Olas, I believe you’re wrong about flex built into racecar chassis. Flex is the enemy because it distorts the settings the engineer is trying to achieve. If you’re racing a production car body (BMW for example) the first thing to do is to intercept the body shell in the construction process and fully seam weld it, rather than use a tack welded production body. This is done exactly to reduce flex and improve stiffness.
That said and done, the race car will still have flex, of course. The engineer/chassis tuner/driver/shaper/glasser/surfer has to rely on the test track data to adjust the chassis (for each track, each surface, each curve, each driver) to achieve the desired result: A vehicle that interacts with the road, through the tire contact patch, in the most effective way. This will require compromises in some parts of the track.
There are useful analogies for surfing and surfboard construction in the above that I’m sure you can see. As an engineer, I side with the stiff board guys. Flex (strain in engineering terms) distributes stress, and I can believe that proper deformation MIGHT allow a stressed board not to snap. But the wave surface is so infinitely variable that introducing flex is too much to expect a surfer to either appreciate or (directly) control. And I don’t believe flex fins are where you wanta be, either. Surely there’s enough “slip” in fin hydrodynamics that one doesn’t need flex to further degrade the efficiency of a fin. But that’s just me, I gotta understand it to believe it.
I think that some forms of flex in surfboards are often misunderstood. The damping effect of uncontrolled (or poorly designed) flex is obvious; Morey Doyle. If flex is used like Greenough’s designs, then the positive aspects negate the minor damping effects. If flex is used to allow portions of the rocker to bend during a turn and become flat after the load is removed, you get a board that turns well and trims well. The rebound effect with this type of flex is minimal (in my experience) at best. The feeling may be more the result of a very efficient bottom contour reducing drag giving you the illusion of a burst of acceleration.
Given the advancement of fin design and clustering, using “Greenough” flex in short boards will not likely enhance the performance beyond what is already available. Flex in longer boards, say 7’6"+ is more of a viable alternative to multiple fins (IMHO).
A properly designed flex fin does not bend, it twists. If combined with the right hull (length, width, outline, rocker, and rail shape) they can be extremely effective. Flex fins do not (again, IMHO) work as well in cluster set-ups (wash-out, stalling, etc.) as stiffer fins do.
Like everything else in surfboard design, there are trade-offs for just about everything. Flex has its place as does stiffness. It just needs to be used in the right context.
I also think of flex in terms of panel stiffness. In my mind the perimeter of the board needs to be stiff while the bottom needs to move a bit. In essence like a tennis racket with a frame and strings. On a surfboard the string part acts as a shock absorber, adding control the way shock absorbers do in cars. This is one issue I think Surftechs have that is holding them back. They don’t move. I, in part, derive this theory from the standard surfboard that is stiffer around the rail and a bit softer in the center because of laps. It’s also harder on the deck and softer on the bottom because of the usual unbalanced laminate schedule. I think this just evolved that way while no one was paying attention and that it is, in essence, right. This also makes use of perimeter weighting also because of the laps. It surprised me when I first started weighing everything that almost 50% of the laminate weight is in the laps. This weighting has a rather large effect on performance.
Now longitudinal stiffness is harder for me to estimate but if indeed there is longitudal flex it’s imperitive that it is combined with positive return as in those racket strings. This is something Greenough obviously knew.
Twist also needs to be addressed as well. This has been done in snowboards and some wakeboards and the effect on performance is dramatic. At any rate I don’t think we’re on the brink of seeing Morey Doyles on the ASP tour either.
By the way JM, nice little primer on composite engineering. EVERY surfboard builder should read it. There was a very similar one printed in Clyde Beatty’s LCS book and catalogue on epoxy board building back in the early 80’s. Not many read it but it contained all the same information written by some Phd from England. That artical was like my bible. I still have a copy of it… somewhere.
Good stuff all around. I “borrowed” the link from Glenn Shotwell who originally posted it near the bottom of page 4 in the “Vacuum forming and sandwich construction” thread that Bert started. I think it, along with the other EPS/epoxy/composite posts, has given several of us a kick in the ass. I know I needed it!
meecrafty , to get results over 1# foam , you need to make the glass job as thick as possible and get maximum compression resistance ,but doing it in such a way as to keep it light as well …so even if you dont vacumn form a sandwich material on , the idea is to create something resembling a sandwich …
a thick but light outer skin …
find a way of doing that , and you will get the desired result with sl grade eps …
Given the advancement of fin design and clustering, using "Greenough" flex in short boards will not likely enhance the performance beyond what is already available. Flex in longer boards, say 7'6"+ is more of a viable alternative to multiple fins (IMHO).
A properly designed flex fin does not bend, it twists. If combined with the right hull (length, width, outline, rocker, and rail shape) they can be extremely effective. Flex fins do not (again, IMHO) work as well in cluster set-ups (wash-out, stalling, etc.) as stiffer fins do.
My intent is to do my best to establish that flex in clusters CAN substantially enhance performance - I’ll probably be riding them in about 2 months.
BTW, flex fins work better is they don’t twist and use an efficient planshape. That’s one of the principle downsides to flex fins a la Greenough (which do work better than rigid). They are convex, on the high pressure side and concave on the low pressure side. They use a very long chord length close to the tip, which is horrible for induced drag. They can washout and oscillate if ridden in faster waves than they were designed for, or under the feet of a more aggressive rider. These downsides are all correctable by putting the flex in the box instead of the fin.
WRT the core discussion, I was of the impression that a core’s tendency to gas under temperature changes posed a significant difficult in core materials. At least, Clark thinks so, and there is a long history of delams with oher core materials that leave too much air inside (including Tom Blake’s wood boards). So the core needs to
not gas under temperature changes
provide vertical support to the laminate so that it doesn’t flex vertically
have good adhesion with the laminate
and that PU was, in general, the winner among uniblank materials. Perhaps someone more educated on the blank materials could be more informative.
Well gents, last night was simply amazing…the creative juices were flowing big time and I’ve gotten some tremendeous inspiration from this thread. After reading everyones input and MJs link I realized there’s soooooo much I DONT know…good time in my life to be learning.
Bert, it took several hours of thought but I now understand some of what your doing and saying…I got it now and I will continue on my current path with some enhancements…btw my local supplier has coremat (dcell too) so Im using that for at least the deck…I dunno if I need it for the bottom since its primarily tensile…???..still thinking and reading…and learning…what a year it has been…learning so much its rediculous!
Thanks MJ…Im looking back on your previous postings…love what youre doing.
Matt, my motivation for this posts was partly ecomomics and performance leap. Economics…#1 foam is available just about everywhere so I’d like to make a better board (than my current std EPS/Epoxy…which by the way is way better than PU/PE) with cheap/available materials…like I said “why the heck not?”
The mass distribution was an technical after-thought…it seemed like such a basic thought/question…most surfers proly dont care…but you know how Swaylockians like us think…gotta move forward and make some progress. Berts recent posts have been very inspirational and ya gotta give the guy mucho credit for being on the forefront…its kind of sad more people aren’t paying attention.
I’ve been wondering since I heard the comment on eps off-gasing at temperature, has anyone tried to heat the uncovered core and force the material to “gas” before the skin is attached? Could you use a heat box for this pupose?
Sorry about the air bladder comments early in this thread…
“… in most sport speed/racing applications strutural flex is a no no. Flex gives control…but I think you can get control from other sources like the static shape/fins itself.”
I respectfully disagree. It`s all about balanced applications. Flex and rigidity can be complimentary aspects of a synergy… no need to be in opposition.
A largely unexplored world of design, construction and performance exists between rigid hard and floppy soft.
Of all people, surfers should be able to see that the ocean (as all of nature) contains myriad examples of adaptive, structural flex… above, on and beneath the water`s surface.
Traversing hard surfaces (esp. if irregular) frequently requires a responsive balance of rigidity and flex for optimum control and speed. Fluid mediums suggest no less.
EPS doesn’t really have any off gassing issues. EPS looses 95% of it blowing agent gasses in the per-expansion process. It looses 95% of what’s left during the molding process so there isn’t a whole lot of blowing agent gasses left. The XPS process traps most of the blowing agent inside the cells and it basically never comes out. That’s why when the cells are damaged by impact or overheating, you get the delams. The higher densitiy EPS is quite stable actually, 2# being very stable and very fool proof for non sandwich construction board building. In the 1# density the real problem is with heat. There can be some issues there with the foam melting in heat extremes. If your sandwiching though, these problems are solved because the outside foam generally insulates the interior. The other issue with 1# is connecting the fin boxes. I generally connect the box to the deck with small holes drilled through to the deck that are then filled with epoxy/cabosil. With 1.5 and up I don’t have to do this.
The sealed skin of a surfboard is designed to keep liquids and gases out. They do a pretty damn good job of it too. But inevitably, water and cool air permiate this protective skin through whatever microscopic pathways it can find. If the temperature of the air and/or water rises they expand. If they expand faster than they egress, pressure builds up and can lead to delamination. Polyurethane blanks are more dense than Expanded Poly Styrene. Consequently, there typically is less volume of gas/liquid trapped within the shell of P/U boards.
Also, I ran across a post in the archives that said Home Depot foam was unsuitable for surfboards and to use architectural foam? Would the 2# Home Depot foam work? Is this the same as Insulfoam - edro eps foam (bun)?
“BTW, flex fins work better is they don’t twist and use an efficient planshape. That’s one of the principle downsides to flex fins a la Greenough (which do work better than rigid). They are convex, on the high pressure side and concave on the low pressure side. They use a very long chord length close to the tip, which is horrible for induced drag. They can washout and oscillate if ridden in faster waves than they were designed for, or under the feet of a more aggressive rider. These downsides are all correctable by putting the flex in the box instead of the fin.”
I believe you are talking about Greenough’s paddle fin. I was talking more about the older, G-4 and Liddle flex single fins where I have never experienced any “speed wobble” or wash-out problems (and I’ve got them up to velocities at or above anything achievable on a tri-fin cluster). The “paddle” is definately a weird bird. I haven’t sorted in out completely but my best results have been at higher speeds…go figure.
I’ve never had really good results with the G4’s. But I think it’s less the flex than it is the surface area. For my size (220#) the tip just can’t hold. The fin slides into a turn but can’t ‘load up’ and finish, so turns start strong & bog halfway through as they start to let too much water slip by. If I take a high line on a wall and move forward, those fins also tend to slide ass under me.
I’ve also got a G6 paddle. I’ve only tried it in a couple boards. Works great on really soft rails & belly bottom, works crappy on late down rails and a flat to concave bottom. On the soft edged board, it whips left-go-right takeoffs, bottom turns off deep fades, and high line cutbacks like a 2+1, but it also has enough surface area to accelerate if you trim it into a line. It also responds like crazy to kick stalls and rail-to-rail pumps to either burn off or gain speed. It does like a wave at least waist high, though, to get going right.
On the board with harder rails, It just feels like there’s too much going on - sheeting water off the bottom of the board that should be moving through the fin. Feels mushy & unresponsive, and then breaks loose just when you don’t want it to. I get the feeling it wants to work with a board that’s moving in the water and not on it. So for some boards, its fantastic, but its not an all-around solution.
The G-4 is more of a power fin that works better being banked rather than whipped or pivoted. They are especially well known for holding a high line. Try moving it up or back in the box. Once you find the right spot, I’ll bet a lot of your problems will go away.
I use the “paddle” in short, edge bottom boards and as long as you are moving at a fairly good clip it works “well”. You are right about the fin being finicky. I can’t imagine it on a longboard although most reports that I’ve heard say it is a great nose riding fin…
All I’ve had either of those fins in are longboards! I had tried the G4 (and a Velzy “classic”, which is pretty much the same fin) in 5 or 6 different boards, all up & down the boxes. The only place I liked it was on a round pin board that’s normally a 2+1, that I surfed as a single with the G4 at a really hollow beachbreak. But that also had to do with the style that wave demanded - drop in, hit your spot, and hang on - which is, like you said, more like banking than pivoting.
The G6 is great in some longboards, not others. A noserider with big concaves & a really wide tail wouldn’t like the fin, that needs something solid to hold down the tail, like a big Tyler or Yater fin. But a big board wil belly & hips or a ‘modern’ longboard with relaxed rocker & rails would go great with it.
Sorry to hijack this thread. Don’t really know how we got here from there
"BTW, flex fins work better is they don't twist and use an efficient planshape. That's one of the principle downsides to flex fins a la Greenough (which do work better than rigid). They are convex, on the high pressure side and concave on the low pressure side. They use a very long chord length close to the tip, which is horrible for induced drag. They can washout and oscillate if ridden in faster waves than they were designed for, or under the feet of a more aggressive rider. These downsides are all correctable by putting the flex in the box instead of the fin."
I believe you are talking about Greenough’s paddle fin. I was talking more about the older, G-4 and Liddle flex single fins where I have never experienced any “speed wobble” or wash-out problems (and I’ve got them up to velocities at or above anything achievable on a tri-fin cluster). The “paddle” is definately a weird bird. I haven’t sorted in out completely but my best results have been at higher speeds…go figure.
All flex fins have long chord lengths near the tip - this comes from having the tip point nearly rearward.
All flex fins are convex on the high pressure side and concave on the low pressure side, exactly the opposite of the standard for rail fins.
Thrusters are used on the highest speed surfboards, currently.
I started making fins flex in toe-in and kept them really loose. And the flex made turning very very easy, but the oscillations when going straight were the killer. So I engineered a way to keep the board from oscillating, and allowed the flex to be tunable. Then it got a lot better, just as easy to turn, but also easy to go straight without wobbling.
Flex fins, a la Greenough and Liddle et al, are stuck in the realm I was in before I made the fin box. If you make the flex work for turning well, you run the risk of washout and wobble. And if you never wobble or washout, you are not getting the most out of the flex. Besides all that, the planshapes and foil (caused by the flex of the fin) are very inefficient.
I’m pretty sure you can verify this with anyone who has made a bunch of different flex fins to play with the flex and see what happens.
i’m very pleased that more than 1 yr after i posted those threads on my homebuilt sandwich, they have been dug up to add to the interest stoked up by Bert’s vacuum thread
meecrafty, my opinion is that the most exciting stuff that Bert presents here is best done by mastering the vacuum bag. like me you are an engineer from a different discipline so should find the pump lots of fun. i realise that economy is an issue for you and there is no getting away from the extra expense of the sandwich materials, but take a look at Brennan’s posts on the pump thread - he built his own vacuum setup!
surftechs problems in a nutshell , there low density core is turned into something more closely resembling a higher density core by resin penatration , combined with cored skins which add stiffness and you get something to stiff , with no shear movement …
without shear movment , you have no flex …
now while the link on sandwich construction was more to do with structural applications …
…
regards
BERT
I built my first sandwich before seeing details of what Bert was up to, so why would i go to all that trouble of vacuum construction?
my experience with my surftech thats why!
thanks for that explanation Bert, i can see that you are taking things well beyond the basic pvc/glass sandwich of the surftech, nevertheless my surftech mini tank is a magic board to me. Even my wife who doesn’t surf noticed that it is lighter than a normal board. its fairly stiff but does flex a bit, but thats the way i like it
slightly off topic but i reminded myself of your front foot V rear foot comparison thread of some time ago. I believed by eyeballing this this board that despite being a tail rider it had relatively more rocker in the rear, however i have since found out that i was mistaken and it is in fact flatter in the tail and that your theory does hold true for this board. so i discovered that just looking at an unfamiliar shape can be deceiving and just not good enough and the shape needs to be measured
