The stringer is the wooden "backbone" of a surfboard, a durable strip of wood down the length of the foam that provides a support beam to make the board stronger and sturdier. Some boards don't have a stringer, while some shapers don't stop at just one. There have been boards produced with as many as seven or nine stringers, though the benefit from doing so is debatable due to the added weight caused by the wood.
LENGTH / WIDTH / THICKNESS
The interplay of dimensions is the determining factor for a surfboard's performance and speed. These specs affect the board in different ways:
The length of a board determines how easy it is to paddle and how it maneuvers. Longboards are much easier to paddle but they make wide turns, while shortboards are more difficult to paddle but can turn on a dime.
The width and thickness of a board determines its buoyancy and floatation. A wider, thicker board is easier to propel across the water and has more stability than a thinner board. Paddling is easier for these reasons, and so is standing up; however, the sacrifice is in maneuverability as wide boards can't react as quickly or as sharply as the thinner ones.
The curvature of a surfboard is known as its rocker. If you look at a board from the side and you'll notice the upturned nose and the concave shape of the deck. The more pronounced the rocker, the easier the board is to turn and maneuver in the water. However, a flatter rocker is better for streamlined speed.
Fins have a big impact on the way a board rides. The more surface area the fins have, the easier it will be to stabilize and control movements. However, big fins will also drag a board down, so surfers should strike a balance depending on the type of riding they intend to do. Additionally, the location of the fin box(es) will also change the performance of a surfboard – fins pushed to the back will make the board feel more steady, but it will be really tough to turn. Fins brought forward will make the board turn more easily, but the board will feel much more wobbly and will require more balance and control. For more in-depth information related to fins, see this article.
The rails are the sides of a surfboard and have an enormous impact on the way it rides. Turning ease is a factor of the rails' angles, which are often tuned to different measurements as they run from tip to tail. A sharper angle means a quicker, tighter turn. A rail with a duller angle will make wider turns, without as much ease. Most surfboards are designed with a sharper angle in the back portion with a more obtuse tuning as the rails edge toward the nose. This allows for sharp turning (turns originate in the rear) with a more forgiving transition.
There is always some type of concave contour on the base of a surfboard. While the degree of the contour varies, the idea remains the same: water is forced into a channel underneath the board, resulting in a faster ride with improved acceleration.