Why Do Boats Float And Not Sink? (Beginner’s Explanation)

Boats can have many different shapes and functions, but they all need to float.

The fundamental design requirement for a boat is to float, although there are many complicated details and specializations beyond that. Since then, boats have gotten more and more sophisticated, and now they can carry huge cargo and safely traverse treacherous waters.

Boats do not sink because they are designed and built to prevent them from capsizing or sinking under their designed weight and cargo.

This is the case for all types and styles of boats, as long as the water is kept outside the hull.

Here is an explanation of what makes a boat float, which is the most basic requirement of boat design:

Why Boats Float, Not Sink:

The displacement of a boat measures buoyancy and flotation. Water displacement occurs when the boat’s force pushes the water out and away from it, keeping it steady in the water and afloat. If buoyancy is more than weight, any size of the boat, from a canoe to an ocean-going freighter will float.

1. Why Do Boats Float and Rocks Sink?

Boats float because they are lighter than the volume of water that they sit in.

In essence, the boat is less dense than the water, so it floats. Rock is denser than water, so it sinks.

Boats are always built with a hull that keeps the water out and the air and cargo in.  The boat obviously has weight and density, but it has less density than the water surrounding it.

Note that a rock is not as heavy when it is underwater because the water is exerting a “flotation” force on the rock, just not enough to hold it above the surface.

How Does Displacement Work?

Displacement actually describes the amount of water displaced by the boat, measured by the boat’s weight, such as in pounds or kilograms.

The water surrounding a boat is exerting an equal amount of flotation force to hold the boat on the surface by pressing inward on the boat, as it attempts to breach the hull.

Think of how a hole in a boat will allow water to flood into the boat.  Flotation force is the force that acts on the whole hull (as though it would if it were flowing in through a crack) to provide buoyancy that keeps the boat floating when it has no cracks.

If the boat fills with water, it no longer has buoyancy and will sink.

A rock will also displace a certain amount of water, but the rock’s weight is more than the weight of the water it displaces, so it will sink.

2. Does Size Affect Buoyancy?

Buoyancy is the amount of the flotation force of the water that is holding up a boat and is equal to the displacement.

Because a larger boat with more weight displaces more water, its level of buoyancy is much higher than a rowboat.

For example, a little rowboat can only contain a limited amount of weight and people before it is pushed down to the water level and then fills up with water and sinks.

If you fill a rowboat with five people and cargo, most likely it will sink depending on the total weight.

However, if you fill a commercial fishing boat with the same amount of weight, it will float because its buoyancy (or ability to carry more cargo and displace more water) is so much higher.

3. How Can BIG Boats Float?

The larger the boat, the higher the buoyancy.

Big boats can float even though they are heavier because they have a higher amount of water displacement.

This means that their buoyancy exceeds their weight by a higher degree, making them float better than smaller boats.

Ocean-going freighters are designed to float very high above the waterline when they are empty, and then when they are filled with cargo, they settle down to their designed, normal waterline.

This is also why larger boats do better in bad weather and with handling bigger waves – they are more likely to remain sturdy and are less affected.

Because they have more reserve flotation the forces acting on them are a smaller percentage of their displacement/ buoyancy.

4. How Can Boats Made From Steel and Metal Float?

Like the hulls of ocean-going freighters and cruise ships, the steel and metal used in boats are just the outside structure that keeps the water from entering the boat.

Obviously, a piece of solid steel is denser than water, so that it will sink.  A steel ship has a structure that displaces a huge amount of water, so an equally huge flotation force makes it float.

As long as the boat’s interior is full of air and the weight of the boat and cargo does not exceed the flotation force as it sits in the water, it will float.

5. Does the Shape of a Boat Help it Float?

The shape of a boat does not really help it float.

For example, a beach ball will float, but a spherically shaped boat would not work well.

However, the shape of a boat is essential for it to function for what it is designed to do and to make it stable and perform as desired.

Speedboats are built for speed while pontoons are built for slow-moving day trips, for instance.

6. What Shape Boat can Hold the Most Weight?

For efficiency, a box shape can hold the most weight for its size.

Think of a barge full of sand and gravel.  Of course, a barge is not known as the fastest type of vessel, and they need to be towed with very powerful tug boats.

Still, they tend to hold the most weight and do it very efficiently even in stormy weather.

Ocean-going freighters are mostly box-shaped, with a pointy front and a sloping shape underwater in the back for better performance.

7. What is the Most Stable Boat Hull Design?

Every marine architect may have a different opinion of the best design for stability and all the other factors that determine a boat’s performance.

For example, a fast boat is more stable when it is shaped like a speedboat at those high speeds, as the sharp nose cuts through the water better. Whereas a pontoon or catamaran is stable because it has a “wide-stance” between its floaters on the bottom. Both are stable designs but have different functions.

Every marine architect will have different designs for different uses, from a fast sailboat to go around the racecourse to a seaworthy sailboat that can sail worldwide.

Some of the factors that affect boat stability include the underwater hull shape, the boat’s shape and height above the water that will be pushed around by the wind and waves, and the propulsion system of engines and sails.

8. How Do you Calculate if a Boat Can Float?

It’s easy to know the weight of a boat, but the trickier thing is to calculate the boat’s volume that will sit under the water and create the flotation force that holds the boat on the surface.

This requires detailed calculations of the boat’s volume at increasing heights of the hull above the keel.

When the volume of the displaced water equals the weight of the boat, that will be the level where the boat will sit in the water.

If the boat is heavier, it will be lower in the water; if it is lighter, it will be higher.

See related article: Why Do Boats Lean to One Side?

Final Thoughts:

A boat floats because it displaces a water volume that creates a flotation force to hold it up.

If a boat fills up with water, the weight of this water adds to the boat’s weight, and there will be less flotation to support the boat. Once the boat is completely awash, most boats will sink because their hull and equipment materials are heavier than water.

If there are built-in flotation compartments or the hull materials are lighter than water (like wood), and no weights are pulling it down (like an engine), then a boat will stay at the water surface when full of water.

This is typical for rowboats and canoes.

Finally, water must be kept outside of the hull for a boat to float. That’s why a bilge pump or bailer is an important piece of equipment for every boat.