How Paddle Boards Float: The Science of Paddle Boarding

When you see a paddle board floating out on the water loaded down with a fisherman and all his gear, it almost seems like magic that a slim piece of foam, plastic, resin, and epoxy could support the weight of a full-grown man without capsizing or tipping over. 

So how do paddle boards float? It’s all about the science of buoyancy: Paddle boards are designed with a hard foam core that has a lower density than the density of water, which means the buoyant force of water pushes it upwards and keeps it on the surface.  

But how do buoyancy and material density have to do with how well a paddle board floats? Read on to find out more. 

The Science of Paddle Boarding is the Science of Buoyancy

Buoyancy, also known as up thrust, is a force in fluid mechanics where the force exerted by a fluid is opposed to the weight of an immersed object. In a volume of fluid, pressure increases with depth (this is why the deep ocean has a higher pressure than the shallows). 

This pressure difference results in a net upward force that is greater than the force exerted by the weight of the object on top of the fluid. The magnitude of this force is directly proportional to the pressure difference and is the same as the volume of fluid displaced by the floating object. 

Because of this, objects that are denser than the fluid they are submerged in will sink, and objects that are less dense than the fluid will float on the surface of it. This scientific principle is dependent on the force of gravity as well and does not stand in non-gravity environments.

Archimedes’ Principle

Archimedes’ principle is related to buoyancy and states that an upward buoyant force exerted on an object submerged in a fluid is equal to the weight of the fluid that is displaced by the object. 

Therefore, the buoyant force exerted on a paddle board is equal to the weight of the water displaced by the paddle board. 

Archimedes’ principle is one of the fundamental laws of physics related to fluid mechanics and was formulated by Archimedes of Syracuse, an ancient Greek mathematician and physicist.

It makes sense that civilization has known about the principles of buoyancy for centuries, however, because these are the driving principles that have facilitated the construction of boats and other floating structures for that same amount of time. 

A SUP or Ballon, This video explains up thrust / buoyant forces

Other Scientific Factors that Affect How Paddle Boards Float

Buoyancy is not the only aspect of fluid mechanics that affects whether a paddle board can float on the water or not. Here are a few other factors that affect how paddle boards float on the surface:

  • Surface tension: The molecules that form a solid body of water are attracted to each other, and this attraction can create a surprisingly strong film on the surface of the water. This surface tension helps the water hold its shape and contributes to the ability of a paddle board to
    stay afloat.
     
  • Gravity: Gravity is the steady downward force pushing a paddle boarder down onto his board and utilizing his center of gravity is what allows him to stay balanced on top of the board without tipping over. Because gravity is stable, it is easy to compensate for.

  • Hydrodynamic forces: Bodies of water are rarely static, and there are other physical forces in the water (such as wakes and waves) that affect a paddle board’s ability to float in a stable way, and also shift the boarder’s balance. This can lead to a capsize. 

Paddle Board Buoyancy: EPS Foam Cores

The material used to construct paddle boards has a lot of forces exerted on it from both the water, the person standing on it, and internalized pressure from air trapped in the foam material.

Paddle boards not only have to balance the downward force of a person’s weight while standing on it, but they also must bear the weight of the construction materials themselves without becoming too dense to float. 

Paddle board material must be able to withstand a person’s weight without breaking while also being lightweight enough to be buoyant. The more a board weighs, the harder it is to keep it buoyant when immersed in a body of water. 

This is why traditional hard paddle boards (those modelled after surfboards) are typically constructed with an EPS foam core. Foam as a material is full of air, which is significantly less dense than water. What’s more, the air that is trapped in a solid structure can provide a very strong buoyant force. 

EPS foam is porous, however, so further construction steps must be taken to avoid it becoming saturated with water. 

If the centre of gravity is moved the SUP becomes unstable.

Waterproofing Paddle Boards for Added Buoyancy

Foam is a good internal material to construct paddle boards with due to its stiff nature and naturally high buoyancy, but foam is a porous material and exposed to water directly will eventually become saturated with water and less buoyant. 

EPS foam blanks in paddle boards and surfboards are typically glassed with an epoxy resin. This resin provides a smooth surface to cover the board and also helps to maintain the water proofness of the foam. 

On top of these foam blanks, paddle boards are covered with a material determined by the type of paddle board:

  • Inflatable paddle boards: Inflatable paddle boards are covered with two common types of materials, CSM (chlorosulfonated polyethylene) and neoprene. CSM and neoprene are combined to form a super-strong, waterproof material called Hypalon.

    Inflatable paddle boards do not have an EPS foam core to facilitate buoyancy for floating, but rather depend on air trapped within the paddle board and operate more like a whitewater raft. Neoprene holds air well and is extremely durable.
  • Hard paddle boards: Hard paddle boards feature an EPS foam core (like the surfboards they originated from) and are typically covered with a thermoplastic or fiberglass epoxy. Fiberglass, epoxy, and other materials must be used over the foam core of a paddle board because foam will eventually become saturated.

Think about a Styrofoam coffee cup used all day–it will begin seeping coffee by the end of the day. This is because liquid can pass through a foam. To avoid this, a waterproof fiberglass covering is used to protect the foam core of the paddle board and maintain its buoyancy. You may also be interested in knowing that all paddle board paddles will float, too. So you won’t lose one if you drop it.

Change the centre of gravity and guess what?

How to Stay Afloat on a Paddle Board using Science

While the material construction of a paddle board handles most of the buoyancy issues for you, the other issue that can affect a paddle board’s ability to float effectively is how the paddle boarder stands on it. 

The key to staying afloat on a paddle board is to use your scientific knowledge of gravity. Being able to stand on a paddle board is dependent on using your core of gravity in conjunction with the center of gravity of the board to keep your balance.

If you tip the board too far left or right of your center of gravity on the board, guess what? You’re going overboard. 

Here are some tips for effectively standing up on a paddle board and keeping it afloat without losing your balance or center of gravity:

  • Take your paddle board out into the water deep enough that your fin is not in danger of hitting on the bottom.
  • From a kneeling position, slowly stand up with one foot at a time, being careful to stay in the middle of the board with your feet parallel to the stringer. Try to keep your feet roughly shoulder-width apart.
  • Keep your knees bent slightly and keep the core of your body centered over the board. It takes some practice to get a feeling for the most stable point in the board. 

If you fall off, just get back on and try again. Like any sport, paddle boarding takes time and practice to master.

However, paddle boards are scientifically designed to float extremely well in both freshwater and saltwater environments, so if you take your time and have a good sense of humour about the process of learning how to use the paddle board, you’ll be stand up paddling in no time. 

Final thoughts

So there’s your science lesson for the week. Here’s your follow up history lesson. Archimedes developed his buoyancy theory, but his initial motives were less pure science and more due to a local goldsmith’s greed. Newly crowned King Hiero of Syracuse wanted to measure the exact amount of gold contained in an irregular shaped crown created for public display that he had commission from a local goldsmith. There were rumors circulating that the king had been scammed by the goldsmith substituting silver for gold.

Long story short, he commissioned his cousin, a promising 22 year old mathematician Archimedes to figure out a way to determine if any of the gold given to the goldsmith had been swapped out for silver. Using the different weights and water displacement of gold and silver Archimedes found that king had indeed been scammed. Some have called into question the exact details, but Galileo addressed how Archimedes could have designed his experiment a few centuries later.

Archimedes continued from this initial entry into the underlying physics to study buoyancy. This initial event is often cited as the discovery point of Archimedes Principle, but that actually came later based on his original assignment. It often seems like scams are something new, but they have existed for centuries. The electronic tools have made it easier though. But that’s an article for a different website.

One thing that we know isn’t a scam, paddle boarding is easy to get started with and a fun activity. Read our numerous paddle board buying guides, peruse our Gear Section or check out the special promotions link below.

Then you can observe and further ponder how Archimedes Principle is keeping you safe while you’re out there having fun!


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Carlo Raffa

Carlo Raffa is a blogger, stand-up paddleboard enthusiast, water lover and local to Brighton city in the South of England. Paddle boarding is my escape and this is only the starting point. Being a larger guy at 260lbs I am finding it very good exercise as well, especially for building core muscles. This is something that believe it or not cycling 16 miles a day at 6 miles per hour doesn't seem to be doing. Paddle Boarding allows me to just grab my board and walk right through the busy bar filled beachfront between the two piers in Brighton and head straight out of shore. It's not long before the shouting and cheering of our buzzing beach fade into just the lapping waves and the people to just small dots of the Brighton shoreline.

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