The History of Gross & Net Tonnage: How Ship Measurement Rules Shaped Global Trade

What is gross & net tonnage, and why does it matter? Learn how the Moorsom system revolutionized ship measurement, why tonnage rules vary globally, and how it impacts canal tolls & shipping costs. Essential for maritime historians, trade professionals, and genealogy researchers!

An American Ship Discharging Cotton Cargo Directly to the Upper Deck of the Transit Shed at Liverpool, circa 1929. The Port of Liverpool, 1929. GGA Image ID # 220ade3a43

Gross and net tonnage are terms applied to vessels and not to commodities. One hundred cubic feet of space is a vessel ton, and the gross tonnage of a vessel is the number of tons of 100 cubic feet within the ship's closed-in spaces.

The cubical contents of the closed-in spaces within any particular vessel would seem to be a fixed quantity definitely determinable, and would supposedly be the same whatever the flag of the ship or in whatever country the vessel is registered; but the several national rules for the measurement of vessels define closed-in spaces differently, and these rules vary as to what spaces shall be exempted from measurement, and thus from gross tonnage.

Moreover, the Suez Canal Co.'s measurement rules are different from nearly all the national rules.

Origination of the Present Rules and Methods

The present rules or methods followed in measuring vessels to determine their gross tonnage originated with Mr. George Moorsom, of England, and were first embodied in law in the British tonnage act of 1854.

The Moorsom rules for the measurement of vessels have since then been adopted by practically all countries of the world and are now everywhere followed, although, as will presently be pointed out, the practice of the several nations of the world as to the exemption and measurement of spaces within vessels is far from uniform.

Prior to 1854 vessels were measured by brief rules which produced only approximately correct results. Those in force in England were established by the "new measurement" law of 1836.

The need for improvement in the rules having become evident, the commissioners of the admiralty, at the request of the Board of Trade, appointed a committee in 1849 to recommend changes in the rules.

The following year this committee recommended that the contents of vessels should no longer be determined by internal, but by external, measurement. Mr. Moorsom was honorary secretary of this commission, but did not approve of its recommendation.

Formulation of the Moorsom Measurement Rules

When it became evident that the report of 1850 was not to be accepted, Mr. Moorsom formulated the measurement rules which now bear his name. These rules were approved by the Board of Trade and made law by act of Parliament.

Mr. Moorsom worked out an exact mathematical method or formula for determining the cubical contents of vessels, and the ships then registered under the British flag were measured by these rules. It was found that the cubical contents of the entire British merchant marine was 363,412,456 cubic feet.

At that time the total registered tonnage of the fleet was 3,700,000. The ratio of the number of cubic feet of contents to the number of tons register was 98.22 to 1.

It was the desire of the British Government to make as little change as possible in the registered tonnage, and it was accordingly suggested by Mr. Moorsom that in order to simplify calculations 100 cubic feet instead of 98.22 should be considered a gross ton. Tnis suggestion was adopted.

Mr. Moorsom's recommendations regarding measurement rules and tonnage were embodied in Rules 1, 2, and 3 of the tonnage act of 1854. These rules, as now in force, are printed in full in Appendix III to this volume.

Rule 1 prescribes a method for measuring empty vessels, Rule 2 states how laden vessels shall be measured, and Rule 3 prescribes the rules to be followed in measuring the space occupied by the engine and machinery of steamships.

In their present form the rules differ only in minor respects from those originally formulated by Mr. Moorsom.

The first country to follow England in the adoption of the Moorsom system of measuring spaces and the Moorsom ton was the United States, which embodied them without change in the act of 1864.

The Suez Canal Co.'s measurement rules were formulated by the International Tonnage Commission which met at Constantinople in 1873. These rules provided that the gross tonnage of vessels shall be determined by the Moorsom system and be expressed in Moorsom tons.

Thus, by the action of the International Tonnage Commission and by the laws of the United States and other countries, a vessel ton is everywhere 100 cubic feet, and the contents of vessels are determined by the Moorsom system of measurement.

Spaces Included in Measurement of Gross Tonnage

The rules, however, concerning the spaces that shall be included in gross tonnage vary with different countries, and the regulations of the Suez Canal Co. are different from nearly all the national rules.

Dissimilarity in the several measurement codes is due to the fact that some include spaces which are exempted by other rules. Thus the same ship would not have the same measured contents and would not have the same gross tonnage by British, American, and Suez rules.

It is obvious that if gross tonnage were made the basis of Panama tolls it would not be possible to accept the gross register tonnage of vessels as stated in their national registry certificates, because ships of the same size having different gross register tonnage would not be treated "on terms of entire equality, so that there shall be no discrimination * * * in respect of the conditions or charges of traffic."

The Hay-Pauncefote treaty would be violated if Panama tolls were collected in accordance with the gross tonnage of vessels as stated in the ship's papers.

"Exemption" and "Deduction"

In discussing the measurement of vessels to determine their gross and net tonnage it is necessary to keep in mind the distinction between "exemption" and "deduction" from measurement.

Measurement rules stipulate what spaces shall be measured to determine a vessel's gross tonnage and what part of the vessel shall be exempted from measurement, while the rules governing the determination of net tonnage specify which of the spaces that have been measured shall be deducted.

Net tonnage, as will be explained later in detail, is ascertained by deducting from the contents of the spaces that have been measured and included in gross tonnage the contents of such spaces as the rules designate shall not be included within the net tonnage of the vessel.

The gross tonnage of a vessel depends upon the spaces exempted from and upon the spaces included within the measurement, while a ship's net tonnage is affected by the specifications of the rules as to the exemption of spaces from measurement and as to the deductions to be made from the spaces included within the gross tonnage.

The rules regarding the measurement and exemption of vessels control the gross tonnage and indirectly determine the net tonnage of vessels.

Johnson, Emory Richard, Measurement of Vessels for the Panama Canal, Volume 2, Government Printing Office, Washington, DC 1913, Pages 45-46

📏 The History of Gross and Net Tonnage Measurements: A Critical Maritime Evolution 🚢

📜 Recap & Summary: Understanding the Evolution of Ship Measurement & Its Impact on Ocean Travel

The article "History of Gross and Net Tonnage Measurements" provides a detailed account of the origins, development, and global variations in tonnage calculations. This is an essential topic for maritime historians, shipping professionals, genealogists, and educators, as it unravels the historical and legal intricacies of vessel measurement that still influence global trade today.

🌊 Why This Article Matters?

For Teachers & Students 📚 – Provides a historical perspective on how the global shipping industry evolved.

For Genealogists 🧬 – Helps in understanding ship records when researching immigrant ancestors.

For Historians 🏛️ – Chronicles the standardization of ship measurement laws across nations.

For Shipping & Trade Professionals ⚓ – Explains why different countries measure ships differently, impacting port fees & canal tolls.

📢 Key Takeaway: Despite global efforts to standardize ship tonnage, variations in measurement laws still create inconsistencies, impacting trade, taxation, and international shipping regulations.

📊 Key Topics Covered: Breaking Down Gross and Net Tonnage Measurement

1️⃣ What Are Gross & Net Tonnage?

✅ Gross Tonnage (GT) – The total enclosed volume of a vessel, measured in 100 cubic feet per ton. This includes all closed-in spaces (cargo holds, passenger areas, crew spaces, etc.).

✅ Net Tonnage (NT) – The usable commercial capacity of a ship after deductions for crew spaces, navigation areas, and machinery rooms.

📌 Why It Matters?

• Determines ship registry, port fees, and taxation.
• Influences shipping costs & cargo capacity planning.
• Impacts tolls at major canals like Panama & Suez.

🔍 Example: A ship with a large engine room but minimal cargo space might have a high gross tonnage but low net tonnage, leading to different taxation and toll fees depending on measurement rules.

2️⃣ Origins of Gross & Net Tonnage: The Moorsom System ⚖️

🛠️ The Moorsom System (1854, UK) – The first standardized system for measuring a ship’s volume, created by George Moorsom in England.

📌 Why Moorsom’s System Changed Maritime History:

• Replaced arbitrary tonnage measurements used before 1854.
• Established the 100 cubic feet = 1 ton rule.
• Became the global standard, influencing ship design & taxation.

📜 Historical Development Timeline:

1️⃣ 1836 (UK) – "New Measurement" Law introduced, but still inaccurate & inconsistent.

2️⃣ 1850 (UK) – The Board of Trade proposed external ship measurement, but Moorsom opposed it.

3️⃣ 1854 (UK) – The Moorsom Tonnage Act passed, making internal volume the basis of tonnage measurement.

4️⃣ 1864 (USA) – The United States adopted the Moorsom System, bringing international consistency.

5️⃣ 1873 (International) – The Suez Canal Measurement Rules were introduced, differing slightly from national rules.

🔎 Why This Matters Today:

Even though most countries use Moorsom’s system, national variations still exist, meaning a ship’s tonnage can differ depending on where it is registered.

3️⃣ Suez Canal vs. Panama Canal Tonnage Rules 🌎

🚢 Not All Canals Measure Ships the Same Way!

• Suez Canal (1873 Rules) – Uses Moorsom tonnage but includes additional spaces in gross tonnage.
• Panama Canal (1913 Rules) – Based on Moorsom but adjusted for more precise cargo capacity calculations.

📌 Why It Matters?

• A ship might pay different toll fees at Panama vs. Suez, even though it's the same size.
• Some ships are designed to "game" the system, optimizing tonnage to reduce fees in certain canals.

🚢 Example:

A cargo ship with large open deck spaces may have a lower tonnage in Panama but a higher tonnage in Suez, leading to different toll rates.

4️⃣ Exemptions & Deductions: What Spaces Are Counted in Tonnage?

📦 What’s Included in Gross Tonnage?

✅ Cargo holds, enclosed passenger areas, crew accommodations.

✅ Machinery spaces (partially included, varies by country).

✅ Navigation areas like the bridge (included in some cases).

📉 What’s Exempted (Not Counted in Gross Tonnage)?

❌ Open decks & superstructures (varies by country).

❌ Certain machinery spaces, ventilators, and fuel storage.

❌ Crew spaces, lifeboat areas, and some non-revenue spaces.

📢 Why This Creates Inconsistencies:

• British, American, and Suez rules all define exemptions differently, meaning a ship can have different gross tonnage values depending on its registry.
• Ships designed for specific routes (like the Panama or Suez Canal) can be optimized to reduce tonnage-based fees.

5️⃣ Noteworthy Image: "American Ship Discharging Cotton in Liverpool, 1929" 🏗️

📸 Key Image: "An American Ship Discharging Cotton Cargo, 1929"

✅ Highlights how cargo capacity & tonnage measurement influenced global trade.

✅ Demonstrates how tonnage regulations impacted transatlantic commerce.

✅ A valuable historical reference for genealogists & maritime historians researching shipping records from the early 20th century.

🔍 Why It’s Important: This image captures a real-world example of how gross & net tonnage measurements were applied in shipping operations.

📚 Additional Reading & Resources

📖 "Measurement and Tonnage Laws" – A deep dive into how ships are classified globally.

📖 "Cargo and Carrying Capacity of Ships" – Explores how tonnage affects cargo storage & efficiency.

📖 "Net Tonnage of a Vessel and Its Computation" – Focuses on how cargo space deductions impact a ship’s earning potential.

🔚 Final Thoughts: Why This Article Matters

Understanding gross & net tonnage measurements is essential for maritime trade, historians, and shipping professionals.

🌎 For historians – Helps analyze maritime records & major trade routes.

📜 For genealogists – Explains why ship manifests list different tonnage figures.

🚢 For trade professionals – Clarifies how tonnage rules affect port fees & global shipping costs.

By mastering tonnage regulations, we gain a deeper appreciation for the historical, economic, and logistical aspects of maritime commerce. 🌊⚓