c h a ra ct e r- i s e d in that the rotor (6) is located upstream of the stator (5).

6. Propulsion system according to any one of claims 1-6, c h a r a ct e r- i s e d in that the stator (5) is attached to and constitutes a support construction to the surrounding duct (7), which in its turn is fastened to the azimuthing pod (2).

7. Propulsion system according to any one of claims 1-6, c h a r a ct e r- i s e d in that the azimuthing pod (2) is designed to be able to be vertically raised and lowered in a tunnel inside the hull (1).