It’s very clear to me that regardless of outline shape, in prone riding all the action takes place in the back third of the board. The link to Gregg Daniels underwater video of body boarders submitted by JJJ is very revealing. With shots taken both above and below water, it seems pretty clear that the traditional “Boogie Board” shape is ridden more off the back, wave-side rail and corner. This video also shows how the riders of these finless, WP-forward (WPF) shapes rely heavily upon body weight placed on the wave-side rail to keep the board engage with the wave face. What interested me was that the yaw angle didn’t change too much, but the percentage of the board (and the riders body) actually penetrating the wave face increased as the steepness of the wave increased. In a couple of frames, the rider’s body has drifted completely off the board and into the wave face, with only the hands in contact with the deck, as if the body board has become a hand plane. In another frame of a drop-knee rider, only the rear, wave-side corner is in the water. This style of riding would be consistent with the need to keep the wave-side rail fully engaged in the absence of fins. The end result is that at times there is a lot more board in the water, than there is with a WP-back design (WPB). (not the extreme “guitar pick” shape, but shapes where the WP is only 6”-8” behind center. This extra “form drag” has got to slow things down. I noticed in Daniels’ video the riders seemed to be going fastest when just the rear corner was engaged.
As I see it, finless body boards rely primarily (but not entirely) upon the rail to hold position on the wave face. They probably perform better with a WPF outline, as this straightens the outline in the last half of the shape, which puts more rail in direct contact with the wave face for better hold. Unfortunately, this also creates more form drag (less than optimum speed), and ultimately is prone to failure in steeper parts of the wave, as the typically hard edge slips out, regaining hold in a less steep (less powerful) part of the wave. Expert riders are intuitively aware of this design flaw/feature and adjust their riding style in an attempt to compensate for it, e.g. drag wave-side leg, pull up on beach-side rail, do a spinner, etc. But the straighter rail and low rocker also has a wider turning radius, forcing the rider to pull up the nose to free the rail for sharper turns. Pulling up the nose causes the board to stall, which may or may not be desired outcome.
I agree with Larry that the extreme WPB shapes (guitar pick) don’t turn very easy. The rail line is too straight and must be lifted out of the water when turning sharply so as not to “catch” (much like a LB). I think the Boogie board outline suffers from the same problem, but in reverse. There, the rail line from WP to the tail is too straight. The combination of straight rail and flat rocker makes these shapes fast, but those two features also make them difficult to turn without raising the nose and pulling the rail out of the water, which I would call a stall. (Note: In DOH or larger waves, Larry’s frame of reference, where projection out of turns especially on the drop-in, is a must, the WPF design combined with additional length is the better choice. The wide turning radius of the longer, straighter rail line helps to project the rider down the line, away from the cascading lip. (This is essentially the difference in outline shape between a gun and a small wave shortboard.)
My design approach has been to capitalize on the fact that, with prone riding, all the action takes place in the rear third of the board. I place design elements, e.g. fins, rail curve, bottom contours in such a way as to leverage this naturally occurring relationship between rider, board and wave. What I like about the finned, WPB shape in OH or smaller waves is that it has a lot of curve where the outline engages the wave. With the fin(s) providing the necessary hold, this curve minimizes the amount of rail in contact with the wave, thus minimizing form drag. Currently, I locate the wide-point behind center a distance equal to 17% of the board's length. In the case of my personal board, that distance is 8", and it coincides with where my hips are located when riding. The leading edge of the fin(s) is/are located an inch behind this point. The "bump" or "hip" created by the WPB design provides a natural pivot point for a more carving turn, which again is enhanced by fin placement. Rocker curve and WPB outline negate the need to pull up on the nose to initiate a turn, so there is less loss of speed through turns. (This same outline shape is seen in contemporary shortboards, especially the Channel Islands Flyer.) The single most consistent report I get from owners of the TBG4 is how fast the board travels compared to other body boards they've ridden, both soft and hard. The speed comes from maximizing hold, allowing the board to stay in the wave's power band, and by minimizing form drag.
- UnderwaterViewOfBoogieGoingRight.png (348.58 KiB) Viewed 2805 times
- TBG4vMach77small.jpg (60.5 KiB) Viewed 2805 times
Note: I highly recommend watching the video. Waves are far from linear, curving both horizontally and vertically, while constantly changing in steepness and speed. Seems folly to even attempt to understand how a surfcraft works. There are good reasons for the prominence of speculation and subjective report in world of surfcraft design.
