Summary: By shifting his weight, a surfer can activity uncouple and/or couple the interaction of the bottom surface of his board with the flow. Noseriding is an exptreme example of this effect, but it’s also probably happening under your 6’6" too.
Warning: Of course given I have no resources which would allow me to actually go out and collect data, the following is pure “best-guessing”. All comments, criticisms and alternative interpretations are encouraged.
… a brief review
[img_assist|nid=1042478|title=Savistsky planing|desc=|link=none|align=center|width=647|height=321]
Figure 1a is taken from Savistsky’s treatment of hydrodynamic planing (see my prior posts on the role of planing in surfing.) In figure 1b, Savistsky’s original diagram is titled in an attempt to suggest the role of planing in surfing. To my knowledge Savistsky did not deal with planing as a source of drive, or a means of propulsion, as is the case in surfing, but more one of simple lift relative to the horizontal plane -i.e. the classical case of a hydrodynamic lift or perhaps a dynamic buoyancy as in the case of a hydroplaning boat propelled by some external means, an outboard motor for example. Of course, surfing actually involves both applications, but maybe that’s another post.
The curve above the plank in both figures appears to have been Savistsky’s best guess at the pressure distribution generated by planing for a particular velocity of flow, angle of attack (tau in the diagram) etc. He has the position of peak pressure over a region marked by what he has called the root spray and stagnation line.
The stagnation line is meant to mark a transitional zone. Not all of the flow which has interacted with the bottom surface proceeds off in the same direction it which is was originally headed; some of the flow is literally sent off in the opposite direction, creating what is called the root spray. The root spray is pretty easy to observe at the beach – its all that foam and spray coming out from under the forward section of the surfboard.
The stagnation zone is where most of the exchange in momentum (genesis of the forces involve in surfing) between plank and flow is taking place. The pressure generated by planing diminishes the farther away from this zone you go, backwards or fowards. Savistsky saw that some contribution would be present however, at least as long as the bottom surface was in a position to do so.
…. shifting the zone.
Let’s assume there is a surfer on the plank, though he’s not shown. His initial position is such that the pressure distribution generated by planing is something along what is shown in figure 1b. Now let’s move him forward. The question I’d like to address (translation: guess the answer to) are what kind of changes in the pressure distribution profile might be expected.
I’m inclined to believe that the location of the stagnation zone will follow him forward. Chances are this hump will likely fall pretty close to the center of gravity of both the surfer and surfboard, give or take a little. Of course the surfer may shift his weight onto his rear leg, in which case, the hump will likely follow and move further back -i.e. he can fine tune the position of the zone to some degree.
Basically I’m suggesting that the surfer actively controls the zone’s position, and by doing so brings more or less different sections of the bottom of his board into play with the flow.
Now walk the surfer right out to the nose. Chances are the zone will follow – its called nose-riding – no big insight there. But now think about what might be happening with respect to the pressure distribution profile behind the surfer
I would suggest, as he moves forward, the rear of the board will rise slightly and potentially change its interaction with the flow. In fact, thats the trick in nose riding, or more generally ‘walking the board’ – it’s about shifting the important interaction with the flow forward - walking it forward and back.
I’m inclined to believe that potentially the interaction of the rear bottom surface of the board can become sufficiently negligible so as to only minimally interact with the flow, especially during nose riding – though you wouldn’t necessarily observe it to be so from the beach, or even feel it while noseriding. So then, what’s it doing?
As it interacts less, that is its contribution to the driving force of planing becomes less, its contribution, if it remains in full contact with the surface of the water, is one dominated by viscous drag.
… the uncoupling.
I call this effect uncoupling. Uncoupling in the sense that its not really contributing to the forces which make the surfboard go. This uncoupling isn’t an all or nothing type of effect, its more a matter of degrees, and is not restricted to noseriding, though noseriding does offer an example of an extreme case.
The effect is not restricted to longboards however, that is, you can uncouple the rear of your short board to a larger or smaller degree by shifting your weight forward. Perhaps you may not achieve it completely, but I believe you can come close.
Perhaps you might argue that, ‘Okay, but that initial interaction with the flow will result in a wake and that wake is going to do something.’ I agree, wakes can cause real drag problems for example. You can see the wake in Savistsky’s diagram. So if you put a concave in the tail of you board and shift your weight forward, what effect will that have on the release of the wake? If anything, in this scenario a concave will likely uncouple the rear section even faster -i.e. minimizing the rear bottom surface interaction with the flow sooner rather than later.
I’m inclined to believe that to a very large degree surfing is tall about planing and controlling the pressure distribution generated by planing and design is about providing the surfer with the right tools to do so.
Also, I’m pretty sure that unless you sitting over your concaves there’s a good chance they’re may not be what you think they are doing, unless when you shift your weight forward, uncoupling the rear section is what you had in mind. Contours do contribute some fin function, but that’s generally not how people have traditionally interpreted their role. Don’t get me wrong, I’m actually all for uncoupling when uncoupling when it’s called for, and concaves or convexes may be just the tool a surfer needs. Not mention that some extra fin plane can come in handy at times.
One more thing about noseriding and nose concaves, which do act somewhat differently than rear concaves. A nose concave is a neat way to amplify tau (see Savistsky’s diagrams above), as would rocker be in general, but unlike simple rocker, the concave also offers some additional forward fin plane -i.e. the walls of the concave. The more root spray you can generate the greater peak pressure will be -i.e. you’re changing the momentum of more water, and as a result generating more force. Of course you can take this too far and just wind up creating a really bad ‘plowing’ effect. .
All this makes me want to go on and on about gizmos like wings on fins, fin cant, fin design in general, noseriders with quads (real quads, as in no big honker of a center fin), etc. but perhaps thats enough for now.
kc