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Powered monofin, what the heck?

April 26, 2019

Ok, I know what’s on your mind. You think that we just got too much of the good stuff and we built something which completely and by all definitions defies reason. Why would you have a fin and a propeller strapped to your feet at the same time? Isn’t the monofin weird enough? Don’t we have enough underwater scooters already? Why, oh why do you complicate things?

Well, let me explain. We have done just the opposite. We simplified the powered underwater propulsion to the ultimate level. Amphi encapsulates two most common personal modes of underwater propulsion in a single device. It contains advanced sensors and electronics to assist you with your dive and after-dive analysis, it works as if it was a part of your body, it needs no attention. You are free to enjoy your dive or swim. The power is with you or to be precise - behind you.

Let me go through the key aspects of the system. I am sure you will be smiling when you’re done reading, even if it doesn’t change your mind.

Hybrid at heart

The central idea behind the Amphi system is a hybrid propulsion powered partially by human muscles and partially by electric thruster. Sounds familiar? Of course, we see the same concept on our streets in the form of an electric bike. But why the heck one would build such a monster for underwater applications? Isn’t it better to have either a good pair of fins or an underwater scooter? “Make up your mind, man!” The correct answer is my favorite one: “it depends”. If your goal is to minimize the weight and complexity, then the fins are way better. The price you pay is your exhaustion and precious air. If you want to maximize your bottom time and minimize effort, a powerful (and heavy) scooter is your choice. But if you are absolutely honest with yourself, you know that most of your dives are somewhere in between. You are almost never forced to be super light and reliable or to be towed by a behemoth without any effort on your part. Again, a balanced midway solution is desired in most cases. In addition, a smooth transition between muscular and artificial propulsion would be something preferred. The Amphi system delivers exactly that. It shares the effort between the diver and the thruster. You decide how much support you summon. The fact that the fin and the scooter are integrated in a single device adds other advantages which are impossible to achieve by fins and scooter being two separate devices. But I will touch on it later.

Mechanical advantage not to be reckoned with

Boats have propellers mounted at the stern or near the stern, torpedoes, submarines, too. Reason? Interferences with the haul. Ideally, you want your thruster to exchange the energy in such a way that it creates thrust not drag. Hitting the haul with the jet coming out of your thruster creates unnecessary turbulence and drag. The same is true for a diver. Holding a propeller in front of you creates interference with your body. You may minimize it by lowering the scooter and riding above it, but in this case you create more drag by being beyond the hydrodynamic shadow of the scooter and in addition, the line of thrust and the line of drag are not co-axial and it creates torque which throws the diver-scooter system of balance and needs to be compensated for, thus causing additional energy losses.

Amphi places the propeller at your feet. Yes, the fin blade is behind it, but the blade is much smaller and smoother than your body and diving gear put together, so the drag created by the fin is way smaller… Actually, this configuration creates an additional hydrodynamic advantage which I will write about later. And yes, you guessed it, the line of thrust and drag are co-axial.

Another mechanical advantage is integration. We use the same mechanical structure which holds and attaches the fin to your feet to support the thruster. We don’t need handles, controls, anchoring points. Amphi v2 weighs 10 pounds. Similar, power-wise, scooters weigh around 16 pounds. Keep in mind that Amphi already has a fin, which for a traditional diver adds another couple of pounds. Pound for pound, Amphi is roughly two times lighter than the scooter-fin combination.

Synergy (come on, you’ve heard the word before)

This is my favorite part. How does the propeller work with the fin? Is it just two separate thrust generators put randomly together, or something else? Well, the answer is: this is something completely else. There is a synergy between the two. The secret sauce of Amphi propulsion, if you will…

I need to be a little technical here, bear with me. I will try to stay on a very basic level. For now, let me just focus on two things: fin’s angle of attack (AoA) and flow velocity through the fin blade. With any fin, the angle of attack is changing throughout the stroke. The closer to the end of the stroke (the place where the fin changes its deflection from one side to another), the greater the AoA. In the end of the stroke, the vertical velocity VFin is the highest. This vertical velocity VFin, together with swimmers velocity Vs contributes to the flow velocity V through the foil (fin), see the diagram. Sadly, with this large angle of attack α > 40 degrees (especially at low swimming velocities and strong kicks), the fin cannot take advantage of the vertical velocity amplitude. It is similar to a wing stall. It generates no lift on the foil, thus no thrust either. So, the potentially most powerful moment of the kick is virtually powerless. In Amphi propulsion, the angle of attack is changed by the thruster’s jet. It decreases it by providing an additional vector Vt parallel (for the most part) to swimming velocity Vs and skewing the overall flow V vector towards lower angles of attack. This prolongs the lift generation phase of the fin. What is even cooler, it decreases drag on the fin too. Drag is usually very small for small angles of attack. Sometimes it feels almost as if the fin wasn’t resisted by the water at all. In addition to the AoA magic, the thruster provides much higher velocities over the fin surface which creates lift proportional to V2 . As I said, I will be talking about this stuff more in later entries. If this was too much for you, then just remember one thing: we can increase thrust created by the fin by roughly 10—20% by providing synergy between the propeller and the blade.

Ergonomics

How about no controls whatsoever? Is there anything more comfortable than “a perfect nothing”? No push buttons, dials, cables, gauges, levers handles? Nothing, nada, zip. You attach your feet to the Amphi and swim. Amphi will sense the strength of your kick and will add or subtract power accordingly. You can change the characteristic of this control mechanism in two ways: by changing sensitivity to your kicks, or by changing for how long the thruster will remain “on” after you stop kicking. This allows you to go between aggressive sport like machine and relaxed cruising pal.

Other aspects of ergonomics are the SPD clips we used instead of traditional foot pockets. The clips achieve three things: 1) allow you to use your favorite cycling shoes – great comfort, 2) they allow for a little bit of play at the ankle – very important for longer swims, 3) you can release your feet at any moment – fantastically useful for a monofin.

Amphi can be disassembled into: a fin blade (2 pounds), bindings (1.5 pound), PowerPack (6.5 pounds). You can place them in separate bags or in a single monofin bag, like those offered by Waterwayfins or similar. Easy to travel with as long as you remember that the bag is 30 x 30 inches.

Mission specific

What’s more important than being able to share the system with your loved ones. Amphi can be turned from a powerful and very dynamic machine to a toy just by its software. Using your phone app you can change almost anything within the system. But this is not where the flexibility ends. One can change the powerpack and blades. Examples: strong powerpack and short fin blade for mostly powered dives or smaller powerpack and long flexible blade for relaxed long swims using mostly your own muscles. And anything in between.

We plan on having 3 different powerpacks and 6 types of blades. The only constant is the bindings, well, you can get different colors, though.

On-board intelligence

By now you know that Amphi has its electronic brain. A pretty powerful one, too. It can take inputs from multiple sensors to control your swimming actions. But it can also track your underwater path (including position, speed and depth) if you want to, and then display it for you on your smartphone, or share it on the Internet.

In the future, we will add an optional sensor suite called MyGaya, to record oceanographic data. But it’s a separate subject. Bottom line, Amphi is an intelligent dive buddy. It’s not a dumb towing aid.

UW to Cloud

The big part of this project has been allowing people to share their underwater experiences online. Thus Amphi is capable of communicating with the cloud via user’s cellphone. One will be able to sync Amphi’s data to user servers, share them on forums and with friends, compete remotely, etc. The possibilities are endless. We did not even scratch the surface yet. The wifi connectivity is there and how people use it will be mostly up to them. We, the Amphi team, will follow the popular vote.

Stay tuned, stay wet. See you underwater!

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