'Sister Ships' are vessels that are built to the same specifications, for similar trades or operations, often in the same shipyard. Most of the time, Sister Ships will also have matching dimensions and equipment.
The first vessel delivered in a series (by date of build) will be considered the leader of the series.
Vessels may be considered to be Sister Ships if they:
- have the same hull dimensions and form;
- are built to common design characteristics within a certain time frame (i.e. 5 years).
While it is difficult to determine how much variation can be allowed before they are no longer considered 'Sister Ships', generally after a period of around 5 years, new design regulations and patterns may dictate variations in design or structure.
Other factors that might lead to vessels being defined as 'Sister Ships' include:
- Ships having consecutive, or close to consecutive yard numbers;
- A series of ships that are varied in terms of main engine, auxiliary engine or accommodation arrangements but are common in size and structure, and are owned by one owner, or a group of owners that are linked to the same yard.