|Part of the common law series|
|Defenses against formation|
|Excuses for non-performance|
|Rights of third parties|
|Breach of contract|
|Related areas of law|
|Other common law areas|
The doctrine of deviation, as it pertains to legal contracts, is a departure from a planned contractual course or design. When a plan has been adopted for a building, and in the progress of the work a change is made from the original plan, the change is called a deviation. When a ship alters her course or remains in port without just cause, the ship's new course or delay is called a deviation. Unless the contract permitted otherwise, in either case there is a breach of contract by the party responsible for the deviation.
Insurance regarding voyages and shipment 
From the moment this happens, the voyage is changed, the contract determined, and the insurer discharged from all subsequent responsibility. By the contract the insurer only runs the risk of the contract agreed upon, and no other; and it is, therefore, a condition implied in the policy, that the ship shall proceed to her port of destination by the shortest and safest course (or usual course), and on no account to deviate from that course, but in cases of necessity.
The effect of a deviation is not to vitiate or avoid the policy, but only to determine the liability of the underwriters from the time of the deviation. If, therefore, the ship or goods, after the voyage has commenced, receive damage, then the ship deviates, and afterwards a loss happen, there, though the insurer is discharged from the time of the deviation, and is not answerable for the subsequent loss, yet he is bound to make good the damage sustained previous to the deviation. But though he is thus discharged from subsequent responsibility, he is entitled to retain the whole premium.
What amounts to a deviation is not easily defined, but a departure from the usual course of the voyage, or remaining at places where the ship is authorized to touch, longer than necessary, or doing there what the insured is not authorized to do; as, if the ship have merely liberty to touch at a point, and the insured stay there to trade, or break bulk, it is a deviation.
The "course of the voyage" is not meant to be the shortest course the ship can take from her port of departure to her port of destination, but the regular and customary track, if such there be, which long usage has proved to be the safest and most convenient.
A deviation that will discharge the insurer must be a voluntary departure from the usual course of the voyage insured, and not warranted by any necessity. If a deviation can be justified by necessity, it will not affect the contract; and necessity will justify a deviation, though it proceed from a cause not insured against. The cases of necessity which are most frequently adduced to justify a departure from the direct or usual course of the voyage, are
- Stress of weather,
- The want of necessary repairs,
- Joining convoy,
- Succoring ships in distress,
- Avoiding capture or detention,
- Sickness of the ship's master or mariner, and
- Mutiny of the crew.
In construction 
When the contract is to build a house according to the original plan, and a deviation takes place, the contract shall be traced as far as possible, and the additions, if any have been made, shall be paid for according to the usual rate of charging.
See also 
This article incorporates text from A Law Dictionary, Adapted to the Constitution and Laws of the United States, by John Bouvier, a publication from 1856 now in the public domain in the United States.