Tugs and towing vessels on AIS

Which AIS codes count as tugs and towing vessels

Three codes land in this category. 52 is "tug" — the vessel itself, whatever it is doing. 31 is "towing" and 32 is "towing, with a tow longer than 200 metres or wider than 25 metres" — both describe an activity, and sit in the 30s range where each code is a fixed meaning.

In practice most harbour tugs report 52 permanently. Some towing vessels switch to 31 or 32 while a tow is connected, which is closer to what the standard intends, and many never change it at all.

How to spot one on the live map

Harbour tugs are short, powerful and busy: quick runs out to meet an arriving ship, a slow escort in alongside it, then back to their own berth. On a live plot they cluster around port entrances and terminal berths.

A tug with a tow often moves at only 4 to 8 knots. The tow itself — a barge, a dead ship, a rig — frequently carries no AIS of its own, so the tug may be the only dot you see for something several hundred metres long.

Open the live map → Positions there are the latest known, with their source and age.

Who has to carry AIS

Large seagoing tugs fall under the SOLAS rule for ships of 300 gross tonnage and upwards on international voyages. Most harbour tugs are smaller and are covered by national rules instead: in US waters, for example, towing vessels of 26 feet or more with over 600 horsepower must carry AIS under 33 CFR 164.46.

The navigational status to expect

Ship type says what a vessel is; navigational status says what it claims to be doing. These are the statuses that normally go with tugs and towing vessels:

• 0 · Under way using engine
• 5 · Moored
• 11 · Towing astern
• 12 · Pushing ahead or towing alongside
• 3 · Restricted manoeuvrability

Where the type code misleads

Statuses 11 (towing astern) and 12 (pushing ahead or towing alongside) were assigned only in a later revision of the standard, and many transponders and crews still use 0 or 3, restricted manoeuvrability, for a tow. Do not read the absence of 11 or 12 as the absence of a tow.

A towed barge with no AIS is invisible on this map — one more reason a site like this must never be used for navigation.

Lights and shapes

Under Rule 24, a power-driven vessel towing astern shows two masthead lights in a vertical line, or three when the tow is longer than 200 metres, plus a yellow towing light above the sternlight. With a tow over 200 metres it shows a diamond shape by day.

Summary for orientation only. The COLREGs text is the authority; this site is not a navigation reference.

Questions

What is the AIS ship type code for a tug?

Code 52. Codes 31 and 32 are "towing" and "towing with a long or wide tow" — they describe what a vessel is doing rather than what it is, and some tugs switch to them while a tow is connected.

What is the difference between AIS type 31 and 32?

Both mean the vessel is towing. 32 adds that the tow is longer than 200 metres or wider than 25 metres, which matches the COLREGs threshold for the extra masthead light and the diamond day-shape.

Does the barge a tug is towing show up on AIS?

Often not. Unmanned barges and dead tows frequently carry no transponder, so the tug can be the only thing on the map. Navigational statuses 11 and 12 exist to signal a tow, but many crews still use 0 or 3 instead.

Further reading

• AIS ship type codes explained
• AIS Navigational Status Codes: All 16 Explained
• How AIS actually works
• All 100 AIS ship type codes, one page each
• Port activity index