Hydronaut Carbon Fiber Freediving Fins

The Hydronaut Carbon Fiber fins might be new to the freediving scene but the manufacturer, Composites Kingdom, is definitely not new to the Carbon Fiber industry. 

There are many manufacturers of carbon fiber freediving fins out there but when a company with years of experience in manufacturing composite aviation, automotive and armor parts start making them we always get excited.

Normally, magic happens when experience and passion come together. That magic is the Hydronaut Carbon Fins.


About Hydronaut Fins

The Hydronaut Carbon Fiber fins are aimed at intermediate, advanced and competitive freedivers and spearo's but have features that even a beginner will appreciate. The freediving fin range comes in 4 different versions: Gold, Silver, Brass and the Pro version which we tested.

Composites Kingdom also make carbon fiber fins for underwater hockey and other watersports but our interest and focus will be on their freediving fins.

As the metal names of the different versions suggest each one has a different stiffness level:

  • The Gold fins have soft blades and suit freedivers up to 165lbs or 75kg
  • The Silver fins have medium blades and are best suited to freedivers between to 165lbs and 187lbs (75- 85kg)
  • The Brass fins have the hardest blades and are ideal for big and strong freedivers that weigh more than 187lbs or 85kg
  • The Pro version has a custom stiffness and are focused on professional and competitive freedivers 

It isn't only the stiffness that sets the Pro version apart from the other.

The tip profile on the Pro blades is a dolphin tail cut which is also completely different to the tip design of the other versions.

Hydrodynamic studies done by the Rensselaer Polytechnic Institute has proved that the dolphin tail cut is the most efficient. The results from fluid dynamic software also show that the Dolphin tail design provides the most thrust with the least amount of energy consumption. That means more forward momentum with every kick that you give.

Hydronaut Carbon Fiber Fins Side View

The fins are made from 100% carbon fiber. 4 Different types of carbon fiber fabric is used and it is laid up in 10 different layers/sections. The top and bottom Carbon fiber layers have a spread tow plain weave which give the blades a distinctive checkered looked similar to some of C4 Carbon's blades .

The spaces between the interlaces of the weave are about  1 inch square (2.54cm²). A spread tow weave gives the blades greater strength than blades with a tighter weave because of the lower crimp angle - the angle of the fiber when woven over/under each other. 

The top surface of the blades have a smooth high gloss finish with embedded logos and not stickers as you would see on some other brands. The bottom surface has a matt finish. 

There are two big advantages of having a matt bottom surface as opposed to a glossy bottom surface:

  1. A matt surface won't reflect as much light and scare off fish which is quite important when spearfishing, and
  2. A matt surface will hide scratches much better


The Hydronaut blades are what we call variable blades. That means that they are less flexible at the base than at the tip. This helps making the blade tips extra flexible and even more responsive. One of the reasons Carbon fiber blades are so much more efficient is because of the kinetic energy that is stored in that quick snap back.

The fact that the blades are made up of 4 different types of carbon fiber material and laid up in 10 different sections with different weight and directions help to give these freediving fins their flexibility profile and variable nature. 

The blade is attached to the foot pockets at a 20-degree angle which is slightly shallower than some of its competitors but that does make it easier on your ankles and to swim with at the surface. A shallower angle is also said to be stronger than a higher bending angle. 

The total blade length is 31.5 inches(80cm) or 26 inches(67cm) from the angle to the tip. We feel this is the perfect length for Constant weight freedivers and spearo's - probably the reason why it is the most common length as well.

Hydronaut Carbon Fins Back View


Composites Kingdom decided to pair their blades with the Salvimar K-Reinforced foot pockets which has a lot of pros and cons. 

When you have carbon fiber blades this good you want to make sure that the energy is not lost in an ineffective footpocket. These Salvimar foot pockets have a couple of unique features that makes it the perfect compliment to the Hydronaut blades and ensures excellent energy transfer from you to the blades.

The first thing that sets these foot pocket apart is the kevlar insert over the foot bridge area which helps to stiffen the area where most of the energy is lost during the down kick.

The foot pockets are made from a natural rubber that is said to be less effected by cold or warm temperatures than other footpockets made from thermoplastic. The rubber is applied in different thicknesses and hardness. Softer where comfort is needed like above the toes and harder were rigidity and strength is required like at the bottom of your foot and heel. 

The tendons have a stiffness of 80 shore while the rest of the footpocket has a stiffness of 50 shore. The long tendons do give extra rigidity to the blades which are already stiff at the base. Smaller built freedivers might not appreciate the extra stiffness and could be a good reason for them to pair the blades with other footpockets.

The Salvimar foot pocket has another unique feature which to be honest is not loved by everyone but does serve a purpose: the dovetail.

The dovetail in the sole does its job by creating a secure lock for the blade and additional stiffness in the sole to prevent loss of energy. 

The downside is that you have to cut a slot in your blade to pair them with these footpockets. This is not a problem with  The Hydronaut blades come with this slot cut in and the blades factory fitted. This slot does not effect the structural integrity or strength of them at all.

Still, the combination of the materials densities and the Kevlar insert results in a comfortable foot pocket that gives excellent performance with these blades.

The most notable features & specifications of the Hydronaut carbon fiber freediving fins include:

  • Variable blades with Soft tips and stiff base
  • Efficient Dolphin Tail tip cut
  • Strong, yet flexible and responsive
  • Blade Length = 36.5in / 80cm
  • Blade width = 7.7in / 19.5cm
  • Blade Angle = 20 degrees
  • 5 Year Warranty


What Others Are Saying

Since this manufacturer and these freediving fins are pretty new to the market there aren't many reviews available yet. 

Recently the Portuguese spearfishing team - Trika Team - started using these fins and judging by their smiles and the big fish they've been landing they love their fins. 

We'll update this section of the article as we learn from other freedivers what they think of the Hydronaut carbon fins.


​About Composites Kingdom

Composites Kingdom was founded in 2012 and specializes in carbon composites part manufacturing, especially for the  automotive, aviation and armor industries. 

In 2019 they officially launched their freediving fin range. This was after many years of design, development and testing. It took them over 15 iterations and a lot of tests before getting to their first commercial version. 

Composites Kingdom is based in Caldas da Rainha, Portugal but sells and ships their products to clients worldwide. Their freedivings fins can be ordered through their website or through the Freediver Shop

Hydronaut Carbon Pro


Final Verdict

Even though these carbon fiber fins isn't from a world renowned dive equipment manufacturer it is from a company with years of experience in the carbon fiber industry. You can see that a lot of thought went into the design and development of these fins and they are definitely not something that was slapped together over the weekend by a group of hobbyists in a garage.

If you are brand conscious then you will do well to remember that C4 Carbon made Bicycle frames and parts long before they started making the freediving fins that they are now so well known for.

These blades are on par with some of the best out there but it is a very competitive market with little that differentiates all of the freediving fin manufacturers. It is simply not enough to compete on quality and price. Composites Kingdom know this very well too and are looking to implement intelligent features in their future production fins that will set them apart from the rest. 

The blades are stiffer than I expected from a soft fin and compared to other soft carbon fiber fins available on the market. This is not something that will bother stronger and more experienced freedivers - the exact target market of these fins - but it is something that small frame freedivers may want to take note off. 

Stiffness does vary from manufacturer to manufacturer and as far as we know there is no real universal yardstick to measure and compare it by. The best way is to have a pair side by side to compare and test. Remember that there is bit of a trade-off between stiffness and strength. For most spearos though strength will rain supreme.  

A good portion of the stiffness can probably be attributed to the Salvimar K-Reinforced foot pockets which have quite stiff tendons. Pairing the blades with a footpocket with tendons/side rails that are shorter or less stiff would be best for smaller or less stong freedivers. 

The 5 year warranty that the blades come with gives you peace of mind that this company is in it for the long-run and believe in the quality of their product.


If you are in the market for a set of professional level carbon freediving fins for deep freediving or spearfishing, then you should seriously consider the Hydronaut fins by Composites Kingdom.


Hydronaut Pro Freediving Fins

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