IMO numbers are (IHS Fairplay, quote:)
... mandatory for all solas vessels under SOLAS regulation XI-1/3.
The Scheme assigns IMO Ship Numbers to propelled, sea-going merchant ships of 100 GT and above, with the exception of the following: vessels solely engaged in fishing, ships without mechanical means of propulsion, pleasure yachts, ships engaged on special service (e.g. lightships, SAR vessels), hopper barges, hydrofoils, air cushion vehicles, floating docks and structures classified in a similar manner, ships of war and troopships and wooden ships.
IHSF has extended the Scheme on a voluntary basis to include some of the above excluded shiptypes, and in particular fishing vessels and commercial yachts that meet solas requirements. (unquote)
see also
http://www.ihsfairplay.com/About/IMO_standards/IMO_standards.htmland
http://www.imo.org/Pages/home.aspxregards
luko p.