Astern means toward, or positioned at, the rear (stern) of a vessel — used both as a general directional term and as part of specific, standardized engine orders.
As a Direction
“A vessel astern of us” simply means another ship is positioned behind yours — a straightforward directional/positional use.
As an Engine Order
“Astern” is also used specifically in propulsion commands — “full astern,” “half astern,” “slow astern” — instructing the engine room to drive the propeller in reverse, either to move the vessel backward or to actively slow forward motion (a critical maneuver in collision avoidance or emergency stopping).
Why “Operating Astern Propulsion” Has Its Own Sound Signal
Under COLREGs Rule 34, three short blasts specifically mean “I am operating astern propulsion” — a distinct, mandatory signal, because a vessel unexpectedly reversing or slowing via astern propulsion is important information for any nearby vessel trying to predict your movement.
Astern vs Aft
- Astern — behind the vessel, in the water or the general direction; also used in propulsion commands
- Aft — toward the rear structurally, within the vessel itself (“walk aft” means moving toward the stern while still on deck)
Crash Stop: Astern Propulsion in Emergencies
A “crash stop” — applying full astern propulsion to stop a vessel as quickly as possible in an emergency — is a genuine maneuver with real limitations (a large vessel takes a significant distance to actually stop even at full astern), factored into passage planning and collision avoidance decision-making.
Exam Relevance
Astern as both a positional term and specifically as part of engine/propulsion commands, plus its associated COLREGs sound signal, are core Navigation and Ship Handling topics in deck officer competency exams.