Definition
Tonnage has two related but distinct legal senses:
1. Vessel capacity. The carrying capacity of a ship or vessel, measured in tons. How that capacity is calculated has varied by country and era: in England, tonnage traditionally referred to the actual weight in tons a vessel could safely carry; in the United States, it referred to the vessel's internal cubic capacity derived from the dimensions of the hold. As a result, U.S. official tonnage figures have historically run below the vessel's actual carrying capacity.
2. Tonnage duty. A charge or tax imposed on vessels calculated according to their tonnage — that is, in proportion to their capacity or the weight of goods carried. The defining characteristic of a tonnage duty is that the charge is measured by weight (either carrying capacity or actual cargo weight), not by the nature of the goods or any other variable.
The constitutional dimension of tonnage duty is significant. Article I, Section 10 of the U.S. Constitution prohibits states from laying "any Duty of Tonnage" without congressional consent, placing tonnage duties alongside imposts and export taxes as instruments of federal commercial supremacy.
Common Language
Modern common usage (Wiktionary): The weight or capacity of a ship's hold measured in tons; the number of tons a vessel displaces; also, total shipping of a fleet or nation; a charge per ton on cargo.
Historical common usage (Webster's 1913): The weight of goods carried in a boat or ship; the cubical content or burden of a vessel in tons; a duty or impost on vessels estimated per ton, or a toll payable on goods per ton transported.
Editorial note: The common and legal meanings share substantial ground, but the legal term carries a constitutional charge the ordinary word does not. In legal contexts — particularly constitutional and admiralty contexts — "tonnage duty" is a term of art subject to the Tonnage Clause of Article I, Section 10, and courts have given it a specific, expansive definition: any levy imposed on a vessel solely according to the rule of weight or carrying capacity qualifies, regardless of how the charge is labeled. A researcher reading "tonnage" in a commercial or maritime source must determine whether the constitutional prohibition on state tonnage duties is at issue, since labeling a charge something other than a "tonnage duty" did not historically insulate it from constitutional scrutiny.
Common Confusion
Tonnage duty vs. other port charges. States and municipalities historically imposed a variety of harbor fees, pilotage charges, and wharfage dues on vessels. Courts have repeatedly had to distinguish these from prohibited tonnage duties. The constitutional test is not the name of the charge but its structure: if the fee is calculated according to the vessel's tonnage or carrying capacity, it functions as a tonnage duty regardless of what it is called. Charges for specific services rendered (pilotage, wharfage, health inspections) have generally survived scrutiny; flat-rate capacity-based levies have not. Researchers encountering 19th-century port revenue disputes should be alert to this distinction, as the terminology in primary sources is often inconsistent.
Tonnage (capacity) vs. displacement. In ordinary maritime usage, "tonnage" and "displacement" describe different measurements. Displacement refers to the weight of water a vessel pushes aside; tonnage in the legal sense refers to carrying capacity. Historical legal documents generally use tonnage in the capacity sense, not the displacement sense.
Why It Matters in Research
The constitutional prohibition on state tonnage duties generates a substantial body of 19th- and early 20th-century case law that researchers must navigate carefully. The Tonnage Clause was an active area of litigation as states sought revenue from commerce moving through their ports, and the Supreme Court developed a line of doctrine distinguishing permissible port fees from prohibited tonnage duties. This doctrine intersects with Commerce Clause jurisprudence and the Import-Export Clause, meaning a research trail starting in one area may quickly require engagement with the others.
The definitional divergence between English and American tonnage measurement is a practical trap in historical sources. A vessel described by tonnage in an English document and the same vessel described in an American document may carry different numbers, reflecting different measurement conventions rather than different ships. Pre-standardization admiralty records, insurance documents, and shipping registers should be read with this in mind.
Anderson's Dictionary entry — as captured in the source material — approaches tonnage in the context of franchise obligations and public accommodation duties, which situates the term in a different analytical frame than pure admiralty or constitutional law. Researchers using Anderson's for tonnage-related questions should note that the entry speaks to the broader category of public-utility-type dues, not solely to the constitutional prohibition or the admiralty measurement question.
The word "tonnage" also appears in tax and customs statutes in senses that may not map neatly onto either the capacity or the constitutional-duty definitions. Early federal revenue acts imposed tonnage duties on vessels entering American ports and distinguished between U.S.-built vessels, foreign-built vessels owned by Americans, and foreign vessels — each category subject to different rates. Reading these statutes requires attention to which sense of tonnage is operative.
Historical Dictionary Support
Black's (1st and 2nd editions) agree on the core definition — carrying capacity of a vessel calculated in tons — and both flag the English/American divergence in measurement methodology. The 2nd edition is more explicit that U.S. tonnage is derived from cubic dimensions of the hold rather than actual carrying weight, and both editions gesture toward the duty definition by referencing the weight-based principle as the "vital" characteristic of a tonnage duty.
Bouvier confirms the measurement divergence and adds the practical observation that official U.S. tonnage figures regularly ran below actual vessel capacity — a point with direct legal consequence in shipping contracts and freight disputes where capacity was the operative term.
Anderson's entry, as excerpted in the source material, does not directly address vessel tonnage or the constitutional prohibition; the excerpt concerns franchise-based public accommodation obligations. This is a meaningful gap: Anderson's does not provide useful guidance on the Tonnage Clause question or the admiralty measurement issue, and researchers relying on Anderson's alone for this term will find it insufficient.
None of the historical dictionaries address the 20th-century standardization of tonnage measurement under international conventions, which introduced gross tonnage, net tonnage, and deadweight tonnage as distinct technical concepts. These distinctions matter in modern admiralty practice but are absent from the historical shelf sources.
Jurisdictional Note
The constitutional prohibition on state tonnage duties is federal and applies uniformly. However, what counts as a "specific service" charge exempt from the prohibition — versus a prohibited capacity-based levy — has been worked out through case-by-case federal adjudication. State courts applying their own port-revenue statutes in the 19th century did not always apply the federal constitutional standard consistently, making state-court admiralty records from that period unreliable guides to the federal doctrine.