To get less stiffness on its bad days should i set it up as a twin fin?
I don't think that's the answer to your problem. Aside from changing your approach to the wave on those days you can't get your board on a rail, you can try a fin with less rake. Those AM fins look like they have a lot of sweep, which draws out turns, but maintains speed through them. A more upright, pivoty fin could be the answer.
But you might try moving your back foot back. Chances are your finding it hard to initiate a turn when the waves are letting you go faster. The faster you go, the flatter those wider boards like to stay. Stepping back with your back foot can help.
it might not be the fins at all (although it very well could be), it could be the rail thickness/shape, foot placement, or even the swell itself. It could be a myriad of things is what i'm trying to get at. Try some future vectors if you're sure it's the fins. I have a quad with the vectors in them and they are amazing, but i never ride it with a twin set up. I have an actual retro style twinny. Even with the inside fins out of my quad, the two boards ride so different. I've had my quad for about 2 years and I will ALWAYS have one in my quiver.
Go to your local shop and demo fins. I have had this debate/issue since I am a life long thurster guy who finally decided last year to tray out a 5'7 quad fish/hybrid thing... I have been given 15 different opinions from creidible guys on here, but honestly, you just gotta try stuff out. I thought of getting some big keel fins and doing the twinny thing, but after demo'ing a bunch of fins, I found out that my stock fins (m5s - Front) (m3s back) were actually the best setup. I think more than anything, it has a lot to do with rail volume, rocker and the fact that it is a completely different fins setup. I am very happy with my quad now, but you have to know what you are getting in to. Before I take that board out, I have to think it through. Cause I am not going to be going aerial on it usually. Im not going to be turning it in the pocket on a dime. Im not going to be blowing the tail out with the fins out.... Its just a different concept. So look at the surf, and if you see a set or two that a rippable on your thursters, take out the thruster. I am usually happier with a handful of rippable high performance waves on the thurster than I am with 20 semi decent waves on my quad/fish... Its just a different concept... fun in both ways. But there is nothing more frustrating than paddling out and realizing that the waves were actually fast and hollow enough to bring out ol thrusty thurster...
But with that being said, I have that board pretty dialed in, and I know what its good at and what its not good at... You need to get some good speed on it, asjust your technique to the slower turning radius and change of direction and surf it accordingly. Setup your bottom turn a second sooner and if the wave is getting hollow, I usually take a steeper bottom turn and jsut throw the thing into the lip. It will bash the lip off, hold all 4 fins locked in nicely and give you a nice smooth landing. Its just a different sensation... You arent going to bury the rail through cutbacks the way you normally would with any kind of fin setup... Its just a different style board and completely different style fins....
My problem was that I tried to turn this quad fish into something that rode like my shortboards... but then, whats the point... Its a different board. It simple acts different. And in the end, I realized that thats the fun part... something different... You can still pound on sections and destroy the lip, you just gotta figure out the speed and transitions....
Zach... well said. I started riding quads i the 80s, and most of them tracked like crazy. Fast, but tracky. The 80's rockers, foils, and rails, along with less than ideal glassed on fin positions, just didn't match up. So I gave them up until about 10 years ago. Now I've had the good fortune to ride many different quads with many different fin positions, cant angles, and fin selections... McKee positions, all four fins out on the rail, Rusty's setup... with different cant inserts on the same board (thanks Probox), and different fins... flat foiled, vector foiled, 80/20 foils... and ALL of them ride differently than thrusters in the manner in which you put perfectly.... "you just gotta figure out the speed and transitions." While each cluster config and fin option has it's pluses and minuses, they all perform differently than thrusters, and you can't make a quad ride like a thruster, no matter what you try.
Finally, several years ago, I finally got the quad thing dialed in, after years of experimentation, and once I did, I was so stoked on them I thought... "that's it... I'm never going back to thrusters." Ha! Well... the novelty wore off I guess, and now out of my last two boards, one's a thruster, and one's a thruster/quad convertible. Why not, right?
there's not really such a thing as an "obsolete" surfboard design...just different ways of riding a wave.
climbing down off my soapbox now...
play around w/ your fins...the stock plastic fins, as crappy as they are, might be too big for you if you're a small guy. & given that you're riding a 5'6", i'm going to assume that you are a smaller guy, so leave the rears in place & grab a set of g-3's or g-5's & swap the fronts out for those. better yet, try to find a shop near you that is an fcs demo center (assuming the board is, in fact, fcs. if it's futures, go find a futures demo center) & try a bunch of different fins in the board.
i have a a 5'8" fishcuit that i ride set up w/ fcs pc g-am (flat foil) in the front spots w/ fcs pc g-xq rears (80/20 foil) in the rear. this is the set up that seems to work best for me, & i arrived at it after trying a bunch of different fins in the board.
quads are not like thrusters & really seem to be picky about what fins work best. i find that thrusters are more "plug & play", meaning you can just about slap any decent fin into one & the board will work well. yes, the proper set of fins will really bring it to life, but a thruster will still work. quads are more finicky IMO.