Abstract:
Spaced cambered wedges having flow diverting surfaces thereon are deployed from retracted positions within the smooth surfaced sides of a marine vessel hull undergoing water travel above a high speed, under which air ventilated cavities are established by the deployed wedges along the sides of the hull, imposing drag on the hull sides of a substantially reduced magnitude as compared to that otherwise imposed directly by the water alone.

Description:
The present invention relates generally to reducing drag resistance on marine vessels during travel. 
     STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore. 
     BACKGROUND OF THE INVENTION 
     During travel of a marine vessel within a body of water, flow of fluid relative to the solid external surfaces of the vessel hull is retarded by frictional contact imposing resistance to movement of the vessel dominated by frictional drag at high speeds. Many methods have been proposed to minimize such high speed drag resistance along the vessel hull, including emission of air bubbles, polymer injection, compliant coating and laminar boundary layer control by heating. While a degree of success may be achieved by such methods, various technical and practical problems are experienced therewith by reason of which high-speed friction drag reduction under efficient vessel propulsion operation remains a most challenging task. It is therefore an important object of the present invention to reduce drag resistance at high speeds without detracting from efficient operation of the marine vessel at all speeds of travel. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, spaced water flow diverting wedges are mounted on the vertical sides of a marine vessel hull for deployment from retracted positions to protract flow diverting surfaces thereon of reduced length extending at a suitable angle from the smooth surfaced sides of the hull in the direction of travel so as to establish air ventilated cavities along the hull sides, within which the water is mixed with air, imposing drag resistance on the hull sides that is substantially less than that imposed directly by the water alone. 
    
    
     DESCRIPTION OF DRAWING FIGURES 
     FIG. 1 is a simplified side elevation view of a typical marine vessel hull, with the flow diverting wedges thereon deployed during travel through a body of water; 
     FIGS. 2 and 3 are respectively top and front end views of the hull shown in FIG. 1; 
     FIG. 4 is a partial section view taken substantial through a plane indicated by section line  4 — 4  in FIG. 1, showing in detail one embodiment of the flow diverting wedges in a deployed position; 
     FIG. 4A is a partial section view corresponding to that of FIG. 4, showing the flow diverting wedge in a retracted position; 
     FIG. 5 is a partial section view corresponding to that of FIG. 4 illustrating a modified form of the flow diverting wedge, pursuant to another embodiment; and 
     FIG. 6 is a partial section view illustrating yet another embodiment of the flow diverting wedge. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawing in detail, FIGS. 1,  2  and  3  illustrate a marine vessel hull  10  undergoing travel while floatingly supported within a body of water  12 , with its top  14  spaced above the water surface level  16 . The opposite vertical sides  18  of the hull  10  have series of laterally aligned cambered flow diverting wedges  20  mounted therein and protruding therefrom when deployed as shown, so as to establish ventilated cavities  22  within the water along such hull sides  18  during travel, as hereinafter explained. Each of the cambered wedges  20  extends vertically from a location slightly above the water surface level  16  to a depth somewhat above the keel  21  of the hull. The number of such wedges  20  on each hull side  18 , the spacing therebetween, their sizes and wedge angles (α) and vertical lengths will vary, depending on the length of the vessel hull  10  and other factors as hereinafter explained. 
     In accordance with one embodiment, each of the cambered wedges  20  as shown in greater detail in FIG. 4, is of a generally triangular cross-sectional shape having a water flow diverting surface  24  that is slightly curved depending on the general side curvature of the hull side  18  and extending at an angle from its smooth surface in the direction of travel, when the wedges  20  are deployed as shown. Under travel at high speeds, above 45 to 50 knots, such wedges  20  generate the ventilated cavities  22  along the hull sides  18  as diagrammed in FIG. 2, filled with a mixture of atmospheric air and water. The drag imposed on the hull  10  by such air ventilated cavities  22  on the hull sides  18  at the high travel speed, is less than the friction drag ordinarily imposed by the water alone on the hull side surfaces. Thus, at the lower travel speeds of 45 knots or less, the wedges  20  are pivotally retracted as shown in FIG. 4A, into reception openings  26  formed within mounting blocks  28  positioned in the hull sides  18 . In order to provide for inflow of additional atmospheric air into the air/water mixture within the ventilated cavities  22 , if needed below certain depths, a recess  30  is formed in the trailing end of the wedges  20  for exposure to the air ventilated cavities  22 , when the wedges  30  are deployed as shown in FIG.  4 . The magnitude of drag (D) imposed by such ventilated cavity  22  under travel speed (V) depends on the associated cavitation number (σ) and the cavity drag coefficient (C D ), so that: C D (σ)=C D (0)[1+σ] when σ is small. For a fully vented cavity  22 , when a σ=0, the ventilated cavity drag coefficient C D  (0)=2(t/C)/π, where (t) is the wedge thickness and (C) is the wedge chord length. The ventilated cavity drag (D), is then given as: D=C D (0)[0.5 ρV 2 ] (t/ 2), while the cavity length (L) is given by:        L   =         8      t       π                   σ   2                  C   D          (   0   )       .                              
     Based on the foregoing formulae, the dimensions, the number and spacings between the triangular type wedges  20  as shown in FIG. 4, may be optimized for a given hull  10 . Also, the value of the high speed of travel velocity (V) above which wedge deployment is to be effected may be determined. The flow diverting surface  24  having a length (lx) then imposes a friction drag (F) having a coefficient (C f ) associated therewith, given as: 
     F=C f (Re) [0.5 ρV 2 ] lx, where (Re) is the Reynolds number. Based on the foregoing, the ratio of ventilated cavity drag (D) to wedge friction drag (F) is estimated as:            D   /   F     =       π                   σ   2         16                   C   f           ,                          
     by setting L=lx, indicating that the friction drag (F) introduced by the wedge surface  24  is minimized relative to the drag (D) imposed by the ventilated cavities  22  as hereinafter pointed out. 
     In accordance with another embodiment as shown in FIG. 5, a modified form of wedge  20 ′, has a parabolic cross-sectional shape. Such wedge configuration provides a 38% reduction in the ratio of cavity drag (D) to wedge friction drag F, as reflected by:          D   /   F     =     0.62            π                   σ   2         16                   C   f         .                              
     Pursuant to yet another embodiment of the present invention, as illustrated in FIG. 6, a piping system  32  is installed so that pressurized air from a diagrammed pump  34 , of 15 psi for example, is introduced and conducted through a triangular shaped wedge  20 ″ from the bottom portion of its trailing end into a ventilated cavity  22 ′, within which pressurized air along with atmospheric air is mixed with the water for drag resistance reduction as hereinbefore described. 
     Thus, in accordance with the present invention a marine vessel or ship is operated under conventional conditions of a smooth surface hull, with the cambered wedges  20 ,  20 ′ or  20 ″ retracted at speeds less than 45 to 50 knots. At higher speeds greater than 45 to 50 knots, the series of wedges  20 ,  20 ′ or  20 ″ are deployed from the hull sides  18  so as to produce the air ventilated cavities  22  or  22 ′, and thereby eliminate the hull surface friction drag resistance heretofore experienced at such high speeds of travel with reduced ventilated cavity drag imposed by the protracted flow diverting wedge surfaces  24 . 
     Obviously, other modifications and variations of the present invention may be possible in light of the foregoing teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.