when "andrew j. barberi" was approximately halfway between the kvk buoy and the staten island terminal, the captain asked the assistant captain to reduce speed. the assistant captain replied affirmatively and began to input reduced pitch commands. as the vessel rapidly approached the slip, the captain ordered the assistant captain to input a full reverse pitch command. the assistant captain complied and input the ordered pitch command. at the same time, the captain increased engine rpm in order to produce more astern thrust. at this moment the assistant captain became aware that the propulsion units were not following his commands, so he repeatedly pushed the control levels downward to repeat his astern command.