The front fin is the fulcrum. The back fin provides resistance so that more weight can be applied aft of the fulcrum.
This is why twins have more yaw, or side to side movement (like a windshield wiper). The addition of a trailer or center fin provides resistance, so that more weight can be applied and trasfered to “drive”.
With quads the rear fins are closer to the front (fulcrum) fins, relative to a thruster, so the lever is smaller and it takes more force aft of the fulcrum fin to change the angle of the board (go vertical).
This is just metaphor I have been thinking about, I know that it is not exactly what is happening, but may be useful for understanding how fin clusters work, and how fins work together. Most of the discussion on fins is about how individual fins work, and not how they work as a unit.
Tear it apart…
I think the angles play a big part too.
I haven’t thought about “going” vert for so long… I wouldn’t begin to think about it, well I have, but probably won’t bother to try, on my quad(s), so I’m not in a position to say anymore.
hmmm how about concaves leading to and within the fin cluster, part of the equation too, right ?
It is all part of the equation (rocker, rails, fins, contours). I think the question about how changes in concave effect the fins is relatively simple. More concave directs more water toward the back, and less concave (or even vee) directs less water out the back, so it follows it’s more ‘natural’ path, off of the rail. This means that boards with more concave (with all other things being equal, especially fin toe and cant) provide the fins with flow at a lower angle of attack, and conversely boards with less concave (or vee) provide the fins with flow at a higher angle of attack.