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Stealthy  Ships

Using Adaptive Water Curtain Technology (AWCT)
Technology, of AWCT

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Proposed is a technology (AWCT) for adding Stealth properties to existing U.S. Fleet surface ships by the use of sprayed sea water.

The Adaptive Water Curtain Technology (AWCT) is intended to deflect and scatter enemy radar waves thus reducing the ship’s radar cross section (RCS). It consists of (highly conductive) sea water sprayed in a fashion that effectively creates an angled radar reflective curtain around the ship.

To reduce the ship’s remaining RCS, the water curtain can be "modulated" such that the returns appear as "Sea Clutter." This could be done by determining the surrounding Sea State--either locally, or from satellite Sea State data, i.e., deriving the Sea Clutter Spectrum; and applying the appropriate coefficients to the modulating process for optimum mimicry.

This approach is suggested as an "Add-On" to existing surface ships, an interim measure until the next generation DD(X) of stealthy surface ships has replaced this class. The Arleigh Burke class Destroyer--which has rudimentary stealth technology, is used as an example of a recipient ship for this technology. Although this class of ship has a reduced RCS over its predecessor, it can still benefit significantly from the proposed technology. 

This is a Work in Progress.
There are fundimental questions yet to be answered.   See List

-Sea Clutter-
Insufficient Threshold -----------------------------Raised Threshold
Optimum Threshold -----------------------------Excessive Threshold
-Illustration of Sea Clutter overlaid by Detection Threshold-

Targets Detected above the Clutter-
-Sea Clutter Noise relative to Target Signal-

 -Sea State
Sea State = Sea Clutter
Sea State Defined:
Sea State refers to the condition of the sea's surface, quantified in terms of wave height, period, and character. The large number of variables involved in describing Sea State such as wedges, cusps, waves, foam, turbulence, and spray, as well as breaking events of all sizes and masses of falling water. Any or all of these might contribute to the scattering of electromagnetic waves responsible for sea clutter. The basic oceanographic descriptor of the sea surface, however, is the wave spectrum, which, while saying little about these features, contains a great deal of information about the sea surface in general and is central to the application of the Bragg scattering hypothesis. 

There is the need to understand the sea surface in order to understand sea clutter and the prominence of the Bragg hypothesis in existing clutter models.

There are basically two types of surface waves, capillary and gravity, depending on whether surface tension or gravity is the dominant restoring force. The transition between one and the other takes place at a wavelength of about 2 cm; so the smaller capillary waves supply the surface fine structure while gravity waves make up the larger and most visible surface structures. Waves have their origin ultimately in the wind, but this does not mean that the "local" wind is a particularly good indicator of what the wave structure beneath it will be. In order to arouse the surface to its fully developed or equilibrium state, the wind must blow for a sufficient time (duration) over a sufficient distance (fetch). That part of the wave structure directly produced by these winds is called sea. But waves propagate, so even in the absence of local wind, there can be significant local wave motion due to waves arriving from far away, perhaps from a distant storm. Waves of this type are called swell, and since the surface over which the waves travel acts as a low-pass filter, swell components often take the form of long-crested low-frequency sinusoids.

State of the Sea
Height of wave
   0    Calm sea    0 or less than 1 foot
   1    Smooth sea    1 to 2 feet
   2    Slight sea    2 to 3 feet
   3    Moderate sea     3 to 5 feet
   4    Rough sea     5 to 8 feet
   5    Very rough sea     8 to 12 feet
   6    High sea    12 to 20 feet
   7    Very high sea     20 to 40 feet
   8    Precipitous sea     40 feet and over
   9    Confused sea     Record chief direction
The 1938 edition of the US Navy Hydrographic Office publication No. 9
The most recent Sea State Descriptors: The Beaufort Scale
One more of the many Sea State Descriptors
-Gradations of Wave Action-
 Smooth -------------------------------------------------Slight
 Moderate -------------------------------------------------Rough
High -------------------------------------------------Very High
Smooth Bore Nozzle for Solid Stream

Typical input diameter to output opening is 2:1
 Remote Controllable Monitors with Nozzles
Monitor using Hydraulics for Fast Response
Monitor with "Dither Modulation Mechanism"
-Linear Nozzle Array-
-Omnidirectional Nozzle Array-
Linear Nozzle Array
Array shown with one section in up, or operational, position. 
The external "central feeder pipe" is an indication of the "add on" nature of the AWCT.
End View Diagram
-Water Pump-

Water Jet Propulsion Pump.
Powered by Jet Aircraft Engine
-Distance Chart  Combination Nozzle 100 PSI

Approximate Effective Stream Trajectory at 30° Elevation in No Wind Conditions. Distance to Last Water Drop Approximately 10% Farther

Curve A B C D E F G
US GPM 300 500 600 750 1000 1500 2000
Nozzle Reaction (LBS) 152 250 303 380 505 758 1010
     Based on nozzle pressure of 100 psi.-
Angle / Distance Table:  Straight Solid Stream
Using Smooth Bore Nozzle Minitors
Angle of Elevation
1½"(38mm) Orifice
668 US GPM Flow
1¾"(45mm) Orifice
918 US GPM Flow
2"(51mm) Orifice
1188 US GPM Flow
PSI Distance in Feet
PSI Distance in Feet
PSI Distance in Feet
32º 100 57 172 100 75 185 100 84 195
150 70 185 150 88 212 150 93 224
200 86 200 200 95 225 200 98 233
250 93 215 300 98 242 250 105 265
45º 100 85 147 100 105 159 100 112 176
150 102 157 150 115 174 150 128 191
200 113 163 200 127 195 200 136 205
250 123 179 250 134 210 250 144 216
75º 100 134 55 100 149 56 100 153 57
150 160 57 150 173 60 150 178 63
200 171 62 200 192 67 200 201 70
250 187 65 250 203 69 250 214 75

Fire (Water) Streams cease to be effective where they lose body, direction or force. Beyond this point the water is in the form of heavy rain and is easily carried away by air currents. The point of effective reach of the fire streams tabulated above was established as the point where the slugs of water which broke away from the main body of the stream were still closely enough grouped to be effective in extinguishing a fire. As these slugs became further separated from the stream and broke into spray the stream was considered ineffective. Tests were conducted under good conditions (5 mph winds). Adverse winds will considerably reduce the effective range of monitor nozzle streams.
Water Stream Velocity verses PSI

Velocity is measured at Nozzle Orifice
DDG-51 Destroyer with Rear-Directed Water Curtain-

Note that the rear-directed water streams augment the ship's propulsion.-
Propulsion Augmentation
Water Stream is Angled Back at 45° and Out at 45°
(only one stream is shown for simplicity)-

Each Stream Contributes to the Ship's Thrust by 
virtue of the Water Jet Reaction of each Nozzle.
Note that each nozzle's net contribution to the ship's 
forward motion is ~50% of the nozzle's reaction (thrust).-
GE LM6000 Gas Turbine 
Horse Power 57,300
Thrust, Lbs. 255,300
GPM 1,079,508
MW >40


Water Stream Reaction verses Flow Rate (GPM)
-Wind Effects on Water Streams-
Bow Stream
Ideal Water Stream, @ <5knts
Straight into the Wind
Wind Affected Water Stream, @ >>5knts
(Note the Nozzle/Monitor is tilted for compensation)
Near Orthogonal Stream
Ideal Water Stream, @ <5knts
Ship Straight into the Wind
Wind Affected Water Stream, @ >>5knts
(Note the Nozzle/Monitor is rotated for compensation)
-Determining Required Water Volume in GPM-
     Ship Circumference = 1,076'
     Splash Circumference = 1,316'
Stream Geometry



Solve for GPM
Gallons per Foot (velocity)
Time to Splash-
Nozzle Spacing-
Width Start-
Width Minimum-
Width Average-
Width at Splash-
Throw Length-
Ship Circumference-
Splash Circumference-
Possible Number of Nozzles Needed
Illustration of 45° Stream Trajectories (Throw Length)
Throw Lengths (TL) are in Red
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 False-Alarm Rates

With these ideas in mind, it is not surprising that the physical area of a target is normally greater than the radar cross section because some the incident energy is scattered and absorbed by the target. Use of radar absorbent material (RAM) and specific shapes and angles helps to minimise the radar cross section. Target aspect also changes the radar cross section markedly and needs to be allowed for in the design of a radar system. Radar designers normally use extremely pessimistic (low) values for radar cross section during radar design and performance calculations to ensure their systems meet minimum requirements when introduced into service.
     The radar cross section of a target is not constant with operating frequency. There are three broad areas of interest with respect to physical target size, operating frequency and resulting radar cross section. These areas are:

    Enhanced backscatter due to coherent combination of signals reflected from a rough surface having features, with periodic distribution in the direction of wave propagation, and whose spacing is equal to half of the wavelength as projected onto the surface.

Stealth technology - attention to details
In order to achieve optimal stealth properties, the following signatures call for special attention:

Radar cross-section (RCS)
Infrared signature (IR)
Acoustic signature (hydroacoustic and airborne noise)
Magnetic signature (DC, AC and eddy current flux)
Underwater electrical potential (UEP)
Pressure signature
Visual signature, including night vision device (NVD) protection
Transmitted signals (control and directivity)
Laser cross-section

Beaufort Scale, short version:
 No:  Wind type:               Knots: 
   0   Calm                          0 -   1 
   1   Light Airs                   2 -   3 
   2   Light Breeze               4 -   6 
   3   Gentle Breeze             7 - 10 
   4   Moderate Breeze      11 - 16 
   5   Fresh Breeze             17 - 21 
   6   Strong Breeze           22 - 27 
   7   Near Gale                 28 - 33 
   8   Gale                          34 - 40 
   9   Strong Gale               41 - 47 
 10   Storm                        48 - 55 
 11   Violent Storm            56 - 63 
 12   Hurricane                  64 - -> 

Continuity Equation:


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