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Why a boat floats—weight and buoyancy

Why does a vessel float? How is it supported by the water and why does a stable vessel tend to return to its initial upright position? All of this can be explained by the two opposing forces acting on the vessel: its weight tending to push it down and its buoyancy tending to push it up. The weight is equal to the buoyancy and the vessel floats.

When a body is put into water, the immersed part of that body, that part that is underwater, will displace its own volume of water. What do we mean by displace? Consider a bath filled to the very brim. We climb into it and an amount of water will overflow out on to the floor. The volume of that water will be equal to the volume of our submerged body. We have ‘displaced’ that water.
It is the water that the vessel is floating in that provides the buoyancy. Sea water contains salts: it is denser than fresh water and provides more buoyancy. Due to this extra buoyancy, a vessel will float a little higher, that is, with less draft, in salt water as it does in fresh water.

So why is it that some objects float and some do not? Suppose we take two blocks of equal size, one made of balsa wood and the other of iron. We put each block under water and let them go. It is pretty obvious that the iron will sink and the balsa will float. Why?

The same volume of water has been displaced in each case, because the blocks were the same size. The amount of material in the iron block weighs more than the water of the same volume, in other words, the block is denser than water, weight is greater than buoyancy and the block sinks.

In the balsa block, the weight is less than the water of the same volume, buoyancy is greater than weight and the block pops back up to the surface. It will come to rest, floating at a level so that the buoyancy equals the weight. Remember that the buoyancy will be less now that the block is only partly submerged, because the amount of water displaced is less.


We have spoken of the water that floating vessel displaces. This gives us the term for one measure of the tonnage of a vessel—its displacement. This is the actual weight of the floating vessel. Other terms you may come across are:

deadweight—the weight of the load the vessel is carrying such as cargo, stores, fuel and so on
light displacement—the weight of the structure of the vessel
load displacement—light displacement + deadweight.