As every submariner knows, the noise radiated from a submarine is a key factor in determining a boat’s detectability. Many noise quieting techniques are employed to reduce this radiated noise and periodic measurements are made to determine submarine noise signatures. In the old days, limited sound measurements were taken alongside a sound pier in a quiet area of a shipyard. Today, more sophisticated measurements are made with the submarine submerged passing by an array of hydrophones on an instrumented range. The limited number of these ranges (the only one on the east coast is at AUTEC) necessitated several days to transit from homeport to the range. The Navy sought to come up with a more economical way to gather noise data-one that could be used at each submarine homeport.
The engineers at the David Taylor Model Basin (DTMB) developed a prototype portable noise measuring system that could be deployed from the submarine. The system was known as FAB-Fly Around Body. It consisted of a buoyant airplane-like device towed from the bow of the submarine, a faired tow/control cable, a neutrally buoyant hydrophone array, and an instrument package to record signals from the array. Once deployed, the towed body (the real FAB) with its trailing array, could be flown 360 degrees around the sub’s hull from inside the boat. The concept was to take near-field noise measurements while turning on and off various pieces of equipment. Once the first set of readings was complete, the array would be moved farther away from the hull by increasing the scope of the tow cable and a second set of readings would be taken. The collected noise data would be processed to determine the boat’s radiated noise signature.
After running some preliminary trials with the system in the Portsmouth, New Hampshire area in the spring of 1967, ALBA-CORE deployed to Fort Lauderdale, Florida that October to run a thorough test of the system. The typical operational sequence went as follows: In port, the FAB was placed in a cradle located just aft of the sail. The faired (for noise reduction) tow/control cable was connected to the sub/s bow tow point by a diver, adjusted to the desired length, and the other end attached to the FAB on deck. A boom and an air-driven cable winch were mounted on the aft end of ALBACORE’s dorsal rudder. A series of electrical and mechanical checks were run on the F AB controls and the winch to ensure proper operation at sea.
At sea, ALBACORE would rendezvous with a support vessel carrying divers and the array. The array connector was mated to the connector on the F AB’s pigtail and continuity checks conducted. The support vessel placed the array in the water and stood clear. The winch lifted the F AB out of its cradle and the dorsal rudder swung the FAB over to the side where the array was deployed. The FAB was lowered until the pigtail connector was submerged. Final electrical checks were made and the FAB was placed in the water. The hoist cable on the FAB was released by a diver, the dorsal rudder was then center-lined and the winch and boom removed and stowed. ALBACORE would get underway in a slow turn toward the FAB to keep the array out of the screws. With several knots of forward motion, the operator could fly the FAB and array away from the sub and the boat could submerge in a normal manner.
Once the sub attained the desired course, speed, and depth, the FAB would be flown into position for the first series of data points. Ship’s course, speed, and depth would be maintained while the FAB was repositioned for the next series of readings. Course and depth changes were coordinated with the F AB operator to ensure that the array was kept clear of the screws.
When it came time to surface, the FAB was positioned on what would be the lee side of the sub to facilitate recovery operations. With the boat on the surface, the boom and winch were set up on the dorsal rudder and the rudder positioned to the lee side. Divers from the support vessel attached the winch cable to the floating FAB and brought the end of the array to the support vessel. Once the FAB was winched out of the water and set on its cradle, the pigtail connector was disconnected and the array was hauled aboard the support vessel. While it sounds like a cumbersome operation, each deployment and recovery evolution was usually completed within a half-hour.
ALBACORE deployed twice to Fort Lauderdale with FAB. The first set of trials ended prematurely when a casualty to the control system caused the FAB to crash into the sub and crush the fiberglass body. On the second deployment, the system worked well and produced the intended results. The Fort Lauderdale noise data taken by F AB was verified when ALBACORE went to AUTEC and made multiple runs first past an array suspended from a barge and then past AUTEC’s (then very new) bottom-mounted sound array.
So why isn’t the FAB system in use today? Probably the major reason is because of the requirement to put divers in the water. We were limited to sea state two or less for diver operations. Many days were spent in port because it was too rough to put swimmers in the water. Those of us who had our families down for the two weeks of trails manfully endured the canceled underway days. After holding a morning clean-up of the boat, a noontime liberty call usually followed. While the Fort Lauderdale Navy League enjoyed having the sub alongside and holding frequent visit ship days, ship’s company suffered ashore, for liberty in Fort Lauder-dale was not inexpensive. The Navy’s bills for having the ship sit in port (including motel rooms rented by DTMB for crew members whose bunks were filled by racks of test instrumentation) mounted up. Deployment of the FAB system was too dependent on the weather and the cost savings of using the FAB system were not realized.
While the FAB system didn’t provide the desired economic benefits, some of the lessons learned were evidently later put to use. In the days before boats were ShipAlted to install reels for their towed arrays, a small boat with divers was used to carry the array to the underway submarine.
So, was the October 1967 deployment of FAB the first submarine use of a towed array.
