I might be late to the party here but just saw a review online about Kai Lenny changing up the quads and putting the big ones on the back.
what's a bit more weird, is I recently picked up a second hand hypernut 7'2 and after its maiden voyage; noticed that the 4.7" quads were at the back and the 4.5"s at the front. Not sure if it's just because the previous owner had no idea or it was purposeful haha.
forum.surfer.com/index.php?threads/large-fins-placed-at-rear-in-quad-setup.242467/
Another fella also mentioned it on a FB group a couple of days ago for his Placid.
I guess it makes sense for Kai surfing bloody big waves. For us normal folk, not so much. Would likely feel too 'locked in and stiff'.
Yeah weirdly It didn't feel too bad, although I also had to wear my moonboot with about 3x heel wedges and a mid-sole support so wasn't exactly ripping turns haha. Pretty impressed I could even paddle it after 5 months of not being in the water and my legs were totally different heights!
When looking for a new quad set up for my gun I saw these futuresfins.com/products/john-john-florence-big-wave-quad
That setup may work better on bigger steeper waves.
I have a 8'6" Hypernut that I tried all kinds of different fin variations (including the one on your board) and landed on a twinfin (using L thruster side fins {Height 127 mm, Base 120mm}) using the rear boxes. I was about to sell the board until I landed on this fin set up. The channel bottom of the hypernut gives extra hold.
I first heard about using the "power fin", aka large fin, in the back by Richard Schmidt.
surfsplendorpodcast.com/085-richard-schmidt/
I saw the Kay Lenny video where he said he liked that setup too:
Select to expand quotesflinux said..
That setup may work better on bigger steeper waves.
I have a 8'6" Hypernut that I tried all kinds of different fin variations (including the one on your board) and landed on a twinfin (using L thruster side fins {Height 127 mm, Base 120mm}) using the rear boxes. I was about to sell the board until I landed on this fin set up. The channel bottom of the hypernut gives extra hold.
I first heard about using the "power fin", aka large fin, in the back by Richard Schmidt.
surfsplendorpodcast.com/085-richard-schmidt/
I saw the Kay Lenny video where he said he liked that setup too:
I have one board that has future boxes where the front and rear box depth is the same (requires spacers to use new futures quad rear fins). I have a 7'6" quad egg that always felt squirrelly with the normal quad setup with the big fins up front. Then one day I tried a setup like Richard Schmidt with the power fin in the back, the board felt perfect. This board has the deep futures boxes in the front/rear and the front/rear quad fin set is for deep boxes, so I can interchange.
Easy to switch if you have FCS boxes.
maybe Kai, but seems he is selling product too
At 226 for the mfc Kai and 250 for the futures jj it has changed surfing forever by making fins twice as expensive
Still I have to wonder if it might not be kind of good in a sup, any volunteers? I like how they reversed the futures base size so they fit the standard box setup.
With fin boxes placed the standard way, having bigger fins at the rear will only work if you are a back foot surfer, otherwise it will just make the board stiffer. I know, I tried. I am a front foot surfer, so on the opposite, I cut my rears to make them even smaller.
Note that if you pause the video of Kai, you can see that the foil of these MFC fins is also convex on the inside, for less drag and a smoother ride, but less hold and a less positive grip feel.
Feels like Kai's setup is designed for very high speed surfing: Tow-in or Windsurfing. Which is logical.
Note: on the previous version of this video I commented that the fins foils were nearly symmetrical, and Kai response was "No they are flat on the inside"... shows you how you world class riders may not be always gear-savvy. Just look to this screen grab and what would you believe? Kai or your very own eyes? (to quote Groucho Marx)
In the video, Kai said, "The problem with quads, is they tend to make the board ride a lot flatter on the water"
Does anyone understand what he means? Flatter rail to rail?
Or flatter nose to tail?
He is using it in his wave sups including Bali video smaller surf, if it would make me surf like that sign me up. This is standard for windsurf quad already for along time. I used to have a twinzer surf board with small canard fins pretty close in front of the rears, it worked great loose and fast like a twin but better grip. Pretty sure the fins were positioned different than boxes on todays quads would be.
Don't forget area gives lift ,I also have noticed my prone surfing mates use bigger quad fin s than I would use
The tail area is a lot smaller on a surfboard as Kia's sups are ,they are almost surfboards and he surfs them well .
if you are using wider Tail Sup and bigger sups you will not need as big a fin as the tail wide gives you life
and with bigger fins and wide tail you will get airborne on big choppy faces ,Colas uses wide tail boards and actually grind his rear fins
to help keep the tail down in the water ,I also prefer smaller rears to keep it down ,with the wider tails ,
sfilnux,fins give lift so that what he means by it flattening out the board ,tail comes up nose goes down ,
That why windsurfers have so many fins ,as to trim the board in different wind strengths ,
Select to expand quoteTardy said..
Don't forget area gives lift ,I also have noticed my prone surfing mates use bigger quad fin s t
Yes, and also the SUPs are more cumbersome than prone surfboards, and smaller rear fins help to turn quickly these (relatively) big boards.
I still prefer big front fins on SUP to provide a definite pivot point. With bigger fins in rear the board was too stiff and the smaller fronts did not offer enough grip, the slippage contributing to the understeering.
For down the line surfing in big waves, I imagine the reverse would be true, I would welcome a stiffer board with dampened reactions in turns for stability at high speeds.
Interesting comments above, I always found a bigger board needed bigger fins? A board with a thinner or more narrow tail needed less fins as it could help itself bigger? in my old jimmy Lewis world wide 7'9, I tried running small thruster fins and it couldn't hold. Big sides and smaller back centre gave hold and control and turning ability
ha ha... I really don't understand why fins work
I just try different stuff until something clicks
Then I come on here and find out why...
Glad someone understands this engineering theory 
I actually had to use my wider tailed quad (c drive small) in big waves after I broke my step up. The rails gripped well due to their thin stepped design and it was fast down the line, the problem is can't get the tail railed in for good carving turns due to too much lift, this is the benefit that I believe Kai is talking about the ability to rail the board and do performance turns even at high speed. This is what the windsurf quad (twinzer) was originally known for being able to go quickly rail to rail without the big center fin. But I doubt any fin can overcome too much tail volume and width. I also prefer thruster with smaller rear or small quad rears for sup, big difference for me.
Select to expand quotesflinux said..
In the video, Kai said, "The problem with quads, is they tend to make the board ride a lot flatter on the water"
Does anyone understand what he means? Flatter rail to rail?
Or flatter nose to tail?
In windsurfing more front fin bias makes the board ride more planted with nose down so I guess flatter.
I prefer a rear bias twizer style quad for sailing which brings the nose up makeing it more reactive .
Experimented with this yesterday in small surf on a wide round tail surfboard and so far prefering it to the regular quad set up.
