Abstract:
The instant invention is a shroud/keel/rudder combination for use on a engine driven marine vessel having a vented tunnel propulsion system. The combination produces acceptable acceleration, minimal loss of speed, resistance to weed entanglement and optimized steering, in both forward and reverse, especially when used with surface piercing propellers.

Description:
PRIORITY APPLICATION 
       [0001]    This application is based upon Provisional Patent Application No. 60/889,596 filed Feb. 13, 2007, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention is directed to the field of watercraft, and in particular to surface piercing propellers in a ventilated tunnel with a protective shroud and keel and steering fins forward of the center-of-gravity of the vessel. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is well known in the industry that protecting the propeller of a vessel by a keel reduces the chances of damage to the propeller. It is also well known in the industry that raising the propeller up and in a tunnel reduces the chance for unplanned underwater impact. It is further known in the industry that a protective shroud provides an added level of protection for the propeller. Unfortunately it is also well known within the industry that keels and shrouds tend to have other negative side effects not the least of which are reduced acceleration and reduced steering performance. In addition, shrouds can become a source of weed entanglement. 
         [0004]    Examples of protective shrouds can be found in U.S. Pat. Nos. 6,544,082; 6,475,045; 5,890,937; 4,957,459; 4,826,461; 4,789,302; 4,694,645; 4,680,017; 4,637,801; 3,859,953; 3,035,538; 2,244,217. 
         [0005]    What is needed in the art is a shroud/keel combination that provides a level of protection for the propeller without hurting steering performance or acceleration and without entangling weeds. 
       SUMMARY OF THE INVENTION 
       [0006]    The instant invention is a keel/shroud assembly around the propeller with greatly increased tip clearance to provide improved acceleration. 
         [0007]    Another objective of the invention is to provide enhanced steering performance by adding fins in front of the center-of-gravity of the boat. 
         [0008]    Another objective of the invention is to provide a means for weeds to shed from the protective shroud. 
         [0009]    Still another objective of the instant invention is to teach the use of two steering rudders located inside the propeller slip stream. 
         [0010]    Still another objective of the invention is to provide simplistic installation by inclusion of a conventional rudder system to the mid-section mounting base. 
         [0011]    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  
         [0012]      FIGS. 1A ,  1 B,  1 C and  1 D are various prospective views of a full shroud with tip clearance greater than 5% of the propeller diameter; 
           [0013]      FIGS. 2A ,  2 B,  2 C and  2 D are a perspective views of a partial shroud showing the areas at each end providing for the passage of weeds; 
           [0014]      FIGS. 3A ,  3 B and  3 C are various perspective views of dual rudders in a mounting plate; 
           [0015]      FIG. 4A ,  4 B,  4 C and  4 D are various perspective views of the steering fins forward of the center of gravity of the hull. 
           [0016]      FIG. 5  depicts the acceleration performance versus propeller shroud tip clearance. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    Now referring to the Figures in general, depicted is a shroud and keel arrangements along with steering fins forward of the center of gravity. The use of a shroud and keel is beneficial to most any type of propulsion systems and has particular benefit for use with a drop-in vessel propulsion module which employs a tunnel created for a surface piercing propeller. The module, or drive gears, are a rigid assembly wherein the shroud protects the module from impact. 
         [0018]    A shrouded propeller of the instant invention may include a keel and can be used with conventional propellers as well as surface piercing propellers. The shroud avoids the necessity for a breakaway gear case such as the Volvo IPS or Mercury Zeus, where breakaway design is critical to vessel safety. The IPS and Zeus extend beneath a vessel making them susceptible to impact with submerged objects. The shroud of the instant embodiment eliminates the need for a breakaway housing thereby providing a low cost, mechanically simple, propulsion system that allows a vessel to impact a submerged object without loss of the drive system. 
         [0019]    It should be noted that the use of surface piercing propellers requires proper distancing of a shroud to the propeller tips to avoid turbulence that can prevent planning of the vessel. Conventional propellers show little acceleration differences upon placement of the shroud near the propeller tip. 
         [0020]    Now referring to  FIGS. 1A through 1D  which set forth is a tunnel embodiment for use with a drop in module, a shroud  10  is positioned around a a tunnel having deep side walls as typically used with a surface piercing propeller. In this embodiment a surface piercing propeller shroud ( 10 ) is created by use of a protective keel ( 20 ) that extends beneath the gear housing ( 30 ) and propeller ( 40 ) with air drawn through a vented tunnel ( 50 ). 
         [0021]    Shroud clearance is determined by a propeller tip clearance that can be determined by a measurement of the propeller diameter. Preferably tip clearance is greater than 5% of the propeller diameter to allow the vessel to achieve planning speeds. A distance between the propeller tip and the shroud greater than approximately 30% propeller diameter does not further increase performance characteristics. The protective shroud ( 10 ) extends from the surface of the hull ( 80 ) to over the propeller tips to protect the propeller and the gear case from impact with submerged matter and attaches to the protective shroud ( 10 ). 
         [0022]      FIG. 1A  illustrates a rear view of the shroud ( 10 ) and the protection provided to the propeller ( 40 ) from impacts. Unlike deep side wall tunnels, the shroud ( 10 ) is opened to water at low speed eliminating propeller slippage at slower speeds. However, similar to a full tunnel, impact to the propeller and gear housing is protected. 
         [0023]      FIGS. 2A ,  2 B,  2 C, and  2 D illustrate an embodiment which utilizes a partial shroud. The hull of the vessel  80  includes a shallow tunnel  90  that extends longitudinally along the length of the vessel and extends from a position approaching the bow of the vessel and extends to the deep walled vented tunnel  95 . The hull also includes a vent tunnel  50  in a manner which is to the tunnel previously described. The surface piecing propeller  40 , the gearing case  30 , and the rudder assembly  35  are mounted in the vented deep wall tunnel  95 . The hull also includes a protective keel  25  that is attached to the hull in a region of the shallow longitudinal tunnel  90  adjacent to the deep wall vented tunnel  95  at one end and attached to the gear housing  30  on the second and opposite end. Also attached to the second end of the protective keel  25  is a partial protective shroud  15 . Shroud  15  is a wing like construction which is mounted on the protective keel  25  and the gear case skeg. The partial shroud has a leading edge facing the bow of the vessel and a trailing edge face the stern. The partial shroud extends from the protective keel  25  equally from side to side on opposite sides of the longitudinal axis of the vessel. The partial shroud  15  also has an arcuate shape from side to side. The width of the partial shroud as measured from side to side is slightly less than the diameter of the surface piercing propeller. The rudder assembly  35  includes two rudders each pivotally mounted on a vertical axis that are equally spaced from the center line of the vessel. The distance between the two vertical axes is less than the diameter of the surface piercing propeller ( 40 ). 
         [0024]      FIG. 5  depicts the acceleration performance versus propeller shroud tip clearance. With tip clearance less than 5% of propeller diameter, planning of a boat is not possible. Greater than 5% of propeller diameter for tip clearance allows for marginal planning of the hull while beyond 30% propeller diameter tip clearance does not provide significant gains in acceleration performance. 
         [0025]    Both shrouds and keels of ANY configuration contribute to decreased steering performance because vessels are normally steered by sliding the transom sideways (slipping). The addition of a keel resists sliding the hull sideways in a turn. In order to compensate for this reduced turning action, it has been determined that two design features can contribute to restoring steering performance. The first is the use of dual rudders in the prop wash which provide a stronger sliding force at the transom and the second being the addition of one or more steering fins, as often found on Inboard ski boats, forward of the L.C.G. (longitudinal center of gravity). It is the combination of these two features (dual rudders and steering fins) that produce sufficient steering force to compensate for the shroud and keel. 
         [0026]    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.