I'm curious about this for a long time...
Ships that were built in the 50s have IMO starting with 5, 60s - with 6, 70s - with 7, etc. That's correct for most of cases & for them if the IMO starts with, say, 73 - then it's safe to say the vessel was built in 1973 or somewhen around that year. Also correct for most of cases. Ships built in the 90s have IMO starting with 9... I thought long ago that for those that will be built in 2000s the IMO will start from 0, for those built in 2010s - from 1, etc. Of course, that's wrong! Even for a 2016 built ship (not a yacht) it would still start with 9. But there are only 7 digits in IMO & what if eventually
there are/were so many vessels built since the 90s, that whatever 7 digit number starting with 9 you can come up with - it's already taken. What then?
Taking IMOs from long ago of vessels that no longer exist? Start IMOs from 0-4? Introduce 8 digit IMOs?