PRIORITY APPLICATION 
     This application claims the benfit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application No. 60/889,592 filed Feb. 13, 2007, entitled MODULAR OCULAR MEASUREMENT SYSTEM, the entirety of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention is directed to the field of watercraft, and in particular to an improved tunnel for housing surface piercing propellers. 
     BACKGROUND OF THE INVENTION 
     The use of surface piercing propellers to increase the efficiency of watercraft is known in the industry. Inventor Small adapted such use into a tunnel design, as disclosed in U.S. Pat. Nos. 4,689,026; 6,045,420; 6,193,573; and 6,213,824, all of which are incorporated herein by reference. These patents claim various tunnel configurations for the use of such propellers in shallow draft vessels. Specifically U.S. Pat. No. 6,213,824 teaches a tunnel that raises the propeller vertically to reduce draft. This patent has an inlet ramp or chute that feeds water flow to the propeller when the craft is moving forward on plane. 
     Surface piercing propellers operate efficiently when a portion of the blade breaks the surface of the water. Shallow draft vessels that employ these propellers housed within a tunnel rely upon a configuration that allows air to be placed in a position directly before the propellers. Through proper tunnel design, the propellers operate as an air pump drawing the air through a conduit. The shape of the tunnel is calculated to provide efficient operation at cruising and/or top speed. 
     In the teachings of Small, the shape of the tunnel around the surface piercing propeller is just slightly larger in width than the propeller diameter. If the tunnel width is too wide then the ability of the propeller to act like a pump begins to decrease. If the tunnel width is too narrow, inadequate water may lead to excess propeller ventilation. Unique to the tunnel shape of Small is an inlet ramp, or chute, along the leading edge which directs water up to meet the propeller. While the prior art tunnels allow for very efficient vessel operation while on plane, the tunnel design does not provide efficient operation when the vessel is traveling beneath planning speeds or transitioning from off plane to on plane operation. More specifically, the tunnel design of Small fails to provide adequate water flow to the propeller during acceleration. 
     When forward motion is inadequate for the chute to direct water into the tunnel, the required water must come from in front of and below or in front of and from the sides of the propeller. The current tunnel design inhibits the flow of water during a transition stage from idle to planning, resulting in poor acceleration. The result is known as propeller blow out, or excess propeller slip. 
     Thus, what is needed is a tunnel configuration that employs the benefits of the surface piercing propellers for shallow draft vessels but addresses the problem of propeller slip. 
     SUMMARY OF THE INVENTION 
     The present invention is an improvement upon the prior art shallow draft configurations such as those set forth in U.S. Pat. Nos. 4,689,026; 6,045,240; 6,193,573; and 6,213,824. The shallow draft configuration employs the use of a surface piercing propeller placed in a tunnel that runs longitudinally in the bottom of the watercraft. The placement effectively eliminating the likelihood of underwater impact and improving shallow water operation without encountering the high efficiency loses normally associated with other shallow draft drive systems or water jets. 
     The improvement of the instant invention is directed to the shaping of the tunnel and in particular to the forming of a chamfered or radiused corner that improves water flow before the watercraft is on plane. The chamfered corner design allows water to flow into the flow field of the propeller disk providing smooth acceleration. 
     An objective of this invention is to teach the use of a tunnel mounted surface piercing propeller wherein the tunnel has a stepped side wall. Above the step the tunnel is 3-10% larger than the diameter of the propeller; below the step the tunnel can widen to any size without affecting operation efficiency. 
     Another objective of this invention is to teach the use of a tunnel mounted surface piercing propeller wherein the tunnel has a generally vertical side wall. The width of the tunnel above and below the centerline of the propeller is about 3-10% larger than the diameter of the propeller. At the intersection of the vertical side wall of the tunnel and the planning surface of the hull we place a radius or a chamfer that is larger than that required to accommodate manufacturing considerations. 
     Another objective of this invention is to teach the use of a tunnel mounted surface piercing propeller wherein the width of the tunnel above and below the centerline of the propeller is about 3-10% larger than the diameter of the propeller and the width of the tunnel aft of the propeller widens to improve the flow of water into the propeller disk when in reverse. 
     Still another objective of this invention is to teach the use of a tunnel mounted surface piercing propeller wherein the roof of the tunnel aft of the propeller slopes down until the trailing edge of the roof is at or below the free surface of the water when the vessel is at rest. The roof serving to stop air from entering the propeller when the vessel is operating in reverse. 
     Still another objective of this invention is to teach the use of a tunnel mounted surface piercing propeller wherein the roof of the tunnel aft of the propeller slopes down until the trailing edge of the roof is at or below the free surface of the water when the vessel is at rest, the tunnel roof being formed by a hinged panel that drops down in reverse and lifts up when the vessel is going forward. The hinged roof serving to stop air from entering the propeller when the vessel is operating in reverse and swings up to reduce drag when the vessel is moving forward. 
     Still another objective of the invention is to teach an improvement to tunnel configuration that allows water entry to the propeller in reverse by adding a second chamfer to the side walls of the tunnel aft of the propeller disk. 
     Still another objective of this invention is to increase reverse thrust by shaping the tunnel roof so as to greatly reduce the amount of air being introduced into the propeller disk when operating in reverse. 
     Other objectives and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a pictorial end view of a prior art tunnel configuration; 
         FIG. 1B  is a bottom view of  FIG. 1A ; 
         FIG. 2A  is a pictorial end view of the improved design tunnel configuration; 
         FIG. 2B  is a bottom view of  FIG. 2A ; 
         FIG. 3A  is a pictorial end view of a prior art tunnel configuration illustrating water flow at acceleration; 
         FIG. 3B  is a bottom view of  FIG. 3A ; 
         FIG. 4A  is a pictorial end view of the improved tunnel design illustrating water flow at acceleration;; 
         FIG. 4B  is a bottom view of  FIG. 4A ; 
         FIG. 5A  is a pictorial end view of a prior art tunnel configuration illustrating water flow at top speed; 
         FIG. 5B  is a bottom view of  FIG. 5A ; 
         FIG. 6A  is a pictorial end view of the improved tunnel design illustrating water flow at top speed; 
         FIG. 6B  is a bottom view of  FIG. 6A ; 
         FIGS. 7A ,  7 B,  7 C and  7 D are various views of the tunnel showing the hull, and vessel propulsion system; 
         FIGS. 8A ,  8 B,  8 C and  8 D are various views of the improved tunnel showing the hull and vessel propulsion system with the chamfered corner design; 
         FIGS. 9A ,  9 B,  9 C, and  9 D are various views of the improved tunnel design showing the hull and vessel propulsion system with a radiused corner design; 
         FIGS. 10A ,  10 B,  10 C, and  10  D are various views of the improved tunnel design showing the hull and vessel propulsion system with a stepped side wall design; 
         FIG. 11A  is a prospective view of the tunnel roof that includes a fixed downwardly sloping tunnel roof aft of the propeller. 
         FIG. 11B  is a prospective view of a hinged panel that drops down when the vessel is operated in reverse. 
         FIG. 12  is a graph of acceleration improvement versus tunnel width. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The instant invention is directed to the shaping of the tunnel used in housing surface piercing propellers to enable water flow into the tunnel during acceleration and reverse. In particular, there are three ways to achieve the required flow improvement to the propeller disk during acceleration: chamfering, radiusing or stepping the side walls of the tunnel starting at a point that is approximately level with the center line of the propeller. This allows the surface piercing propeller to function well when on plane and moving forward at speed as an air pump, with all the same advantages as described by the prior art. In addition, since the preferred embodiment of the shallow draft tunnel configuration can result in the propeller blades actually being out of the water when the craft is at rest there is a need to find a way to reduce the flow of air into the propeller disk. The instant invention teaches a tunnel roof that can be either fixed or pivotal in nature and extends below the static waterline of the vessel. 
     Referring now to the figures,  FIG. 1A  depicts a vessel ( 10 ) having a surface piercing propeller ( 12 ) placed within a tunnel having a width X as disclosed in the prior art. “X” varies from a few percent larger than the diameter of the propeller to approximately 10 percent larger than the diameter of the propeller. Conduit air vents ( 14 ) extend from the transom ( 16 ), or any other suitable location on a vessel, to a position in front of the propeller effectively filling the vacuum formed in front of the propeller allowing the water level in the tunnel to drop so the propeller can operate in the surface piercing mode where it is most efficient. The side walls ( 18 ) of the tunnel extend in a straight and approximately vertical wall along each side of the propeller.  FIG. 1B  is a bottom view further depicting the side wall ( 18 ) as a straight side wall, the propeller ( 12 ) coupled to a drive source by shaft ( 20 ) 
       FIG. 2A  depicts the use of a vessel ( 30 ) having a surface piercing propeller ( 32 ) placed within a tunnel having width D of the improved design where the side air vents are “stepped”. It has been found that the width of the tunnel in a plane above the centerline of the propeller should be 3 to 10 percent larger than the diameter of the propeller. The width of the tunnel in a plane below the center line of the propeller can also be 3 to 10 percent larger than the diameter of the propeller or even larger dependent on desired performance characteristics. In this configuration, a shortened air vent conduit ( 34 ) again draws air from the transom ( 36 ) of a vessel to a position before the propeller in a similar manner to the prior art. As shown in this embodiment the tunnel has a width located in a plane beneath the center line of the propeller X as disclosed, where “X” varies from ten percent larger than the diameter of the propeller to approximately two times larger than the diameter of the propeller. The propeller ( 32 ) continues to operate as an air pump in a similar manner as disclosed in the prior art. However, the disclosed shape further allows water to be drawn to the propeller by use of the stepped wall depicted by width X. In this configuration there is no need to chamfer or radius the lower corner of the tunnel side wall because the widened tunnel alone is sufficient to provide water flow to the propeller during acceleration. A disadvantage of this embodiment however is that the stepped air vents reduce planning surface in the aft section of the hull and while this may not be a detriment, and may even be an advantage on some hulls, other hull shapes may find this loss of planning area unacceptable and so in those cases it is preferable to bring the lower surface of the air vent down to the planning surface of the hull and chamfer or radius its&#39; inside edge. 
       FIG. 2B  depicts the position of the stepped wall through radius ( 38 ) with the upper side position of the tunnel conforming to the teaching of the prior art and depicted by wall ( 40 ). The propeller ( 32 ) remains within the tunnel, the upper portion of the propeller surrounded by the tunnel shape disclosed in the prior art with a modification to the air vent conduit and stepping of the walls along the lower portion of the propeller. The result has been proven to provide the water flow necessary to provide smooth acceleration, and lessen the planning transition period. 
       FIGS. 3A and 3B  depict a tunnel of the prior art with an illustration of water flow during acceleration. Water flow blockage ( 13 ) results in turbulence to the propeller ( 12 ) as a result of the straight vertical side wall ( 18 ) inhibiting water flow. At the lower speed, the chute forming along the leading edge of the tunnel is ineffective, the hull design actually prohibiting debris, as well as water, from reaching the propellers. The lack of water resulting in a turbulent flow along the tips of the propeller, resulting in slippage and poor acceleration. 
     As depicted in  FIGS. 4A and 4B , the use of the stepped tunnel allows water flow to carry past the corner radius ( 38 ) and flood the tunnel with sufficient water to eliminate the turbulent flow area caused by the sharp tunnel walls. 
       FIGS. 5A and 5B  depict the efficiency of the invention of the prior art at speeds where a flow of water is delivered through the chute ( 19 ) directly to the propeller ( 12 ) and the efficiency of the super cavitating propeller is allowed to operate accordingly. 
     Similarly, as depicted in  FIGS. 6A and 6B  the water to the propeller ( 32 ) of the instant invention tunnel shape provides the same efficiency, wherein the upper portion of the tunnel maintains the shape necessary for the propellers to operate as an air pump. 
       FIG. 7A  is a rear view of a marine vessel having a surface piercing propeller  32  mounted on a unit  31 . The unit is positioned aft an angled front wall  33  which extends to a fixed tunnel roof  35 . Depicted is a transom  36 , with the tunnel  42  further formed by opposition vertical side walls  37  &amp;  37 ′ and angled transition walls  39  &amp;  39 ′.  FIG. 7B  shows a side view of the marine vessel  30  showing the relationship between the propeller  32  and the tunnel  42 .  FIG. 7C  is a bottom view of the vessel showing the propeller  32  the tunnel  42  and the transom  36  of the vessel  30 .  FIG. 7D  is a perspective view of the hull bottom showing the relationship between the hull bottom the tunnel  42 , the propeller  32 , and the transom  36 . The exact positioning of the propeller in relation to the top and each side wall is dependent upon the size of the vessel and the power plant. It has been discovered that optimum efficiency is possible when the tunnel is 3-10% larger than the diameter of the propeller. 
       FIG. 8A  is a rear view of the marine vessel showing the transom  36 , the tunnel  50  and the propeller  32 . The tunnel  50  has opposing side walls  52  and chamfered transition sections  54  that extend from the side walls  52  to the hull bottom. As shown at  56  the tunnel is widened aft of the propeller to facilitate the flow of water to the propeller disk when operating in reverse.  FIG. 8B  shows a side view of the marine vessel  30 , shown in  FIG. 8A , showing the relationship between the propeller  32 , and the tunnel  50  with the chamfered transition section  54 .  FIG. 8C  is a bottom view of the vessel showing the propeller  32  the tunnel  50  with the chamfered transition section  54 .  FIG. 8D  is a perspective view of the hull bottom showing propeller  32 , and the tunnel  50  with the chamfered transition section  54 . Optimum efficiency is possible when the tunnel is 3-10% larger than the diameter of the propeller, or expressed in the range of in the range of 1.03 to 1.1 times the diameter of the propeller. 
       FIG. 9A  is a rear view of the marine vessel showing the transom  36 , the tunnel  60  and the propeller  32 . The tunnel  60  has opposing side walls  62  and curved or radiused transition sections  64  that extend from the side walls  62  to the hull bottom. As shown at  66  the tunnel is widened aft of the propeller to facilitate the flow of water to the propeller disk when operating in reverse.  FIG. 9B  shows a side view of the marine vessel  30 , shown in  FIG. 9A , showing the relationship between the propeller  32 , and the tunnel  60  with the curved or radiused transition section  64 .  FIG. 9C  is a bottom view of the vessel showing the propeller  32  the tunnel  60  with the curved or radiused transition section  64 .  FIG. 8D  is a perspective view of the hull bottom showing propeller  32 , and the tunnel  60  with the curved transition section  64 . Optimum efficiency is possible when the tunnel is 3-10% larger than the diameter of the propeller. 
       FIG. 10A  is a rear view of the marine vessel showing the transom  36 , the tunnel  70  and the propeller  32 . The tunnel  70  has opposing side walls  72  and stepping transition sections  74  that extend from the side walls  72  to the hull bottom.  FIG. 10B  shows a side view of the marine vessel  30 , shown in  FIG. 10A , showing the relationship between the propeller  32 , and the tunnel  70  with the stepped transition section  74 .  FIG. 10C  is a bottom view of the vessel showing the propeller  32  the tunnel  70  with the stepped transition section  74 .  FIG. 10D  is a perspective view of the hull bottom showing propeller  32 , and the tunnel  70  with the stepped transition section  74 . Optimum efficiency is possible when the tunnel is 3-10% larger than the diameter of the propeller. 
       FIG. 11A  shows a fixed sloping tunnel roof section  80  located aft of the propeller. The trailing edge of section  80  is at or below the free surface of the water when the boat is at rest. This roof section  80  stops air from entering the propeller when operating in reverse. 
       FIG. 11B  shows an alternative embodiment to the tunnel roof section shown in  11 A. In this embodiment the roof section aft of the propeller includes a hinged roof panel  82  that is pivotally coupled to the roof  81  by a hinge. The hinged roof panel drops down when the vessel is operated in reverse and is lifted up when the vessel is operated in the forward direction. This hinged roof panel  82  serves to stop air from entering the propeller when reversing and swings up to reduce drag when going forward. In the preferred embodiment, the hinged roof panel operates under water pressure provided as the vessel moves forward, forcing the hinged panel upward or when the vessel is moved backward, forcing the hinged panel downward. Alternatively the hinged roof panel can be operated by an electric or hydraulic ram. 
       FIG. 12  is a graph of the acceleration improvement of the instant invention versus tunnel width. As the tunnel is widened, acceleration begins to drop off or decrease. This is illustrated by test numbers  412  in  FIG. 12 . The instant invention is illustrated by test numbers  2  and  3  in  FIG. 12 . The optimum efficiency, as demonstrated by acceleration, of the instant invention is achieved when the tunnel is 3-10% larger than the diameter of the propeller. This relationship between the diameter of the propeller and the tunnel width is illustrated in  FIG. 12  by the scale on the left side of  FIG. 12 . The instant invention, test number  2  and  3 , has a tunnel width of 103-110% of the propeller diameter. As illustrated in  FIG. 12  the acceleration of the instant invention is 21-22 seconds. The prior art acceleration is far less than these times. The best acceleration of the prior art is about 28 seconds which is significantly slower that the acceleration of the instant invention. The best acceleration of the instant invention, test number  2 , is 33% faster than the best acceleration of the prior art, test number  6 . This demonstrates a significant improvement over the prior art. 
     It is to be understood that while I have illustrated and described certain forms of my invention, it is not to be limited to the specific forms or arrangement of parts herein described and shown. It will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is shown in the drawings and described in the specification.
An improved tunnel configuration for tunnel mounted surface piercing propellers. The improved tunnel configuration provides a flooding suction to the tunnel to allow flooded propeller operation at speeds below planning. The tunnel is stepped whereby an upper portion of the tunnel is sized to allow the propeller to draw air at high speeds. The lower portion of the tunnel is sized to allow the propeller to be flooded resulting in smooth acceleration, improved handling in forward and reverse and a reduction of the transition period.