Abstract:
A passive air induction system for boats that includes a wedge member and air induction tubes. The system is installed within the propeller tunnel of high-speed planing vessels to reduce or eliminate noise and vibration caused by cavitation vortices created by rotation of the propeller blades. The air induction tubes include air inlets that receive ambient air, which is delivered to the wedge member to be released from a plurality of distribution slots on the wedge member as air bubbles. The air bubbles released from the distribution slots provide a cushioning effect against contact between the cavitation vortices and the vessel hull, thereby reducing vibration and noise that has been heretofore common at high speeds in planing vessels.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to passive air induction systems, and more particularly to advanced marine vessel tunnel designs incorporating air induction tubes and distribution slots about the perimeter of the tunnel for marine propulsion systems. 
     2. Description of Related Art 
     During rotation of a boat propeller, the hull bottom of the vessel is impacted by energy in the form of shock waves due to the phenomenon referred to as tip cavitation. Cavitation occurs when spiraling low-pressure cavitation voids (vortices) are created by each blade of the propeller during rotation. As these voids migrate to a zone of higher pressure the cavity suddenly collapse leading to a violent “implosion.” This “implosion” produces noise and erratic vibration in the vicinity of the propeller that propagates into the hull structure and eventually onto the boat decks and upper structures of a vessel. As propeller loading increases to meet the demands of heavier boats and increased horsepower these cavitation voids increase in size, the implosion becomes more violent, and thus, the impact on vibration becomes more significant. 
     U.S. Pat. No. 6,406,341, issued to Morejohn on Jun. 18, 2002, discloses a shallow draft boat hull which incorporates a vent tube in an angled tunnel for the purpose of creating a low pressure area, and dissipating air bubbles in the tunnel. Air bubbles are sucked into a recess and travel upwardly through a tube to be exhausted behind the transom. 
     U.S. Pat. No. 6,213,824, issued to Small on Apr. 10, 2001, U.S. Pat. No. 6,193,573, issued to Small on Feb. 27, 2001, and U.S. Pat. No. 6,045,420, issued to Small et al., on Apr. 4, 2000, teach a system of hull design, which incorporates centrally placed air induction plenums having planar walls running longitudinally along a section of the hull. Air is introduced into adjacent tunnels which house struts and propellers. The designs are based on critical angular and area formulas for attempted linearization of the relationship between static and dynamic water levels, vessel velocity and engine revolutions per minute. 
     U.S. Pat. No. 5,957,078, issued to Van Tassel on Sep. 28, 1999, discloses a marine propulsion system and hull design, which includes air ducts, air valves or a combination of both, to introduce air in the propeller tunnel, before the propeller itself. This design is specifically for the purpose of controlling the level of water in the tunnel to optimize the performance of surface-piercing propellers. 
     The prior art, however, fails either alone or in combination with other references, to teach or suggest the instant engineering designs for an advanced arcuate or semi-circular boat tunnel design incorporating air induction tubes and a complementary curved slot distribution wedge with air outlets, nor any similar or related structure, which was designed for creating a layer of air to absorb energy or shock waves caused by cavitation. The prior art does not disclose or illustrate the components of the instant invention, and likewise does not address the particular problems solved with this marine propulsion system. 
     SUMMARY OF THE INVENTION 
     The passive air induction system comprises an innovative engineering design for marine vessel tunnel designs, which are the tunnels housing the drive shaft and propeller mounting structure, struts, and/or rudder assemblies of a vessel. The instant invention comprises in one embodiment a semi-circular tunnel design incorporating air induction channels and the complementary curved distribution channel or chamber with orifices constituting air outlets, and any similar or related structure, for creating a layer of air to absorb energy or shock waves caused by cavitation. 
     While the vessel is moving, this passive air induction system provides a small layer of compressible air between the hull bottom and the propeller wake developed in advanced tunnel designs found on shallow draft, high speed planing vessels. This compressible layer of air is capable of absorbing and thus reducing the energy (shock waves) exerted against the hull bottom due to the phenomenon referred to as tip cavitation which is caused by low-pressure cavitation voids (vortices) created by each blade of the propeller during rotation. Since this system is not active until the vessel is on plane the slow vessel wake and steering performance are unaffected. 
     The required air for this system is drawn in through induction channels or tubes which run longitudinally from the transom of the vessel to the distribution orifices. These orifices are distributed evenly around the semi-circular perimeter of the tunnel. 
     An objective of this invention is to reduce noise and vibration caused by cavitation vortices created by the rotation of the propeller blades of a vessel within the boat propeller tunnel. 
     Another object of this invention is to provide an efficient, low-energy means for reducing or eliminating noise and vibration caused by cavitation vortices by passively drawing ambient air into a channel installed within the boat propeller tunnel to be exhausted just aft of propeller as air bubbles, thereby forming a cushion against the impact or “implosion” of said cavitation vortices. 
     An additional objective of this invention is to avoid any impact on slow vessel performance. 
     In accordance with these and other objects which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a perspective and isometric view of typical installation of the system in a modern propeller tunnel in conjunction with the induction tubes and distribution slots. 
         FIG. 2  shows a rear view of the system illustrated in  FIG. 1 . 
         FIG. 3A  shows a cross-sectional view of a tunnel and air induction system incorporating the instant invention. 
         FIG. 3B  shows a cross-sectional, detailed view of the arcuate channel and flange of the wedge member. 
         FIG. 4  shows a perspective and detailed view of the transition from induction tube to distribution slots. 
     
    
    
     DETAILED DESCRIPTION 
     The air induction system  10  as stated in this invention is a passive system developed to provide a uniform layer of compressible air, capable of absorbing the cavitation energy that is found in propeller tunnels. As illustrated in  FIGS. 1-4 , the passive air induction system  10  comprises an air distribution channel  12  and at least one air induction channel  14  in gaseous communication with the air distribution channel  12 . In a primary embodiments, the air distribution channel constitutes a wedge member  12  that is preferably curved and generally semicircular in shape. As seen more particularly in  FIG. 4 , wedge member  12  includes a bottom surface  12   a,  a top surface  12   b,  and a flange  12   c  connected to and projecting at an angle upward from the lower surface of said wedge member. Preferably, the flange  12   c  is oriented perpendicularly with respect to a wall  16   a  of a boat propeller tunnel  16  that forms part of a boat hull bottom  20  of a vessel. As illustrated in  FIG. 3B , the top surface  12   b  of the wedge member  12  includes a free end  12   b ′ that is angled upward toward the wall  16   a  of the boat propeller tunnel  16  so as to form an arcuate channel  12   e  with the flange  12   c.  The free end  12   b ′ of the wedge member top surface  12   b  may contact the wall  16   a  of the boat propeller tunnel  16 . 
     The wedge member  12  also includes one or more air distribution orifices  12   d  for exhausting air from said wedge member around the periphery of the wall  16   a  of the boat propeller tunnel  16  aft of the propeller  18 . As illustrated in  FIGS. 1 and 2 , the wedge member  12  is attached to a wall section  16   a  of the curvilinear boat propeller tunnel  16 , which is incorporated as a section of the boat hull bottom  20 . The wedge member  12  is shaped and sized for secure and complementary installation on the wall  16   a  of the boat propeller tunnel  16 . The wedge member  12  may be an integral component of the boat hull, molded as unit in the manufacturing process. Alternatively, the wedge member may be attached to the wall  16   a  using any suitable hardware or fastener that is preferably corrosion-resistant and water-resistant or insoluble. 
     The distribution slots  12   d  of wedge member  12  comprise openings spaced evenly and at regular intervals about the edge of the flange  12   c  to provide air flow and entrained bubbles  24  around the arcuate periphery of the wall  16   a  of the boat propeller tunnel  16 . 
     Although the primary embodiment of the air distribution channel is a wedge member as discussed herein, one skilled in the art would appreciate that the channel could be any cross-sectional geometric shape incorporated about the periphery of the propeller tunnel. The significant characteristic is that the channel is a conduit for gaseous introduction into the propeller tunnel when the vessel is operating at higher speeds or on plane. Similarly, in alternate embodiments of the passive air induction system  10 , the distribution slots  12   d  may comprise any series of spaced orifices or apertures perforating the flange  12   c.  The shape and size of the orifices is also a matter of design choice. 
     As illustrated in  FIGS. 1 and 4 , the air induction channel can constitute a generally tubular member  14  which supplies ambient air entering through an air inlet  14   a  near the transom of the vessel to said distribution slots  12   d.  Preferably, the system  10  includes at least two air induction tubes  14 , which run longitudinally from the transom  32  of the vessel at the stern end  16   b  of the boat propeller tunnel. The air induction tubes  14  may be integrally formed or molded within the hull and arm members  22  that connect to the wedge member  12  and extend aft of the wedge member. Air enters the air induction tubes  14  through the air inlets  14   a  and is carried or suctioned into the arcuate channel  12   e  of the wedge member  12 , which forms part of a chamber or cavity  26  between the top surface  12   b  of the wedge member and the wall  16   a  of the boat propeller tunnel  16 . Note also that arm members  22  running fore and aft can be “shelled” in the manufacturing process to the same wall thickness as wedge member  12 , such that induction tubes  14  take the shape of the outside surfaces of members  22  with any appropriate offset inwardly to match that of wedge member  12 . 
     In the preferred embodiment of the passive air induction system  10 , the air induction tubes  14  and the wedge member  12  form a integrated system and unitary device. The system typically includes two air induction tubes  14  each of which is seated at opposite sides to the propeller tunnel, run generally longitudinally thereto, and are parallel to one another. However, it is contemplated that the air induction channels may run from other locations on the vessel, and the conduits need not be symmetrical. Similarly, although the channels  14  are depicted as generally tubular, they can be of any appropriate geometric shape and size for a particular vessel design. 
     During manufacturing, whether integrally with the hull molding process or installation as separate induction system, wedge member  12  is fitted and secured around the periphery of the propeller tunnel and aft of the propeller blade  18 . The wedge member or channel  12  defines the cavity  26  between the top surface  12   b  of the wedge member and the uppermost wall  16   a  of the boat propeller tunnel  16 . Ambient air received into the air inlets  14   a  of the air induction tubes  14  flows and is transported through the induction tubes and supplied into the distribution cavity  26  formed between the wedge member  12  and the wall  16   a  of the boat propeller tunnel  16 . The air entering into the cavity  26  is then exhausted as air bubbles  24  through the distribution slots  12   d  of the wedge member  12 . As shown in  FIG. 3A , the air bubbles  24  form a boundary layer and cushion between the wall  16   a  of the boat propeller tunnel and cavitation vortices  28  created by rotation of the propeller  18 , thereby reducing noise and vibration caused by implosion of the cavitation vortices on the boat propeller tunnel. 
     The inventions described herein also include a method for reducing noise and vibration caused by cavitation vortices  28  created by the rotation of a boat propeller  18  within a boat propeller tunnel  16  of a hull bottom  20 . The method comprises the steps of installing a distribution member  12  to the hull bottom  20  within the boat propeller tunnel  16  aft of the propeller  18  and supplying air to an enclosed cavity  26  that is exhausted from the enclosed channel around the periphery of the wall of said boat propeller tunnel aft of the propeller. When installed within the vessel hull about the propeller tunnel, the distribution member  12  defines the space or cavity  26  which receives the air supplied by and through the air induction channels. The cavity is formed in part by the arcuate member  12   e,  perpendicular flange  12   c  of the wedge member  12  and a top surface  12   b  of the wedge member having end  12   b ′ that is angled upward toward the wall  16   a  of the boat propeller tunnel  16 . The air is exhausted through a plurality of distribution orifices  12   d  in the face of the distribution channel, causing air bubbles  24  to be released around the periphery of the walls of the propeller tunnel, thereby forming the protective cushion and boundary layer between the bottom of the hull and the deleterious cavitation vortices. 
     The high velocity water passing over wedge  12  creates a region of low pressure slightly aft of the distribution slots  12   d,  as shown in  FIG. 3A . Ambient air from the atmospheric pressure region at the transom  32  of the vessel is sucked through air induction tubes  14  into the negative pressure region created when the boat is on plane. This air is then discharged back into the boat propeller tunnel  16  through the distribution slots  12   d  of the wedge member  12 . The result of this discharge of air is the creation of a uniform layer of air that forms a cushion at the boat propeller tunnel surface or wall  16   a  and the stern end  16   b  of the boat propeller tunnel  16 . This cushion, due to the compressibility of air, is capable of absorbing the cavitation energy and insolating the boat hull bottom  20  from the shock waves created by the implosion of the collapsing low pressure voids of the cavitation vortices  28 . Testing of the system  10  has produced results showing extremely substantial reduction in vibration levels in the hull structure, exceeding 100%. The cushioning effect of these air bubbles  24  reduces or eliminates the noise and vibration normally caused by the implosion of the cavitation vortices within the boat propeller tunnel  16 . 
     The instant description, drawings and artistic renditions illustrate to one of ordinary skill in the art, how to manufacture, assemble and utilize the instant passive air induction system  10  for marine vessels. 
     The instant invention has been shown and described herein in what is considered to be the most practical and preferred embodiment. It is recognized, however, that departures may be made therefrom within the scope of the invention and that obvious modification will occur to a person skilled in the art.