At OceanCraft we have constructed our own database to remove the confusion over IMO numbers. We always work with the six-digit number as the key field in the database regardless of whether it is an LR number or an IMO number. This is a problem with the Miramar database in that they chose to use the seven-digit IMO as a key field and then were stuck when they came to the six-digit numbers. They chose to add a 5 at the beginning of the number as the 55xxxx series of numbers was not used. This then causes problems within the ShipSpotting website as some posters choose to use the Miramar version and other use the six digit (IMOised) with a check digit at the end which is not strictly correct (and which is the point raised by Phil English).
From the OceanCraft database, the six-digit series appears to be in two primary groups, 50xxxx series and 54xxxx series.
The 50xxxx series (six-digit IMO) introduced in 1963 was applied to the list of ships in Lloyds in alphabetical order of the ship's name it carried in 1963. This series ranged from 500001 to 539965, so this covers almost 40000 ships.
The second series of 54xxxx was used and this appears to be for ships built in 1963 after the application of the 50xxxx series and ships which experienced a name change in 1963. This runs from 540001 to around 542950, so about 2950 ships and is applied in five alphabetical A-Z groups. The only explanation we have for this is that the 50xxxx series was implemented in, say, July 1963 and the 54xxxx series was applied at the end of each month remaining in 1963 to those ships which either entered service or changed name within that month. From 1964 the 64xxxx series was used.
Every number of both series was used and covers all ships extant in 1963.
There is evidence of 56xxxx, 57xxxx, 60xxxx series, (and possibly 61xxxx series, but this could be another Miramar 'invention') but we are still collating and evaluating these.
We hope the above info will be of some use but we do not know what the solutions is for the ShipSpotting website. It is looks as if ShipSpotting is a gallery of photos trying to have some structure without a database, where it really needs to be a database with structure linking to a gallery of photos. Discuss.OceanCraft Modelswww.oceancraft.co.uk