Question: How can bi metal ships float if they are thousands of tonnes

Susan Cartwright answered on 15 Jun 2015:

The key quantity isn’t mass, it’s density (mass divided by volume). Any object will float if its average density is less than the density of water. This can be achieved by making the object out of very low-density material: icebergs float because ice is less dense than liquid water, and the largest icebergs are much larger than the largest ships – the largest iceberg recorded in modern times was 295 km long by 37 km wide! But a low average density can also be achieved by making an object hollow, which is how metal ships work: most of the interior of a ship is just air. If you took all the metal of a big ship and compressed it into a solid ingot, obviously it would sink like a stone, but if you make it into a big hollow boat shape, it floats quite happily. Iron has a density of nearly 8000 kg per cubic metre, whereas water has a density of about 1000 kg per cubic metre (a bit more for seawater) and air only about 1 kg per cubic metre – so if 1 cubic metre of iron is made into an empty box containing 8 cubic metres of air, its average density will be 8008/9 kg per cubic metre, which is less than 1000 – so the box will float. Ships are much bigger, but work on the same principle.