Register Length is the length of ship measured from the fore-side of the head of the stem to the aft side of the head of the stern post or, in the case of a ship not having a stern post, to the fore-side of the rudder stock.
If the ship does not have a stern post or a rudder stock, the after terminal is taken to be the aftermost part of the transom or stern of the ship. This length is the official length in the register of ships maintained by the flag state and appears on official documents relating to ownership and other matters concerning the business of the ship. Another important length measurement is what might be referred to as the IMO Length.
This length is found in various international conventions such as the Load Line, Tonnage and SOLAS conventions and determines the application of requirements of those conventions to a ship. It is defined as 96 per cent of the total length on a waterline at 85 per cent of the least moulded depth measured from the top of keel, or the length from the fore-side of stem to the axis of rudder stock on that waterline, if that is greater. In ships designed with a rake of keel the waterline on which this length is measured is taken parallel to the design waterline.