It’s a funny one to think about, but do you actually know why your board shoots you forward across the face of a wave? Most people don’t think about their board in this way, even the sort of person who does take note of tail shapes and concaves, is often not aware of the actual physics that are taking place under their feet.
Now to start with, Gravity of course plays a big roll. When you paddle into a wave and drop down the face, it’s Gravity pulling you down and making you accelerate, and even when you angle across the face of the wave, gravity still plays a roll. But if Gravity was the only force in play, you should go fastest when you run straight into the beach. In reality the opposite is true, and your board can travel way faster by setting a good trim line across the face of the wave, and the reasons for this lie in the way your board interacts with the flow of water on the wave face.
It’s a tricky concept to get your head around, but when a wave rolls through the ocean, it doesn’t actually move the water much. A rubber duck floating on the surface would pretty much bob up and down as the waves passed. However, from the perspective of a surfer riding the wave, this effectively creates a flow of water running up the face of the wave.
When we set our rail and ride across the wave, the rail of our board interrupts this flow of water. Some of that water will flow around the soft, rolled rail of your board, and it’s this flow that grips onto the board and stops it slipping down the face.
The rest of the water hits the bottom of the surfboard, turning the flow, and is pushed backwards through the tail. Now by applying Newton’s 3rd Law of Motion (every action causes an equal and opposite reaction) we can see that turning this flow of water will not only generate lift, but also forwards thrust in direct proportion the the volume of water that is displaced. This is why most modern surfboards incorporate some form of concave bottom, to help channel the water backwards. It’s also why it’s so important to learn to lock in your rail, and keep a high line on the wave face.
The image above shows cross sections of 3 surfboards on a wave. Surfboard 1 at the top of a wave has a 90° angle to the effective flow, this means that lots of the water is re-directed backwards, creating lots of speed. Surfboard 2 at the bottom of the wave has a much softer angle, so most of the water will flow past the rail, meaning that the board will be going much slower. Surfboard 3 is in the middle of a bottom turn, also re-directing water, and therefore also generating extra speed.
This is why good surfers spend so much time learning to put their board on rail, as by doing so they can create more speed, and therefore do bigger turns. So next time you look at a photo of a surfer with their board on rail, remember, since water weighs 1kg per liter, for every liter of water flying off the board, they’re creating 2.2lbs of lift and thrust, setting them up for their next maneuver!