Why does a metal ship float but a metal needle sink?

Answer 1

According to Archimedes principle and we can note that the amount of water displaced by a needle is lesser than the amount of water moved out by the needle (that is displacement of the needle). So the needle is no longer stay on the surface of the water and sink into the water. As it sink into the water means below the water it can not be float again because of it density (mass per volume) is heavier than the density of the water. But in the case of iron ship, if the shape of the iron is just a flat and it can be sink like a needle. But making the Iron by forming a shape causing her displacement (it displaces on the water) more than weight of water she move out (immersed in) the ship can stay on the water. Yes, as we say if the displaces by ship is equal to the water that she move out is equal then the ship is afloat (at the certain draft). For the ship which design to afloat on the water until extra particular amount of cargo is loaded on the ship (dead weight ton). She can remain afloat until that amount of weight is reached (that is displaces by ship is equal or more than the displaced water. but if she has lesser amount she subject to sink.) These are the basic and other factors to be taken into account as per their structural design by NA.

That is my idea but please overwrite mine if your answer is good enough.

Toe Wai Win (Singapore Maritime Academy) COC 1&2
Ships float because they are hollow on the inside, so the air inside keeps it floating, because air is lighter than water.

Needles sink because they aren't hollow, so there is no air inside to keep it a float.

Hope this helps :)
We actually can get a needle to float on water if we put it in the water in such a way as to allow surface tension to keep it afloat. Setting that idea aside, a piece of steel (a needle is made of steel) will sink in water because it is more densethan water. Drop a needle in water and it will sink to the bottom. But a steel ship will float on water because of the buoyancy of the water. The ship displaces the water and will stay afloat. Let's look at the situation. If you're getting into a bath tub full of water, gravity is pulling down on the water, and also on you. Gravity continues to pull down on you as you enter the water. As you are pulled down into the water by gravity, your body has to move some of the water out of the way so that it can "get into" the water. The moving of the water out of the way is called displacement. The water level rises in the tub because you displace some water getting in. The net result of this displacement is that buoyancy takes effect. You know that when you are immersed in water, you feel "lighter" in it. This is because the water is trying to push you back out, and this the source of the force of buoyancy. Just like you in the tub, when a ship is in water, it displaces a volume of water that equals it own mass. That is, when a ship is in water, it pushes a certain volume of water out of the way. This is the vessel's displacement. The mass of this volume of water that it displaces equals the mass of the ship. Once the ship displaces a mass of water equal to its own mass, it will then not sink into the water any more. Buoyancy has taken hold. The ship is made of steel, but is "hollow" inside. The extra space inside the ship gives the ship a lower average density than the density of the water itself. The "empty" spaces inside the ship are the key to its ability to float. A solid piece of metal, like the needle, will sink if just dropped into the water. Yet the steel ship will float.
This happens according to Density : Volume where the large volume off water over comes the Volume of Ship. therefore the Density of water with its Volume have greater ratio then Ship and its Volume which is why ship does not sink.

Or to put it another way, the ship is designed to be buoyant; the needle is not.

there is no need to put in other way the needle can also float if you can manage to put it on surface of water without any external down force... or say tension less

Yes, a needle can float on the surface tension of water, if you lower it in carefully enough (for example, cradled in a fork). But the question involved the reason a needle *sinks* when a ship would float. This has nothing to do with surface tension.

Let's put it this way: the needle weighs more than the water it displaces; the ship weighs less than the water it displaces. If the needle were as hollow as the ship, it would float too.
It's about density, how much something weighs by volume. Things with lower density floats in liquids with higher density.

The needle, although it's a tiny thing, is solid metal, so it's still heavier by volume when compared with water.

The ship, despite being heavier in total, is basically a shell with lots of air in it. This air make the ship as a whole have a lower density than water, so it floats.
a stone sinks in water but an iron ship floats in it.why does it happen? is it due to the different properties of iron and stone? is it due to the shape of ship or stone?

Ships are hollow. Needles are solid. The air in the ship is what's holding it up.

Nail is a solid body because it does not contain air whereas a ship is a large hollow body because it contains air. Density of nail is more than density of water. Anything whose density is more than that of water will sink into it. While the density of a ship is less than that of water. Hence a nail or pin sink but a ship floats on water.

Answer 2

the boat is hollow and is lighter than a big solid piece of steel that weighs 100 ton
Metal boats and ships float because they are designed to displace water in a particular way, utilizing the surface area of the materials used to build the boat or ship, the interaction of the surface tension of the water and the materials, and the displacement of air within the volume of the interior of the vessel. Metal coins were not meant to float, having none of the aforementioned qualities (i.e., its materials are relatively heavy, its flat surface does not displace water easily, and it contains virtually no air within it). As an experiment you can try at home (minors: get your parents' or guardians' permission, first!), tear off a 1" x 8" sheet of aluminum foil, and roll it into a ball as tightly as you can, then drop it in a clear glass of water (at least 8 ounces). Despite the fact that it's made of metal, it floats. Of course, if you made a big enough ball (don't try, as you'll just waste the aluminum!), it would sink. Why? Because the volume of the small ball is easily displaced by the water, whereas the water cannot displace the larger ball (there is also the fact that, no matter how hard you try, you'll never be able to squeeze out all the air from the small ball, so, like an air-filled balloon, there are some air pockets that will keep it afloat). For further information, click the "Brain Pop Jr" link, below.
The total weight of the surface area of a boat is less than than the weight of a body of water of the same size (buoyancy), so that is why it floats. A needle sinks because a body of water with the same size is lighter than the needle.

it is designed in such a way that the buoyancy remains greater then its weight.

Answer 3

The needle has a greater density than water, a ship does not.