July 24, 2020
The Amphi team has simplified the powered underwater propulsion to the ultimate level. Our powered bionic monofin encapsulates two 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 and 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. In this blog, we are going to go through the key aspects of the Amphi.
The central idea behind the Amphi system is hybrid propulsion powered partially by human muscles and partly by electric thruster. Does this concept sound familiar? We see the same approach on our streets in the form of an electric bike. You may be wondering why one would build a powered bionic monofin. Wouldn’t it be better to use a good pair of fins or an underwater scooter? Well, the answer to this question is: it depends. If your goal is to minimize the weight and complexity, then fins are the way to go. The downside to using a pair of fins is that you are quickly exhausted and less accessible to air.
If you want to maximize your bottom time and minimize effort, a powerful (and heavy) scooter is the best choice. Realistically, dives are somewhere in between. You are rarely forced to be super light and reliable or to be towed by a behemoth without any effort on your part. Ideally, you want something in between, a smooth transition between muscular and artificial propulsion. The Amphi delivers exactly that. It shares the effort between the diver and the thruster, and you can decide how much support you need. The fin and the scooter are integrated into a single device, adding other advantages that are not possible when using fins and scooters.
Boats have propellers mounted at the stern or near the stern. The reason for this is that it interferes with the haul. Ideally, you want your thruster to exchange the energy so 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 the 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 generate more drag by being beyond the hydrodynamic shadow of the scooter. 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. 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. The Amphi weighs close to ten pounds. Similarly, 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.
How does the propeller work with the fin? It is just two separate thrust generators put randomly together, or something else? Well, the answer is: this is something entirely 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 at a fundamental level. For now, let me focus on the 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 (where the fin changes its deflection from one side to another), the higher the AoA. At 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's 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 attack angles. This prolongs the lift generation phase of the fin. What is even cooler, it decreases drag on the fin too. Drag is usually minimal 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 remember one thing: we can increase thrust created by the fin by roughly 10—20% by providing synergy between the propeller and the blade.
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 character 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 machines and relaxed cruising pal.
Other aspects of ergonomics are the SPD clips we used instead of traditional foot pockets. The clips achieve three things:
Amphi can be disassembled into a fin blade (2 pounds), bindings (1.5 pounds), PowerPack (6.5 pounds). You can place them in separate bags or 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.
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 shortfin blade for mostly powered dives or smaller powerpack and long flexible blade for relaxed long swims mainly using your muscles. And anything in between.
We plan on having three different power packs and six types of blades. The only constant is the bindings, well, you can get different colors, though.
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. The bottom line, Amphi, is an intelligent dive buddy. It’s not a dumb towing aid.
The significant part of this project has been allowing people to share their underwater experiences online. Thus Amphi is capable of communicating with the cloud via the 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.
If you are interested in learning more about the Amphi, connect with us today!