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The short answer
A ship has three names made of numbers and letters, and they are easy to mix up. Her MMSI is the nine digits her radio sends, and its first three digits show her flag. Her IMO number is seven digits that belong to her hull and stay with it. Her call sign is a short run of letters and digits that is her name on the radio.
If you can choose, search with the IMO number. Shiptrail's search box takes any of the three, typed by itself, and so it does a name, but a name can belong to many ships. The rest of this page says what each one is, how to check an IMO number you have been given, and what it means when a number finds nothing. How to track a ship has the search itself, and What AIS is says where these numbers are sent from.
What each one is
An MMSI, a Maritime Mobile Service Identity, is nine digits that identify a ship's radio. A map goes by it to tell one ship's reports from another's. The first three digits are the country code of the administration that issued it, so a flag is written into the number. QUEEN MARY 2 has MMSI 310627000, and 310 is Bermuda's.
An IMO number is seven digits that go with a ship's hull, written IMO 9241061 for the same ship. It is not sent with every report but with her identity, about every six minutes.
A call sign is the ship's name on the radio: letters and digits, four to seven of them for a ship. Its first characters show the country that gave it, as an MMSI's first digits do, and it is sent with her identity too.
Which one stays with the ship
Her IMO number is the one that stays. It is never reassigned to another ship, and it does not change when the ship moves to another flag. It does not change when she is sold or renamed either.
Her MMSI is the one that moves. It follows her flag, and when a ship changes flag her MMSI is reassigned, so the same hull can be heard under one number one year and another the next. A ship's page says so too: a new flag brings a new number. Her call sign carries a country in its first characters, so it is tied to her flag as well.
That is why a name is never proof of which ship you are looking at. Names are reused, and at one point the map held a dozen ships called EAGLE. A name finds candidates and a number settles it. Shiptrail's list of famous ships, for one, goes by IMO number where a ship sends one and by MMSI where she does not, never by name.
Which to search with
Use her IMO number if you have it, because it names one hull and nothing else. An MMSI is nearly as good for a ship that is sailing now, and a call sign works too. A name is the weakest, for the reason above, though it is often all a booking gives you.
Type the number by itself, or with IMO or MMSI in front of it, and the box reads either. If the text does not find a ship, the box says what it looks like, and that is usually the quickest way to see what went wrong.
How to check an IMO number
The seventh digit of an IMO number is a check digit, there to catch a typing mistake. Multiply the first six digits by 7, 6, 5, 4, 3 and 2, add the results, and the last digit of the total should be the seventh digit of the number.
Take IMO 9481805, the number painted on the ferry in this page's photograph. Its first six digits give 63, 24, 40, 4, 24 and 0, which add up to 155, and the last digit of 155 is 5, which is the number's last digit. Change one digit, to 9481905, and the sum becomes 158. The last digit of that is 8, which does not match, so the number fails. The check catches most single-digit typos, though not every one.
Shiptrail uses the same check in the search box and on ship pages. It also turns down 1234567, which passes the check: transmitters that have not been set up send it, and it was once heard from four ships at once.
When a number finds nothing
Usually it is one of three things. The first is a typo. If an IMO number fails its check, the box says it is not a valid IMO number and that its last digit does not match the other six, so you can look for the mistake.
The second is a ship Shiptrail has not heard. It knows the ships its receivers heard in the past year, and a ship far from a coast, or where there is no receiver on land, may never have been heard. Then the box says that no ship with that number has been heard. Far from land, the open ocean is dark to it.
The third is a number that only looks like an MMSI. A nine-digit booking or reference number is easy to take for one, but the first three digits of a real MMSI are a country code. A made-up number such as 150234567 starts with 150, which is no country's, so a reference number that starts that way is not an MMSI.
Where to find them
Your booking or bill of lading names the ship, and may give her IMO number as well. With only the name, search for it and open her page. Its section called Her MMSI shows the nine digits with the country code picked out, and her IMO number if she sends one. When a name matches several ships, the IMO number is what tells them apart.
Whatever you find, the position beside it is a report from a ship's radio, heard by a receiver on land. Positions are not for navigation.
Questions people ask
- Can two ships share an IMO number?
- They should not: an IMO number is never reassigned to another ship. But a crew types it into the transponder by hand, and Shiptrail has heard the same valid number from two ships at once. A ship's page lists the other MMSIs her IMO number was heard under, under the words "was also broadcast by". That can be the same hull under an earlier MMSI after a change of flag, or a second ship sending a wrong number. The list leaves out any MMSI that is on the map now, and it lists nothing at all when more than three others share the number, since that is a wrong number and not one hull.
- Does a boat without an IMO number have a page?
- Yes. Any ship or boat Shiptrail has heard in the past year has a page by her MMSI, and one with no IMO number has a page that says she broadcasts none.