Take your hand and draw it through water in a pool. You can feel resistance---this is hydrodynamic drag. It is the effective force created by the interaction of your body and the water. Swimmers push against water to move forward, and water pushes back to slow them down. Olympic swimmers try to maximize useful drag and minimize burdensome drag.
Let's start with a discussion of forces. The act of swimming essentially uses just four forces:
- Gravitational force. This is a downward force dependent upon on the swimmer's mass.
- Buoyancy force. The water pushes up on the swimmer with a value proportional to the volume of water displaced by the swimmer. If the swimmer stays at the surface, buoyancy force must be equal in magnitude to gravitational force.
- Thrust force. Something must push the swimmer forward to balance the drag force. In this case, the thrust is a combination of a swimmer kicking the water with her feet and pulling it with her hands.
- Drag force. As the swimmer moves forward, he or she pushes water. This water pushes back, producing drag. The drag force depends upon the shape and size of the swimmer and his or her speed relative to the water.
"Good" drag comes from thrust force. It is created by the swimmer's hand moving backwards through the water, and their kicking feet. It's amazing that this forward drag can be equal to the backward drag caused by the forward motion of the rest of the body---but it can.
But the real question is how do you minimize the "bad" drag? Here is a model you can use to find the magnitude of this drag force.
A couple of the terms in this expression are beyond anyone's control. The first is the density of water (ρ)---that's a fixed variable unless you're swimming in something other than water. The second is the cross sectional area (A), which is related to the underwater profile of the swimmer. Yes, you can alter this a bit by changing how you move your arms and rotate your head. That could lead to slight decreases in drag force.
The last term to consider is the drag coefficient (C). This variable takes into account how water interacts with the object. It depends on the shape and the surface of that object. A rough brick would have a different drag coefficient than a smooth brick of the same size. A sphere would have a different coefficient than a cylinder.