is a nautical expression
indicating a point that is behind a given part of a boat
. For example, "abaft the funnel" means behind the ship's funnel
(chimney). (If not qualified, abaft
means "behind the mid-point of a ship or group of ships".)
Note that unqualified or in the "abaft the beam" form, abaft does not necessarily refer to a point actually on the ship; it may refer to a point some significant distance away.