Thruster position and rear quad fin boxes are usually placed in boards with “toe in”. That means those boxes are not oriented straight, as in the center fin box, but rather pointed several degrees inward toward the center line/nose of the board. In good wave conditions “toe in” is beneficial, but under most, average conditions it can create drag slowing the board. Speed Fins are designed to reduce that “toe in” angle and add significant speed to your board.
The result of the average “toe in” angle measurement is ¼”. That means if you put a fin in the side box position and measure the trailing edge to the center line, and then the leading edge to the center line of the board, the average difference between the two is ¼”. Speed Fins will reduce, or negate the “Toe In” angle of your board’s side fin boxes, and turn your board into a much faster board…..and, it’ll still rip turns.