Free ships

ships of neutral nations, which in time of war are free from capture even though carrying enemy's goods.
- Burrill.

See also: Free

References in classic literature ?
Here, having made a good acquaintance at Manilla, he got his ship made a free ship, and the governor of Manilla hired him to go to Acapulco, on the coast of America, and gave him a licence to land there, and to travel to Mexico, and to pass in any Spanish ship to Europe with all his men.