A ship is “open” when it is located at the end of a shipping route. As soon as a shipping route connects two settlements (or cities or for Cities & Knights also knights), the shipping route is considered as closed. A ship in a closed shipping route cannot be moved. The same rule applies for roads.
This rule also applies if an opponent builds a settlement on an intersection of your closed shipping route. In this case, your continuous trade route is considered as interrupted; however, the two settlements of the shipping route are still connected, meaning that the shipping route remains closed.
Special Case: If a ship's line starts at a settlement or city and returns to this settlement or city without a building between the individual roads, one of the two ends of the ship's line bordering the settlement or city is considered open. Thus, any of the two ships that border the settlement or city may be moved.
Special Case 2: If a ship's line describes a circle without connecting a settlement or city, then each of the ships in this circle is considered to be a open.
In the upper picture, any ship other than ship A may be removed.
- If e.g. Ship F is taken away, only ships G and E are considered open.
- Instead, if between ships C and D a settlement would be built, then none of the ships would be considered open anymore, since now two settlements would be connected.