Hydraulic and earth engineering – Bank – shore – or bed protection
Reexamination Certificate
1998-09-29
2001-01-09
Lillis, Eileen D. (Department: 3673)
Hydraulic and earth engineering
Bank, shore, or bed protection
C405S303000, C037S195000
Reexamination Certificate
active
06171021
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the operation of high-speed vessels in shallow waters, and, more specifically, to a method and apparatus for reducing wake wash produced by high-speed vessels in the course passage through shallow waters in bodies of water such as harbors, rivers, canals and the like.
BACKGROUND OF THE INVENTION
High speed vessels, including military craft, ferries and pleasure boats, are steadily increasing in number in many areas of the world. The development of water jets and light weight construction methods has made it both possible and economical to transport people and goods on the water at increasingly higher speeds. Unfortunately, such higher speeds have also created problems with wake wash in relatively shallow waters near the shoreline such as in harbors, rivers, canals and other estuaries.
Studies have been conducted to determine the effects of running a vessel in shallow waters. One important parameter is known as the depth Froude number, which is a function of vessel speed, the water depth and gravitational acceleration. It has been found that a depth Froude number of unity corresponds to the maximum speed at which free harmonic water waves can travel undisturbed on the surface of a body of water. Vessels operated at a speed in shallow waters which produces a depth Froude number of about unity develop moderate size waves which can travel long distances at high energy. As these high energy waves approach a shoreline, where the water depth continues to decrease, the wave periods become shorter causing the wave height to increase. In turn, these larger waves can be hazardous to other users of the body of water and can severely damage the environment and/or man-made structures along the shoreline.
The speed at which a vessel produces a depth Froude number of unity, for a given shallow water area such as a harbor, river or canal, is known as the critical speed. Modern high speed vessels are operated at subcritical speeds in deep waters, but once entering shallow waters the same vessel speed over ground can be critical or supercritical. The problem of excessive wake wash mainly occurs when a high speed vessel transitions between supercritical speed and subcritical speed in the course of passing through a shallow water area. For example, a high speed ferry must decelerate from supercritical speed to subcritical speed in the course of entering a harbor to unload passengers, and then accelerate from subcritical speed to supercritical speed on the return trip. The longer it takes for the ferry to accomplish these transitions, the more wake wash is created, fuel is wasted and time is lost.
Another problem associated with transitioning between subcritical speed and supercritical speed, particularly for slower vessels, results from the increase in wave making as the vessel approaches critical speed. The larger waves formed by the vessel near the critical speed act, in effect, as a barrier and resist acceleration of the vessel which slows it down. Consequently, additional fuel and energy area required to overcome this wave resistance in the course of accelerating the vessel from subcritical speed through critical speed to supercritical speed.
The problems with vessel operation and unacceptable wake wash noted above have been investigated, but no viable solutions have been proposed. Although a vessel can be operated at reduced, subcritical speed before reaching shallow waters, this substantially increases transport time and can waste fuel. Additionally, while breakwaters have been employed in some areas to reduce the effects of wake wash, this is expensive and often cannot be employed in smaller bodies of water such as river, canals or other estuaries.
SUMMARY OF THE INVENTION
It is therefore among the objectives of this invention to provide a method and apparatus for reducing wake wash in shallow water areas which avoids operation of high speed vessels at the critical speed, which permits a transition directly from supercritical speed to subcritical speed, which is effective in virtually all types of shallow water areas, which preserves the shoreline and which increases the economies of high speed vessel operation.
These objectives are accomplished in a method and apparatus involving forming a transition area along the bed of a body of water in which the natural water depth is altered, and operating a vessel in the course of passage over the transition area such that the vessel speed instantaneously changes from supercritical speed to subcritical speed without passing through critical speed.
One aspect of this invention is predicated upon the concept of changing the configuration of the bed in a discrete area of shallow waters within a body of water over which vessels can be decelerated and accelerated without passing through the critical speed. In one presently preferred embodiment, the transition area is in the form of a dredged pit having a bottom wall, opposed side walls and opposed end walls collectively defining an interior having a depth greater than the normal or natural depth of the water at that location. The length of the dredged pit, or distance between the opposed end walls, is preferably about two to five vessel lengths. The distance between the two side walls, or width of the pit, is preferably on the order of about one to five times that portion of the width of the vessel which is submerged in the water.
In an alternative embodiment, the transition area comprises a ramp having a first end, a second end spaced from the first end, a top wall extending between the first and second ends and opposed side walls located on either side of the top wall. The ramp has a height dimension, measured from the bed of the body of water in an upward direction, which increases from the first end to the second end at which the water level is less than the natural depth of the body of water. The length and width dimension of the ramp of this embodiment are substantially the same as the area of the interior of the dredged pit described above.
In a still further embodiment, the transition area is formed from a combination of the ramp and dredged pit discussed above. Preferably, a ramp and dredged pit are located immediately adjacent one another with a combined length in the range of about two to five vessel lengths or more, and an overall width in the range of about one to five times the width of that portion of the vessel which is submerged in the water.
It is contemplated that the transition area utilized in a particular location will be dependent upon the configuration of the existing bed of the body of water, with a view toward minimizing the amount of construction required to build the transition area. Regardless of the type of transition area employed, an important aspect of this invention involves operating a particular vessel in such a way as to “skip” or transition between supercritical speed and subcritical speed in the course of passage over the transition area, without passing through critical speed. In the presently preferred embodiment, the vessel speed is controlled to decelerate from a depth Froude number of about 1.4 to a depth Froude number of about 0.8 as the vessel passes over the transition area. Conversely, when accelerating the vessel, the speed over the transition area is increased to transition from a depth Froude number of about 0.8 to about 1.4. As noted above, and described in detail below, such vessel speeds are a function of gravitational acceleration and the water depth of a particular body of water in the region of the transition area. In practice, the location of a transition area within the shallow waters of a body of water will be marked with buoys or the like, and operators of vessels will be assigned specific speeds to be observed upon entering and leaving the transition area depending upon tidal conditions. By avoiding the critical speed in areas close to the shoreline, damage to the environment and man-made structures caused by wake wash is substantially reduced.
REFERENCES:
patent: 1314743 (1919-09-01), Groat
Holland & Knight LLP
Lagman Frederick L.
Lillis Eileen D.
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