Hail The Rail
There are three types of rails to consider when deciding what works best for you.
Creating an overall blueprint for a surfboard integrates features such as fins, the tail, and bottom contour according to a surfer's needs and physical structure. Combined, overall design is like the engine room of the surfboard–it provides optimal design performance on a custom level.
Rails are a vital part of that combination. When deciding what rail works best, the most important thing surfers and shapers need to ask themselves is how much volume they want. Three basic rails are used in modern surfboard design: low, medium, and boxy rails. Each rail type is distinguished by the amount of flotation and sensitivity it provides. Most rails go from a soft rounded curve into a sharply defined edge on the back two-thirds, because this design gives what's referred to as “bite and release.” The round curvature, or gradual tuck/roundness, on the bottom of the rail gives a board bite–water will wrap around the curve while the more defined corners toward the back release it.
For example, take the foil (contour) of a fin. It goes from thick to thin because water will bite at the front and flow off the end. A rail line works the same way. When you set it on edge, the thicker part grabs and releases as water flows down the edge. Imagine if the board you were riding was one soft curve from front to end. What would hold it in? The defined, sharper edge cuts, and cutting through the wave keeps the board from sliding out.
This seems like a lot of jargon, but the reality is that you should always consult your local shaper to get the most out of your money and your surfboard. The following are descriptions of the three basic designs:
Low–This type of rail has a low volume and less flotation. The less foam on a rail means a higher level of sensitivity–it's more lively. The lighter weight allows the board to be thrown around a lot easier. If it's too thin, it'll catch on turns because it's sinking too much–it won't have enough volume and may fail to release.
Medium–The most common rail used in designs. Medium is what it is–a halfway point between boxy and low-volume rails. Again, individual needs including performance and weight create the overall feel.
Boxy–The fuller volume of a boxy rail is simply used for heavier surfers. A boxier rail allows bigger guys to ride shorter boards. A good example of this is a fish: the smaller, wider size means there has to be more volume in the rail. And boxy rails are more forgiving.