How much would the sea level fall if every ship were removed all at once from the Earth’s waters?
Archimedes’ principle tells us that the water displaced by a ship weighs as much as the ship itself. If we can figure out the total weight of all the world’s ships, we can figure out how much water they’re displacing, then divide that volume by the surface area of the ocean to figure out how much the water level would drop.
Weighing ships is confusing. There are a bunch of different measurements of the size of a ship, and many of them, like gross tonnage, are actually measures of the volume of the ship’s rooms and other internal spaces, not its weight.
The UN Conference on Trade and Development publishes estimates of the size of the world shipping fleet.
What the UNCTD publishes is “deadweight tonnage”, which is the maximum weight of the ship’s fuel, cargo, and crew. What we want is “displacement”. Unfortunately, comprehensive numbers for displacement are harder to find.
Fortunately, we can estimate it. Brian Barrass’s book Ship Design and Performance for Masters and Mates gives a table of ratios of deadweight tonnage to displacement for different types of ships.
Extrapolating from the last few years of UNCTD data, and using the coefficients from the book, suggests that the world fleet weighs about 2.15 billion tons when fully loaded.
A ton of water is about a cubic meter. 2.15 billion cubic meters divided by the surface area of the oceans equals about 6 microns (0.006 mm).
So there you go, if every ship were removed all at once from the Earth’s waters the sea level would fall by about six microns—slightly more than the diameter of a strand of spider silk. Learn more here.