I don't believe in Flex !

[BWD]

those things would be even better to have than a magic board...

[jonysan]

I suspect you have some already.

I'm going to think of some other topic to question soon !! probably do beginners want to ride strapless too soon,

[heemskn]

Personally I believe in flex, but not in the explanations by most board builders/marketeers of why flex works.

Over the last 50 or so years board builders and surfers have figured out the best shapes and construction methods through trial and error. It is a beautiful thing, and we should all be thankful for that. But most builders and surfers lack the engineering/material science knowledge to understand why some things work and others don't. So they have come up with a whole lot of theories that are nonsense. Storing energy in a flexed board or fin and releasing this as forward thrust are two of these. But there are others that are even more clearly nonsense, for example how epoxy boards ride worse because they are lighter and therefore "ride higher in the water and don't hold turns as well" and a whole lot of theories about fin configurations (thruster vs. quad anyone?). As an engineer, reading some of the surf board building forums can be funny or tragic, depending on how cynical you are. But at the end of they day these board builders have figured out what works, so let's just take advantage of these magic boards, and ignore the flawed marketing hype.

[Da Yoda]

I am probably completely wrong, but think flex in relation to surfboards, and particularly fins is marketing hype,

I encourage you and anyone reading this thread to listen to this interview of Larry Allison on fin design and construction.
https://soundcloud.com/stand-up-paddle- ... acing-fins

Yes this interview is focused on SUP fins, but he touches on the comparisons of surfboard fins in relation and also fin constructions which is applicable to both SUPs and surfboards. This interview OPENED my eyes up big time to all of the "marketing" hype. I now use one of Larry's fins on my SUP and it's definitely a noticeable difference vs the carbon fins.

Good "fin flex" tech starts at 19:00

[jonysan]

You guys would understand the words, "I can't see a thing , till you opened my eyes" really interesting to get these opinions,

[NHKitesurfer]

I watched the video and he lost me when he started saying something to the effect that the top and bottom of the board slide like sheets or something like that. I don't even think this is possible given how surfboards are built. But I've never built one and could be completely wrong.

What I do agree with is that nearly everything flexes but question how much of that stored energy is then used when released and whether or not it makes any difference. Yes, on the extreme ends of the bell curve but in the middle where most boards are constructed I just don't know. I am open to learning more and seeing someone's theory proven.

[iriejohn]

A fin is a vertical wing and when that wing is moving through fluid at higher angles of attack the tip of that wing should twist to reduce drag.

[TomW]

Hej Jony-san,
I'm wondering also about this " flex" in a surfboard( for kitesurfing). Seems that flex in the tail would be interesting if you could tune it to a specific flex. But that's not what I'm seeing in boards. Not much science in it.

[jonysan]

Interesting idea, what about a twintip construction with that sort of spring back flex, so when you carve hard into a turn the tail and back half of the board attains a curve that matches the wave , then springs back to it's normal flattish rocker as the load comes of it, like a snowboard, it would need quite a bit of nose rocker. also a lot of torsional stiffness so the rail edge in contact would keep pressure into the water

[Flyfish]

this is exactly why Sunova stopped using glass-ons and future boxes, why they don't anchor the boxes through to the deck, and why some other surfers and builders still prefer the original FCS to futures or FCS-II.
It allows just a little more flex through the tail.

Huh, I never really thought about how glass on fins might change the flex in the tail of the board. interesting. Unfortunately I hate how FCS always pop out or crack.

I'm not really sure I understand the OP's original question, but I'm enjoying learning some about fins. Good post.