That’s what I’m talking about when I say you have to keep both factors in mind… volume and shape… because they have, to a great degree, independent effects. Add to that the FLOW of water… and not only that, the SPEED of that flow. For example, a low volume rail that’s knifey will release water like a hard edged modern rail, but only at certain speeds.
Here’s something I wrote about water flow…
Water flows in almost all
directions along a surfboard’s rail. First and foremost, water flows along the
rail from nose to tail as the board slices through the water’s surface while in
trim or during a turn. In fact, while water may flow along only a small portion
of the rail at any given time, namely the back half or third of rail leading
into the tail, this lateral flow makes up the primary movement of water along
the rail/water interface. Water may flow only under this portion of the rail,
or may wrap part way, or in some cases, fully around the rail and onto the
deck, depending upon the rail’s shape and volume. Still, the flow of water
along the rail from where the rail first engages the wave face, until it
releases from under the board out the tail area, must be designed so that
turbulence, drag, and release are all controlled and manipulated to attain the
desired goal.
Water also flows upward under
the bottom of the rail as water molecules rise up the face of the wave, which
creates a lifting force under the rail. This water interacts with the rail at
varying vector angles, depending upon the speed of the board and wave. In
sailing, the angle of the wind interacting with the sail is changed by the
speed of the moving boat. This combination of relative motions produces what is
referred to as “apparent wind,” and may be very different from the “true wind”
if the boat is traveling fast enough. A similar condition exists between water
flowing along the rail, and the speed of the board and wave. If, for the sake
of illustration, we hold the speed of the wave constant, we can see how at
higher **board **speeds, the net vector
angle of the upward flow into the rail’s bottom is tilted at a greater angle
toward the tail, and becomes closer to parallel with the plane of the rail. At
lower board speeds, the net flow is closer to vertical, or more perpendicular
to the plane of the rail. However, if we hold board speed constant, we will see
an opposite effect when we look at wave speed. At higher **wave** speeds, the net flow vector upward into the bottom of the rail
is more vertical and perpendicular to the plane of the rail – faster wave
speeds require water to rise up the face more rapidly. But at lower wave
speeds, the net flow is more horizontal along the rail while the board trims,
as slower waves require water molecules in the face of the wave to rise more
slowly.
Water also flows into the
rail, from apex toward the stringer. If the path of a single water molecule is
followed as it first interacts with the rail apex, it leads from that point to
some other point on the deckside of the rail if it rises, or some other point
below the rail apex if it sinks, until it is released. Again, the water flows
at some angle along the rail, but still moves inward from the rail apex. Here,
the shape of the rail, particularly on the deck side, helps determine the path
water will follow as it travels at some angle inward and back along the rail.
Water also travels inward from the rail apex as a board moves laterally, or
slides, particularly ahead of the widepoint as the tail is pushed ahead of the
nose, and the nose is forced to slide into the wave face laterally as the board
pivots under the rider’s front foot. During these types of snaps or tail
slides, the nose rail is abruptly forced into the wave face, and both the shape
and volume of the rail determines, at least to some degree, what happens next –
rail shape may force the nose to rise or drop as it slips into the wave face,
while rail volume determines the amount of resistance the nose will incur, as a
larger volume rail along the nose will not allow the nose to sink as deeply as
a thinly foiled nose rail. Similarly, a thicker nose rail will tend to pop back
out of the face easier than a thinly foiled nose rail, as the added volume translates
into added buoyant force.
The flow of water along,
into, and up under the rail, combined with the varying speeds of the wave and
board, creates a complicated and dynamic condition that is for all practical
purposes in constant flux. Still, among all of this seeming chaos, there can be
order. The challenge of the designer-shaper is to create a sense of
“equilibrium within the chaos” for the rider, where water flowing in a
multitude of directions around the rail can be used deliberately to achieve a
desired result… to make the board rise, fall, trim, change speed, release,
slide, or even fly. Therefore, when designing a rail for a given board, the
shaper must keep in mind the type of wave – is it a thick, ledgy peak, or a
thin, peeling wall? Is it a slow, mushy sandbar wave, or firing pointbreak? The
shaper also needs to consider the goals of the rider - does the rider want a
board that is stable and catches waves easily, or one that is more loose, and
suited more for vertical, aerial-oriented surfing? Does the rider want to set a
line and walk to the tip, or do hard, driving turns out on the open face?
