1. Why Newspapers Called an Iron Ship Floating in 1787 "Strange"
Around late July 1787, a ship built from British iron by Wilkinson appeared in the Birmingham canal [1]. Reports at the time stated it carried about 22.75 tons of cargo [1]. This vessel is considered one of the earliest candidates for an iron-hulled ship, though historians debate what exactly counts as the "first" iron boat, so we cannot say for certain that this 1787 ship was the very first [1].
Separate from reports of its arrival, a newspaper dated August 7 noted that it might seem strange for iron to float [1]. The article explained that if iron is spread out thinly, it can float [1]. A solid lump of iron sinks, yet a ship made of the same material floats even with cargo. This contradiction is what led newspapers to call the phenomenon "strange" [1].
2. The Rule of Floating: A Ship Pushes Aside Water Equal to Its Own Weight
Any object that floats pushes aside water equal to its own weight [2]. The weight of the water pushed aside by a ship is called displacement, and this amount equals the actual weight of the ship [3].
Inside an iron ship, there is a large space filled with air. Because of this, even if the total weight of the ship remains the same, its volume becomes much larger, making its average density lower than that of water [2]. The iron itself does not become lighter; rather, the shape allows it to displace enough water to support its weight. A solid lump of iron has a small volume, so it cannot push aside enough water to equal its weight, and it sinks. A ship spreads out its shape to displace the necessary amount of water [2].
3. Why the Same Ship Sinks Deeper in a River Than in the Sea
If a ship must push aside water equal to its own weight, what happens when the type of water changes? In calculations for load lines, fresh water density is set at 1,000 kilograms per cubic meter (1,000 kg/m³), while standard seawater is 1,025 kilograms per cubic meter (1,025 kg/m³) [4]. Seawater is heavier because it contains salt, making it about 2.5% heavier than fresh water for the same volume [4].
When a ship of the same weight floats, it needs less volume of heavy seawater to displace enough weight, but more volume of lighter fresh water. Therefore, the same ship sinks deeper in fresh water than in seawater [4]. For example, if we calculate for a 1,000-ton ship, it displaces 1,000 cubic meters of fresh water, but only about 976 cubic meters of seawater (1,000 ÷ 1.025) [4]. The difference is about 24 cubic meters, which is equivalent to a cube with sides of about 2.9 meters. However, this is just an example calculation; it does not mean the sinking depth increases by exactly 2.5%. How much deeper it sinks depends on the shape of the ship [4].
The difference in volume caused by water density affects safety margins, which leads to important rules for loading ships.
4. How Water Density Changes with Temperature and Salt, Even Within the Sea
The density of surface seawater is roughly 1.025 kilograms per liter (about 1,025 kg/m³) [5]. When water temperature rises, density decreases, and when salt content increases, density goes up [5]. This means seawater density changes depending on location and season.
Maritime resources explain that a ship moving from cold, salty sea water into warm, less salty water will displace more water to balance its weight, causing it to sink deeper [3]. People loading cargo onto ships must consider these differences in water when deciding how much to load.
5. How the Plimsoll Line on a Ship's Hull Shows Safe Sinking Depths
On the side of a ship, there is a symbol with a circle and a horizontal line through it. This is called the Plimsoll line or load line, developed in 19th-century Britain to prevent disasters caused by overloading [6].
Next to this mark, lines are carved for different conditions, such as Summer (S), Winter (W), and Fresh water (F) [6]. Because it is harder to get buoyancy in fresh water than in seawater, a specific line for fresh water exists [6]. The differences in water weight we have seen are directly represented by the position of these lines.
Samuel Plimsoll campaigned to require ships to display load limit lines. In 1876, the Merchant Shipping Act made displaying these marks mandatory for applicable British ships [7]. The concept existed before this law, so Plimsoll did not invent it alone, but his efforts were crucial for safety regulations.
6. An Experiment: Compare Sinking Depths of a Plastic Bottle in Different Water
You can try an experiment to see how water density affects floating. Put a few coins into a small plastic bottle and close the lid tightly. Float it in a deep container of water and mark the water level on the bottle with a permanent marker.
Next, dissolve plenty of salt into the same container's water, then float the same bottle in it. How does the position change? Since salt water is heavier, you can expect the bottle to rise slightly higher out of the water. The difference between real seawater and fresh water is small (about 2.5%), so using very salty water makes it easier to see the difference. Be careful not to get salt water in your eyes.
If you see a large ship at a port, look for the circle and line marks on the hull. How many lines are there, and what conditions do they represent? To learn more about the history of these marks, you can read explanations from the Royal Museums Greenwich [7].