Observe and identify
Advanced level, about 5 min. International COLREGs / IRPCS.
The situation
You are under power, heading north at six knots in good night visibility. Two groups of lights are moving together across your starboard bow. Inspect both groups and their changing bearings. There is room to stop well short of their track.
What do the lights imply for your first action?
- A long astern tow: stop short of the whole track
- Two independent vessels: keep speed through the gap
- A large power-driven vessel: turn behind the leading lights
Answers and explanations
Lookout: Read the vertical masthead stack, then find the trailing light group. A changing bearing on the leading vessel does not establish that the whole operation is clear.
When the situation changes
The leading light group has now passed west of your intended track and its bearing is opening. The trailing light group remains east of that track and continues moving west. The space between them still crosses your route.
Keep stopped until the entire tow is past and clear
The tug being clear does not establish that the tow is clear. Continue observing the whole operation and resume only after a safe passage is established. The replay keeps you stopped while the trailing vessel passes; its elapsed time is an exercise value, not a prescribed waiting interval.
Resume now because the tug is clear
The trailing vessel has not cleared your route. Resuming now puts your hull across the towline even though the tug itself is already clear.
Accelerate to pass ahead of the trailing vessel
The space ahead of the trailing vessel contains the towline. More speed does not turn that space into a safe passage.
The sound answer
A long astern tow: stop short of the whole track
The leading vessel’s three vertical white masthead lights indicate a tow exceeding 200 m, measured from the tug’s stern to the after end of the tow. The following vessel shows sidelights and a sternlight. You have the crossing tug to starboard and must keep clear; stopping well short here leaves the whole tow room. Towing alone does not make every tug restricted in her ability to manoeuvre.
Why the other options fall short
Two independent vessels: keep speed through the gap
These are connected vessels in a long astern tow. The apparent gap contains the towline. A passage that misses both hulls can still strike the connection.
A large power-driven vessel: turn behind the leading lights
Three lights in a vertical stack indicate the long astern tow, rather than fore-and-aft mastheads on an ordinary large power-driven vessel. Avoiding only the tug leaves the trailing vessel and towline in your intended passage.
Rules 7, 8, 15 & 24(a), (e) · Keep clear of the whole tow
An astern tow over 200 m is marked by three vertical masthead lights on the towing vessel. The ordinary vessel being towed carries sidelights and a sternlight. Assess the entire operation: collision risk may remain despite appreciable bearing change when approaching a tow. Take early, substantial action and check it until finally past and clear.