Setting the rail to Redirect Waterflow
When the surfboard is set on rail, the water pressure that hits under the surfboard gets redirected backwards, which creates lift and forward drive.
Watch at 1:02
If you were to stick your hand out of a car window that’s moving forward and then turn your hand upwards, your hand would get lifted upwards. This lift is the result of the air pressure under your palm that’s increasing.
Although surfboards work a bit differently as they move over water, a similar thing happens when surfers put their board on rail. The water pressure that hits under the surfboard gets redirected backwards, which creates lift and forward drive. Think about if you were on a boat trying to move forward. You would have to use a paddle to redirect water backwards, in order to bring the boat forward. The same goes for boat propellers. A big boat propeller can eject a larger cylinder of water backwards than a small one. The more water that gets redirected backwards, the more forward drive is created.
When motorboats accelerate and reach a certain speed, you can notice they lift higher on the water and start planing. When most of the hull comes out of the water there isn’t as much water resistance, allowing the boat to move faster while using less fuel.
When surfers dig their rails, the result is forward thrust and upwards lift. Just like the example of the motorboat, this lift makes your surfboard move faster as it planes over water with less drag.