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more comparable to the SWATH (i.e., the FF-1052 and the FFG-7 classes) run a very poor second in the competition with the SWATH design. The greatest difference between the monohull and SWATH concepts appears to be in the lower speed regime. Such speeds are frequently required when towing surveillance arrays or employing underwater sweeping systems. They are also the speeds for which mechanical motion damping devices (e.g., fin stabilizers) are usually ineffective. A detailed analysis of the data also revealed that the dominant factor in determining the seakeeping qualities of the ships was the consideration of helicopter compatibility, although the incidence of motion sickness was also important. We might reasonably expect that the motion envelopes for vertical/short takeoff and landing (V/STOL) aircraft will be at least as restrictive as those currently implied by helicopter operations.
Although ship motion data are not available to reflect the potential impact of stabilization devices on the seakeeping qualities of the monohulls, it is instructive to point out that only roll can be significantly reduced by motion damping devices, and that these systems have enjoyed only limited success. Thus, while both monohulls and SWATH can receive some benefit from mechanical motion reducing devices, the relative difference between the seakeeping qualities of the divergent design concepts would probably be similar to those shown in Table 4.
The significance of the seakeeping qualities of naval combatants will continue to increase in the years ahead in light of the growing role that helicopters are playing as sensor and weapon platforms. Seakeeping will become even more important if the Navy continues to place strong emphasis on the development of V/STOL aircraft. If sea-based V/STOLs are to be operated from small- and medium-size combatants, these ships must be able to provide for the maximum availability of these aircraft in all ocean environments. There are, of course, many other compelling arguments for a good seakeeping platform, such as the employment of towed sonar arrays for ASW. The effectiveness of these arrays is highly dependent on their orientation with respect to the submarine target. A poor seakeeping platform therefore may be forced to forgo an optimal course if the motion of the platform is excessive. Finally, we have entered the age of the cruise missile and supersonic missile warfare. Now, and for the foreseeable future, the ability to rapidly react and defend against attack will determine the survivability of the warship. It seems clear that both the ships’ crews and their systems will be more alert and ready if they are functioning in an environment where the debilitating effects of ship motion are reduced to their lowest possible level. Thus, the effectiveness of the warship in performing a variety of missions, particularly when the support of embarked air vehicles is required, can be significantly enhanced if the platform possesses superior seakeeping qualities.
It is unfortunate that the small waterplane area twin hull has appeared on the scene in the same time as the surface effect ship and the hydrofoil’ While these latter concepts offer major increases in potential speed for the small combatant, they also involve considerable technological risk and raise the nagging question of the util' ity of speed in the naval warfare environment. Conversely, SWATH does not offer a dramatic calm water speed improvement over the conventional monohull, nor does it require a sophisticated technology. SWATFl simply possesses better seakeeping qualities and potentially greater deck area to sustain air operations. Because of the chronological coincidence of SWATH, surface effect ships, and the hydrofoil, these differences are frequently ignored, and the concepts are throw'n into the same bag, labeled "new technology” platforms.
SWATH is a low-risk concept that offers a dramatic improvement in seakeeping qualities when compared r° conventional monohull designs.
Unfortunately, the Navy has not yet decided to do so, although there are at least two mission areas—mine countermeasures and surveillance towed arrays—where a SWATH platform could have offered a significant improvement over the conventional monohulls that are currently planned.
SWATH is ready for the Navy- When will the Navy be ready for SWATH?
Tactical ASW & Ocean Acoustic Forecasting
By Lieutenant Commander Alden B. Chace, Jr., U.S. Navy. Oceanography Department, Naval Postgraduate School, and Lieutenant George V. Galdorisi, U.S. Navy. Helicopter Anti-Submarine Squadron Light Thirty-Two
. . . even with our great modern strides in technology—lasers, homing and guided ordnance, sonar, electromagnetic devices of many kinds, and their countermeasures —further significant improvement in the performance of these devices and weapons
The ability of naval operating forces to detect, localize, and track submarines is mainly dependent on their proficiency in predicting the behavior of underwater sound and then employing their acoustic sensors based on this
may only come from an increased knowledge of the environment in which they operate. (Rear Admiral C.O. Holmquist, U.S. Navy [Retired], former Chief of Naval Research, November 1973 Proceedings)
knowledge. ASW tactical planning ultimately depends on the environment. How far away and under what conditions can you expect a given probabil- 'ty of detection? Are you limited to direct-path sound transmission in the 0cean’s upper mixed layer, or is sound propagation in the convergence zone likely to permit longer range detection? Ocean acoustic forecasting answers these and other tactical questions.
The properties of sound transmission through the ocean are determined ln two steps. First, at what speed does sound travel at every point in the vertical and horizontal space between your sensor and the target submarine? Second, how do you convert sound speed to expected sensor performance?
The speed of sound in sea water is a function of temperature, salinity, and pressure. Most U.S. Navy antisubmarine warfare (ASW) ships and air- ctaft use the expendable bathythermograph (XBT or AXBT) to determine cke temperature at various depths. Salinity is measured by few Navy ships ari(l must be taken from data collected °Ver many years by research vessels. Pressure is primarily determined by the vertical distance below the sea surface.
The next step is to use the sound sPeed distribution as the entering argument for one of the acoustic al- 6°rithms which will determine, for y°ur sensor and target, how far away y°u have a given probability (50% is standard) of detecting the submarine.
Acoustic forecasting is conducted at the Fleet Numerical Weather Central (FNWc), Monterey, California, for fleet users worldwide, and, increas- lngly, current operations are being analyzed on board ship or at patrol squadron (VP) home bases. FNWC uses large computers to work the global forecasting problem. About 200 BT reports per day are received from naval and civilian sources. This information Is fitted into a Northern Hemisphere grid of 2,450 ocean points which are about 200 nautical miles apart at the middle latitudes. FNWC estimates a sound speed profile for a given geographic position based on that point’s Proximity to surrounding grid points. This method of analysis is well suited
for planning a mission.
However, this method does not provide the tactical ASW commander with adequate acoustic predictions. Mesoscale features, such as eddies with diameters less than the grid spacing, are lost in this analysis. Additionally, the BT data received by FNWC are generally clustered in normal naval operating areas and merchant shipping lanes. As a result, there are large global areas where there is little information upon which to base acoustic forecasts.
One way to achieve improved acoustic forecasts is for the user to provide its own current BT information to FNWC and to request acoustic forecasts based on it. The resulting acoustic products are more accurate than those generated from estimated or "blended” BTs. However, this process is not without problems. First, there is a significant lag between the time the BT inputs are submitted to FNWC and an acoustic forecast is received by the user. As a result, transiting units may have moved into different waters before the requested forecast can be received. Second, EMCON (emission-controlled) conditions preclude afloat units from requesting FNWC support. Third, EMCON conditions also limit the meager supply of BT data provided to FNWC. A fleet unit may find itself with only an historical estimate (climatology) of the thermal structure for a designated place at a specified time.
Calculating acoustic performance on location immediately after the BT data are taken overcomes the time-lag problem. And, increasing numbers of ships and tactical support centers (TSCs) have computers capable of executing the required acoustic algorithms. The Naval Sea Systems Command’s acoustic performance prediction (APP) program is designed to provide this increased capability. Efforts under this program are directed toward two formal systems. The integrated command (or carrier) ASW prediction system (ICAPS) is used primarily for VP and VS (antisubmarine warfare) squadrons. ICAPS is managed by the Naval Oceanographic Office. There are currently more than 20 installations in this system. Aircraft carrier installations can be used to support escorting ships. Some installations are in TSCs ashore. The other afloat installation planned under the APP program is the sonar in situ mode assessment system (SIMAS) which may be added to surface combatants having AN/SQS-26 sonar. This system is managed by the Naval Underwater Systems Center (NUSC), New London, Connecticut, and is not yet widely installed in the fleet. NUSC is also charged under the APP program to provide an on-board prediction capability for submarines. Additional efforts have been made, notably in the VP community, to use hand-held calculators for portions of the acoustic prediction problem.
Briefly, each of the aforementioned systems requires in situ BT information and some easily obtainable onscene information (such as wave height and water depth) to produce a sound propagation loss curve (range versus decibel loss). This evolution takes several minutes. Thus, these systems circumvent the problems inherent in both user-provided BT products and blended products. They also are not subject to transmission delays.
Effective use of acoustic sensors depends upon knowledge of the behavior of sound energy in the sea. The Navy currently has only a small number of professionals who have a detailed understanding of how the environment affects our primary ASW sensors. Spreading this knowledge to each fleet unit holds the potential for significant ASW improvement.
EDITOR’S Note: Lieutenant Commander E. W. Shaar, Jr., U. S. Navy, makes a case for training naval officer oceanographers and for sending them to sea in his February 1977 Proceedings article, “ASW and the Naval Officer Oceanographer.” Commander Shaar examines how an on-scene oceanographer could take advantage of natural sound transmission paths such as surface duct, convergence zone, sound channel, and bottom bounce to enhance tactical antisubmarine warfare. Sound propagation graphics and a practical problem are presented in the article.