Displacement Tonnage Explained: How Ships Are Measured by Weight & Water Displacement

What is displacement tonnage & why does it matter? Learn how ships are weighed, how cargo capacity is calculated, and why displacement affects stability & fuel efficiency. Essential reading for maritime historians, engineers & ship enthusiasts!

Before discussing dead-weight tonnage, which is applied to the weight of cargo and fuel vessels can carry, it will be best to explain displacement tonnage.

The displacement ton is a unit applied to vessels and not cargo. Still, to ascertain the dead-weight tonnage a ship can carry, it is first necessary to determine the vessel's displacement tonnage.

The displacement tonnage of a vessel is its weight in tons of 2,240 pounds avoirdupois. It is equal to the weight of water the ship displaces when afloat. Unless the term is qualified, the displacement tonnage of a vessel is the weight of the ship with its crew and supplies on board but without fuel, passengers, or cargo.

This is a vessel's displacement "light." The weight of water a ship displaces when loaded to its "deep-load line" is its displacement " loaded." The difference between the displacement tonnage of a vessel when "light" and when loaded to its "deep-load line" is its dead-weight tonnage, which is the maximum weight of fuel, cargo, and passengers that a vessel can carry.

An Historical Example of Displacement

Displacement is often confused with tonnage. It has a distinctively different meaning. When we say that the Mauretania displaces 44,540 tons or that the Olympic has a displacement of 62,000 tons, the ship, with everything on board and laden to her maximum loadline, or draught, displaces these respective weights in avoirdupois tons of water.

In plain English, these figures represent the total weights of the respective vessels, including their loads. Displacement is an elastic term because it varies from hour to hour. As the furnaces eat up the coal or Oil and the passengers consume the provisions, the ship becomes lighter and lighter, so that in the case of an Atlantic greyhound, the difference in the displacement when arriving at New York, as compared with that when leaving Europe, may vary between 5000 and 7000 tons.

Calculating the Displacement Tonnage

A cubic foot of sea water weighs 64 pounds or one-thirty-fifth of an English long ton of 2,240 pounds avoirdupois. Thus, the contents in cubic feet of that part of the vessel's hull below the water line divided by 35 equal the vessel's displacement tonnage.

If a ship were box-shaped—that is, if it were a parallelepiped—the product of its three dimensions in feet, its length, breadth, and depth below the water line, divided by 35 would be the displacement tonnage, but, as vessel hulls are not parallelepiped, the cubical contents of the hull of a ship have to be calculated using special, mathematical rules, such as Simpson's rules or the trapezoidal rules.[1]

The ratio of the actual contents of the submerged portion of a ship's hull to the contents of a parallelepiped having length, breadth, and depth corresponding to the length, breadth, and draft of the ship is the vessel's "block coefficient" or its "coefficient of fineness."

A full-shaped, slow freight steamer has a " block coefficient " of about 0.8—i. e., the submerged portion of the hull has a volume equal to 0.8 of the volume of a parallelepiped with equal dimensions.

The "block coefficient" or "coefficient of fineness" of the average freight steamer varies from 0.7 to 0.75, while the coefficient of a combination freight and passenger steamer is about 0.65; that of a fast passenger steamer is about 0.6, while racing yachts may have a coefficient as low as 0.4.

When the "coefficient of fineness" of a vessel is known, its displacement tonnage is determined by multiplying its length, breadth, and draft by its "coefficient of fineness" and dividing the product by 35.

In commercial practice, it is desirable to know a vessel's displacement tonnage at any given draft between its "light" and "loaded" lines because the difference between the displacement of a ship's "light" and the tonnage of its actual displacement indicates the weight of what the ship contains other than a crew and supplies.

Displacement Tonnage or Weight of a Ship

The displacement tonnage or weight of any particular ship at any given draft is shown by the vessel's "displacement curve" and scale. Figure 27 reproduces a typical displacement curve.

 

Fig. 2. Displacement Curve and Scale. Includes Vertical Displacement and Deadweight Scale. GGA Image ID # 2000f2a326. Click to View Larger Image.

The length, breadth, and draught of a ship cannot be multiplied together for displacement, but, by the application of a simple method known as Simpson’s rules, the volume of the immersed portion of the ship can be ascertained, which, if considered as water, and divided by 35, will give the displacement in tons. (See Chapter X. for Simpson’s Rules and Calculations.)

To make a separate calculation whenever the displacement is required at any particular draught would entail considerable labor and inconvenience. This is avoided by using what is termed a Displacement Curve, by means of which, in a moment, the displacement can be read off at any draught. It is constructed in the following manner (fig. 2).

Draw the vertical line A B, and upon it construct a scale at, say, 2 inch=l foot, indicating the draughts up to the load line, exactly as read upon the stem or stem of a vessel, inserting in each foot space twelve equal divisions for inches. From the top of the line A B draw the horizontal line A C. Divide this line into 1-inch spaces, each one representing 100 tons of displacement. Subdivide the spaces again into tenths, each of which will, therefore, represent 10 tons.

Supposing our vessel be of box form, 100 feet long, 20 feet broad, and 10 feet draught, then by calculating the displacement at a series of draughts, say 2, 4, 6, 8, and 10 feet respectively, we are in a position to find a number of points by means of which the displacement curve is constructed.

Figure 27 presents the displacement scale for a small vessel that draws but 7 feet of water when light, its displacement "light" being 550 tons. The ship may load to a maximum draft of 14 feet, at which draft its displacement is 1,400 tons.

The dead-weight capacity of the ship is 850 tons. It may be noted in passing that the vessel is permitted to be loaded so that it has but 2 feet of freeboard, the freeboard being the distance between the level of the upper deck and the "deep-load line." Vessels engaged in the overseas trade would not be permitted to have such a small freeboard.

Drawing the Ship's Displacement Curve

The figure also gives the ship's displacement curve. The curve is drawn as follows:

At the left, the draft of the vessel and its freeboard are given in a perpendicular scale, which may be assumed to have been drawn to a scale of 1 inch to 1 foot. A horizontal scale is so constructed from the top of this vertical scale that 1 inch equals 100 tons of displacement.

By drawing horizontal lines through the points indicating the draft of the vessel at different drafts from zero to 14 feet and by drawing vertical lines through the points in the horizontal scale corresponding to the number of tons of displacement at various drafts from zero to 14 feet, and by drawing a curve through the points of the intersection of the horizontal and vertical lines, the curve of the ship's displacement is located.

With this displacement curve known, the vessel's displacement at any given point in its draft can be read off from the displacement scale.

[1] Mathematical' rules for the calculation of the contents of the halls of ships are explained, among other places, In Chapter X of the book Know Your Own Ship, by Thomas Walton, London, 1909.

Johnson, Emory Richard, Measurement of Vessels for the Panama Canal, Volume 2, Government Printing Office, Washington, DC 1913, Pages 36-37

Thomas Walton, "Chapter I. Displacement and Deadweight," in Know Your Own Ship: A Simple Explanation of the Stability, Trim, Construction, Tonnage, and Freeboard of Ships,.together With a Fully Worked Out Set of the Usual Ship Calculations (From Drawings). Specially Arranged for the Use of Ships’ Officers, Superintendents, Engineers, Draughtsmen, and Others. Fourteenth Edition. Revised by John King. London: Charles Griffin and Company, Limited, 1918, pp. 4-6.

⚓ Displacement Tonnage: Understanding a Ship's True Weight & Capacity 🌊📏

📜 Recap & Summary: The Role of Displacement Tonnage in Maritime History, Trade, and Ship Design

The article “Displacement Tonnage” (1913 & 1918), sourced from maritime engineering publications, explores the concept of displacement tonnage—one of the most fundamental measurements in shipbuilding and navigation. This article is particularly valuable for teachers, students, genealogists, maritime historians, naval architects, and shipping professionals as it explains how displacement tonnage is calculated, its significance in understanding ship weight, and how it impacts ship design, cargo capacity, and ocean travel.

Unlike gross and net tonnage, which measure internal volume, displacement tonnage measures the total weight of a ship, including the water it displaces when afloat. This metric is essential for determining cargo capacity, understanding how ships float, and assessing their efficiency in different conditions.

💡 Who Should Read This?

📖 Teachers & Students – Gain insights into shipbuilding physics and maritime trade.

🧬 Genealogists & Passenger Researchers – Understand how ships’ weight influenced their historical voyages.

⚓ Maritime Historians & Enthusiasts – Discover how legendary ocean liners like the Mauretania and Olympic were measured.

📦 Shipping & Naval Professionals – Learn how ship displacement affects cargo loads and fuel consumption.

📊 Key Topics Covered: The Science of Ship Displacement

1️⃣ What Is Displacement Tonnage? Why Is It Important?

Displacement tonnage is the total weight of a ship, including everything onboard, measured in long tons (2,240 lbs).

This measurement is not about cargo space, but about how much water the ship displaces when floating.

Ships have two types of displacement tonnage:

• “Light” Displacement – The ship’s weight with only crew and basic supplies, but no cargo, fuel, or passengers.
• “Loaded” Displacement – The ship’s weight when fully loaded with fuel, cargo, and passengers.

The difference between light and loaded displacement is the ship’s deadweight tonnage, indicating its carrying capacity.

🔍 Why It’s Important: Displacement tonnage affects ship stability, fuel efficiency, and maritime regulations.

2️⃣ How Displacement Tonnage Is Calculated

💡 Key Formula:

📏 Displacement Tonnage = (Volume of submerged hull in cubic feet) ÷ 35

A cubic foot of seawater weighs 64 pounds, meaning that dividing a ship’s submerged volume by 35 gives its weight in long tons.

📐 Factors Affecting Displacement Calculations:

✅ Block Coefficient (Coefficient of Fineness) – A mathematical ratio comparing the ship’s submerged shape to a simple box shape.

✅ Ship Type & Purpose – Different ships have different coefficients:

Freight Steamers – 0.7 to 0.8 (fuller hulls for cargo).

Passenger Liners – 0.6 to 0.65 (sleeker hulls for speed).

Racing Yachts – As low as 0.4 (thin, streamlined hulls).

🔍 Why It’s Important: Shipowners and naval architects use displacement calculations to optimize ship design, fuel efficiency, and stability.

3️⃣ Noteworthy Example: The Mauretania & Olympic 🚢✨

The legendary ocean liner Mauretania displaced 44,540 tons.

The Titanic’s sister ship, Olympic, displaced 62,000 tons.

These numbers represent each ship’s total weight, including cargo and passengers, at full capacity.

🌊 Did You Know? Displacement changes throughout a voyage! As ships burn fuel and passengers consume supplies, their displacement tonnage decreases—by as much as 5,000 to 7,000 tons on a transatlantic crossing.

🔍 Why It’s Important: This explains why ship captains must constantly adjust for balance and stability during long voyages.

4️⃣ Displacement Curves: How Ships Track Their Weight at Different Drafts

A Displacement Curve is a graph used to quickly determine a ship’s weight at any given water draft (depth in water).

📊 Key Image: "Displacement Curve and Scale"

✅ Shows how displacement tonnage changes as a ship is loaded or unloaded.

✅ Helps captains and engineers read the ship’s weight without recalculating formulas.

✅ Used to ensure safe loading and stability at sea.

🔍 Why It’s Important: Instead of doing complex calculations, ship operators can use a pre-drawn curve to instantly read a vessel’s weight at any water depth.

5️⃣ Displacement Tonnage vs. Deadweight Tonnage: What’s the Difference?

💡 Key Takeaway: Displacement tells how much a ship weighs, while deadweight tells how much a ship can carry.

🔍 Why It’s Important: Knowing both is crucial for ship safety, efficiency, and legal compliance.

📸 Noteworthy Images & Their Importance

🖼️ "Displacement Curve and Scale"

✅ Helps visualize how ship weight changes at different drafts.

✅ Essential for quickly checking displacement without doing complex math.

✅ Used by ship captains and naval architects to optimize loading and stability.

🌎 Global Impact of Displacement Tonnage in Maritime Operations

⚓ Shipbuilding & Engineering – Displacement tonnage influences ship design, stability, and fuel efficiency.

📦 Trade & Cargo Management – Helps determine how much cargo can be carried safely.

🚢 Passenger Safety – Ensures ships are not overloaded, preventing accidents.

🌍 Environmental Impact – Affects fuel consumption and carbon emissions in global shipping.

By understanding displacement tonnage, shipbuilders, merchants, and historians can better analyze historical voyages, shipping economics, and naval engineering advancements.

📚 Additional Reading & Resources

📖 "How A Ship’s Gross Tonnage Is Computed" – Learn how volume-based tonnage is calculated.

📖 "Net Tonnage of a Vessel and Its Computation" – Understand how net tonnage affects taxes and canal tolls.

📖 "Cargo and Carrying Capacity of Ships" – Explore how displacement affects real cargo loads.

🔚 Final Thoughts: Why This Article Matters

Understanding displacement tonnage is key to deciphering maritime history, naval engineering, and global trade.

🌎 For historians – Helps analyze historical ship records and famous liners.

📜 For genealogists – Offers insight into passenger ships and their weight capacities.

🚢 For ship enthusiasts – Provides in-depth knowledge of ship stability and design.

📦 For trade professionals – Explains how weight limits affect cargo transport.

By mastering displacement tonnage, we gain a deeper appreciation of how ships operate and navigate the world’s oceans. 🌊⚓