High speed drag reducing ventilation for marine vessel hulls

The present invention relates generally to reducing drag resistance on marine vessels during travel.

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.

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.

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.

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.

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.