Why Do Rafts Float?


Large raft floating on river

Rafts are the earliest and most basic form of boat the world has known. With its simplistic, flat-bottomed design many people wonder how a raft floats especially with people on it. Let’s investigate this now.

A raft, like all boats, floats due to the displacement of water that occurs when it is placed in the water. The large surface area of the raft’s flat bottom displaces enough water to keep it buoyant due to the principle of flotation. The principle of flotation states that when an object displaces the same amount of weight in water than the weight of the object itself then the object will float. As a raft is lighter than the weight of the water that it displaces, it easily floats.

Why traditional and modern rafts float

The reason any boat floats is due to its buoyancy and the principle of flotation (more on that principle later). A raft is no different.

Buoyancy is a term used to refer to the force that exists which supports things in a liquid (and a gas also).

When a raft floats on calm water gravity forces the boat down. When this happens the raft displaces the water below it pushing it away and the part of the raft in the water takes the place of the displaced water. In turn, the water below the raft pushes back up. This pressure pushing upward against the bottom of the boat is the force we call buoyancy.

How buoyancy works

Although we talk about buoyancy when we are dealing with boats and rafts the actual term used to describe how well a boat can float is called net buoyant force.

The net buoyant force exerted on an object is responsible for its buoyancy and its ability to float or inability to float. The net buoyant force exerted on an object is the difference between the push upwards from the liquid that supports the object and the gravitational force that is pushing the object down.

This is how net buoyant force works:

  • If the gravitational force is greater than the push upwards from the water then the net buoyant force will be negative and the object will sink.
  • If the gravitational force is equal to the push upwards from the water then the net buoyant force will be zero and the object will float.
  • When the gravitational force is less than the push upwards from the water then the net buoyant force will be positive and the object will rise (obviously this only happens when the object in submerged under the water).

So what does all this information have to do with a raft’s ability to float?

Well, in order for any object to float the net buoyancy force must be zero, i.e. the force pushing the boat up must be equal to the force of gravity pushing the boat down. Easy so far?

The force that pushes the boat up out of the water is known as buoyancy force. (I know … you would think they would give these things completely different names to avoid confusion but who am I to argue with scientists?!).

So to work out the net buoyancy force of an object (how well it can float) we use a simple equation.

The equation for the net buoyant force of a boat is:

Net buoyant force = buoyant force – the mass of the object

So, when we know how much water is displaced by an object, like a raft, and we know the mass of the object, we can tell if it will float or sink. With this knowledge we can also design boats that are guaranteed to float in the water regardless of what they are made from. Hence you can heavy military vessels made of steel and weighing as much as 97,000 tons floating in water that would sink a small steel bar.

We can ensure any boat will float in water through the principle of flotation.

The principle of flotation states that any object will float if it displaces the same weight in water that equals the object’s weight.

This can be stated as follows:

If an object displaces water with a weight that is equal to its own weight, the object will float.

Some examples to illustrate this principle:

  • A raft weighing 2 lbs that displaces 2 lbs of water will float.
  • A raft weighing 40 lbs that displaces 40 lbs of water will float.
  • A aircraft carrier that weighs 97,000 tons that displaces 97,000 tons of water will float.

On the flip side if the object is heavier than the weight of the water it displaces then the object will sink.

Example:

  • An object weighing 2 lbs that only displaces 1 lb of water will sink.
  • An object weight 40 lbs that only displaces 39 lbs of water will sink.

This is why many different types of boats, from drift boats to canoes and kayaks to Jon boats and skiffs, can float even though they have completely different hull designs and completely different weights.

Why a flat surface floats on water

A raft is somewhat different from other boats when it comes to using the principle of flotation because a raft is usually lighter than the weight of the water it displaces because it has a completely flat surface on its bottom and top.

We know that an object will float if the weight of the water it displaces equals the weight of the object, but flotation also occurs when the weight of the object is less than the weight of the water it displaces.

So, the principle of flotation also states that:

If an object displaces water with less weight than its own weight, it will float.

As a raft is a lightweight flat-bottomed watercraft, with a large bottom surface area, it displaces a lot of water. But because water is heavy and a raft is light the weight of the water that is displaced to make room for the raft is much lighter than the raft, so the raft floats.

Of course this principle is just as hard-at-work when you overload a raft and make it heavier than the water it is displacing. Overload a raft so it becomes heavier than the water it displaces and … you guessed it … the raft will sink.

So now you know why a flat raft will float on water even when used in extreme activities such as river rafting.

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