If a piece of metal sinks in water as a rock does, how in heavens name does a multi tonned ship float.
the weight of the water it displaces is less then the total weight of the ship+cargo
It displaces more water than it weighs. (it’s not solid)
Because of the concept of displacement. For an item to be able to float, it must be able to displace at least as much water as the object itself weighs.
Or to be more straightforward, say you have a piece of iron, about 1 foot by 1 foot by 1 foot in a cube. And say this piece of iron weighs about 10 times the amount of a 1 foot cube of water. If you drop the iron cube in water, it will sink, because it will not at any time be able to displace 10 times its weight in the water.
Now take the same cube of metal, and make a boat out of it, with a large flat bottom and sides. As the boat sinks in the water, it starts to displace a much larger volume of water than it did as a cube, and when the volume of water displaced by the boat is equal to the volume of 10 times the original cube of metal, it will reach equilibrium. So long as the sides are high enough that water does not spill over the top before the displacement depth is reached.
This is an easy experiment to try at home - take an empty plastic butter container, and a rock. Fill a sink with water, and place the butter container in it so it floats. Now place the rock in the butter container, and see how the water in the sink rises, as the butter container and rock must now displace a greater amount of water before they reach equilibrium, and stay afloat.
This of course is all an oversimplification, but close enough for most non-science people to understand I hope.
You need to displace water. A metal ball will sink as will a rock. Hollow out the ball (or rock for that matter) and it will float.
As long as the object in question displaces more water than it weighs it will float regardless of what it’s made of. IIRC a gallon of water weighs 8 pounds or so. By that count a 100,000 ton boat would displace roughly 25 million gallons of water to float. A standard swimming pool holds about 50,000 gallons of water.
BTW…rock boats have been made. An Empress of China had a marble pleasure barge. I’ll post a link about it if I can find one.
The hull a ship is metal, but the inside is mostly air. Try it in the kitchen sink with a mixing bowl or a glass. Full of water, they’ll sink. But if they’re empty, it can take a bit of force to press them down far enough for the water to start flowing in.
Add to that the fact that water is pretty heavy itself. If you had a multi-tonned ship in the water, and you added about 83 billion boxes of Jell-O to the water until it hardened, you could lift the ship out and still see the hole where it had been. It would take multi tons of water to fill in that hole (in fact, it would weigh just as much as the ship did).
It was for Empress Dowager Cixi.
Here’s a link http://www.clubi.ie/muirghen/sum/index.html . A picture of the boat is at the bottom of the page.
I ment the ship+cargo weighs less then the total water it displaces
From the official United States Merchant Marine site:
Concrete Ships
Concrete ships, most of which later became breakwaters, (including two at Normandy), came in the following four types:
[ul]
[li]C-1-S-D1, 366 feet long, 54 foot beam, 3 cylinder triple expansion steam engine, 1,300 horsepower, 4,825 gross tons. Speed 7 knots (24 built)[/li][li]B7-A1 (barge) 366 foot long, 55 foot beam, 4,968 gross tons (11 built)[/li]B7-A2 (barge) 375 foot long, 56 foot beam, 5,410 gross tons (22 built)
[li]B5-BJ1 (lighter) 265 foot long, 48 foot beam, 2,630 gross tons (22 built) named after chemical elements[/li]B5-BJ2 (Refrigerated lighter) 265 foot long, 48 foot beam, 2,630 gross tons (3 built)
B5-BJ3 (lighter) 265 foot long, 48 foot beam, 2,630 gross tons (2 built) similar to B5-BJ1, specially fitted for Army use
[li]B7-D1 (barge) 366 foot long, 54 foot beam, 4,338 gross tons (20 built)[/li][/ul]
These were all commissioned during WWII when steel was at a premium.