1. Why Does Iron Sink as a Nail but Float as a Ship?
If you drop an iron nail into a basin of water, it sinks to the bottom immediately. Iron has a specific gravity of about 8, meaning it is about 8 times heavier than the same volume of water, so it cannot float as a solid mass [2]. However, a ship made of the same iron does float. One might think "the iron became lighter," but that is not true. The inside of the iron vessel is filled with light air. The volume increases, but the mass of the vessel itself does not change. As a result, the mass per unit volume, or density, becomes smaller [2]. What changed is not the amount of iron, but the average when the "whole ship," including the air inside, is viewed as one block. So, what conditions of this average determine whether it floats or sinks?
2. How Does the Weight of Pushed-Away Water Hold a Ship Up?
When you submerge your body in a bathtub, the water level rises. The water pushes your body up with a force equal to the weight of the water your body pushed aside. This upward force is buoyancy. Archimedes' principle states that the buoyancy an object in water receives is equal to the weight of the water it displaces [1]. Applied to a ship, the ship floats when the weight of the water displaced by the part of the ship underwater (the buoyancy) balances the total weight of the ship [2]. Because the ship's interior is mostly air, the average density of the "whole ship" is kept lower than water [2]. "Whole ship" includes not just the iron hull but also the air inside. With the same iron, if you roll it into a small ball, it displaces little water, and the buoyancy is insufficient, so it sinks. If you spread it into a large vessel, it displaces more water, and the buoyancy can support the weight. So, how thick can the iron vessel be and still float?
3. How Thick Can an Iron Box's Plates Be and Still Float?
Let us try a simple calculation to find the limit. Imagine making a cubic box with sides of 1 m from iron plates. Assume the inside is air and it has a lid. Taking iron's density as 8 times that of water (based on specific gravity of about 8), we can calculate. The weight of 1 m³ of water is about 1000 kg. When the box is just fully submerged, the buoyancy is equivalent to about 1000 kg. If the mass of the iron in the box is less than this, it will float. If the iron plate thickness is 2 cm, the volume of iron is about 0.115 m³, and the mass is about 920 kg, which fits within 1000 kg. If the thickness is 2.5 cm, the volume of iron is about 0.143 m³, and the mass becomes about 1140 kg, so even if fully submerged, the buoyancy is insufficient. Thus, the upper limit for thickness is just over 2 cm. This figure is a rough upper limit based on the moment just before the entire box sinks. Actual ships are not cubes and have frames and machinery. However, the numbers show that "an iron box floats if the plates are not too thick." The plates of the earliest iron ships are said to have been about 6.35 mm (1/4 inch) [3]. This is about one-third the thickness of 2 cm. Did a ship with such plates really go to sea?
4. Did the Iron Steamship Aaron Manby Really Sail in 1821?
According to Grace's Guide, the Aaron Manby was a steamship built in 1821, considered the first iron ocean-going steamship [3]. It was about 36 m (120 feet) long. The flat-bottomed hull was made by fastening iron plates 1/4 inch (about 6.35 mm) thick to an iron frame made of angle iron [3]. Grace's Guide states that contrary to the common belief of the time, this iron ship not only floated but traveled at 9 knots (about 17 km/h) and could navigate in water about 30 cm shallower than any other steamship [3]. Since this is how one source describes it, we cannot say that everyone at the time believed "iron does not float." It is reported that in 1822, it crossed the English Channel and sailed up the Seine River to Paris [3]. In the previous calculation, we saw that a box with 6 mm iron plates could float. This ship demonstrated that with an actual ocean-going steamship. Did iron ships come to Japan?
5. The Meiji Maru: An Old Iron Ship Preserved in Tokyo
Japan also has an old form of an iron ship remaining. The Meiji Maru is preserved at the Etchujima Campus of Tokyo University of Marine Science and Technology [4]. The Meiji Maru is an iron lighthouse inspection ship built in 1874 (Meiji 7) at the Napier shipyard in Glasgow, Scotland. It is 68.6 m long and 9.1 m wide. It is considered the oldest surviving iron ship in Japan and was designated an Important Cultural Property in 1978 [4]. Its length is about 1.9 times that of the Aaron Manby. The materials also mention a gross tonnage of 1027.57 tons. Gross tonnage is not the weight of the ship, but an indicator of size determined by the volume inside the ship, so care must be taken not to confuse it with tons of weight [4]. A floating ship is not just about floating. The more cargo you load, the deeper the ship sinks. How deep is the ship allowed to sink? If you load too much cargo, the ship sinks deep, and at some point, it becomes dangerous.
6. What Does the Load Line Mark on a Ship's Side Show?
On the sides of large ships, a mark combining a circle and horizontal lines is painted. This is the load line (freeboard mark), which shows the limit to which the ship can safely sink [5]. The more cargo there is, the deeper the hull sinks, and the load line gets closer to the water surface. If the mark is exceeded, the ship is more likely to take on waves, increasing the risk of sinking [5]. Also, fresh water provides less buoyancy than seawater, so the ship sinks a little deeper for the same weight. Therefore, in addition to sea areas and seasons, separate lines for fresh water are provided [5]. The UK Merchant Shipping Act (1876) made these marks mandatory. However, the position of the lines was fixed by law in 1894. The mark, as a symbol of a circle and line, is now common worldwide [6]. Recalling the calculation from the previous chapter, the thickness just before the box sank completely was the "upper limit." The ship stops well before that point; this way of allowing for safety is close to the idea of this line. Can we check this floating condition at home? If the mark is exceeded, the ship is more likely to take on waves, increasing the risk of sinking.
7. How Many Coins Can an Aluminum Foil Boat Hold?
What you can do today: Float a boat made of aluminum foil in a basin or bowl of water. Prepare two pieces of foil about 20 cm square. Roll one tightly into a ball, and fold the other into a box shape with raised edges, then float them to compare. Fold the box so the bottom is 5 cm square. Place 1-yen coins on the box boat one by one. A 1-yen coin weighs 1 g. If the bottom is 5 cm square, the water displaced when it sinks 1 cm is 25 cm³, and the weight of the water is about 25 g. Subtracting a little for the weight of the foil itself gives a rough estimate of how many coins can be added. Compare which number of coins causes it to sink with your prediction. Be careful not to spill water, and wipe the floor if it gets wet. Do not put coins in your mouth. What you can do by going out: The Meiji Maru is preserved at the Etchujima Campus of Tokyo University of Marine Science and Technology [4]. The days and times for visits may change, so check the university's guide before going. For further study: NASA's explanation of Archimedes' principle [1] is in English but is short and readable. Ikeda Yoshio's Blueback article [2] explains why ships float in Japanese.