One guy mentioned he was confused about wings and fuselage and how that relates to surfboards. I used wing shape as an analogy so one can get a feeling for how shape affects aspect ratio from familiar airplane wings.
There were a few requests for a diagram. This is very simplified but should shed some light on the discussion. It is important to keep in mind that as the speed changes the wetted area changes. The slower you go the more area is needed to plane/support the weight. As the area increases or decreases with speed, the wetted area aspect ratio changes as well.
I think it is almost impossible to know what the wetted area is at any given time. It might be possible to get a reasonably accurate estimate of the wetted area/aspect ratio if you knew the speed at any given moment. Add to that the directional changes of the flow which changes the span value and you get some pretty serious variables that are way over my ability to understand.
This is math emulating art so there is a lot of room for discussion about the nuance of the influence of aspect ratio on surfboard performance. Given the vagaries of wetted area size and shape it is probably best to stick with the known outline area if you want to use aspect ratio as a data point for surfboard design. However shapers have been doing a pretty damn good job for a long time without worrying about all this crap. Another consideration is the fact that it is very likely that one aspect ratio might be perfect for one guy and a total dud for another.
Jay Davies conducting a bit of R&D. Photo: Brody
Probably the only way to get a handle on if aspect ratio is a valid data point would be to do a study of one rider's boards (good and bad) to see if there are trends that can be used effectively to predict how aspect ratio affects performance.
In the end aspect ratio is simply a description. We might find that aspect ratio is as relevant as "blue boards go faster than red boards". Until all of the design factors within a surfboard can be quantified this will remain an intellectual discussion. If that discussion reveals that aspect ratio is valid or not as a design data point we will have learned something. Hopefully it will be justifiable ignore all the math and just go surfing.