# What (if anything) is the difference between the displacement of a ship and the weight of a ship? If they are the same why are there two different terms?

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What (if anything) is the difference between the displacement of a ship and the weight of a ship? If they are the same why are there two different terms?

In: Engineering

If I’m not mistaken (which I can be), the weight is how much it weighs, measured in kilograms or tons (or pounds or whatever) and the displacement is the amount of water it moves when it is put in water. For the same weights, but different shapes, one ship could displace more water than other.

Once again, I might be wrong, but from what I recall, it should be that.

In addition the weight of the water displaced must be greater than the actual weight of the boat for there to be buoyancy.

If the boat weighs more than the water it displaces does, it will sink.

Weight is weight. Displacement is taking a bath tub and filling it to the very edge. Then getting in. The water that spills out is what you displaced. That weight of water is your displacement.

The ship will displace water equal to its weight so a floaty box of metal that weighs 10,000 tons will displace 10,000 tons of water

But “displacement” of a ship isn’t just about the ship. The displacement is the amount of water the ship pushes away *when loaded to a specific level*. This actually turns into several different numbers depending on the era and method of measuring but things like Net Tonnage aren’t actually about the mass of the ship, it’s an administrative measure that goes into port duties. Deadweight tonnage is about the total mass of cargo the ship can carry

Regardless, basically never will you see something that actually measures the mass of the metal of the ship. It almost always includes at least drinking water, fuel, provisions, and minimum ballast. And yeah, it’s confusing

Well, let’s look back at the ancient Greeks – no, hold on, it’s actually relevant, come back here.

Archimedes was the first person to fully realize and use this principle. He was given the problem of figuring out whether a particular crown was pure gold, or whether the maker had fudged by including some silver. Without melting it down or otherwise breaking it.

He had to think about this for a while. The solution came to him while he was in the public baths – you’ll see why – and, famously, it caused him to leap OUT of the bath and go streaking through town yelling “Eureka!”, Greek for “I have got it!”

The problem boils down to finding the volume of the crown. Its weight is easy to find. He realized that if you filled a pot exaaactly full of water, and then dropped the crown in? The amount of water that came over the edge would exactly equal the VOLUME of the crown… its “displacement”, for the water volume it displaces.

Once you have the weight and the volume of the crown, its density is easy, and then compare that to pure gold’s density and voila! the answer.

So the ship’s weight is how much it weighs (duh), and usually you have values for “empty” and “fully loaded”. Its displacement is how much water it displaces for either of these, and by extension the weight of that water. If things are going well, the displaced water weighs exactly as much as the whole ship, but has a smaller volume … which means there’s ship sticking out above the surface of the water, and the ship doesn’t sink. Because only as much VOLUME of ship goes underwater as the volume of the displaced water.

Dave, ship captains don’t go streaking about the ship. usually.