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.