I realize the approach taken (so far in this thread) tends to be the phenomenological one, but if you dare lift the lid on on that fuzzy box called dynamics, a number of interesting design directions present themselves.
Background...
We can't say for sure exactly what the profile of pressure developed under a board is during surfing – both over time and space. But we have a crude picture as offered to us by Savitsky in his classical analysis of hydroplaning.
Savitsky didn't address surfing in his model of planing, just the phenomena itself. In order to apply it to surfing you need only rotate his classic diagram, see figure at bottom of post.
… towards the topic at hand.
Notice where the pressure profile peaks, its basically a little back from where the bottom of his plank meets the flow. But just as importantly, as it relates to the topic of this thread, notice how it tapers off rapidly after the peak. When you're surfing, that peak of pressure is likely to be somewhere between your two feet, in fact its palpable -i.e. its presence, detected through your feet provides critical information during surfing.
Now imagine a little tail rocker on Savitsky's plank. The effect of the rocker, would be to diminish this profile even further -i.e. cause it to drop off even faster, at least under certain circumstances - like when the peak is a little more forward than usual, say when you shift your weight forward.
...at least some evidence.
We've got some real evidence that this is so from some of the experimental designs that have been reported here on Swaylocks. I'm of course referring to You? when you carved an exaggerated swallow tail into one of your boards, and wound up with the potential to generate a 'rooster tail' as a result. That is, the water tended to squirt up in forward section of the exaggerated swallow tail.
But this isn't true for swallows in general, so one might conclude that a point exists where the pressure profile tapers off, and it seems to happen quite rapidly after the peak, at least on a short board, or a board that being surfed from the tail (as opposed to a longer board in which the surfer is surfing the board more forward.) But this is where the fins are? That is, the pressure profile stands to be very different, remarkably so between where the fins are located and where the of peak pressure is.
Consider an experiment, you stick your flatten but vertically held hand in a stream. Now let the height of the stream oscillate a bit, that is let the level of the water go up and down a bit, but you keep your hand steady. As the water level changes so does the wetted surface of your hand, or 'emulated fin'. And on a surfboard as the peak pressure shifts forward, so stands the effectiveness of the fin, or at least that's what I'm proposing. Mind you this example exaggerates the effect, in particular it suggests that the base of the fin becomes less effective under such circumstances... oh, wait, that is what I'm saying. Of course, whether you believe it or not, well that's another matter.
Could this be happening during surfing? For longboarders, at least for those who tend to walk about a bit, I would argue that it surely does. Popping a shallow fin while on the nose happens all the time. Though there are other parameters involved in good fin design, I would nevertheless suggest that the general design of longboard fins, and in particular the depth of longboard fins tends to account for this effect.
For shorter boards, or boards designed to be surfed from their tail, the effect is less obvious and dramatic, but I would suggest this effect is very much present too.
(By the way, if you take the argument a little further, you wind up arguing that a board with less rocker can get away with a shallower fin... among other strange conclusions?)
… the classical design of fins.
Without much thought, if asked to make a fin, most of us (maybe not, but that's another thread) would likely copy that classical tapered profile – a larger base tapering off to a tip, with a little sweep backward from base to tip. But given what I've just argued, one can perhaps understand why those strange fin design outliers, like the Dewey Weber's Performer, or some those Rice take-offs, or a lot the latest shortboard fins have tended to diverge from that classical base-to-tip profile taper. Surely in at least some of those cases, the designer, I would assume, wanted to maintain effective fin surface when conditions were such that tail of the board was experiencing virtually no, or little planing pressure -i.e. back to the stream analogy, when the water level dropped a bit. Whether or not the designer thought along these lines I don't know, but intuitively he may have realized as much,
Of course, there are other design considerations to be accounted for here, but for now I'm focusing on just this one. That is, as the rear or tail section bottom interacts less with the flow and stands to lift itself virtually, in the dynamical sense out of the flow, or interact less with the flow, under those circumstances, a little more surface towards the tip of the fin will enable the fin to continue to function effectively.
Summarizing...
Or as the tail interacts less, the base region of the fin interacts less, and more fin is required further out towards the tip to maintain fin function.
… back to the 'finless'.
In a nutshell, as your weight shifts forward, especially on a finless board, the effect of the tail contours is diminished given their inherent shallowness. And it doesn't take much to diminish it to virtually nothing.
… a solution?
Given you haven't got 'fins', move the critical contours forward. Some have already started to do this, if the pictures are any indication of a trend. Really, move whole mess forward, and sacrifice that central flat region in the process if you must.
Another solution, might be to back off on the amount of tail rocker, maybe completely - in which case you may not have to move the critical contours all that much forward.
If you're retaining the rocker and just moving the critical contours, I would concentrate them very close to the expected placement of the forward foot. As for what you do to the bottom of the rest of the rear section? You could do what you what's being done now, or nothing at all. But you might want at least a simple concave or vee or something - something to have that you can bring to bare when you sink the tail a little -.i.e shift the peak pressure towards the rear. I, personally would also change the design of the contours, moving away from the rail-exterior-rounded design. (Then again maybe not... that is, the precise nature of the contours as well as there exact position, is something that likely needs to be experimented with, so for now I'm inclined to just suggest moving the contours forward if you want to keep a lot of rocker - added Jul 26/10.)
… Sorry but a few more notes on longboards.
If you've followed my argument so far, than nose-riding tends to become a slightly different dynamical event then its generally understood to be.
Its not about wings on fins, nor about tail flips, nor curious slots or contours in the nose, though these kinds of novelty design tweaks (cheats?) can work to a point, or enhance whats really required. Its about understanding that as you move, that is walk along your board, the peak pressure also moves. Now if there is sufficient pressure for the bottom surface area being presented, you can walk up right up to the nose.
Forgive me, I know this thread wasn't about nose-riding, but it is all sort of related. So on that note, for those who have made it this far, here's a bonus - one of my favor poems... (Warning: Don't repeat it a parties, people will avoid you.)
A system is a big black box
Of which we can't unlock the locks,
And all we can find out about
Is what goes in and what comes out.
Perceiving input-output pairs,
Related by parameters,
Permits us, sometimes, to relate
An input, output and a state.
If this relation's good and stable.
Then to predict we may be able,
But if this fails us – heaven forbid!
We'll be compelled to force the lid!
- Kenneth Boulding, economist.
kc
