Abstract:
A momentum altering system comprises a transportation device configured to transport the momentum altering system towards an object moving through water. An engagement device is configured to attach to the object when the momentum altering system is transported sufficiently near the object. At least one decelerating device is connected to the engagement device. At least one decelerating device is deployed by the engagement device after the engagement device attached to the object. At least one decelerating device includes a plurality of parachute sea anchors that produce drag when pulled though water thereby altering momentum of the object.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a utility application claiming priority to U.S. Provisional Application Ser. No. 61/635,052 filed on Apr. 18, 2012 entitled “NET ENGAGEMENT WITH PARACHUTE SLOWDOWN (NEPS) SYSTEM FOR NON-LETHAL MOBILITY HINDERING OF MARITIME VESSELS,” the entirety of which is incorporated by reference herein. 
    
    
     GOVERNMENT RIGHTS IN THE INVENTION 
     This invention was made with government support under grant number N0014-08-C-0329 awarded by the U.S. Navy, Office of Naval Research (ONR). The government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to systems for altering the momentum of vessels. More specifically, the invention relates to reducing the momentum of maritime vessels using parachute sea anchors (PSA). 
     BACKGROUND 
     Large maritime vessels have considerable momentum while in motion. Stopping these vessels quickly and over a short distance is of particular interest, for example when intercepting hostile vessels engaged in sea piracy. A drogue chute is a canopy shaped device that is used by mariners in a storm to keep the bow of the vessel pointed in the direction of the prevailing waves. 
     The publication “Concept of Using Drogue Chutes as a Ship Decelerator System” describes the use of a series of equal sized drogue chutes to decelerate a ship but fails to provide a complete solution to remotely intercepting and decelerating a vessel (see Chiang, L., Dunker, S., “Concept of Using Drogue Chutes as a Ship Decelerator System,” Waterside Security Conference, Marina di Carrara, Italy, November, 2010). Indeed, this publication describes this well recognized and long standing problem in its conclusion by stating “However, more testing and development would be required when sizing the system to full scale as the system would have a considerable increase in volume and weight, that could make it more difficult to maneuver and position than subscale systems. Attaching the system to oncoming vessel would be another challenging development to address. [sic]” 
     Deploying a decelerating system is further complicated by the variety of bow shapes, and potential misalignment between the ship trajectory and the deployed system. In addition, there are considerable forces involved in decelerating a ship with a hull displacement up to and exceeding 300,000 tons at 10-20+ knots without resorting to excessively bulky or heavy materials. A system is required that can deploy a lightweight and small form factor device remotely towards a hostile vessel, attach to the vessel and then decelerate the vessel in a short period of time. 
     BRIEF SUMMARY 
     In one aspect, the invention features a momentum altering system comprising a transportation device configured to transport the momentum altering system towards an object moving through water. An engagement device is configured to attach to the object when the momentum altering system is transported sufficiently near the object. At least one decelerating device is connected to the engagement device. At least one decelerating device is deployed by the engagement device after the engagement device attached to the object. At least one decelerating device includes a plurality of parachute sea anchors (PSAs) that produce drag when pulled though water thereby altering momentum of the object. 
     In another aspect, the invention features a momentum altering system comprising a transportation device configured to transport the momentum altering system from an aircraft towards an object moving through water. The transportation device includes a parafoil. An engagement device is configured to attach to the object when the momentum altering system is transported sufficiently near the object. The engagement device comprises a load bearing line in communication with a one or more self-tensioning loops, the one or more self-tensioning loops are in communication with a base net based on a tensegrity structure with a lasso. The self-tensioning loops distort the base net to increase a contact area between the base net and the object upon contact of a portion of the base net with the object. At least one decelerating device is connected to the engagement device. The at least one decelerating device is deployed by the engagement device after the engagement device attaches to the object. Deploying the at least one decelerating device includes deploying a plurality of parachute sea anchors (PSAs) at a preset time by a programmable time release unit (PTRU) that includes a timer. Each of the plurality of PSAs is deployed with temporal separation from another of the plurality of PSAs sufficient to alter the momentum for the object within a load limit of each of the plurality of PSAs. Each of the plurality of PSAs produces drag against a progressively larger volume of water than a previously deployed PSA of the same deceleration device. 
     In another aspect, the invention features a momentum altering method comprising transporting a net toward an object moving through water. The net has a lasso-based structure connected to a plurality of parachute sea anchors (PSAs) by a plurality of self-tensioning loops. The object is engaged with the net. Each of the plurality of PSAs is deployed into the water with temporal separation from another of the plurality of PSAs. Each of the plurality of PSAs resists a larger volume of water than a previously deployed PSA. The net tightens to substantially conform to a feature of the object by causing at least one of the plurality of self-tensioning loops to move thereby distributing a load of the plurality of PSAs to the net. The object is decelerated by resisting a flow of water. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1A  is an elevation view of a parafoil-guided NEPS system deployed from an aircraft. 
         FIG. 1B  is an elevation view of the NEPS system in  FIG. 1A  prior to engaging a vessel. 
         FIG. 1C  is an elevation view of the NEPS system in  FIG. 1A  after engaging the vessel. 
         FIG. 2A  is elevation view of a rocket-propelled NEPS system deployed from a towable platform. 
         FIG. 2B  is an elevation view of the NEPS system in  FIG. 2A  prior to engaging a vessel. 
         FIG. 2C  is an elevation view of the NEPS system in  FIG. 2A  after engaging the vessel. 
         FIG. 3A  is an elevation view of a rocket-propelled NEPS system deployed from a helicopter. 
         FIG. 3B  is an elevation view of the NEPS system in  FIG. 3A  prior to engaging a vessel. 
         FIG. 3C  is an elevation view of the NEPS system in  FIG. 3A  after engaging the vessel. 
         FIG. 4A  is a plan view of a self-tensioning engagement net. 
         FIG. 4B  is schematic view of a portion of the net shown in  FIG. 4A  before load equalization. 
         FIG. 4C  is a schematic view of the portion of the net shown in  FIG. 4B  after load equalization. 
         FIG. 5  is a plan view of a lasso engagement net. 
         FIG. 6  is a plan view of an embodiment of NEPS system as shown in  FIG. 2B  and 
         FIG. 3B . 
         FIG. 7  is a plan view of an embodiment of a tensegrity-expanded engagement net as shown in  FIG. 1B . 
         FIG. 8  is a schematic view of a deceleration device. 
         FIG. 9  is a schematic view of test setup of the NEPS system. 
         FIG. 10A  is a graphical view illustrating the rate of deceleration of the vessel shown in  FIG. 9 . 
         FIG. 10B  is a graphical view illustrating the distribution of force over time for the parachute sea anchors shown in  FIG. 9 . 
         FIG. 11  is a graphical view illustrating the rate of deceleration of a full-scale vessel. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of systems described herein provide for the efficient deployment of a decelerating device, the attachment or engagement of the device to a maritime vessel and the deceleration of the vessel within a short period of time. The decelerating device is launched from a variety of platforms including, but not limited to, aircraft, ships, rapid inflatable boats (RIB), helicopters and drones as further described in the embodiments herein. The launching or deployment of the device, the attachment to the vessel and the timed opening of each of the PSAs operates as an integrated NEPS system allowing for the effective interdiction and deceleration of maritime vessels. In another embodiment the NEPS system is used to decelerate runaway vessels arriving at a port of call. In other embodiments, the NEPS system provides a differential drag on a vessel to alter its trajectory. In another embodiment, the NEPS system alters the trajectory of an iceberg. 
       FIG. 1A ,  FIG. 1B  and  FIG. 1C  show an embodiment  10  of a NEPS system  12  deployed from a fixed wing aircraft  24  (e.g. C130). As illustrated in  FIG. 1A , the NEPS system  12  includes an engagement net  14 , a first pair of small decelerating devices  16   a  and  16   b , a second pair of medium decelerating devices  18   a  and  18   b , and a third pair of large decelerating devices  20   a  and  20   b . Each decelerating device includes a parachute sea anchor (PSA) in a deployment bag. Each PSA is released after a time delay determined by a timer contained in the deployment bag. In other embodiments, the NEPS system  12  includes more than three PSAs on each side for decelerating vessels with more momentum due to higher hull displacement or velocity. In another embodiment, the NEPS system  12  has fewer than three PSAs on each side (e.g two PSAs) and multiple NEPS systems are deployed to stop a larger vessel. The use of multiple NEPS systems with two PSAs allows the rate of vessel deceleration to be modified for each encounter with a vessel. In one example, the use of multiple NEPS systems is used to decelerate vessels with very large hull displacement. 
     The engagement net  14  and decelerating devices are bundled together and attached to a parafoil (e.g. JPADS 2K)  22  coupled with a controller  23  that releases the parafoil  22  from the NEPS system  12  and provides guidance to steer the parafoil  22  towards the bow of the ship  26 .  FIG. 1B  shows the NEPS system  12  being steered towards the bow of a vessel or ship  26  by the parafoil  22  and the parafoil  22  subsequently detaching from the NEPS system  12 . The parafoil  22  then drifts away. Alternatively, the parafoil  22  is maneuvered away from the ship  26  by the controller  23 . In one embodiment, the parafoil  22  is steered towards the bow of the ship  26  using a GPS guidance system in the controller  23 . In another embodiment, the parafoil  22  is maneuvered with an optical guidance system with pattern recognition capability in the controller  23  to detect the bow of the ship  26 . In another embodiment, the parafoil is steered by remote control by a datalink between the aircraft  24  and the controller  23 . 
     When the parafoil  22  guides the NEPS system  12  sufficiently close to the ship  26 , the controller  23  detaches the parafoil  22  from the NEPS system  12  and releases the bundled engagement net  14 , and deceleration devices on a trajectory towards the bow of the ship  26 . The parafoil  22  and controller  23  drift away to be recovered at a later time. In an alternate embodiment, the controller  23  is attached to the NEPS system rather than the parafoil  22 . 
       FIG. 1C  shows the NEPS system  12  after the engagement net  14  has captured the bow of the ship  26  and the PSAs have been released from their respective deployment bags. In the embodiment  10  shown in  FIG. 1A ,  FIG. 1B  and  FIG. 1C , the engagement net  14  is based on a tensegrity structure that expands after it is released as shown in  FIG. 1B , while it free-falls onto the bow of the ship  26 . For ships with a bulbous bow, the engagement net  14  need only land in the water in front of the ship to engage the bulbous bow directly. Although  FIG. 1C  shows a hexagonal-shaped engagement net  14  other shapes that support a tensegrity structure are envisioned within the scope of the NEPS system—for example, a pentagon or octogon. 
     After the engagement net  14  captures the bow of the ship  26 , the first PSA  28   a  is released from the decelerating device  16   a  after a time delay. The first PSA  28   a  remains connected to the engagement net  14  with a rode line  27   a . After a second delay the second PSA  30   a  is released from the decelerating device  18   a  and is connected to the engagement net  14  with the rode line  27   a . Subsequently, the third PSA  32   a  is released from the decelerating device  20   a  after a third time delay and is also connected to the engagement net  14  with the rode line  27   a . The staged deployment of the PSAs ensures that the design limits of each PSA are not exceeded. For example, the diameter of PSA  27   a  is less than the diameter of PSA  32   a  thus providing less drag force against the ship  26  while being able to withstand a higher speed through the water. In one embodiment, the ship  26  is decelerated by two groups of PSAs, one on the port (shown in  FIG. 1C ) and the other on the starboard side (not shown), thereby exerting more drag force on the ship  26  without substantially altering the ships trajectory, increasing the side loading on the engagement net  14  or increasing the risk of the PSAs getting tangled in the ships propellers. In another embodiment, the engagement net  14  connects to a group of PSAs on only one side of the ship  26  to change the ships trajectory. 
     The deployment of PSAs each with a progressively larger diameter reduces the time required to decelerate the ship  26  without unduly increasing the volume and weight of the NEPS system. This reduction in weight and volume in turn enables the use of a parafoil  22  to transport the NEPS from the aircraft  24  to the ship  26 . In other embodiments, a different number or PSAs are used to decelerate ships of different hull displacement and velocity. While the PSAs are shown with round canopies, other shapes are contemplated, for example an elliptical or square canopy. In one embodiment, the PSAs are of different shapes so that each subsequently deployed PSA has a higher drag cooefficient than the previously deployed PSA without necessarily using a circular canopy with a larger diameter. 
       FIG. 2A ,  FIG. 2B  and  FIG. 2Cs  show an embodiment  40  of a NEPS system deployed from another maritime vessel.  FIG. 2A  shows the NEPS system  42  on a towable platform  44  being towed by a coast guard patrol boat (CPB)  46 . In one embodiment, the towable platform  44  is stored remotely from the CPB  46 , in a shipping port for example, and quickly attached to the CPB  46  when needed. Alternatively, the towable platform  44  is attached to a rigid inflatable boat (RIB). With reference to  FIG. 2B , the NEPS system  42  includes an engagement net  48  that is propelled from the platform  44  towards the ship  26  by one or more rockets  50 . The rockets  50  are on the leading edge of the engagement net  48 . In one embodiment, drag-chutes  52  are on the trailing edge of the engagement net  48  to keep the engagement net  48  substantially opened prior to capturing the ship  26 . Similar to the transegrity-based net  14  shown in  FIG. 1A ,  FIG. 1B  and  FIG. 1C , the engagement net  48  is attached to decelerating devices  16   a ,  18   a  and  20   a  that will ultimately deploy on one side of the ship  26  and a second chain of decelerating devices  16   b ,  18   b  and  20   b  (not shown) that will deploy on the other side of the ship  26 . 
       FIG. 2C  shows the NEPS system  42  after the engagement net  48  has captured the bow of the ship  26  and the PSAs have been released from their respective deployment bags. The engagement net  48  is secured to the bow of the ship  26  with self-tensioning lines that equalize the force of the PSAs  28   a ,  30   a  and  32   a  on the engagement net  48 . In a manner similar to that described for  FIG. 1C , PSAs  28   a ,  30   a  and  32   a  connect to the engagement net  48  through a rode line  27   a  on the port side of the ship  26 . A set of PSAs  28   b ,  30   b  and  32   b  (not shown) also connect to the engagement net  48  through a rode line  27   b  on the starboard side of the ship  26 . In other embodiments, a different number of PSAs are used to decelerate ships  26  with different hull displacements and velocities. For example, two PSAs are used for smaller or slower ships in one embodiment and four PSAs are used for larger or faster ships. 
       FIG. 3A ,  FIG. 3B  and  FIG. 3C  show an embodiment  60  of a NEPS system  42  deployed from a helicopter  62 . With reference to  FIG. 3A , the helicopter  62  carries the NEPS system  48  on a detachable line connecting a net container  64  including the engagement net  48 , the rockets  50  and the drag-chutes  52 . The net container  64  further connects to deceleration devices  16   a ,  18   a  and  20   a  to be deployed on one side of the ship  26  and similar deceleration devices  16   b ,  18   b  and  20   b  (not shown) to be deployed on the other side of the ship. 
       FIG. 3B  shows the NEPS system  42  after being released by the helicopter  62  and the net container  64  being opened to deploy the engagement net  48 , the rockets  50  and the drag-chutes  52 . In one embodiment, the net container  64  is opened by a datalink with the helicopter  62 . The trajectory of the rockets  50  in  FIG. 3B  differ from the trajectory shown in  FIG. 2B  because the NEPS system  42  will be deployed from a greater height. In one embodiment, the trajectory of the rockets  50  in  FIG. 3B  is determined by the weight, balance and aerodynamics of the overall NEPS system  42 . In another embodiment, the trajectory of the rockets  50  in  FIG. 3B  is controlled by a guidance system including in the rockets  50 . In another embodiment, the system is deployed by a parafoil, similar to that shown in  FIGS. 1A-C , from the helicopter  62 . 
       FIG. 3C  shows the NEPS system  42  after the engagement net  48  has captured the bow of the ship  26  and the PSAs have been released from their respective deployment bags in a manner similar to that shown in  FIG. 2C . The staged deployment of progressively larger diameter PSAs and the load equalization of the engagement net  48  permits the use of lighter weight materials which enables the use of multiple launching platforms, a few of which have been shown by example in  FIG. 1A  through  FIG. 3C   
     High-strength engagement net systems have been developed that can be used with any of the launching platforms shown in  FIG. 1A  through  FIG. 3C . One embodiment of an engagement net  70  is shown in  FIG. 4 . The engagement net  70  includes a webbing  72  connected to a plurality of small diameter self-tensioning loops  74   a - p . Each of the small diameter loops are connected to one of a plurality of medium diameter self-tensioning loops  76   a - f . Each of medium diameter loops are connected to one of plurality of large diameter self-tensioning loops  78   a - b . Loop  78   a  is connected to a rode line  82  that connects to a group of PSAs. Loop  78   b  connects to a rode line  80  that connects to another group of PSAs. When the engagement net  70  is used to alter the trajectory of a ship, an iceberg or other maritime objects one of the two rode lines is left unconnected and the webbing  72  will capture an extruding surface—in the case of a vessel the surface is the bow. In one example, when the engagement net  70  is used to attach to an iceberg to alter its trajectory, the engagement net  70  further includes protrusions capable of penetrating the iceberg to secure the engagement net  70  thereto. 
       FIG. 4B  and  FIG. 4C  further illustrate the operation of the self-tensioning loops shown in  FIG. 4A .  FIG. 4B  shows a portion of the net  70  prior to contacting the object whose momentum is to be altered. In one example, the net  70  contacts the bow of a ship as shown in  FIG. 1C ,  FIG. 2C  and  FIG. 3C .  FIG. 4C  shows the net  70  distorted to conform to the irregularities and non-planar surface of the bow of the ship. As the net  70  distorts, the self-tensioning loops  74   d  and  74   e  each rotate to equalize the load on webbing  72 . Similarly the loop  76   b  rotates to equalize the load on the self-tensioning-loops  74   d  and  74   e . The self-tensioning loops provide a more even distribution of the load imposed from the PSAs across the net  70 , thus permitting the webbing  72  to be made of lighter weight material with lower load bearing capability. The resulting lighter net  70  enables more efficient methods of launching the NEPS system as shown in  FIG. 1A  through  FIG. 3C . 
       FIG. 5  illustrates another embodiment of an engagement net  90  based on a lasso structure. The net  90  includes a base net  92  connected to top load bearing lines  94  and bottom load bearing lines  96 . In one example, four lines are used for the top lines  94  paired with four lines for the bottom lines  96 . The set of top lines  94  pass through a bottom loop  98  formed by the bottom lines  96  and then connect to a set of self-tensioning loops that form the connection to a rode line. The set of bottom lines  96  pass through a top loop  100  formed by the top lines  94  and then connect to a set of self-tensioning loops that form the connection to another rode line. Specifically, for four load-bearing lines, three total sets of self-tensioning loops are needed, with two sets connecting to the load-bearing lines and one set connecting those two sets to the rode line. 
     When the base net  92  contacts the bow of a ship or other maritime object and the PSAs are deployed, force on the rode lines will cause the top lines  94  and the bottom lines  96  to pull together and cinch around the bow of the ship, substantially conforming to the shape of the bow to securely attach the PSAs to the ship. The bow would thus be inside what would otherwise be a square knot. 
       FIG. 6  illustrates an example of a NEPS system  110  as used in the embodiments shown in  FIG. 2B  and  FIG. 3B . The NEPS system  110  includes a base net  112 , which is based on the net structure shown in either  FIG. 4A  or  FIG. 5  in alternative embodiments. The net  112  is propelled towards a maritime object (e.g. a ship) in one example using rocket motors  114   a  and  114   b . The rockets  114   a  and  114   b  are preferably set at a divergent angle of 25 degrees to each other to facilitate keeping the net  112  open prior to capturing the maritime object. The rockets  114   a  and  114   b  are connected to the net  112  by a harness  116 . In one embodiment the net  112  is also kept opened by drag-chutes  118   a  and  118   b  connected to the trailing edge of the net  112 . In another embodiment, a break-away line is attached to the deceleration devices  18   a  and  18   b  instead of using drag-chutes  118   a  and  118   b . The net  112  connects to deceleration devices  16   a ,  18   a  and  20   a  on one side of the net  112  and to deceleration devices  16   b ,  18   b  and  20   b  on the other side of the net  112 . 
       FIG. 7  illustrates an example of a NEPS system  120  using a tensegrity structure as used in the embodiment shown in  FIG. 1B . In one example, load-bearing lines  122  are formed by cables under tension that surround the outside of the tensegrity structure. For a hexagon tensegrity structure the cables would connect the end points of every other rod  124 . The embodiment  120  is shown using the lasso structure of  FIG. 5  with a top load bearing line  126  connected to a rode line  27   a  and a bottom load bearing line  128  connected to a rode line  27   b . The rode line  27   a  connects to three PSAs,  28   a ,  30   a  and  32   a  respectively. In contrast to the embodiment  110  in  FIG. 6 , the NEPS system  120  using the tensegrity structure relies on a guided parafoil, rather than rockets, to propel the NEPS system  120  towards the bow of a ship. As illustrated in  FIG. 1A  and  FIG. 1B , the tensegrity structure remains compact while attached to the parafoil to reduce the aerodynamic drag. After the tensegrity structure lands on the bow of the target vessel, the PSAs are deployed. When the PSAs create a drag force under water, the resulting force on the rode lines  27   a  and  27   b  breaks the tensegrity structure and causes the load bearing lines  122  to cinch around the bow of the vessel. 
     The dynamic load equalization of the engagement nets afforded by the use of movable self-tensioning loops shown in  FIG. 4A  and a lasso shown in  FIG. 5 , significantly reduce the NEPS system volume and weight. Synergistically, the staged release of progressively larger PSAs, permits the use of smaller and lighter weight PSAs, which when used with the smaller and lighter weight engagement nets enables the efficient placement of the engagement net on the bow of a ship or other maritime objects. 
     In a preferred embodiment, the deceleration devices  16   a - b ,  18   a - b  and  20   a - b  include mechanisms for the timed release of PSAs in an aerodynamically efficient enclosure as further detailed in  FIG. 8 . The deceleration device  130  is enclosed in a deployment bag  132  held closed by a webbing  134 . The deployment bag  132  connects to either the engagement net or another deployment bag with a rode line  136  that also connects to a PSA  138 . A programmable time release unit (PTRU)  140  releases the webbing  134  at a time predetermined based on the anticipated loading on the PSA  138  by the maritime object that the NEPS system is designed to decelerate. In one embodiment, the PTRU  140  timer is activated and starts the time interval when the pressure on the rode line  136  exceeds a threshold. After the webbing  134  is released by the PTRU  140 , an exposed drag-chute  142  will pull the PSA  138  out of the deployment bag  132  and allowing the PSA  138  to inflate. 
     In one embodiment, the PTRU  140  includes an electronic time clock that activates a piston actuator that releases a clamp after a preset time interval. The clamp then releases the webbing  134  allowing the deployment bag  132  to open. The piston actuator optionally includes mechanical leverage to allow the clamp to open when the webbing is under tension. For example, mechanical leverage is used to drive a clamp loaded with several thousand pounds of force imposed by the webbing  134  with a piston actuator only capable for providing five pounds of force. In another embodiment, the PTRU  140  uses a dissolvable salt tablet, instead of an electronic time clock, to determine when the piston actuator should be activated. 
     The performance of the NEPS systems shown in various embodiments of  FIG. 1A  through  FIG. 3C  was tested under various conditions and test setups, an example of which is shown in  FIG. 9 . The test setup  150  used a scaled model of a ship  152  to verify the performance of the PSAs and to extrapolate the performance of the NEPS system  42  shown in  FIG. 2C  and  FIG. 3C . The PSAs  28   a ,  30   a  and  32   a  are connected to a load cell  154   a  used to monitor the total drag force provided by the PSAs  28   b ,  30   b  and  32   b . The PSAs  28   b ,  30   b  and  32   b  are connected to a load cell  154   b  used to monitor the total drag force provided by the PSAs  28   b ,  30   b  and  32   b . The load cell  154   a  is connected to the engagement net  48  with a rode line  27   a . The load cell  154   b  is connected to the engagement net  48  with a rode line  27   b.    
       FIG. 10A  and  FIG. 10B  further illustrate the performance of the test setup  150  shown in  FIG. 9 . A ship  152  with 99 tons of displacement, measuring 24 meters in length, with a beam of 6 meters and maximum velocity of 13 knots was tested and the results showed that the ship  152  was stopped within 30 seconds.  FIG. 10A  shows the deceleration of the ship  152  from an initial forward velocity of 12 kts with staged deployment of PSAs to maximize the deceleration of the ship  152  without exceeding the design load limits for each PSA. 
     The first set of PSAs to deploy are PSA  28   a  and PSA  28   b , each having a 1.5 meter diameter and deployed approximately 2 seconds after the engagement net  48  contacts the bow of the ship  152 . The second set of PSAs to deploy are PSA  30   a  and PSA  30   b , each having a 2.5 meter diameter and deployed approximately 5-7 seconds after the engagement net  48  contacts the bow of the ship  152 . The speed of the ship  152  has decreased to 8 knots by the time the second set of PSAs are deployed. The third set of PSAs to deploy are PSA  32   a  and PSA  32   b , each having a 4.5 meter diameter and deployed approximately 15 seconds after the engagement net  48  contacts the bow of the ship  152 . The speed of the ship  152  has decreased to 4 knots by the time the third set of PSAs are deployed. The test results shown in  FIG. 10A  and  FIG. 10B  show a rapid and smooth rate of deceleration of the ship  152  with a relatively uniform load (e.g. force) on the NEPS system. 
     Subsequent to testing a scaled model as shown in  FIG. 9 ,  FIG. 10A  and  FIG. 10B , a full-scale vessel was tested with deceleration results shown in  FIG. 11 . The full-scale vessel had 3,568 tons of displacement, measured 75 meters in length, with a beam of 18 meters and a maximum velocity of 15 knots. The NEPS system used for the full-scale vessel used three PSAs with canopy diameters of 4.5 meters, 7.5 meters and 12 meters respectively.  FIG. 11  shows the successful deceleration of the full-scale vessel from 13 knots down to 4 knots within 60 seconds, consistent with estimates extrapolated from scaled model tests shown in  FIG. 9 ,  FIG. 10A  and  FIG. 10B , thereby demonstrating the maturity of this technology. 
     While the invention has been shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the following claims.