This invention relates to submersible propulsor units, and is specifically concerned with a high thrust, primary integral-motor type propulsor unit for water vehicles that provides low noise emissions, high shock resistance, and easy maintenance.
The primary propulsor systems used to drive large surface ships and submarines have, in the past, generally comprised a fossil fuel or nuclear powered prime mover that powers a propeller located on a shaft that extends through the hull of the vehicle through a water-tight seal. A gear train is typically provided between the output shaft of the prime mover and the shaft connected to the propeller.
Unfortunately, there are three major shortcomings associated with such primary propulsor systems that limit their usefulness in military applications. First, the shaft seals necessary to keep water out of the hull of the ship are relatively delicate structures which are highly vulnerable to damage when subjected to the kind of mechanical shocks that may be expected under combat conditions. Second, the use of a gear train generates a relatively high level of noise that may render the vehicle easily detectable by the sonar equipment of hostile nations. Thirdly, the prime mover, gear train, and propeller shaft must all be located in alignment with one another at the rear of the vehicle for the efficient transfer of power, which in turn limits the design options for the vessel designer.
Electric motor type propulsor units for water vehicles are also known in the prior art. While such propulsors may be used for surface vessels, they find their primary application as secondary drive units for submarines where reliability, control, high thrust coupled with low noise emissions, and shock resistance are at a premium. In the prior art, such propulsor units have typically comprised a "canned" electric motor having an output shaft that is connected to a propeller. Such propulsor units advantageously eliminate the vulnerable seals and noisy gear trains associated with conventional primary propulsor systems. They also afford the designer of the vehicle some liberality with respect to the design of the vehicle, as such propulsor units may be located at any one of a number of locations along the hull of the vessel, and not necessarily at the rear of the vessel. Unfortunately, such electrically-powered propulsor units also have certain drawbacks. For example, because the "canned" motor must be disposed either directly in front of or behind the flow of water generated by the propeller, the location of the motor creates obstructions to fluid flow that tends to reduce the effective thrust that can be generated by these units while at the same time creating unwanted noise. Of course, the thrust may be increased by increasing the rotational speed of the motor. However, this may create cavitation in the water surrounding the propeller which creates even more noise.
To overcome these shortcomings, the Westinghouse Electric Corporation developed an integral motor propulsor unit that is disclosed and claimed in U.S. Pat. No. 4,831,297. This particular propulsor unit generally resembles a jet engine in structure and comprises a cylindrical shroud having a water inlet and a water outlet, a propeller having a hub rotatably mounted within a shroud on a shaft that is concentrically mounted within the shroud by a plurality of support vanes, and an electric motor for driving the propeller that includes an annular rotor mounted around the periphery of propeller blades, and a stator that is integrated within the shroud of the unit. The advanced design of this particular prior art propulsor unit substantially increases the thrust output for a propulsor for a given weight and size while simultaneously reducing the noise generated by the unit due to the largely unencumbered flow of water that the propeller of the device can force through the fluid-dynamically shaped shroud, and the relatively large-diameter propeller that this design is compatible with. The quietness of the unit is further improved due to the noise-blocking characteristics of the shroud.
While the aforementioned integral motor propulsor unit represents a substantial advance in the art, the applicants have noted a number of limitations associated with the design of this device which might impair its ability to fulfill certain applications. For example, while the thrust output per unit weight ratio is associated with this particular prior art propulsor is very high, the absolute amount of thrust that can be generated by this propulsor might not be high enough for certain applications. Of course, this prior art propulsor unit could be upscaled in all dimensions to produce more power. However, for certain submarine and military surface ship applications, there are limitations with respect to the width of the propulsor unit which might not allow such an overall upscaling of the device to solve the problem of the need for increased thrust. As the width of the propulsor unit increases, the unit as a whole exposes more and more area to fore and aft shock waves that military submarines and surface ships might be exposed to during combat. Still another limitation associated with such prior art propulsion units is caused by the arrangement of the thrust bearings used in these units. These bearings must be routinely serviced, and the difficult access caused by the manner in which these bearings are arranged necessitates either the complete removal of the propulsor unit from the vessel whenever the bearing assemblies must be repaired or replaced, or the dry docking of the vessel itself which, of course, requires considerable effort and expense. After such removal or dry docking has taken place, a large amount of disassembly of the unit is required to access the bearings.
Clearly, there is a need for a submarine propulsor unit that maintains all of the advantages associated with the latest prior art propulsor units, but which is capable of generating larger amounts of thrust with a mechanism which does not exceed the maximum width limitations associated with submarine applications. It would further be desirable if the bearing assemblies could be easily accessed in the event that a repair or maintenance operation were necessary without the need for removing the unit from the vessel or dry docking the vessel, and without the need for a large amount of disassembly of the unit. Ideally, such a propulsor unit would be even more durable and reliable than prior art units, and would possess even greater shock resistance.