Did you know? The modern design of the fin was invented by a Lieutenant Commander in the French Navy, Louis de Corlieu (source). Inspired by how the webbed feet on frogs and ducks propel them through water, de Corlieu designed wearable swim fins that work in a similar manner.
He demonstrated his first prototype to the public in 1914 and later left the navy to refine the design, patenting it in 1933. By 1939, the propulseurs de natation et de sauvetage (swimming and rescue propulsion device) had entered mass production.
Since then, the design of the humble swim fin has seen countless iterations, styles, and designs to suit many different kinds of activities, from snorkeling to SCUBA.
But what made Louis de Corlieu’s basic design from the early 1900s so effective?
The physics of how swim fins work
The basic function of the swim fin is to generate forward propulsion or thrust by exploiting the resistance of water against anything moving through it.
Variations on this theme are pretty much how any animal or watercraft (with the exception of sailboats) moves through or over water. Animals use fins, flippers, or webbed feet. Boats and submarines use oars, paddles, and propellers. Swimmers use their arms and legs. Divers and snorkelers use swim fins!
Resistance and thrust
Water pushes back against anything moving through it (resistance). That means anything moving through water has the potential to generate propulsion.
Your legs generate propulsion when you do a flutter kick, for example. Moving your legs up and down pushes the water off and around your feet, creating thrust. Thrust is the result of Newton’s Third Law, which is often simplified to the catchy phrase, “For every action, there is an equal and opposite reaction.” So, as your legs push water behind you, there is a corresponding force that pushes you forward.
Specifically, it’s the downward motion of your legs combined with the flexing of your ankles, that provides most of the thrust. You might be surprised to learn that much of a swimmer’s thrust is not generated from the flutter kick, but from the arm strokes (e.g. the scooping motion of the front crawl). That’s because the human foot is not particularly suited to moving large amounts of water. Compared to say, a frog’s webbed feet, the comparatively narrow human foot is not very good at generating thrust.
So how do you generate more thrust with your kicks? Increase the surface area of your feet! Just like the webbed feet of a frog, or the flippers of a sea lion, a wider, flatter area can move greater amounts of water.
Enter the swim fin!
How swim fins work to optimize thrust efficiency
Thrust efficiency is how much of your kicking energy is converted to forward movement. No propulsive method can have perfect efficiency, but that doesn’t mean we can’t optimize! The engineers who design swim fins can adjust certain variables (e.g. blade length, shape, stiffness) to give you more bang for your buck as it were.
That being said, thrust efficiency is highly variable from person to person regardless of the fins you use! We all have different body shapes, and kicking styles. Your kicking technique, leg strength, depth, and the equipment you’re wearing all affect your thrust efficiency.
That means manufacturers typically design fins with specific activities in mind. A SCUBA diver won’t use the same fins as a casual snorkeler and a freediver will use something very different from either. Likewise, a fin suited to an advanced diver will not be a good choice for a beginner.
Related post: How are SCUBA fins different from snorkeling fins?
Generally speaking, the following factors affect how swim fins work:
- Strong legs (and good technique!) = stiffer fin
- Deeper activity (SCUBA, freediving) = stiffer and longer fin
- Heavy equipment (SCUBA) = stiffer and bulkier fin
Thrust efficiency is important so you don’t tire yourself out. Using fins that are too stiff for your strength or activity will give your leg muscles cramps and waste your air supply. Similarly, using fins that are too flexible will also waste air and energy.
Long blades give you more thrust but are harder to maneuver in
First, let me define what I mean by “long” fins. I distinguish this based on what activity the fins are used for. So, a “short” fin is under 25 inches, as these are mostly designed for snorkeling and swim training. Conversely, a “long” fin is over 25 inches, as these are mostly designed for SCUBA diving. The very long fins you may have seen that are 40 inches plus are used near exclusively by freedivers and spearfishers. If you’re curious, you can learn more about freediving fins with this great guide from Deeper Blue.
Longer fins generate more thrust. Now, it’s possible to generate the same thrust with either long or short fins, but longer fins are more efficient at doing so. Simply put, longer fins can move more water with each kick.
On the flip side, shorter fins are easier to maneuver in. Not only do long fins take up more space, but because they move so much water, even small kicks can generate a lot of movement. That means longer fins can be a liability for moving through caves, wrecks, or closely along the sea floor.
Related post: Long vs. short fins for snorkeling, which is best?
Moving more water with each kick also means you need more power from your legs. If your kick is weak or your kicking technique is incorrect, then you won’t get the benefits of the long fin’s thrust efficiency.
Longer fins also tend to be stiffer…
Stiff blades give you more thrust but require more leg strength
Finding a good balance between stiffness and flexibility on the blade is an important engineering consideration. Flexibility allows the fin to “scoop” the water and adds snap to the kick. But stiffness is also necessary for applying force to the water in the first place.
All fins must hit a “sweet spot” between too stiff and too flexible. Too stiff and the fin will take too much energy to move and won’t “scoop” and “snap” the water effectively. Too flexible and there will be very little thrust.
Where that sweet spot is depends, in part, on the length of the blade. Because long fins need to move more water, they experience greater resistance from that water. A stiffer blade can handle that resistance without bending overmuch. However, the same stiffness on a short blade would be inefficient. Conversely, the ideal flexibility for a short blade would make a long blade too floppy to be effective.
The fin’s stiffness largely depends on what materials it’s made of. In the old days, fins were made from natural rubber. Today, fins are made from silicones and thermoplastic elastomers (silicone-plastic hybrids). Silicone gives modern fins a suppleness that not only improves the fluidity of the blade’s movement, but also makes for more comfortable foot pockets. Thermoplastic elastomers are also very versatile and can be engineered with varying stiffness even along the same blade. Conversely, natural rubber is harder to precisely engineer, is less comfortable on bare skin, and is prone to hardening and cracking over time. Advanced freediving fins are often made from carbon fiber reinforced polymers, which have exceptional strength and stiffness, while also being lightweight.
How swim fins work with proper kicking techniques
Underwater flutter kick
The key to kicking with fins properly is to move your legs at the hip. Your knee should bend only slightly on the down stroke. Focus on keeping your movements slow and smooth, and your ankles flexed. If you’re kicking properly, you’ll notice that you need to do very little with your arms, which you should keep at your sides or gently clasped in front of you.
This hip-generated leg movement will encourage the water you displace to run off the edge of your fin blades and out behind you in a jet stream. New divers often bend their knees too much and look as if they’re climbing stairs or riding a bicycle. This movement generates a lot of turbulence, which means displaced water is moving downwards and outwards, not just backwards. Excessive turbulence decreases your thrust efficiency because less of your energy is contributing to forward propulsion.
For most folks, the down-stroke of the flutter kick is more powerful, while the up-stroke is weaker (often with reduced range of motion). This is because people in general have weaker back muscles on their legs (glutes, hamstrings) relative to the front thigh muscles (quads). Add stiff hip flexors to the equation and you have the perfect recipe for imbalanced kicks! This can lead to a tendency towards upward movement and bad trimming (body position not horizontal and streamlined).
In addition to training with fins in a pool, strength conditioning for the glutes, hamstrings, and hip flexors will help improve your finning technique!
Kicking for surface activity and snorkeling
Many casual snorkelers will be accustomed to a swimming flutter kick, which requires short and fast strokes. That means kicking with a short fin will feel more familiar, and thus easier to get used to. In fact, short fins are a great way to train your legs for the flutter kick and are routinely used by swimmers for this purpose. These shorter kicks with a more limited range of motion will keep you from splashing the water’s surface too much.
Related post: Ideal snorkeling fins for beginners
I highly advise practicing with your new fins in a pool prior to using them in open water. Not only will your leg muscles become accustomed to them, but you’ll be able to maneuver in them more intuitively. Not having to think so much about how to move your legs means you can focus more at looking at all the cool sea life around you!
To avoid too much splashing, kicking up silt, or accidentally damaging coral, I recommend casual snorkelers learn how to frog kick. Though only SCUBA divers tend to learn this kick, I personally believe it would solve many beginner snorkeler problems!
Other kinds of kicks
Because SCUBA fins tend to have better thrust efficiency, even small kicks have the potential to create a lot of movement. To move finely and precisely with long fins takes practice using a variety of techniques beyond the simple flutter kick.
For example, SCUBA divers tend to use long fins to give them the thrust they need while wearing heavy equipment. For fine and precise movements, SCUBA divers learn alternative finning techniques such as the frog kick, helicopter kick, and reverse kick.
Related Post: How to Kick When Snorkeling (It’s Not a Flutter Kick!)
The frog kick is ideal for conserving energy (and air!) when there is little current. This kick will also keep you from disturbing silt and sediment while moving close to the seafloor. A modified version of the frog kick with bent knees is ideal for caves and other tight spaces. The helicopter kick involves moving one leg with a frog-like kick to pivot in place. The reverse kick is essentially the frog kick done backwards.
Design features on fins that improve thrust, comfort, and maneuverability
Channels and rails
Many fins feature bolsters, rails, and channels that have been engineered to encourage water to run off the fin’s edge. Channeling the water off the fin in one direction improves thrust efficiency. More force applied into the water backwards means more forward motion for you (without killing your legs to achieve it)!
Because they require more complex engineering, you’ll primarily see channels and rails on SCUBA fins. These features add to the bulk and weight of the fin, but are useful for improving thrust without overly stiffening the blade. That means the diver can move effectively in deep water while carrying heavy equipment and conserving energy and air.
Curved edges
Swim fins are designed so that the water they displace as you kick runs off the blade’s edge, maximizing your thrust. The broader the edge, the more area for water to run off it. That’s why many fin designs feature a convex or concave edge, rather than a straight edge.
The curved blade edge increases the surface area for water to run off the fin. More water moving off the fin means more thrust! These designs are often inspired by the shape of fins on fish and whales. Natural selection is the engineer of life, and we humans do well to learn from it!
Split fins
Split fins use an innovative design that increases the surface area of the blade’s edge and reduces the power needed for each kick. That means they can accommodate shorter, faster strokes, and work especially well for snorkelers with weaker leg muscles or knees. But, relative to standard (paddle) fins, they have poorer acceleration and maneuverability. As such, some snorkelers and SCUBA divers find they don’t perform as well in strong currents or in environments that require fine and precise movements.
Related post: How do split fins work?
Vents
A vent is any hole or opening in the fin blade. Vents allow some water to pass through the blade during the kick. This reduces the overall resistance of the water against the blade, making it easier to kick. If designed and positioned well, such vents will not overly compromise thrust. In some cases, the vents are angled to allow more flow-through on the up-stroke of a flutter kick. That makes the up-stroke easier. Because most folks have a relatively weak up-stroke, the reduced resistance can help to correct an imbalanced kick.
Hinges
From what I’ve seen, hinges are a fairly new design on fins that serve one of two purposes:
- Give a stiff blade a more fluid “snap” during the kick
- Position the blade up against the shins for shore entries
In the first case, a flexible hinge between the blade and foot pocket (in theory) allows a relatively short blade to still generate a decent amount of thrust. Because these designs are so new, I haven’t got a good feel for whether they’re all they’re cracked up to be or just kind of gimmicky. If anyone has experience with fins like this I’d love to hear about it!
In the second case, hinged fins (sometimes called flip fins) are a great option to make diving more accessible to folks with mobility issues. Shore entries from awkward terrain or steep boat steps are tricky while wearing fins. For folks who have difficulty putting on their fins in water or who otherwise have their hands occupied, these can be a great option. Once in the water, a simple kick will snap the blades into position for swimming.
Spring straps
Spring straps are another relatively new design that has taken the SCUBA world by storm. Rather than the standard buckle most open-heel fins have, these fins have a strap reinforced with a stainless steel spring. Once you adjust this strap to your foot plus dive boot, you don’t need to mess with it ever again! To put on and take off the fin, you simply pull the strap over your heel. The spring ensures it’s always at the correct tightness. For deep dives, the spring will also adjust to the compression of your dive boot, ensuring you don’t end up with a loose strap at depth!
Related Post: 5 Reasons to Upgrade Your Fins With Spring Straps
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