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HomeFreedivingThe End of the Hyperfin? The DOL-Fin X-20 Review

The End of the Hyperfin? The DOL-Fin X-20 Review

After using the DOL-Fin X-20 for only a few months, I’m willing to predict that within ten years, most competitive and many recreational freedivers will be using a design based on this exceptional piece of freediving technology.

Why? Because it is simple, robust, works amazingly well and, best of all, promises to deliver consistent performance over a long life span.

DOL-Fin stands for Dynamic Oscillating Lateral Fin, the creation of Ron Smith, chief engineer, owner and inventor at Smith Aerospace. Smith, as the company name implies, is a former aerospace engineer and knows what he’s doing. He dove with the first prototype of the DOL-fin for over ten years, a long and fruitful R&D process which led to the present models on the market.

The DOL-fin X-20 and Orca are the first type of single blade hydrofoil fins designed expressly for freediving. And it shows.


I hope that there are more design innovations to come, but I think the DOL-Fin X-20 represents a surge forward in fin technology.

This is the fin I’ve wanted for years, and I’ll tell you why, with lots of calm, scientific reasoning and some unrestrained jumping up and down.

First Impressions

The DOL-Fin X-20 arrived at my door in a small box in mint condition.

This alone was impressive.

I’ve received shipments of single and multiple monofins from China, Europe and the Ukraine that have arrived in various states of undress and slapdashery, the cardboard boxes in tatters. Often the fins themselves had small chips in them.

This time, I opened a pristine box about 80 cm x 20 cm and found the X-20 neatly wrapped in its component parts: blade, struts, cycling shoes (I bought two sizes), screws, flex pads, and extra parts. With a screwdriver and wrench, it took me about 20 minutes to assemble it, after I made sure I wasn’t installing the blade upside down.

The X-20 comes in six possible configurations. There are three different trim plates, which limit the angular movement of the fin blade and two positions for the blade, either close or further away from the body.  Additionally, there are three standard fin sizes available from the factory and custom sizes are apparently also available from Smith Aerospace.

For my first few dives I used the DOL-fin on the default setting (trim plate 3 and the closer position) and then made my last dives with the trim plate 7 and in the extended position. The latter position is by far the best setting for me, the one that feels the most like a hyperfin –??that is to say, an extension of my body.

I had taken the precaution of ordering two sizes of cycling shoe, wanting to try out both a larger shoe for thicker wetsuit socks for cold water diving (6-8 Celsius) and a thinner more snug fit for performance.

The X-20 is a gorgeous piece of hardware once assembled.

It’s much more James Bond than any monofin. It’s black and gold and silver. You can fit it into a suitcase, and it is lightweight.

I tried the fin on. It was so easy. Not like the time I tried to put on a sprint monofin with a hard rubber footpocket on my bathroom floor. The rubber was so stiff and the fit so tight, I ended up with cuts, broken fingernails, and pulled a muscle in my neck straining to get it on.

The X-20 by contrast, is a breeze. Open the double velcro on both shoes, pull apart the shoe opening, insert feet, pull shoe straps tight, close velcro.


The Cycling Shoes Work. Really.

The first time I tried the Lunocet, I snorted in derision. I’ll admit it, it made me a freediving snob for an afternoon.

Cycling shoes? Really? They looked so bulky and non-streamlined. How could they possibly work?

After that, I had already made up my mind about using cycling shoes. Not gonna happen.

So when Ron Smith showed up in Vancouver last summer to have me try an X-18 prototype, the precursor to the X-20, I was skeptical and thought he had made an error in judgement in going with the cycling shoes.

But darn it all if it wasn’t better.

Now I like them better than the foot strap system used in the DOL-Fin Orca. Why? Because you can more likely get a consistent and easily adjusted fit. All you need to do is find the best shoe and sock combination for the job and stick with it. Tight as vice or loose and comfy, it’s entirely up to you. The best part is that you can get the fit you like consistently.

The best thing is that the X-20 provides great performance with a comfortable fit. This may be hard to accept for all you hyperfin masochists out there who sniff at freedivers who own comfortable fins.

On one recent line diving session, my diving partner had to remove her very beautiful
custom hyperfin after only two dives and no more than 15 minutes in the water. Half an hour
later her feet were still sore.  Sound familiar?

The conventional wisdom goes, if you want a competition fin, you need a super-tight fit. But if you want to use that same super-awesome monofin for fun diving or a long training session, your feet hurt.  The DOL-Fin X-20 simply allows you to dive or train for much longer without sacrificing performance.

DOL-Fin X-20 Performance

Okay, now we’re getting down to brass tacks.

It is unfair to compare the X-20 or the Orca to a hyperfin, simply because DOL-Fins are different technologies. There will be differences in fin behaviour to get used to and other idiosyncrasies.

This is not a cop-out or an attempt to hide anything.  Weigh this article, watch the growing number of videos online, and read the Community Forums for personal reviews that are sure to follow to form your own opinion. And if possible, try one.

I know that the monofin fanatics (myself included) on the forum would love some sort of side by side comparisons of the X-20 and a top hyperfin, preferably in a flume with some for some scientists in white lab coats taking detailed measurements of electrical muscle activity, vortices coming off the fins and metabolic waste products. That would be great.

A more practical head-to-head test is a difficult analysis to do objectively, as there are too many variables.

Also, in terms of efficiency, while it is possible to cover 25m in a few monofin strokes
with a super long glide (weights help, of course), there is an energy cost to that which is near impossible to measure, given that it also depends on the stroke preference of a given athlete and his or her apnea capacity.

I will be doing some Distance per Stroke, Stroke Count and Time per Length comparisons in the coming months, as I am curious to find out if there’s any significant difference between a top hyperfin and the X-20. What I’m really looking for as a freediver is whether the DOL-Fin X-20 can become a fluid extension of my body when swimming underwater.

Although the DOL-Fin X-20 is not expressly designed for dynamic apnea, I will be using the  X-20 for my dynamic training, to explore its potential and to learn its special powers and any flaws. So far, I certainly don’?? notice any performance loss from using the X-20 instead of a hyperfin.

Where the X-20 is different and stands out from a hyperfin is that the X-20 gave me a
clear and obvious propulsion on the upstroke of the kick, whereas a hyperfin, especially with a kick, kick and glide style, does not tend to offer that for propulsion, at least not without excellent technique and continuous strokes.

As with any fin, I believe that there are slight shifts in monofin technique needed to get the most out of the X-20 and that takes time. I’m looking forward to a year of diving with the X-20 in the ocean and in the pool to further refine my technique and see how far it can take me.

However, after a few months of use, I think the X-20 matches or surpasses most hyperfins in a couple of clear categories: cold water freediving, dynamic apnea, versatility, and a whole new category, fin modification.

Freediving with the DOL-Fin X-20


My first X-20 trials were in cold water in March 2012.

Diving with a 7mm suit and lots of weight is never ideal, but for many divers, it is a necessity. What I enjoyed for my first dive was how the X-20 was able to move me around with ease, despite the inertia and bulk of my dive configuration.

In cold water, a tight hyperfin offers great power, but after a while the stress that firm strokes put on the feet makes them sore after a long session, not to mention the reduced blood flow and numbing cold.

With the X-20, it didn’t matter how hard I had to kick, my feet never suffered. And I still got excellent propulsion underwater, against currents, and on the surface swimming into wind chop.

For cold water diving, you really cannot beat the X-20. Even with 3mm socks, I was not cold after a 90 minute session in 8C water. I have never been able to dive with a monofin for that long without letting my feet go partially or fully numb.

The X-20 helps eliminate one specific winter diving obstacle: if you like to sit down in your fancy soft-as-butteropen cell wetsuit, you need to find a rock clear of major barnacles and ideally sheltered from the wind chop and swell. Otherwise, putting on a tight hyperfin a stressful operation, and often results in holes in the seat of your pants and over-exertion.

Not so with the X-20. I simply walked in using both legs and one hand to brace myself
on rocks until I was in deeper water. I then floated face down, put on one foot at a time in about 10 seconds.

There’s only thing I need to warn you about if you live in a winter climate with sub-zero
temperatures: Don’t lick your X-20. It’s aluminum.

Not only is is rude to show such affection in public, you’ll be stuck with a piece of excellent freediving tech stuck to your face until someone shows up with some warm water.

Don’t say I didn’t warn you.


I was more than pleased to find that the descent at every weight configuration was stable, from fully loaded in my 7mm to diving with a triathlon suit and only a neck weight. The one major advantage of the X-20 is that the blade is so thin and small compared to a monofin, that adjusting your trim, posture or changing from a small to a large amplitude kick is very easy. There’s just no resistance.

The DOL-fin responds to traditional high amplitude monofin strokes, small ankle flicks, toe flicks from the knee, full body undulations (classic monofin technique), upstrokes (by pushing the fin to the posterior)….the whole gamut. I also experimented with surface swimming, swimming on my side, and on my back.  Most of these are also possible with a monofin, but with the DOL-Fin X-20 the hydrofoil does much of the work for you and your propulsion doesn’t change if you get tired, as it can with certain types of monofin.

The only thing that you have to watch out for with the X-20 is the spatial awareness of a fin that does not have a triangular leading edge profile. This means for line-diving you have to position yourself and your duck dive such that the X-20 doesn’t get hooked behind the line.

This happened to me once or twice.

It didn’t take me long to debug that: Simply do a surface dive parallel to the line but slightly in front of it, then do a one-quarter turn to face the line again.

This is also true underwater when freediving around rocks, pilings and kelp. Unless you pull 90 degree rolls like an X-wing in a narrow canyon, you may find slipping through a kelp forest somewhat challenging with the X-20.

If we’re talking about the downside and possibility of entanglement with the wide and narrow blade of the X-20 and Orca for recreational diving, the main benefit of the X-20 design is that you can quickly get out of the fin by undoing the velcro straps with one hand.


The ability to generate good propulsion, whether with the lower legs and ankles or with a full body undulation, works very well with your feet fit in the cycling shoes.

If you must jam your feet into something super tight, you can purchase smaller shoes or modify the shoe for your needs.

Tightening the footstraps as much as possible is an obvious step, but for even more connection, I found I could grip the fin with some inward pressure against the inside wall of the shoe, which brought my legs closer together, and an even greater precision of the fin stroke, leading to improved stability and thrust.

While keeping your legs together is important for monofin use, pressing inward against the frame is not something I’ve felt before, since it isn’t really possible to the same degree in a rubber foot pocket.

Performance in the Pool


I first tried swimming with the X-20 in a pool with no wetsuit or ballast. It was a little twitchy and I side-slipped a few times, whereas on my very first ocean dive with lots of weight and a thick wetsuit the X-20 felt very stable and solid.

With my first hyperfin, I could flex each individual foot differently, which resulted in uneven foot pressure and some side-slipping/twisting at first.  It took me time to calibrate my technique for the hyperfin, as I recall.

The X-20 also responds to uneven foot pressure, especially so without the added stability of ballast. This also means that once you figure out how to balance the foot pressure, you can use it to fine tune your fin stroke for steering, attitude change or better streamlining.

In that sense, the X-20 feels quite lively and responsive, even though it has no flexible parts except for the rubber than holds the fin to the struts.


Most recently, I swam with the LZR suit, a 2kg neckweight, and the X-20 with half socks, typically used for monofins.

After sorting out my weighting, I felt exactly like I was diving with a hyperfin. On the first try, I covered 25m lengths in an average of three to four easy cycles of kick- kick- glide, allowing for variations in push offs.  My average speed per 25m was about 17-18 seconds. Unfortunately, I did not have access to a 50m pool for testing, which would have provide more interesting and accurate results.

Again, I was surprised by the smooth strokes possible with the X-20. I do prefer a more body driven monofin stroke for dynamic and I’ve worked hard on my technique for years. The X-20 still allowed me to streamline well and cover the distance with good speed.


I’d already seen Wes Lapp’s excellent US Dynamic record with his underwater flip turns and seen how Eric Fattah uses a classic lateral turn (rotating 180 degree on the horizontal plane) with the DOL-Fin Orca. I was hoping neither type would be necessary with the X-20, as both tend to be more energy intensive than I would like.

As it turns out, most of the regular turn styles that freedivers use with a monofin can be used with the X-20. Pushing off is not as intuitive as first, but that’s a minor issue.

For the X-20, the distance from foot to fin is actually shorter than most hyperfins by about 20-22 cm. So, planting your feet and getting a good push works with practice.

The main thing to get used to is that you don’t have the same sense of planting your actual feet against the wall, instead you connect with the wall with the end of the heel of the cycling shoes and the front end of the metal struts.

Swimming with the X-20 in a lane with other swimmers might be something to be careful with. While the fin tips are not sharp like some older monofins used to be, they are pointed on the ends.

I’m guessing most new users of the DOL-fin X-20 will take some time to become aware of the dimensions of the fin. No doubt there will be some Ben-Hur fun to be had if you share a lane with others.

Hyperfins vs. The DOL-Fin X-20

Quite frankly, I have always loved the springy, slippery finesse of a hyperfin. The power
and the glory of a descent with a springy blade, the smooth acceleration just before the sink phase, and the loaded up, rhythm of a long ascent.  A good hyperfin can feel like an extension of the body.

But these fins also have their drawbacks.


How well your hyperfin performs is directly related to your technique. How the fin blade bends depends on your skill, strength and flexibility, and, of course, the construction of the blade, its cut and stiffness, and the rubber components.  But you are mostly responsible for the propulsive and drag forces at play.

If you have bad technique, you won’t be able to unlock the true potential of your hyperfin, and especially a classic monofin, until you improve considerably.

By contrast, the X-20’s blade movement is controlled by the fin’?? suspension system. It is consistent and the angle of attack of the blade is designed to always in the best possible orientation. In some ways, it is idiot proof.


The number one problem with ordering a monofin or hyperfin over the internet is you have really no idea if it will be right for you. How can you really tell how stiff the blade is, how well the foot pockets will fit you and how it will perform?  Yes, there are some amazing fin builders out there and they do great work, but there are definitely times when it is hit and miss.

Once you receive it, there’s not much you can do to change the fit or performance, short of sanding the blade down or cutting and re-gluing the foot pocket, which I’ve tried. Not easy and seldom does it improve the fin.


When you find a hyperfin that fits perfectly and performs well, you may also face the problem of inconsistency. Once you find the right hyperfin with the perfect combination of fit, blade stiffness, flex profile, buoyancy and snazzy good looks, it inevitably breaks or wears out, because you love it and use it all the time.

You contact the manufacturer for another fin to replace the one you had. It comes in the mail and you discover that it is not the same as the previous fin. The blade itself is different, or the well-meaning fin-maker has changed glues, rubber and construction in an effort to make it better. Well intentioned, but you had your fin dialed in and that’s why you wanted the same one.

Now you have a fin that you can’t use, and as a custom fin, it might not work for someone else.

With the X-20, not only does it have a much longer theoretical lifespan, being made of sturdy aluminum and replaceable components, if you want a second one, its repeatable CNC manufacturing techniques will be pretty much guarantee identical function.


Some freedivers have asked about wear and tear on the fin. The only real dangers are that of the anodized finish being scratched off by hard surfaces. Just as any monofin can accumulate scratches, the X-20 blade can also get scratched.

But unlike a monofin, it is near impossible to actually damage the fin beyond cosmetics. The plastic fin tips are extremely flexible and can be bent back into alignment – the same fin tips are used for the Orca, and this has been Eric Fattah’s experience after more than a year of use.

The aluminum blade itself is indestructible. Whereas a monofin can be cracked by dropping it on its edge, stepping on it, or bending it in the wrong way, as I did once in hefty shore break, good luck trying to do so with the X-20. The fin may get scraped by the pool wall, but it certainly won’t get functionally damaged without a fight. This means you’ll be able to use your investment for longer and get more value from it. No doubt this will keep the resale value high for used DOL-fins as well, especially since the shoes can be easily swapped for a buyer with different size feet.

Another common issue is the sometimes rapid breakdown of the hyperfin materials themselves.  Known issues include bad glue for the rubber parts of the foot pocket, cracked or nicked fin blades, deep scratches, and stretched foot pockets.

None of these issues are relevant to the DOL-Fin X-20.


No hyperfin on the market can beat the range of versatility of the DOL-Fin  X-20. Here’s just a few things to consider:

Buoyancy: You can change X-20’s buoyancy by using socks and shoes of different sizes.  Thanks to the good heel support of the shoe, there is no noticeable loss of energy transfer to the fin with thick socks.

Distance from the feet: Change the trim plates and screw position to your preference to suit your size, weight, technique, activity (DYN vs. CWT), and water conditions.

Pool and winter diving. Think about it. You could use the same fin for dynamic apnea
training (for hours on end) without pain as you would for cold water freediving with a thick suit.  The cost? An extra $100 for an extra pair of shoes and some thicker neoprene socks.

Own a quiver of fins: The X-20 is available with 3 different sizes of fin blade, so you can
experiment with which one suits you best for different applications. Ron lists a recommended size based on your height and weight, but you could play with that recommendation for other uses (a short blade for dynamics or subsurfing in waves for quick directional changes or a longer blade for cold water depth diving). You could build a quiver of fins that would still fit in one suitcase.

To match the DOL-Fin X-20’s range of applications and fittings, you would need at least two different hyperfins to even come close, but you would need at least one more for cold water freediving.

DOL-Fin Modifications

Standardizing the parts list for the X-20, its technical specs will allow future customers to depend on a consistent level of quality and also use that standard to develop hacks and add ons that further extend performance.

The X-20 could start a whole new movement: DOL-Fin Mods.

You could select your own shoe sizes, you can pad the shoe with incompressible material, buy a smaller shoe size or whatever you like. Since you have control over the footpocket, you are in control of the fit.

This is revolutionary –??even though most of us have thought of this idea. Smith Aerospace has made it a reality.

You could, in theory, build your own shoe or chose a different shoe model, as long as it
mates with the frame for the X-20. You could also design your own trim plates or extend the fin struts further if you understand the physics behind this fin and the true meaning of the Strouhal number.

Summary: Swim Like A DOL-Fin

The DOL-fin X-20 by Smith Aerospace is a new fin that gives you excellent fit, amazing durability, and high performance.

I am especially excited about the potential for modifying the X-20 to get the very best out of this amazing fin.

  • Is the DOL-fin at least as good as a hyperfin for recreational diving?
    Yes, great performance combined with true the comfort and warm feet make it so much better.
  • Is the DOL-fin easy to use and still able to offer great performance for a range of skill levels?
    Yes, I think that the learning curve for the DOL-Fin is actually shorter than for a hyperfin. The diver has to kick properly to create the foil deflection that drives the hyperfin. With the DOL-Fin, that foil is rigid and only rotates on its suspension. All the new diver has to learn is how to maintain even foot pressure.
  • Will it last? Is it well made and durable?
    It looks sturdy and its construction is such that if any parts wear out, they can easily be swapped out for new ones. Try that with a hand glued hyperfin.
  • Is it ready for the freediving market or does it need further improvement?
    More than ready. You will see more and more divers show up with them at competitions and dive trips in the very near future.
  • Will it replace my hyperfin?
    That’s up to you. But I think that as a first entry on the market the X-20 offers some really good benefits to freedivers at any level and it is the wave of the future.

You can find out more about the DOL-Fin X-20 on the Smith Aerospace website.

Here are some video clips to check out to see the X-20 and Orca in action.

Line Diving

Free Gliding

Wayne Judge X-20 trials

Orca 150m Dynamic

Recreational Dive

Peter Scott
Peter Scott
Peter Scott freedives in British Columbia, Canada. After competing in the World Championships for Canada in 2001, he has continued his exploration of the ocean through writing, art, photography, freediving, swimming, surfing, windsurfing, and travel. Visit his website at