Physics- Drag, Power and Pools

Physics- Drag, Power and Pools

Yes… I understand it is counter-intuitive to have a physics chapter in a book called The BIOLOGY of Swimming, but we aren’t going to let a little word play in the title get in the way of some important information that every swimmer and coach should know. After all, this chapter is all about the environment your Swimming Machine operates in, and understanding the habitat you live in will help you conquer it (there…Now it’s a biology chapter again!).

The world of computational fluid dynamics can be very scary and physics for many is not a strong subject. Our goal is to highlight the important concepts and avoid getting drowned in math. All we want to know is what slows us down and what speeds us up. Sounds like something we have talked about before: The Power to Drag ratio. Remember that the only way to go faster in swimming is to increase power or decrease drag. Truly improving power is what all the chapters before this one have talked about, but decreasing drag is probably even more important. Even how we apply our power in the water is just another form of drag. Let’s start with the drag that slows us down.

Drag: Frontal, Wave and Skin Drag

In the 1988 Olympics, a famous swimmer by the name of Matt Biondi won 5 gold medals, one of which was in the 100m freestyle. Video analysis later showed that during the race, Biondi had the lowest average power output compared to anyone in the field and still managed to win the race by almost half a second! While that is just one example, and one example is never a good basis for hard science, the story does help illustrate the power of reducing drag in your stroke. You don’t have to be a monster to swim fast, you just have to work with the water.

The three forms of drag that slow us down are frontal, wave and skin drag. Let’s start with the simple and obvious one: frontal drag. Water has inertia, meaning if it is not moving, it don’t wanna move. As your body pushes through the water, all those water molecules have to be pushed out of the way and you have to overcome the inertia of the water. The same thing happens on land when you push through the air, but you hardly notice because air is so light and fluffy.

Tangent: A bow wave forms at your head as you move through water. The lowest point of this wave is near your mouth which helps you breathe when you turn your head.

Now that you managed to push all that water out of the way, this kind of creates an empty space of water behind you. Like all empty spaces, it wants to be filled in, creating a negative pressure and sucking anything nearby into it. One thing nearby is your tail end and if you have a lot of frontal drag (pushing a lot of water out of the way) you will by definition have a large sucking hole behind you pulling you backwards. This is sometimes called eddy drag.

Frontal and eddy drag can be combined into what is called “form” drag. Between pushing your way though the water and keeping yourself from being sucked backwards we already have our work cut out for us. It gets worse. The faster you move through the water; the more form drag there will be. But not just more… more more! Form drag increases exponentially to the second power with regards to speed. That means if you double your speed in the water, form drag quadruples. At this rate, it won’t matter how big and strong you are, a lot of form drag means you won’t be all that fast.

So, how do we minimize the effect of these drag forces? It’s simple: don’t push around so much water. The only two things we can actively change to reduce form drag is swimming speed and exposed cross-sectional area. And since we don’t plan on swimming slowly anytime soon, we just have to talk about cross-sectional area. As you can imagine, if you expose less of your body to the oncoming water, the less water you will be running into. The amount of exposed cross-sectional area is partly how your body is shaped and is generally not changeable either and is sometimes called Passive drag. Your swimming technique plays the biggest role in increasing or decreasing form drag and is sometimes called Active drag.

Tangent: Large container ships have a protrusion sticking out of the front of their hulls. This “bulbous bow” on ships help decrease the bow wave and frontal drag leading to about 12-15% better fuel efficiency!

Let’s pretend your Swimming Machine is just a long cylinder. As it floats at the surface of the water in a perfectly horizontal position, the cylinder is exposing the least surface area to the oncoming water.

Because buoyancy is a fixed force, if any part of the cylinder starts to go up, the other parts have to go down. This is equivalent to raising your head in freestyle, tucking your chin in backstroke and not getting back to a streamlined position on fly and breast. The extra part of the cylinder that is sinking to keep your head up is newly exposed to the oncoming water you are trying to swim through. Now, your exposed surface area is growing, meaning you have to push through more water and are creating more frontal drag in front of you and more eddy drag behind you. This is why coach bugs you so much about getting your head down!

This concept can be applied to every little detail of any stroke: too deep an arm reach, too wide a kick, bending at the hips instead of the knees in breastroke, diving too much on the fly press… Always be thinking and challenging yourself to expose as little of yourself to the oncoming water to keep that form drag low low low!

Let’s talk about Skin drag. Obviously, having rough skin or a practice suit on is going to create more drag than being smooth and slipping through the water like an oiled fish. But why? As an object, like the Swimming Machine, begins to move through the water, the interaction between the water and the exposed surface of the object creates something called the Boundary Layer (aka, the water that “bounds” the surface). As the object moves through the water, it pulls the boundary layer along with it.

The thicker the boundary layer, the more water molecules that are being moved along with the object, increasing the drag on that object. Let’s go back to our cylinder analogy for a bit. A smooth surfaced cylinder will have less surface area exposed to the water than a rough surfaced cylinder and will therefore have a thinner boundary layer. That means having a smooth surface will physically pull along less water molecules than a thick boundary layer. That’s why you feel so light after you shave and suit up for taper!

There is another effect that accounts for skin drag. First off, we need to understand how water likes to move…in water. If we zoomed into the near molecular level of an empty pool, we would see that water stacks like a zillion pieces of paper stacked on one another in parallel. This is called Laminar fluid. In contrast, Turbulent fluid is when the layers are disrupted by something passing through the water creating swirls and vortices of near random motion and energy.  

As a swimmer moves through the water, the surface nearest the front of the swimmer (their fingers) pulls along a thin laminar boundary layer. This has very low drag because laminar water easily slips past itself. As the water moves down the length of the swimmer, it continually slows down and causes adjacent water molecules to slow down too. This thickens the boundary layer causing more drag. At some point down along the swimmer, the laminar boundary finally detaches from the surface and swirls away as turbulent water. Now we are in some serious trouble because turbulent water creates a lot of drag and sucks away the forward energy of the swimmer.

Tangent: Spoilers on cars and planes “spoil” laminar flow to great drag.

The closer to the front of the swimmer that this transition occurs, the more turbulent water and drag the rest of the body has to pass through. Not to mention, the thickened boundary layer now acts as its own exposed surface area to the oncoming still water, further increasing the form drag we talked about. Smoother and longer swimmers prevent the boundary layer from separating too early. Shaving, wearing a cap, compression from tight suits preventing jiggly skin from going rough are all ways to decrease skin friction. Not to beat a dead horse… but minimizing exposed cross-sectional area by tucking your chin helps too because it also minimizes how much of your skin is exposed to the oncoming water and therefore the total boundary layer you are pulling along.  

Tangent: Re-circulation flow behind the object due to turbulence is called the wake which is the wave pattern on surface. This was described the Lord Kelvin and is sometimes called the Kelvin wake and is always angled 39°.

The last form of drag that all great swimmers need to minimize is wave drag. This is potentially the most devastating of all the drags and can account for up to 50% of the drag we make swimming freestyle. Wave drag is what it sounds like, it is the drag caused by creating waves in the water. So… what’s a wave again? A wave is nothing more than energy passing through a fluid. Energy has to come from somewhere. Remember that before you came along and splashed your way through the lane, the water was perfectly still and motionless, no energy. After you passed by, waves started going everywhere: forward, side to side and downwards towards the black line.

That wave energy had to come from somewhere and since you are the only source in the lane, you bet it came from you. The energy in the waves was transferred from your own kinetic (movement) energy. Every time your arm entered the water or your body came crashing down after a breath or you wiggled your hips sided to side you transferred your own forward motion speed into waves and slowed down. With increasing swimming speed, wave drag also increases exponentially… to the THIRD POWER! That means if you double your swimming speed, your wave drag increases by a factor of 8! Ever hear your coach say “Swim gentle” or “quit splashing so much?” It’s because they want you to minimize wave drag. You should listen, it will help.

Tangent: Some early research shows “the more curved female form showed higher drag at depth but, due to a lower wave drag contribution, lower drag near the surface when compared to the less curved male form!”

 

Power and the Good Drag

All the drag forces we talked about earlier can be overcome with the proper application of…well more drag. But this drag is good. It’s how we “grip” the water. Every arm stroke and kick is the way the Swimming Machine takes all the drag forces from above (form, skin and wave) and uses them to push water backwards. There is nothing fancy or complicated going on here, this is just Newton’s third law in action.

Everything that moves has to apply a force in the opposite direction. Rockets use controlled explosions that propel energy towards the Earth to push the rocket away from the surface. Car’s tires push backwards into the grown to push the car forward on the road, and swimmers do the same with the water. The more water you push behind you at a greater speed, the faster you will go.

There are a few ways to increase how much energy you put into pushing water behind you. Increasing tempo is probably the simplest. Doubling your stroke rate (assuming the same amount of power in each stroke) will double the amount of water being pushed. Of course, that doesn’t double your speed because of the bad drag we talked about before. Another way to increase how much water you are pushing is by increasing your distance per stroke. Early vertical forearm, flexible ankles, rotating and reaching out in front of you are all ways coach tries to teach you in order to maximize the drag on your arms and to increase the amount of time that drag (or arm pull) is being used.

Tangent: Interference drag is caused when the water displaced by two objects run into each other and interfere and cause extra drag because the waters from each object need to occupy the same space. This can happen between fingers that are barely open and let their boundary layers interfere with each other, effectively increasing the size of you hand! But it would require very stable fingers that aren’t too far apart…

Another way to increase distance per stroke is by using an “S” pattern pull with your arms. Most swimmers already do this naturally, but that still leaves the question of “why does it work?” If you pulled your arm straight through the water, you would be applying the maximum amount of drag on your arm and therefore transferring the maximum amount of power from your arms to the water in the time it takes to perform that stroke. But it would be for the shortest time because the shortest distance between two points is a straight line.  

If you pull with an “S” pattern, you can greatly increase the distance your arm travels between the starting and ending point of the stroke. This will take a longer time, which is good when we want to apply our good drag for the longest time possible.

I know what you are thinking. If I’m not pushing directly back on the water, aren’t I just pushing water sideways and not applying the maximum power to the water with my arm? Yes, that’s true. But the drag of an arm traveling at an angle through the water is actually still very high. Obviously, too much “S” will take you nowhere (like skulling drills). We can actually run the numbers to get an idea of how much more total energy we applied to the water with a straight vs. “S” pattern pull. All we have to do is multiply the total distance by the average power. Let’s assume pulling straight through applies an average power of 1, and pulling with an “S” averages a power of 0.8 (20% less drag, a conservative estimation).

Simply put, increasing the distance of every stroke taken by “S”ing through the water outweighs the drop off in power and drag produced by a hand moving at an angle through the water.

Tangent: At first, coaches and scientist thought that swimmers created power with their arms by the use of lift forces described by Bernoulli a long time ago. These are the same forces created by airplane wings and boat propellers to create pressure differences across an object. The problem is you need perfectly laminar flow and very smooth hands, which the Swimming Machine doesn’t have.

 

Pool Physics- Getting Rid of Waves

“Wow, that’s a fast pool” said every swimmer at one point in their careers. But how can that be? How can mere water act differently in different tubs? As all things that affect swimming speed, it comes down to the Power to Drag ratio. Pools and how they are constructed can affect both a swimmer’s power and drag based on how the pool manages the energy bouncing around inside them. Remember that waves are pockets of energy traveling through the water. Just like sound waves, these water waves can bounce, reflect and come back and hit the swimmer who made them.

Now, you are trying to push through water that is actively pushing back. You are not just trying to overcome the inertia of the water, but it’s kinetic energy as well. To make matters worse, waves made by multiple swimmers that interfere with each other cause the laminar water in the pool to become turbulent. Traveling through turbulent water is like being attacked on all sides and sucks your speed as water that is traveling in every and all directions starts bouncing off of you and stealing your kinetic energy.

Luckily, the constructors of high-end competition pools know this and try to build in features that allow waves to dissipate and/or continue carrying their energy away from swimmers in the pool. Deep pools with infinity gutters, wide and thick lane lines and empty outside lanes all help with letting waves keep traveling away to dump their energy somewhere other than a swimmer.

Tangent: The standard depth of the pool at the Olympics is 3m, or almost 10 feet.

But there is more we can do to avoid waves and turbulent water. Think about all the turbulence and waves you make behind you. All that water is traveling with you now (which is what makes drafting nice in practice) and it wouldn’t be a problem if you didn’t have to turn around and go through it. This is all wave drag that will be encountered if you choose to push off the wall right into it.

But this wave drag can be avoided… simply go under it. At about half a meter deep, the wave drag decreases by about 20% and at about 1.5 meters, the wave drag is almost completely gone. This is why many swimmers who rely on their dolphin kicking tend to push off deep and then let their buoyancy work them back up to the surface.

Tangent: When studying fish and how wave drag affects them, research found they need to be at least 3 body diameters deep to the surface to reduce drag. At the surface, 70% of the drag they experienced was wave drag!

That wasn’t as painful as you (or me) thought it would be. The big takeaway is what you already knew: drag sucks, don’t make any more than you have to and use the good drag as much as you can. Every drill, every set, every race you should be thinking about how to maximize the Power to Drag ratio!

Karl Hamouche- Swim Smart founder
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