The Call at 0200
You are the master of a 380 GT motor yacht, three days out of Palma, bound for Antigua. Your chief engineer knocks at 0230 and tells you the second engineer is ill — genuinely unfit for watch. You have one qualified engineer and two engine room ratings. The CE wants to go to a machinery-space periodic unmanned (UMS) arrangement for the Atlantic crossing. You need to decide — not advise, decide — whether that is safe and lawful, and what engineering watch arrangements will be in force for the rest of the passage.
The Governing Framework
For a yacht under 500 GT operating on a commercial basis under the Red Ensign Group Yacht Code (REG YC), engineering watchkeeping standards derive from STCW, the MCA's safe manning principles, and the yacht's Safe Manning Document (SMD). The SMD is the key instrument: it specifies minimum safe manning levels and the permitted modes of operation, including whether UMS is authorised. You cannot simply decide to go UMS — it must be authorised by the flag state and recorded in the SMD. If your SMD does not authorise UMS, the answer to the CE is no.
Engineering Watch: Sea, Anchor and Port
At sea — crewed machinery space: A qualified engineer officer must be in charge of the engineering watch. STCW requires watchkeepers to be competent, rested, and properly handed over. The watch officer must not leave machinery spaces until properly relieved. Hours of rest rules apply equally to engineers: minimum 10 hours in any 24-hour period, no more than two separate periods, one of which must be at least 6 hours.
UMS operation: If your SMD and flag state authorise UMS, the duty engineer must be available and able to respond immediately to alarms. Alarm systems must be operational and capable of alerting the duty engineer to their cabin. The OOW on the bridge takes on additional responsibilities: they must be familiar with engineering alarm response procedures and know when to call the duty engineer. You as master set the UMS threshold in your standing orders.
At anchor: Engineering watch requirements do not automatically reduce at anchor. The risk assessment changes — propulsion is not in active use — but auxiliary systems (generators, bilge, fire detection) remain live. Your SMS and SMD will specify minimum engineering watchkeeping at anchor. Many vessels maintain a duty engineer available call. If you are anchored in an exposed or congested anchorage, you may decide to maintain a full engineering watch: that is a master's command decision.
In port: The duty engineer arrangement must cover safe operation of all systems in use: shore power changeover, gangway lighting, fire detection, AC systems, sewage. A full in-port engineering watch is not always required, but an engineer must be contactable and able to respond. Port state requirements may impose additional conditions.
The Decision in the Vignette
Check the SMD first. If UMS is not authorised, you maintain a crewed watch and manage the workload with the CE and ratings within legal hours of rest — which may mean reducing speed, seeking a port of refuge, or requesting flag state dispensation. You brief the OOW on their additional responsibilities, increase your own presence on the bridge, and record all decisions in the log. You do not override the SMD on your own authority.