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Why Do Steel Boats Float?

Small boats and personal watercraft may have lightweight hulls made from plastic or fiberglass, but larger vessels require stronger and more rigid materials. Cargo ships, military ships and other large boats are made primarily from steel. Steel both reinforces the structural integrity of a boat and offers it some protection against collisions, but steel is heavy and will cause an improperly constructed boat to sink.

Density
- Density is determined by measuring an object's mass and volume.
  
  Density is a measure of the amount of mass a given volume of an object has. Two objects can be the same size, yet have different densities, and gravity pulls the more dense object harder toward the earth. This principle is relevant to objects floating in water or in the atmosphere. A hot air balloon, for example, floats above the ground because its density is less than the air below it. Icebergs and ice cubes float on the surface of water because frozen water is less dense than its liquid counterpart. Steel, however, is more dense than water. If you measure the mass in a cup of steel it is significantly greater than the mass in a cup of water. As such, boat designers consider some additional factors when designing a boat that will float.
Displacement
- Displaced water rises proportionally to the volume of a sinking object.
  
  When steel sinks, the water level rises slightly as the water is displaced to make room for it. For a ship to float, it must displace a significant amount of water. If the amount of water displaced by the ship weighs more than the steel lowered into the water, the ship achieves buoyancy and stays afloat. Solid steel does not achieve buoyancy in water because the steel is much heavier than the water it displaces. However, a hollow steel container, filled with something much less dense than water, combines the respective volumes of the steel and its lighter cargo to displace the requisite volume of water.
Total Density
- Air trapped in water wings works like air trapped in ships' hulls.
  
  When a boat is made from steel and other materials, the density of the individual materials remains the same, but the structure itself adopts a measurable density. The total density of the structure is the average density of its parts, so if enough of the ship's components have a low enough density, the ship itself is less dense than the water. Therefore, when engineers design a boat, they need to measure the mass of all its parts and compare that to the amount of water it will displace. They can then use the water's density to determine whether or not enough water mass will be displaced to support the boat.
Designing Boats
- An unbalanced boat can quickly take on water and lose buoyancy.
  
  Engineers must consider weight distribution as well. Ideally, the craft's weight is distributed evenly along its surface. If the boat's weight distribution is irregular, one end will sit lower in the water. When one part of the boat sits too low, water may seep in over the top and into the boat. As water replaces the air within, the boat's total density rises and it begins to sink, which is why an otherwise buoyant boat will sink if it tips to the side.

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