The phrase "fall astern" can be used to describe a ship or boat that is moving slowly behind another vessel. There are a few different synonyms for this phrase, each of which can be used to convey a slightly different meaning. For example, "lagging behind" implies that the slower vessel is having trouble keeping up with the faster one; "trailing" simply means that one vessel is following another; and "bringing up the rear" can be used to describe the slower vessel as part of a larger group, such as a fleet. Other synonyms for "fall astern" might include "fall behind," "trail behind," "linger," "dawdle," and "wander".