Abstract:
An air encircled marine thruster cowling includes a cowling that surrounds a propeller and provides an annular blanket of air around the slipstream from the propeller. The cowling includes an inner conical section around the propeller and an outer tubular section around the conical section to form an annular space around the conical section. A duct leads exhaust gas from the engine into the annular space, and an additional duct leads moving ambient air into the annular space, as when a boat is in motion.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus for improving the thrust produced by a marine propeller. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Marine propulsion systems generally operate to move a vessel through the water by producing an accelerated column of water. The column of water, known as the slipstream, provides a thrust against the propeller, nozzle or other propulsive device to push the vessel through the water. 
     The present invention, generally, provides an apparatus for increasing the thrust of a marine propulsion system without increasing the load on the engine. 
     More particularly, the present invention provides an apparatus for increasing the thrust of a conventional marine propulsion system by providing a blanket or layer of air encircling a slipstream produced by a propulsion system. According to a preferred embodiment, the invention comprises a cowling for a conventional marine propeller that radially encloses the propeller and extends downstream of the propeller to encircle the slipstream of the propeller. An inner part of the cowling is shaped as a section of a cone, with the propeller disposed in a front end of the cone section, and the cone section axially converging (or narrowing) in the rearward direction. An outer part of the cowling is tubular shaped and is longer than the inner part. A front end of the outer part is joined with a front end of the inner part so that the inner and outer parts form a rearward opening annular chamber. Means for providing air to the annular chamber is included to form a blanket of air around the propeller slipstream. 
     The means for providing air comprises at least a conduit leading engine exhaust to the annular chamber. Additional means includes a duct to lead ambient air to the annular chamber. Movement of the slipstream causes a vacuum in the annular chamber that draws air through the ambient air duct. Other suitable means may also be provided. 
     The invention is described in terms of a conventional propeller, however, it is understood that any apparatus which produces an accelerated column of water to move a vessel could be substituted with advantageous results in the invention, for example, a bow or stern thruster on the side of a vessel used for positioning the vessel for docking. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     The present invention can be further understood with reference to the following description in conjunction with the appended drawings, wherein like elements are provided with the same reference numerals. In the drawings: 
     FIG. 1 is a sectional view of an air-encircling thruster cowling according to the present invention; 
     FIG. 2 is a rear view of the thruster cowling of FIG. 1; 
     FIG. 3 is a graph of static thrust tests comparing a thrust produced by an engine having a conventional propeller and the same propeller equipped with an air-encircling thruster cowling according to the present invention; and, 
     FIG. 4 is a graph of dynamic thrust tests comparing the speed of a boat with an outboard motor having with a conventional propeller and the same propeller equipped with the air-encircling cowling. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a sectional view of an air encircling marine propeller apparatus, or thruster cowling, according to the present invention. A propeller cowling 20 is shown installed on a conventional outboard motor. FIG. 2 is rear view of the propeller 10 and cowling 20 of the present invention. Only as much of the outboard motor apparatus as is necessary to describe the invention is illustrated. The direction of flow of water and air are indicated by arrows. 
     Although the invention is described in conjunction with an outboard motor having a propeller, it is understood that the invention may be used with an inboard motor, or any other propulsion device that produces a column of water to provide thrust to move a vessel in the water. 
     The outboard motor includes a propeller 10 supported in a manner as is known. The propeller gearbox and rudder are not illustrated. The cowling 20 is a generally cylindrical body having an inlet end 22 and an outlet end 28. The cowling 20 is mounted to a suitable support structure 12 of the outboard motor adjacent to the propeller 10, so that the propeller is positioned in the cowling 20 adjacent to the inlet end 22. 
     The cowling 20 comprises two coaxially arranged, hollow elements, an inner body 24 and an outer body 26. The inner body 24 surrounds the propeller 10 and has a predetermined length extending downstream of the propeller. The inner body 24 is formed as a section of a cone, or frustoconical, shape that narrows in the downstream direction. A slipstream produced by the propeller, indicated by arrows 40, moves downstream of the propeller within the inner body 24. 
     The outer body 26 is formed with a cylindrical shape, and the inner body 24 is disposed within the outer body. The outer body 26 is longer than the inner body 24 extending in the downstream direction a predetermined distance farther than the downstream length of the inner body. 
     The inner 24 and outer 26 bodies are joined at a common front end 28 to form an annular space 30 the encircles the inner body 24. Means for providing air in the annular space 30 is arranged so that a blanket or layer of air, indicated by arrows 45, flows from the annular space 30 and encircles the slipstream 40 of the propeller 10. 
     Means for providing air in the annular space 30 includes a conduit 50 that carries exhaust from the outboard motor engine (not illustrated) to the annular space 30. The exhaust, indicated by arrows 52, passes from the conduit 50 to a chamber 56 through a series of openings 54. The chamber 56 communicates with the cowling 20, and the exhaust air 52 then travels to the annular chamber 30. An additional duct 60 is provided to lead ambient air into the chamber 56. Movement of the slipstream past the annular space 30 results in low pressure in the annular space, and consequently, the chamber 56. Air drawn by the duct 60 passes through the chamber 56 to the annular space 30, as indicated by the arrows 62. 
     The annular stream of air 45 surrounds the slipstream 40 of the propeller 10 as the slipstream exits the inner body 24 of the cowling 20. The encircling air is believed to facilitate the movement of the slipstream 40 into the surrounding water, increasing the thrust generated by the slipstream. 
     A series of tests has indicated that the present invention can significantly increase the thrust produced by a conventional outboard motor propeller at the same engine speed. The tests were performed for both static thrust and speed through the water for a 15 foot bass boat equipped with a 70 horsepower outboard motor. 
     In the static thrust test, the boat was attached to a set of hydraulic scales mounted on a fixed support to measure the thrust produced by the propeller. The engine was run through a range of speeds and the resulting thrust recorded. The test was repeated with the propeller equipped with a cowling according to the present invention. The tests were again repeated with the air injection means blocked to test the effect of the cowling alone. 
     The results of the tests are shown in FIG. 3. As can be seen, the propeller with a thruster cowling produced thrust significantly higher than a conventional propeller throughout the range of speeds tested, and was greatest in the upper engine speed region. The use of a cowling without an annular air stream around the slipstream increased thrust, but as can be seen, the increase was less than for the cowling with air injection according to the present invention. 
     For the dynamic tests, the speed of the boat at various engine speeds was measured. The engine was taken through a range of engine speeds and the speed of the boat in the water was measured. The tests were conducted with the conventional propeller and with the propeller equipped with the thruster cowling both with and without an air stream. The results of the dynamic test are shown in FIG. 4. As can be seen, the thruster cowling-equipped propeller produced greater boat speeds throughout the range of engine speeds. 
     The foregoing has described the preferred principles, embodiments and modes of operation of the present invention; however, the invention should not be construed as limited to the particular embodiments discussed. Instead, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations, changes and equivalents may be made by others without departing from the scope of the present invention as defined by the following claims.