In international law in the 18th and 19th Centuries, cartel ships were ships employed on humanitarian voyages, in particular, to carry prisoners for exchange between places agreed upon in the terms of the exchange. While serving as a cartel a ship was not subject to capture. However, if it engaged in commerce or warlike acts (such as carrying official dispatches or messengers) it lost its character of inviolability. It would then be subject to capture. The cartel protection extended to the return voyage. Furthermore, the prisoners being taken for exchange are under an obligation not to engage in hostilities towards their captors. If they were to capture the cartel ship, they would have no rights to salvage, and the owner of the vessel, assuming it was a ship from their country, would have no right to reclaim the vessel.
- Maxey, Edwin (1906) International law with illustrative cases. (F.H. Thomas Law Book Co.), p.500.
- Upton, Francis Henry (1863) The law of nations affecting commerce during war: with a review of the jurisdiction, practice and proceedings of prize courts. (J.S. Voorhies), pp.25-27.