Data are presented on the performance trials of the USNS SUMNER with a comparison of ship performance before and after the addition of a skeg. The USNS SUMNER (T-AGS 61) was one of six T-AGS 60 (PATHFINDER) Class oceanographic survey ships operated by the Military Sealift Command (MSC) for the Naval Oceanographic Office (NAVO). The primary mission of these ships is to conduct surveys of ocean bottom regions. The T-AGS 60 Class ships are mono-hulls with wide, flat stern areas that provide very little lateral projected surface area. Previous model tests and ship trials of USNS MARY SEARS (T-AGS 65) demonstrated that the ship is directionally unstable. The addition of a skeg improved directional stability of the model and MARY SEARS. Spiral maneuvers with the USNS SUMNER demonstrated similar results in pre- and post-test trials. Details of the performance calculations of the USNS SUMNER are provided with uncertainty estimates. By comparison of the ship data without the skeg to the data with the skeg, the relative difference in power averaged 28 % higher without the skeg over a speed range of 4 – 15 knots. The average speed is 6.5 % lower for the ship without the skeg at the same shaft speed.

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