Rails Plane

Surfboard design elements or components interact differently with the different flows.

That a rail, in particular a wave-side rail, when a surfer is traveling transversely across the wave-face will plane along the wave-face, would seem to be an important consideration in the design of surfboard rails.  And, as outlined in prior posts ("The Deceleration Wave" and "What Makes Surfboards Go") different sections of the wave-side rail will be subject to different flows, and therefore experience different levels of planing.

When traveling transversely, the wave-side rail will plane.

As discussed in a prior post ("What Makes Surfboards Go"), planing will occur whenever you move a partially submerged somewhat solid object through a liquid, or move a liquid by a partially submerged object. So if you are traveling transversely and on one side you're moving partially submerged through a vertical face of water (or at some other less than vertical angle) that portion which is partially submerged will experience a planing force directed out of the face of that wall of water.  It well appreciated by surfers that “boxy” rails (excessively thick rails which present a large surface to the wave-face) will make transverse motion across a wave-face more difficult to maintain. The dropped-rails seen on SUPs or even the rolled decks seen on some surfboards are often used to get around this problem – an attempt to reduce the planing surface of the rail.

The rail profile and surface area requirements presented to the fluid (upon and in which planing occurs) tends to vary as you move from nose to tail on a surfboard. For example, the mid-section rail, when moving transversely will experience a greater forward flow than the tail region (see, post "Drag Keeps Surfers Connected to A Decelerating wave") and will often appear to be buried far deeper in the face of the wave than that of the tail section. Each section having its own combination of flows to deal with.