Article I, Section 10, Clause 3:
No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.
The purpose of the Tonnage Clause is
to 'restrai[n] the states themselves from the exercise' of the taxing power 'injuriously to the interests of each other.' . . . In writing the Tonnage Clause, the Framers recognized that, if 'the states had been left free to tax the privilege of access by vessels to their harbors the prohibition [in Article I, § 10, clause 2] against duties on imports and exports could have been nullified by taxing the vessels transporting the merchandise.' 1 The prohibition against tonnage duties embraces all taxes and duties, regardless of their name or form, whether measured by the tonnage of the vessel or not, that, in effect, are charges for the privilege of entering, trading in, or lying in a port. 2 The Tonnage Clause, however, does not ban all
taxes which fall on vessels that use a State's port, harbor, or other waterways. Such a radical proposition would transform the Tonnage Clause from one that protects vessels, and their owners, from discrimination by seaboard States, to one that gives vessels preferential treatment vis-à-vis all other property, and its owners, in a seaboard State. 3 But it does not extend to charges made by state authority, even if graduated according to tonnage, 4 for services rendered to the vessel, such as pilotage, towage, charges for loading and unloading cargoes, wharfage, or storage. 5
For the purpose of determining wharfage charges, it is immaterial whether the wharf was built by the state, a municipal corporation, or an individual. Where the wharf was owned by a city, the fact that the city realized a profit beyond the amount expended did not render the toll objectionable. 6 The services of harbor masters for which fees are allowed must be actually rendered, and a law permitting harbor masters or port wardens to impose a fee in all cases is void. 7 A state may not levy a tonnage duty to defray the expenses of its quarantine system, 8 but it may exact a fixed fee for examination of all vessels passing quarantine. 9 A state license fee for ferrying on a navigable river is not a tonnage tax but rather is a proper exercise of the police power and the fact that a vessel is enrolled under federal law does not exempt it. 10 In the State Tonnage Tax Cases, 11 an annual tax on steamboats measured by their registered tonnage was held invalid despite the contention that it was a valid tax on the steamboat as property.