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Eye on the Navy boats. Navy ships operate very close to one another when conducting underway replenishment, something merchant ships don't do. Even when they are in the middle of the ocean, ships conducting an "Unrep" must get within 80 to 180 feet of each other while steaming at 13 knots. The delivering ship is the "guide," and maintains course and speed, while the receiving ship must make an approach that brings it up and next to the guide. If not careful, the two ships underway alongside can be "sucked together" as the pressure areas between the two ships changes. "Underway replenishment probably accounts for more collisions by naval vessels than any other evolution," Barber says. "The approach is the most hazardous part, and it is important to have multiple ways of determining separation as you make your approach." In 2000, a Fast Combat Support Ship (AOE) was refueling a destroyer, when the destroyer ran into the starboard side of the oiler. "The size and hull shape of the AOE creates pressure differentials that require strict attention to the conning officer of the receiving ships. This is particularly true when the destroyer must maintain station at the forward replenishment stations where the bow wave of the AOE tends to push other ships away," Ramsey says. MSI's Brian Boyce says one must understand how ships interact when alongside, as well as fundamental vector math. In this case, the conning officer took the conn as close quarters developed and wrongly assumed that by steering his stern away from the oiler (and consequently moving his bow toward the oiler creating a converging situation) that somehow the bow pressure effects or other physics would hold his bow off. "It didn't work. He should have known that steering away from the oiler, even though it would place his stern temporarily closer, if done aggressively enough, would create a diverging vector that would overcome whatever was pulling the ships together," Boyce says. An amphibious dock landing ship (LPD) way was making an approach on a Military Sealift Command (MSC)operated oiler in the Western Pacific, when it miscalculated its approach and ran into the oiler's stern. The LPD's bow was seriously damaged. This was caused by a series of compounded mistakes, what Boyce refers to as the "error chain." According to Boyce, the CO of the LPD was too personally engaged in supervising the conn to be able to objectively assess a developing dangerous situation. "The main error was failure to assess a continuing CBDR (Constant Bearing Decreasing Range) situation. Others saw the problem but put their trust in their captain and failed to aggressively warn him." Barber agrees that this is an issue. "It has always surprised me how often when investigating a collision we discover that someone in the watch team knew that a problem was developing, but because they figured the Captain/OOD/someone else knew better they didn't speak up. Sometimes this is the Captain's own fault because he has on previous occasions intimidated people who brought him unwelcome news." Edward Lundquist is a retired U.S. Navy captain who is a senior science advisor for Alion Science and Technology. Lundquist supports the U.S. Navy's Surface Warfare Directorate.