…or lucky you, it’s raining.
There's a kind of rule-of-thumb when consider the force developed by moving fluids impacting solid objects – the direction of the force developed is perpendicular to the surface upon which the fluid is impacting. It's not a hard and fast rule, but it's unlikely to steer you wrong when dealing with water. Admittedly, factors such as viscosity act to make the net force somewhat less than perpendicular, but for water, it's 'more perpendicular' than not, so to speak.
An Occam's razor approach
Put aside turns, for that matter put aside most of the more interesting radical maneuvers in modern surfing, and consider a case where the direction of the incident flow across a toed/canted multi-fin system is close to parallel to the center line of the board.
Under these conditions the direction of the net force developed by the laterals of a toed/canted mult-fin system, as they are commonly installed, is both backwards towards the tail and down into the wave. The backwards towards the tail is the result of the toe, the down into the wave, a result of the cant.
Neither of these contributions is perfectly back or down, but they are sufficiently so, so referring to them using these terms is not unreasonable here. The direction of the incident flow will vary of course, but the proposed case covers a large range, plus of minus a few degrees kind of thing. The point being this is not an unreasonable departure point, it kind of sets the stage for consideration of other flow conditions.
… back and down?
“Back” here amounts to resistance to the forward movement of the board. The question is then, “Why would you ever want that?”. The “down” amounts to sinking the tail of the board, which also begs the same question.
… model building
The magnitude of the force by any single fin in the system, is basically a function of three things – the actual area of the interaction with the flow, the speed of the flow and what is commonly referred to as the angle-of-attack of the flow. In fact you could model the magnitude as being directly proportional to the area and the speed squared, for a given angle-of-attack. The relationship between the magnitude and angle-of-attack is not as simple, but we can at least say, with increasing angle-of-attack the magnitude of this force will also increase. Please note: This isn't a lift/drag argument. It's about the net characteristics of the net force developed on an object by a moving fluid, in this case water.
So for a given flow speed and angle of attack, the force will increase linearly with fin area. For a given fin area and angle-of-attack, the force will increase as the square of the speed of the flow. And for a given area and speed of flow, the force will increase with increasing angle-of-attack, not exactly in a linear manner, but nothing too crazy either.
We now have a crude modeling of the direction and magnitude of the net force developed by a given toed/canted multi-fin system. Hence when faced with a given set of conditions for which a board is to be used, the builder can make his adjustments accordingly. It's definitely a simple model – let's call it Occam's model
… back to “back” and “down”.
… “down” first
At a sufficiently high enough speed a surfboard will appear to want to leave the water. In fact as the speed increases the amount of bottom surface generally contributing to the force of planing is decreasing. This generally occurs as a direct result of the increasing force of planing itself – pushing the bottom up and out of the water.
Obviously, in the extreme case of a high flow speed, staying “connected” to the surface would be of some benefit. Then again, at the other end of the spectrum, “down” begins to work against you. But then there's that pesky middle range of conditions, and getting the amount of “down” right is kind of a crap shoot. But luckily for us we've got something called the marketplace, and from it we get information like “what works, most of the time.”
… now “back”
Surely speed is critical, so why slow a board down? I would suggest that speed alone is not critical, being able to remain in the more critical section of the wave is however, and that's at whatever speed that requires. Skill can get and keep you there, but maybe a little design help wouldn't hurt. Think of the small amount of resistance produce by modern fin systems, the “back” to the forward motion of the board, as a “tether” to the wave.
By the time a wave becomes interesting to a surfer, it's virtually all acceleration, or if you prefer acceleration and deceleration. The curl is decelerating faster than the shoulder as it continues to move toward and over the bar or reef. The flow up the face in the curl region is accelerating much faster than that out on the shoulder. It's very easy to get too far out ahead or behind all of this change, and having a little help from good design surely couldn't hurt, even if its just a little tether, helping to inform you and adjust to these changes.
… adjustments
But once you install a fin-system for a given average set of conditions, the deed is done. And though it's functional properties are somewhat fixed, you can usually compensate here or there by bringing on line more or less bottom surface area to adjust the force being developed by planing. This is generally done with adjustments in posture or position on the board. Or by using other slight modifications and adjustments to technique. If you're surfing the board with a given installed fin system in conditions close to those optimal for which it [fin system] was designed, it's likely that these kinds of changes will be made automatically and go virtually unnoticed. If they become too noticeable, then you're likely to want to ride something else because fin-system is well outside the conditions for which it was designed. (We're assuming the builder knew what he was doing, of course.)
I'm inclined to believe that modern toed/canted multi-fin systems are all about allowing the surfer to move beyond the continual preoccupation with the rudiments of the sport -i.e. it makes the boards more 'user friendly' if you like. It allows for greater opportunity to be more creative on a wave. It does this by both enhancing sensitivity and connection to the flow in the wave though this notion of a 'tether', as well as the more traditional application of fins - control and stability - see below.
The amount of force that's being created usually isn't that great. Actually comparing it to a tether is probably pretty actuate. Tethers can offer some resistance, but they can also be broken. Their real value is often one of transmitting 'information', telling you how the motion of the tethered thing is changing. Here, that thing being the flow in the wave face.
You really don't get this kind of helpful information from a single fin. You can get your information about the changing flows from other clues of course.
… note on stability and control
The direction of the net force produced by a modern fin-system is perhaps not the best way to stabilize a craft. But then again, when you think about it, this choice of less than the maximum probably works for surfing. Too much stability would be simply a burden. But something more stable that mere a single fin, for example, but less than the maximum, kind of makes sense.. It allows for greater ease in control, yet another trade-off, if you like.
kc
ps
Given the "no edit" on the opening post of thread policy, my apologies for any errors.