The doctrine of deviation is a particular aspect of contracts of carriage of goods by sea. A deviation is a departure from the "agreed route" or the "usual route", and it can amount to a serious breach of contract.

The consequences of unjustified deviation can be very grave for the carrier, who is thereby prevented from relying upon exclusion clauses within the contract limiting his liability; nor will the carrier be able to rely on statutory protections, such as Art. IV of the Hague-Visby Rules.

The scope of deviation.

The "agreed route" is identified from the contract of carriage, as evidenced by the bill of lading. The ports of origin and destination (for example "Piraeus to Liverpool"') define the route. The "usual route" is not necessarily the shortest route, but is a version of the agreed route, taking into account safety issues, distances, seasonal conditions and war zones. For instance, the route "Auckland to London" could be via Suez, Panama, the Cape of Good Hope, or Cape Horn, but the usual route will be identified by reference to current practice or to any previous dealings of the parties.

Although a deviation is primarily a departure from the agreed route or the usual route, the concept of deviation extends beyond this. Deviation may include any unjustified delay such as remaining at authorized ports longer than necessary or acting beyond the agreed scope of the voyage. For example, if a ship makes an authorized call at a port, but stays on to trade or break bulk, this action may amount to a deviation.

The concept of deviation has been extended (sub nom “quasi deviation”) to cover wrongful transshipment, careless stowage, and stowage in the wrong part of the ship (such as on deck, rather than below decks). The consequences of quasi-deviation are the same as for ordinary deviation.

Article IV Rule 4 of the Hague-Visby Rules provides: "Any deviation in saving or attempting to save life or property at sea or any reasonable deviation shall not be deemed to be an infringement or breach of these Rules or of the contract of carriage, and the carrier shall not be liable for any loss or damage resulting therefrom". The Hague-Visby Rules do not define deviation but rely on existing common law definitions. Nor do the Rules define “any reasonable deviation”, but in decided cases the phrase has been taken to include deviation…..

1.    to avoid foul weather,

2.    to join a convoy,

3.    to avoid capture or detention,

4.    to take stricken passengers or crew for medical attention,

5.    to effect necessary repairs, (though this may be evidence of unseaworthiness),

6.    mutiny of the crew.

If a justified deviation is followed by an unjustified deviation, the doctrine of deviation will apply from the unjustified deviation.