I’ve always wondered and never understood how buoyancy works, especially with huge metal ships that I think should surely sink.
In: Physics
An object will float as long as it displaces an equal amount of water than it weighs. When a ship floats, a large portion of it is underwater. That underwater volume of the ship displaces the water. Let’s pretend we have a large tub of fresh water filled right to the top; if we add anything to the tub, the water will overflow. If we put a object that weighs one tonne in the tub, the water will overflow from the tub. If we collected the water that overflowed, it would weight exactly one tonne, the same as the object. One tonne of water was displaced.
For the object to float, the underwater volume has to equal the displaced water. One tonne of fresh water is one cubic metre, this means the object needs an underwater volume of one cubic metre. If the object is 1 metre long, and 1 metre wide, the object will sink 1 metre deep before it floats. Salt water is more buoyant, with a relative density of 1.025. The same vessel floating in salt water would only be 0.976 metres underwater (1/1.025).
So, in a sense, ships sink until they are deep enough to float, if that makes any sense. Ships are usually very deep; a loaded ship will have more of the hull in the water than out of the water. My ship has a loaded draft of 8 metres, while a tanker could go down to 25 metres or so. A box shaped vessel in fresh water that is that 225 metres long, 30 metres wide, and a draft of 25 metres would weight 168,750 tonnes and still float (225*30*25).
Latest Answers