USS Mobjack (AVP-27/AGP-7) was a motor torpedo boat tender in commission in the United States Navy from 1943 to 1946. She saw service in the Pacific theater during the latter portion of World War II.
After the conclusion of her Navy career, the ship served in the United States Coast and Geodetic Survey from 1946 to 1966 as the survey ship USC&GS Pioneer (OSS 31), the third Coast and Geodetic Survey ship of the name. Pioneer operated in the Pacific Ocean during her career, making an important discovery of magnetic striping on the ocean floor that made a major contribution to the development of the theory of plate tectonics. She also took part in the International Indian Ocean Expedition, becoming the first ship in the history of the U.S. Coast and Geodetic Survey to visit the Indian Ocean.
Construction and commissioning Mobjack was laid down as a seaplane tender, AVP‑27, by Lake Washington Shipyard, Houghton Washington, on 25 February 1942 and launched on 2 August 1942, sponsored by Mrs. H. R. Peck. She was reclassified as a motor torpedo boat tender and redesignated AGP‑7 on 11 March 1943 and commissioned on 17 October 1943.
United States Navy service
World War II Following shakedown off Southern California, Mobjack departed San Diego, California, for the Southwest Pacific on 14 December 1943. Steaming via New Caledonia and the New Hebrides, she arrived at Rendova, British Solomon Islands, on 14 January 1944 to begin duty with Motor Torpedo Boat Squadrons, South Pacific (SOPAC). She trained and supplied patrol torpedo boats (PT boats) in the New Georgia area until 6 March 1944, when she steamed to the Treasury Islands and thence to the motor torpedo boat base, Emirau, arriving on 23 March 1944. In mid‑May 1944 she returned to the Treasury Islands, where she overhauled PT boats and repaired and tended the motor gunboats (PGMs) and landing craft infantry gunboats (LCI(G)s) of Task Group 30.3 into July 1944.
Employed briefly in transporting spares and other materiel, she departed for British New Guinea on 23 July 1944, arriving at Dreger Harbor on 27 July 1944 to begin service under Commander, Motor Torpedo Boat Squadrons, 7th Fleet. During the greater part of August 1944 she operated at Aitape Harbor, tending Motor Torpedo Boat Squadron (MTBRBon) 33 and providing assistance in salvage work. On 28 August 1944, she got underway for Mios Woendi, Netherlands New Guinea, in company with MTBRBon 33. Arriving there on 31 August 1944, she was engaged in fueling, provisioning and making final repairs to the PT boats of the squadron preparatory to sailing for Morotai as a unit of Task Group 70.1.
Morotai Arriving at Morotai on 16 September 1944, the day after the American landings there, she commenced tending the boats of her squadron and Consolidated PBY Catalina flying boats assigned to air-sea rescue of pilots downed en route to and from raids on Truk. For the first three days she went about her duties unscathed in spite of Japanese air resistance.
At sunrise on 19 September 1944, however, a smoking Japanese fighter emerged from a heavy overcast and dove for Mobjack. Unable to crash into the ship, the pilot dropped two bombs which exploded close alongside, holing the deck and wounding one of her crew.
Making temporary repairs, she remained at Morotai, tending PT boats of Task Unit 70.1.2 and PBYs now ranging over the Netherlands East Indies on relief and intelligence and reconnaissance missions, until February 1945.
The Philippines During February and March 1945, Mobjack transported materiel, spares, and advanced base personnel as she accompanied motor torpedo boat squadrons to forward areas. At Mios Woendi on 1 April 1945, she took on base force personnel of MTBRons 9 and 10 and on 2 April 1945 got underway for Samar in the Philippine Islands. Arriving on 11 April 1945, she disembarked her passengers and steamed on to Palawan, where she relieved motor torpedo boat tender USS Willoughby (AGP-9) as the repair unit for MTBRons 20 and 23. She moved on to Mangarin Bay, Mindoro, in mid‑May 1945 to overhaul and repair motor torpedo boats for use in the Philippines and in the upcoming Borneo operations.
The Borneo campaign On 8 June 1945, Mobjack steamed to Samar to stage for the landings at Balikpapan on Borneo. Departing on 21 June 1945, she moved, with MTBRons 10 and 27, to Basilan Island, thence, on 26 June 1945, further down the Sulu Archipelago and into Makassar Strait. On 27 June 1945, she joined the minesweepers, at work since 1 June 1945, and the ships of the bombardment group, which had been pounding the Japanese-held oil center since 17 June 1945, in preparing the way for the Australian assault force. For four days Mobjack fueled and sustained the motor torpedo boats assigned to night patrol off the coast to prevent the Japanese from replacing naval mines, restoring obstacles blown by underwater demolition teams, or disturbing channel markers planted by the minesweepers. On 1 July 1945, these extensive preparations and precautions proved to have been well executed as wave after wave of personnel of the Australian 7th Division of the Australian I Corps went ashore without a casualty.
Mobjack, harassed by Japanese night air attacks until Royal Australian Air Force night fighters stopped them, remained outside the harbor for the next nine days. On 11 July 1945 she stood into the harbor, where, on 15 August 1945, she received a dispatch directing the cessation of all offensive action against the Japanese. World War II was over.
Honors and awards Mobjack received three battle stars for World War II service.
Post-World War II On 12 September 1945, Mobjack returned to the Philippines and for the next two months engaged in decommissioning motor torpedo boats under Commander, Motor Torpedo Boat Squadrons, Philippine Sea Frontier. On 10 November 1945 she headed eastward, arriving at San Francisco, California, on 29 November 1945.
In December 1945, stripping of Mobjack began, and on 21 August 1946 she was decommissioned. The Navy transferred her the same day to the United States Department of Commerce.
U.S. Coast and Geodetic Survey service
After the ship underwent conversion into an "ocean survey ship" (OSS), the Department of Commerce commissioned her into service with the United States Coast and Geodetic Survey as USC&GS Pioneer (OSS 31), the third Coast and Geodetic Survey ship to bear the name.
Pioneer spent her Coast and Geodetic Survey career on duties in the Pacific Ocean.
Early operations Pioneer departed the U.S. West Coast in April 1947 for her first assignment, which was hydrographic survey work in the Aleutian Islands.Annual Report of the Director of the Coast and Geodetic Survey, 1947, p. 112, at noaa.gov NOAA Central Library: U.S. Coast and Geodetic Survey Annual Reports She spent the field seasons of 1947, 1948, 1949, and 1950 in the Aleutians, also running sounding lines from points in the Aleutians across the Gulf of Alaska and North Pacific Ocean to points on the U.S. West Coast.Annual Report of the Director of the Coast and Geodetic Survey, 1948, p. 128, at noaa.gov NOAA Central Library: U.S. Coast and Geodetic Survey Annual Reports Annual Report of the Director of the Coast and Geodetic Survey, 1949, p. 10, at noaa.gov NOAA Central Library: U.S. Coast and Geodetic Survey Annual Reports Annual Report of the Director of the Coast and Geodetic Survey, 1950, p. 12, at noaa.gov NOAA Central Library: U.S. Coast and Geodetic Survey Annual Reports
Discovery of magnetic striping During the 1950s, Pioneers operations moved beyond the Aleutians into the broader Pacific Ocean, and by 1955 she was involved in a joint survey effort with the U.S. Navy.The Integrated Undersea Surveillance System (IUSS)/Caesar periodical The Cable (Spring 2006) has "Recollections on the Successful Implementation of the Portion of Brick Bat 03-Titled Project Caesar Ii-Pacific (PART 1)" by Robert Kneedler who was at the time part of the Navy management team at Commander, Western Sea Frontier. He notes that the survey was actually for the then very closely held Sound Surveillance System (the name itself then classified) installation under the very high priority (Brickbat 3) installation program given the unclassified name Project Caesar. The associations and details of SOSUS/Caesar remained classified until 1991. At the time the Navy's Hydrographic Office managed the project's survey efforts. Also under that office was Project Magnet, an aeromagnetic survey flown by specially equipped Navy aircraft. Within the same division of that organization was a shipboard magnetic data collection effort. The USC&GS was also collecting magnetic data and beginning shipboard collection in U.S. waters. In cooperation with the Scripps Institution of Oceanography the ship towed the first marine magnetometer, developed by Victor Vacquier of Scripps, to study underwater magnetism.
Roger Revelle heard of the Pioneer survey and saw a chance to obtain data more detailed than the ocean tracks collected in other Scripps magnetic collections. Revelle proposed that Scripps provide the magnetometer and an operator, Ron G. Mason, on sabbatical at the California Institute of Technology, so that towed magnetic collection could be included in the Pioneer's collection. His initial proposal to the Navy was rejected due to the high priority of the program and difficulties with towed devices. Revelle approached John Lyman, then Director of the U.S. Navy Hydrographic Office's Division of Oceanography. Lyman took the proposal to the Hydrographer of the Navy who approved the idea under strict rules of no delays due to towing Vacquier's ASQ-3A fluxgate magnetometer and cutting the tow cable and losing the magnetometer if it interfered with the priority survey operations. Mason had previous experience with magnetic data collection on the 1952 Capricorn Expedition and was able to find a way to overcome an issue with security classification of such data by the Navy by subtracting the average signature from the observed publishing only the difference that satisfied the Navy.
In August 1955, Pioneer began the west coast survey off southern California at an east–west line spacing of with Mason and the magnetometer aboard. The magnetometer was described by Mason as a "gimbal mounted flux gate oriented in the direction of the Earth's magnetic field by two other flux gate elements which control orienting servo motors." all was contained within a streamlined "fish" to be towed about behind the ship. Two other veterans of the Capricorn Expedition, . Arthur D. Raff and Maxwell Silverman, alternated aboard for the magnetic survey. The resulting magnetic intensity charts showed north–south lineations and a lateral offset at the along the Murray fracture zone. The magnetic data revealed structural trends not evident in the undersea topography and horizontal displacements greater than any then found on continents. Mason published results of the Murray fracture zone in 1958 in the Geophysical Journal of the Royal Astronomical Society titled "A Magnetic Survey off the West Coast of the United States Between Latitudes 32° and 36° N and Longitudes 121° and 128°". Pioneers west coast survey – which went down in history as "The Pioneer Survey" – continued with this pattern observed throughout the survey area. The stripes indicated both periodic reversals of the earth's magnetic poles and the steady creation of new rocks which spread across the sea bottom as the ocean floor moved, making a record of these changes in polarity. Masons work aboard Pioneer with the fluxgate magnetometer thus revealed "magnetic striping" on the floor of the Pacific, the first time it had been noted anywhere and a major discovery in the development of the theory of plate tectonics.
Operations, 1957-1960