Underwater Propulsion

I met Lance Rennka in the 1970's at the Divers Den dive shop in Santa Barbara and we have been good friends ever since. He
started SCUBA diving in 1957 (Back in O2, When There Were Wooden Tanks and Steel Men is one of his published books). He became a NAUI SCUBA Instructor in 1965, trained divers and instructors for NAUI, YMCA and PADI, and was an Instructor Trainer for over 20 years retiring in 2003. His books are not only interesting eye openers, but inspirational. He is a truth seeker and an adventurer with a universal view. He was nice enough to allow us to post this writing of his and we hope enjoy the read, especially if you dive underwater.

Underwater Propulsion
By Lance Rennka, Ed.D.
In Billions of years of evolution, Mother Nature has developed a variety of techniques to provide the aquatic creatures mobility in water. When man began to enter the water and attempt to move over and through it, adaptations had to be made. Our human ability to problem solve allowed us to build floating devises from natural materials, i.e. logs, rafts, boats, canoes, skin boats, etc. Trial and error provided our ancestors many opportunities to modify and improve their creations. The first motives to venture into the water were primarily to harvest the bountiful variety of food the waters offered.

Learning to swim in the water was a survival exercise when someone accidently fell into the water and had to keep their head above water to breathe and reach safety. The learned skills of paddling with the hands and kicking the feet provided a certain amount of propulsion, but required a lot of effort and wasn’t very fast. With practice, we learned to swim efficiently and in order to increase our endurance, we came-up with a variety of different kicks and strokes – each with their own effort requirements and cruising speed. Learning to breath-hold allowed the swimmers to dive underwater and hunt and do a certain amount of work. By being observant of how other creatures moved in the water, adaptations were made as new material was developed.
One of the first attempts was webbed gloves to provide a bigger surface area of the hand – make it a bigger paddle – frogs, gators, etc. The next obvious attempt was to make the powerful legs work better by making the feet provide more propulsion. The first attempt to develop modern fins was simply nailing shingles to the bottom of tennis shoes. Theoretically, this should work, because if you increase the surface area of a stick by adding a paddle, you get more propulsion per stroke.
From here-on, Physics and human Physique take over. If we’re going to problem solve multi-billion year aquatic propulsion evolution, we should consider the aquatic creatures that are good at moving through the water and follow their “designs” – does this make sense?
One of the fastest creatures in the sea is a tuna fish, which pushes a huge body through the water at tremendous speeds – over 60 miles/hour. Whales and dolphins push huge bodies through the water with relatively small surface area tails. So maybe, just maybe if we’re going to design and build devises to help humans be more efficient in the water and move faster, we should consider Natures solutions and then apply science to understand how we can adapt the techniques so we can move better in water. I suggest the solution may be counter-intuitive, especially if you’ve been exposed to the large variety of fins available on the market today.
The first assessment needs to be taking an inventory of the human Physique, which is designed as a terrestrial creature that walks upright. To stand upright, Nature designed us with feet so we could walk and stand upright without falling over. To understand what this means, find a pair of crutches, and try to stand still and upright with both of your feet off the ground – got it?
Next lay down on your back, extend your legs and point your toes as straight as you can, note that your feet will not make a straight line with your body, unless you’re a ballerina who learned to walk on her toes. Now relax your muscles and notice that your feet point almost straight-up. If you could point your toes straight, it would take a certain amount of effort to do so and hold them straight while you kicked your legs up and down. This is why the tennis shoe – shingle thing didn’t work, the blade was impossible to get into a propulsion angle with your legs straight. So obviously fins needed to wait for a flexible material – rubber – to be invented.
The human leg is designed to walk – basically to climb stairs. Our weight is pulled down by gravity, so the strongest muscles in our body are used to hold our body up right and provide forward or backward movement on land.

Now get your fins and put them on, lay down on a bed on your back with your legs extending past the bed, keep your legs straight, toes pointed and kick your legs up and down. Look at the “angle of attack” the fins make in the air and you’ll see the problem of trying to get efficient propulsion out of fins in the water. If the fins didn’t bend, you’d actually move backward. If you don’t bend your knees, the efficiency of the fins, even if they bend, is very limited.
The “angle of attack” of a fin blade needs to be 45 degrees to provide optimum propulsion when moved in the water. This is the same angle your hands subscribe when you tread water on the surface. Now, with the fin on your foot, bend the fin so as much of the blade as possible is at a 45 degree angle. Note that only a small portion of the fin blade bends sufficiently to provide forward propulsion. Most of the fin actually tends to pull you backward. The only time the fin actually provides the optimum forward propulsion with the toes pointed and legs held straight is when we kick backwards from the centerline of our body. This is impossible to perform in the face down position on the surface of the water. All of my diving students learned to swim on their backs on the surface.
If you’re on the surface on your stomach, to get the fin to provide a modicum of forward propulsion, you must drop your knee down and then kick. If you’re moving forward on the surface and want to “put-on-the-brakes” you drop your knees. So in an effort to position the fins to give you propulsion, in the face-down position on the surface, you put on the brakes and then kick – doesn’t make sense.
The Ama Divers – female breath-hold divers of Japan – don’t use fins and when underwater they have developed a unique kick, which actually makes sense. They hold their thighs straight, relax their foot, bring their heel to their buttock, then extend the foot and kick the leg to the straight position. This is easy enough to prove, get in the pool, lay on your back, then practice the kick until you realize you’re probably moving faster than any other leg kick would move you. Now pay attention to the angle of attack your foot is making and how little effort you’re exerting.
If you use your fins when doing a similar kick, you’ll realize this is how you actually kick with fins when submerged. Note: this type of kicking will require building the musculature in the front of your thigh – the ones that bring your leg forward from the flexed position. This is not something we normally do against resistance except on a certain exercise machine. To move the thigh forward of the centerline of our body requires strengthening the stomach muscles, but remember, moving the thigh puts-on the brakes.

Physics
Physics is the study of how and why everything in the Universe works. The Laws of Physics provide us with the factors involved with movement and propulsion in a fluid environment. These factors include:
Drag – the friction of moving through water – shape, size and configuration.
Vacuum – created behind an object when moved through a fluid, i.e. stepping in mud and trying to pull your foot out . . . without losing your shoe.
Angle-of-Attack – the angle of the paddle, oar, fin, propeller, sail, etc. to provide propulsion in a fluid
Effort – the amount of energy needed to move at a given speed in the fluid
First, let’s look at a tuna fish. Huge body, real narrow base of the tail, and real narrow fins, vertical to the movement in the water – and really FAST. Why? The tail fin is moved rapidly from side to side and because the blades are narrow, it can change the angle of attack quickly. The slowest fish have rounded tails and fairly long blades. This means the base of the tail has to move a certain distance before the fin can reach the optimum angle of attack.
Put the tip of your fin on the floor and perpendicular to it. Move the fin body (foot-pocket) back and forth and see how far it has to move before the blade reaches a 45 degree angle. This is the transition (null movement) and there is very little propulsion produced during this movement. The longer and stiffer the fin the greater the “null” movement required to get the blade into a power producing angle. Don’t get pissed at me because you have a pair of fins with three foot blades. The assumption with the long blades was the greater the surface area the greater the propulsion. In fact, if you look at the long fins in action in slow motion, you’ll notice that there is an “S” curve in the fins. Any engineer will tell you an “S” curve is inefficient – you can out-swim eels.
What this means is there is a critical length for a fin. In wind tunnel experiments (which can be extrapolated for fluid applications) the ideal length for the blade of a fin is approximately 12 inches. Any blade longer than that and there is turbulence – disruption of the water flow off the tip of the fin causing drag.
The vacuum factor has to do with the flow of water filling in “behind” the fin as it moves through the water. The reason it’s difficult to pull your shoe out of mud is that “something” has to fill-in behind the shoe before it will come loose – try removing a suction cup without breaking the seal. The next time you’re going to be walking in mud, tape a piece of PVC pipe to your leg with one end at the bottom of your shoes, in this way, air/water will have access to the bottom of the shoe and it will come out easily – you’re welcome.
What this has to do with fin propulsion, is that there needs to be a “channel” to allow the water to easily move from the front of the fin to the back of the fin easily. The best way to see this effect is to sit on the side of the pool and kick your fin up to the surface. Notice if the water is moving off the sides of the fin to fill in behind it. If it is moving off the side and not off the tip the fin is inefficient. This is especially evident in the “effort” (energy) needed to move the fin through the water. The same “size” blade (surface area) “should” require the same effort to move it through the water. Try different types and configuration of fins with different stiffnesses “against” each other sitting on the side of the pool. Then put one type of fin on one foot and a different kind on the other foot. Try kicking the fins the same way underwater and see if you tend to spiral as you swim. One fin will have more propulsion than the other.
We were taught that you could “feel” the power in certain fins. In fact, they were harder to move through the water. They were not providing more propulsion, the “power” we felt was the water trying to get from the front to the back – the amount of effort it took to move the fin.
OK, I know about the leaky fins, the ones with holes in them. Someone was paying some attention. The idea of holes in the blades was to let the water from the front of the negative portion of the blade move easily to the back of the blade thus reducing the vacuum effect. If there’s any question if it worked, plug the holes on one fin and see (feel) what happens. There’s another leaky fin, and I’ll only ask you one question, “If you split a fish’s tail will it swim faster or slower?”

One of the biggest problems for divers is fins, i.e. putting the fins on before entering the water, removing the fins after a dive and the inefficiency of the fins to propel them through the water. Also, the Foot Pocket has to fit over and around the foot wearing a booty. Do you get raw or blistered toes after long use? If so, the fin leverage is at the weakest part of your foot – the toes – instead of the strongest area near your arch. Most divers are additionally handicapped because they’re wearing gloves, carrying a lot of weight and pushing a large cross-section.
Heel attachment – Here’s a real good test to see how efficient your fins are – remove the heal straps and go for a swim. If the fins come off your feet, its “drag” – friction – that’s causing the problem.
If the “bend part” of the fin started where your toes attached to your foot instead past your toes, would you have better leverage? Could your fin be shorter and thereby make the transition to the power stroke quicker – more power, less null? Would there be less muscles used – your calves – if you weren’t working your fins with your toes? If the major propulsion stroke involved only the thighs, would you exert less energy for the same speed in the water – say 40% less? If the water coming off the tip of the fin produced a jet of water and boil three feet off the tip rather than off the side, would that indicate better propulsion? If you can leave the heel strap off and the fins will stay on your feet, does that indicate the fins have little drag in the water? If the fins look like a dolphin tail would it make sense that some human paid attention to the billions of years of natural evolution and the human physique needs? And if the Seal Teams tested this fin and proved it worked and it was faster than any other fin on the market, would you be interested in owning a pair?

If your answer was, “Of course!” or “I’d at least like to try a pair.” Then check-out: www.ForceFin.com for a dealer near you.

Lance Rennka started SCUBA diving in 1957 (Back in O2, When There Were Wooden Tanks and Steel Men is one of his published books). He became a NAUI SCUBA Instructor in 1965, trained divers and instructors for NAUI, YMCA and PADI, and was an Instructor Trainer for over 20 years retiring in 2003. Lance’s contact info – scubalust@aol.com