BACKGROUND OF THE INVENTION

This invention relates generally to the field of hydroelectric turbines, where a water stream is passed through a rotating turbine to produce electricity. More particularly, the invention relates to such devices where the flow of water through the turbine is controllable by water flow control means. One of the problems inherent in a low head (run of the river, tidal, etc.) turbine for producing electricity is the relatively slow water velocity; usually from 1 to 5 knots. Using a venturi device, a conical device having a larger forward opening than rear opening, to increase the velocity of water flow into and through a hydroelectric turbine is known. Such venturi devices are typically constructed of a rigid material such as metal or concrete, with gates provided to control the water flow into the turbine. This ability to increase low head water velocities to those of high head applications has inherent advantages of cost, size, efficiency and overall maintainability. Since horsepower output is on an exponential curve with water velocity, it is expedient to keep the blade diameter as small as possible to reach the goal power output. The turbines are typically mounted in a fixed manner due to their size and weight. It is an object of this invention to provide a hydroelectric turbine wherein the venturi is a conical shroud composed of a flexible and collapsible material, and wherein water flow into the turbine is controlled by closing the forward opening of the shroud. It is a further object to provide such a turbine that is relatively lightweight such that it can be positioned in a flowing stream of water using a flotation chamber. It is a further object to provide such a turbine wherein the forward opening of the shroud is controlled by a draw line that easily closes and opens the forward opening. The embodiment has the following aspects which the inventor believes are new: 1) The front shroud is collapsible on demand. 2) The front shroud can be pursed (drawn) together to stop water from flowing through it and the turbine. 3) The purse line can be released to allow water to flow through the front shroud and turbine. 4) The turbine is a combination of a flexible front and a rigid rear shroud used together as one unit. 5) The flotation chamber is used to position the turbine during operation and to raise the turbine to the surface to be serviced. SUMMARY OF THE INVENTION

The invention is a hydroelectric turbine wherein water flow is used to cerate electricity, the turbine comprising a venturi device comprised of a forward shroud composed of a flexible and collapsible material, whereby the flow of water into and through the turbine is accelerated by the tapered conical shape of the shroud, the front opening of the shroud being larger than the rear opening, and wherein water flow into the turbine can be stopped completely by closing the forward opening of the shroud. In the preferred embodiment, a draw line is passed through purse rings disposed on the forward opening of the shroud, such that the shroud is closed by drawing or pursing the draw line to bring the purse rings together. Release of the draw lines allows the shroud to open. A rear shroud also of conical shape but having its rear opening larger than its forward opening is also provided. The turbine and shroud combination is joined to a flotation chamber by multiple cables extending from the purse rings. The flotation chamber is anchored to maintain the turbine in a generally stationary position relative to the water flow. The height of the flotation chamber and the turbine is controllable. The flotation chamber can be used to bring the turbine to the surface of the water for servicing when the front shroud is closed. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the front and rear shroud with the turbine section between. The cables to the flotation chamber are cut at match line (A). Figure 2 shows the flotation chamber with anchor line (10). The cables to the shroud are cut at match line (A). DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, the invention will now be described with regard for the best mode and the preferred embodiment. In general, the invention is a hydroelectric turbine having a front shroud (4) composed of a flexible and collapsible material, the front shroud (4) having means to close its forward opening to stop flow of water to the turbine, referred to herein as turbine housing (1). As the water flows through front shroud (4), its velocity and pressure increases. When the flow enters turbine housing (1), it has reached maximum velocity and pressure. It is in the area (2) that the work is being done of rotating the turbine blades. As the water passes through the rear shroud (3), which is preferably composed of a rigid material, its velocity and pressure is decreased continually until it merges again with the outside flow. A plurality of purse rings (5) are disposed on the forward opening of the front shrough (4). The draw line (6) which passes through all the purse rings (5) and is attached at one end to the front of the shroud (4) has the ability when pulled, to purse (draw together) the purse rings (5) and stop the water flow through the turbine. When the draw line (6) is released, the shroud (4) will again open, allowing water to pass through the turbine. The cable (7) attaches the front shroud (4) to the flotation chamber (8). The scope angle adjustment (9) is where the anchor line (10) is attached to the flotation chamber (8). By adjusting the attachment point of the anchor line (1) fore or aft, the scope angle can be altered, thereby altering the floating height by raising or lowering the flotation chamber (8) and the turbine. The embodiment is designed to be pulled below the water surface by the increased amount of drag created when the turbine is operating. When the turbine is switched off or the front shroud (4) is pursed, drag will decrease and the entire embodiment will float to the surface of the water body.