Net Tonnage Explained: How Ship Capacity & Taxes Are Computed in Global Trade

What is net tonnage & why does it matter? Learn how ships are measured for commercial capacity, how deductions affect taxation & port fees, and why net tonnage is crucial for shipowners, trade professionals, and maritime historians!

In a recent issue, we took up the measurement of the ship for gross tonnage. We now come to net tonnage. Generally, net tonnage may be described as that portion of the ship's internal capacity devoted to commercial uses—the carriage of passengers and freight—and is the tonnage left after certain deductions have been made from the gross tonnage.

The deductions allowed by our laws are as follows: Crew's quarters, master's cabin, navigation spaces, donkey boiler and engine, sail locker, and machinery space.

However, it must be borne in mind that space may only be deducted if it has been measured and included in the gross tonnage. When bounded by flat surfaces, the deducted spaces may be calculated by taking the product of the three dimensions. Still, when one or more curved sides bound a space, the cubical contents of such space are to be measured in the same manner as deck erections—by using the Moorsom system.

Crew’s Quarters

In crew spaces may include the necessary passageways, provided such passages are exclusively for the use of the crew—and the necessary mess rooms, bathrooms, and toilet rooms, for the exclusive use of the officers and crew.

In addition, there may also be included spaces reasonable in the extent necessary to shelter the cook when employed in the preparation of food, the engineer when used in condensing water for the crew, the chief engineer's office, and the wireless operator when engaged in his work or where he may be berthed.

A hospital space for the crew, which shall be separated from other spaces, is allowed as a deduction. A deduction may be made if there is permanently cut into a beam and over the doorway of every such space the number of men it is to accommodate or the purpose for which it is to be used.

Navigation Spaces

The spaces allowed as deductions under this head include any space reasonable in extent exclusively used for the working of the helm, the capstan, and the anchor gear (which consists of the chain locker) when below decks or for keeping charts, signals, lamps, and other instruments of navigation and boatswain's stores. Words indicating the use of any of the deducted spaces must be cut in a beam and over the door of each space.

 

The Diagram Shows Spaces to Be Included in the Machinery Space and the Points from Which Measurements Are Taken. GGA Image ID # 2000e16831

Donkey Engine and Boiler

Where the donkey engine and boiler are situated within the machinery space, it should not be the subject of a separate allowance, as it would then be included in the machinery space deduction. It will not be allowed as a deduction in a house above the upper deck and, therefore, not measured in ascertaining the gross tonnage. In all other cases, the space occupied by the donkey engine and boiler, if connected with the main pumps of the ship, is to be allowed as a deduction, provided the place is reasonable in extent and properly constructed. It is to be marked as other deducted space.

Sail Locker

A deduction is allowed for necessary space to store sails in a vessel propelled wholly by sails, at most two and one-half percent of the gross tonnage. The area must be reasonable in extent and properly constructed.

Machinery Space

A deduction for the space occupied by the machinery must be made for vessels propelled by steam or other machinery. This deduction is not to be taken as the actual tonnage of the machinery space but is determined in the following manner:

• In paddle steamers, where the tonnage of the space occupied by the machinery and necessary for its proper working is above 20 percent and under 30 percent of the gross tonnage, the deduction shall be 37 percent of the gross tonnage.
• In screw vessels where the tonnage of the machinery space is above 13 percent and under 20 percent of the gross tonnage, the deduction shall be 32 percent of the gross tonnage.
• In measuring machinery spaces, shaft trunks shall be included as a part thereof.
• In paddle vessels where the tonnage of the space occupied by the machinery is 20 percent or under of the gross tonnage, the deduction shall be one and one-half times the actual tonnage of the machinery space.
• In the case of screw vessels, when the machinery space is 13 percent or less of the gross tonnage, the deduction shall be one and three-fourths times the actual tonnage of the machinery space.
• But if the actual machinery space is so large as to amount in the case of paddle vessels to 30 percent, or above, and in the case of screw vessels to 20 percent, or above, of the gross tonnage, the deduction shall consist of 37 percent, of the gross tonnage in the case of a paddle vessel and 32 percent, of the gross tonnage in the case of a screw vessel.
• Or, if the owner prefers, there shall be deducted from the gross tonnage the tonnage of the space occupied by or required to be enclosed for the proper working of the machinery, with the addition in the case of vessels propelled by paddles of 50 percent, and in the case of screw vessels of 75 percent.

Spaces in Engine Room

Within the meaning of our measurement laws, only those spaces occupied by the engine and boiler rooms, together with the space necessary for the working of the machinery, may be deducted, and are as follows:

• Space below the crown of the engine room.
• Light and air spaces for the machinery, framed in between the crown of the engine room and the upper deck.
• Light and air spaces for the machinery, framed above the upper deck.
• Shaft alleys or trunks and escape trunks.

After all the allowed deductions have been made, the amount of tonnage remaining shall be deemed the net or register tonnage, which shall be permanently marked on the main beam of the vessel.

A method of measurement similar to ours having been adopted by Great Britain, Belgium, Denmark, Italy, Sweden, Norway, Spain, Holland, Russia, Finland, Portugal, Japan and France, the tonnage of their vessels in our ports must be taken to be the tonnage expressed in their documents, as a like courtesy has been extended to our ships in their porta. Vessels of other foreign countries are to be measured according to our laws.

Phillips, Robert, “Net Tonnage of a Vessel and Its Computation,” in The Nautical Gazette: An International Weekly Chronicle of Shipping, Volume 99, No. 15, Whole No. 2565, New York, Saturday, 9 October 1920, P. 456-457

📏 Net Tonnage of a Vessel and Its Computation: Understanding Maritime Measurement & Trade 🚢⚖️

📜 Recap & Summary: The Importance of Net Tonnage in Ocean Travel, Trade, and Ship Documentation

The article "Net Tonnage of a Vessel and Its Computation" (1920) by Robert Phillips serves as an essential guide for understanding how ships are measured for net tonnage and why it matters for trade, taxation, and ship classification. This detailed explanation is crucial for maritime historians, genealogists, teachers, students, ship enthusiasts, and professionals in global shipping.

The net tonnage of a ship represents the actual space available for passengers and cargo after deducting certain areas like crew quarters, navigation spaces, and machinery rooms. This measurement determines port fees, canal tolls, and taxation in maritime trade, making it a fundamental aspect of ship ownership and operation.

💡 Who Should Read This?

📖 Teachers & Students – Learn how ship tonnage laws shaped maritime trade and engineering.

🧬 Genealogists & Passenger Researchers – Understand historical ship records and passenger accommodations.

⚓ Maritime Historians & Enthusiasts – Discover how net tonnage influenced naval architecture and global commerce.

📦 Shipping & Trade Professionals – Gain insights into how port fees and taxes are calculated based on net tonnage.

📊 Key Topics Covered: How Net Tonnage Is Computed & Why It Matters

1️⃣ What Is Net Tonnage & How Is It Calculated?

Net Tonnage (NT) is the remaining internal volume of a ship after deducting non-commercial spaces (crew quarters, machinery rooms, etc.).

It is a crucial factor in taxation, port charges, and canal tolls (such as Panama & Suez Canals).

Only spaces included in gross tonnage can be deducted from net tonnage—ensuring accuracy in calculations.

🔍 Why It’s Important: Net tonnage directly impacts a ship’s profitability by determining how much cargo or passengers it can legally carry.

2️⃣ Key Deductions from Gross Tonnage to Determine Net Tonnage

✅ Crew’s Quarters – Includes mess rooms, bathrooms, and necessary passageways used exclusively by the crew.

✅ Navigation Spaces – Covers chart rooms, signal storage, anchor gear, and boatswain’s stores.

✅ Donkey Engine & Boiler – Deductible if used for ship pumps and measured as part of machinery space.

✅ Sail Locker – Allowed only on sailing ships, up to 2.5% of gross tonnage.

✅ Machinery Space – Complex deductions based on whether a vessel is a paddle steamer or screw-driven ship.

🚢 Why It’s Important: These deductions reduce taxable ship tonnage, lowering operating costs for shipowners.

3️⃣ Understanding Machinery Space & Engine Room Deductions

For paddle steamers:

• If machinery space is 20-30% of gross tonnage, deduction = 37% of gross tonnage.
• If under 20%, deduction = 1.5× actual machinery tonnage.

For screw steamers:

• If machinery space is 13-20% of gross tonnage, deduction = 32% of gross tonnage.
• If under 13%, deduction = 1.75× actual machinery tonnage.
• If machinery space exceeds 30% (paddle) or 20% (screw), owners may choose:
• 37% deduction (paddle) or 32% deduction (screw)
  OR
  Actual machinery tonnage + 50% (paddle) or 75% (screw)

🔍 Why It’s Important: Ships with larger machinery spaces get better tax deductions, affecting their design and profitability.

4️⃣ Net Tonnage & International Trade Regulations

🚢 Thirteen Countries Have Adopted the American System of Measurement

✅ Great Britain

✅ Belgium

✅ Denmark

✅ Italy

✅ Sweden

✅ Norway

✅ Spain

✅ Holland

✅ Russia

✅ Finland

✅ Portugal

✅ Japan

✅ France

Ships from these nations have their net tonnage accepted in U.S. ports, and vice versa.

Other foreign ships must be re-measured according to U.S. laws before trading in American waters.

🔍 Why It’s Important: Standardized measurement simplifies international shipping and avoids unnecessary taxes or re-measurements.

📸 Noteworthy Images & Their Importance

🖼️ "The Diagram Shows Spaces to Be Included in the Machinery Space and the Points from Which Measurements Are Taken."

✅ Visually explains how machinery spaces impact net tonnage calculations.

✅ Highlights how deductions are determined for different ship types.

🌎 Global Impact of Net Tonnage in Maritime Law & Trade

⚓ Shipbuilding & Engineering – Tonnage laws affect ship design, efficiency, and financial feasibility.

📦 Economic & Trade Policies – Net tonnage influences taxation, canal tolls, and customs duties.

🌍 Maritime Infrastructure – Ports and canals use net tonnage to regulate global shipping operations.

By understanding how net tonnage is computed, shipowners can optimize profitability while complying with international regulations.

📚 Additional Reading & Resources

📖 "How a Ship’s Gross Tonnage Is Computed" – Learn how gross tonnage is calculated before deductions.

📖 "Measurement and Tonnage Laws" – A deep dive into the legal framework of ship measurement.

📖 "Cargo and Carrying Capacity of Ships" – How tonnage affects cargo space and profitability.

🔚 Final Thoughts: Why This Article Matters

This in-depth guide to net tonnage computation is essential for anyone studying maritime history, trade, or ship design.

🌎 For historians – It explains how international laws standardized maritime trade.

📜 For genealogists – It helps decode historical ship records & passenger accommodations.

🚢 For ship enthusiasts – It offers insights into how tonnage affects ship classification.

📦 For trade professionals – It clarifies how port fees & canal tolls are calculated.

Understanding net tonnage computation unlocks a deeper knowledge of maritime law, engineering, and commerce—making this an essential read for those interested in ocean travel and global shipping. 🌊⚓