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The above picture is of Slater hauling ass. He’s obviously on a gun. I can’t say for sure, but its likely a safe bet that he’s got a tri-fin setup.
There are a few things about this shot which I find interesting (aside from the fact that it’s unlikely I’m ever gonna be in a similar position.) In particular they related to the spray or wash emanating from under his board.
The Spray Root
The spray root is that part of the flow that is stopped dead or sent off in the opposed direction than that of the incoming flow. You can see it in the diagram of planing below.
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Below I’ve drawn a line in the Slater shot right across the board to indicate what I believe marks the beginning of the wetted bottom surface of Slaters board, or what you could call the spray root line. Not much surface area is in play. Which is consistent with the high flow rates -i.e. higher flows, less surface area required to generate a given force.
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Again this is my take on this snapshot, but if you look hard (blow up the shot if you can) you can see that the wash coming out from under the board is literally dropping straight down the face. Poor wash, Slater killed it!
But what's up with that next line of wash as you move back from the spray root?
Lateral Fin Mayhem
Mayhem may be a bit dramatic, but I’m inclined to argue that the toed lateral is shaving off a shit load of flow and that’s what is producing the effect. I’ve drawn another line in the picture below to indicate which wash line I’m referring to.
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What is neat is that the crest of this wash, I guess it’s a wake of sorts, is also dropping pretty much straight down the face, though it seem to dance a little before doing so. Which is pretty cool, if my ‘why’ is correct (otherwise things will get uncool fast.)
The flow up the face will change depending on where you are on the face – whether you are out more on the shoulder or closer to the peak. Closer to the peak the flow is more upwards than forwards, out on the should its less so – how much depends on how far out you are. The changing nature of the flow is continuous. The flow being cut off by his lateral has more forward velocity than the flow it then meets as it comes off his fin, the consequence is that it literally breaks like a little wave. Okay, maybe a strange little wave, anyway it shears off. Chances are the laterals are also canted to some degree, so you've got that contributing too.
Big Finish? … hardly.
All that fin interaction costs something. That is, shearing all that flow and sending off in another direction cost something - and I believe that something is drag. The question then is why would that be desirable?
Surely running a single fin would be less of a drag? And some do exactly that. But I would contend that if they do, their posture and position on the board would be noticeably different from that of Slater's – and not just because Slater has a natural body genius. Sure, Slater is hauling ass, but I suspect that his brain is telling his body that you don't want to get to far ahead of 'it'. He riding his back foot, but my guess would be a lot less than someone trying to achieve a similar position on a single fin.
Its amazing what a little fin drag can do.
kc