Deviation. Under the contract of affreightment, if a vessel be compelled by stress of weather, or by a foreign enemy, or by any inevitable accident, to deviate from the appointed voyage, or to take refuge in a port not included in the destination, no claim thence arises at the instance of the freighter against the owners; and where the ship, in consequence of such accident, requires to be refitted, the master may now detain the cargo until the necessary repairs be executed, although formerly the rule was, that he was bound to find another ship in which to embark the cargo. Where the deviation is imputable to the fault of the master, the freighter has a claim for damages, in satisfaction of which he may retain the freight.

Definition source
Bell, William, A Dictionary and Digest of the Law of Scotland, 7th ed. rev. George Ross (Edinburgh: Bell & Bradfute, 1861), s.v. “Deviation.” Public domain.

Legal Subject: Deviation

Case Date Legal Subject Abstract
Cunningham & Co. v. Craigie 1789 Insurance, Arrestment, Deviation, Creditor, Debt, Prize William Cunningham and Company, merchants in Glasgow, pursued Laurence Craigie and other insurers to overturn Lord Dreghorn's interlocutor that assoilzied the defenders, seeking recovery under a 1783 policy. The defenders answered that the policy was void for concealment of a voyage to Charlestown carrying British troops and for material alteration of the terms from a voyage to St Thomas. The pursuers subsequently petitioned the Court to alter its recent interlocutor, while the respondents denied any knowing intent to deviate to Charlestown and maintained that the policy covered only the voyage to St Thomas with a convoy call serving as a risk-reducing clause.
John Steven and Company v. John Douglas 1772 Deviation Defender John Douglas provided an insurance policy to pursuer John Steven and Company for a shipment of goods on the Belfast Trader. The planned shipment route was from Belfast to Greenock or Port Glasgow. Before the ship set sail, however, merchants in Stranraer requested that the ship transport some goods from Belfast to Stranraer. The owner of the Belfast Trader agreed to transport these goods. In the course of trying to reach Stranraer, the ship encountered a storm and sank off the coast of Girvan in Ayrshire. Douglas refused to reimburse Steven and Company. Douglas claimed that the voyage taken by the Belfast Trader was different than the voyage provided for in the insurance policy. Steven and Company sought reimbursement, arguing that the voyage to Stranraer on the way to Greenock or Port Glasgow was not a significant deviation from the original route.
Mackies v. Houston and Others 1774 Deviation Pursuers Alexander and John Mackies obtained insurance for their ship, the Marshall of Gardenston, and its cargo. Although the ship was bound for Dundee, it docked at Down (now known as Macduff) during the voyage, and the cargo was stolen by a mob. The pursuers applied to the insurance underwriters for indemnification and eventually brought suit against them. However, the underwriters argued that they were not required to pay because the ship-master had deviated from the insured voyage by going to Down and taking on cargo there. Relevant documents are reproduced in an appendix to the defenders’ court filing.