It would be quite a bit more useful, to me, tom, if in addition you posted the relative wide point positions as a percentage of fore-aft, and relative thickness as a percentage of chord length.
I have total respect for using foils from the masters, and considering them the “gold standard”, but that doesn’t mean you cannot also study their statistics, and try to determine why they vary from the two dimensional foils used to develop the theory of aerodynamic foils.
I’ll try to get around to it. But, I’m kind of buried trying to keep the doors open around here. That’s why I tag in and out of these threads with such brevity.
what kind of sponge/cleaner combo do you guys reccomend for getting brains off a monitor and keyboard, my head just popped again…
Lawless, I see what you are saying, the double ought series is way more forward…
There are, however, other differences that may influence lift and drag, such as the leading edge radius, no?
The 6x series seems to be a very narrow range of shapes, differing only in wp aft as a % of chord…and further back than a surfboard fin…
I suppose the thing to do would be to get data on more of the NACA foils and see if there are any trends in the data, maybe some further forward foil-series with varying LE radii to compare…but there’s the head pop thing…
Again, please excuse if I speak out of turn, I have no real experience, esp. regarding sailboat hull design, but sometimes I feel that getting a read from someone who “doesn’t know the math” but has some intuition, fiziks background, and a little real world experience can be helpful in the evolutuion of thought…Don’t take them as challenge, just questions as the thread goes on…if my interdictions chafe, tell me to cheese it and I’ll resume reading…
wells
PS I’ve made some tubercles…I’ll try and borrow a friend’s camera tonite and post method and methodology. The razzing alone should be entertainment enough for a few days…I’m looking forward to a good thrashing.
in the shortboards there was a 50/50 split some said the futures were faster others said the fat foils were faster???
the guys who said futures were faster also said they could feel the extra drag from the fuller foils,
the crew who said the fat foils were faster said that futures didnt respond and nothing happened when they tried to turn , they sliped out easier , lost speed out the end of deep turns…
Bert & others
In trying to understand your test results better, particularly the bold statement above…where there any other correlations within each 50% group (other than what you have already mentioned)?
I have attached your pic from the original fin theory post. The first thing I noticed is the bluff leading edge. Could this, and to a lesser degree, the combination of the other key foil factors be the BIG deciding factor between the two groups? Meaning, does the Futures group feel more drag because of the bluff leading edge (or was it thickness?)
What are the key differences between the stock Futures and your customs? Is the pic one of the foils tested?
I would guess that a thicker foil with the “correct” leading edge would impact your test results. What was the leading edge radius as a percent of foil thickness?
BTW gents, I’m on the side of reducing drag at the expense of lift so my favorite foil in the group is 66.
Question to the foil lift proponents: how much lift would a board receive from a rail fin with zero degress of cant?
PS - Bert, your tests reminds me of something called “Design of Experiements”…aka DOE. A technique used in design/engineering/statistical circles that takes a bunch of different design variables, changing only one variable at a time, to polar extremes, testing and repeating the process for each variable, while holding all other variables constant. The goal is to find the key design attributes that contribute most to performance or quality or whatever your doing. Don’t think you can really do it with surfboard riders cuz the testers have too much subjectivity.
Tom,
No offense man, but your R value seems off (100k is very low) so I did a little self refresher.
Re = (Velocity * Length) / n
where n is the kinematic viscosity - n for water is .000001. L is measured in metres and V in m/sec. L is average cord length which should be less than the base length for your typical board fin - I used 3.4 inches or 0.08 meters. I assumed both 5 and 10 m/s for V which seems reasonable. R is 400,000 and 800,000 respectively. The R values are even higher for L = 4.7 in.
Well after all this discussion, I took a close look at the typical FCS side fin and its UGLY man… I mean full-on joke. There’s actually a flattened region behind the entry starting about 5/8 aft of the leading edge and extends over an inch so its difficult to identify the location of max thickness - its around 33%. The entry of the foil is a joke…it looks almost angular. The thickness is 6% of CL. At least they’re easy to install which is ok for a occasioanal builder like myself. I’m definitely going to make some mods.
I’m not yet ready to accept the idea that common surfboard fins are too thin…
If thicker fins offered a competitive advantage, why are all the pro surfers riding thin fins? Many many people have noted that the thin fins used in surfboarding are suboptimal according to two dimensional, linear, fluid theory. Why can’t some nobody just use 12% thick fins and blow everyone else away???
There is more here than meets the eye. It could be that the range of AOAs used in surfing are not reaching max stall angles, or it could be that low pressure vortices are responsible for a lot of the lift and the foil thickness is thus largely irrelevant (and the changes noted by people altering thicknesses of side fins occurs from changes in the fin directionality at the leading and trailing edges).
Fins are easy to make. At least with single fins, there is not a night and day difference going from 6% to 10%. And after thousands tried their hands at fin making, 4-5 inch chord lengths come with max width close to 0.375, and 3.5 inch chord lengths come with max width close to 0.25 inches.
The leading edge, however, I am ready to believe is a joke. sharp leading edges in general are horrible for flow separation even at small AOA changes, and you can easily feel the difference when you ride them - if you then go home and radius them as I used to do to mine. I’m not sure how they got in the fin mold in the first place.
BTW, your Re calculations were pretty close to mine…
“I’m not yet ready to accept the idea that common surfboard fins are too thin…” ----------------- Nor am I.
“sharp leading edges in general are horrible for flow separation even at small AOA changes” ------------- Right on!
“I’m not sure how they got in the fin mold in the first place.” ------------------- Could it have been laziness?
A few reasons why common fins aren’t any thinner than they are might be. Loss of strength in the materials used, expediting the construction process, following the crowd, force of habit , fear of change, & and reservation about have to market a product that is a departure from the norm.
Mahalo, Rich
Is it indeed a certainty that water will flow along the concave inner surface of a side fin, especially when pressed with any AOA? I know this assumption is key if one is to conclude that the pressure difference and lift will be more with a thicker foil coupled with a flat inner surface, vs a foil coupled with a concave inner foil, since the distance the water will travel on both surfaces is now more similar as a result of the inner curve, but something in my head wants me to question that. At almost any speed at all with almost any AOA, won’t the water flow bypass the inner concave curve mostly and tend to want to flow rather straight? (maybe with a bit of turbulence?) (I apologize if I don’t bring the physics background that most in this great thread have). But I do have a rather sensistive feel for equipment and the curiosity and joy with trying new things.
Is it indeed a certainty that water will flow along the concave inner surface of a side fin, especially when pressed with any AOA?
Recall that when a rail fin is engaged, the water is flowing INTO the concave surface, so there is little chance it will flow straight instead.
However, for the OUTSIDE rail fin, the chance of flow separation is enhanced relative to a double foiled fin.
The punchline is no one gives a crap what the outside rail fin is doing. Except, I have noticed some brilliant fin makers increase fin depth and reduce chord length when they use concave rail fins, to reduce chances of flow separation at negative AOAs.
"I’m not yet ready to accept the idea that common surfboard fins are too thin… "
Blakestah,
coincidentally, I traced a 10% thick foil on top of the std FCS foil trace and it just seemed too thick…didn’t look right. So I settled on a .35 thick foil modification compared to the stock .26. My plan is to simply refoil/smooth the curve similar to 66 and add a sheet of thick laminate (about 1/16 or so) to the flat side. Im simply looking at a NACA 66 foil in the thread and using one half of it.
What I understand (I’ll have to check this assertion) is that thinner foils tend to prevent seperation (wake) at slow speeds
like when paddling and surfing slow weak waves. There are advantages…but I’d like to make a set of thicker foils with an improved curve just to keep me playing as usual.
The thicker is better proponents are on the side of lifting - Im just not “on-board” so to speak. The main reasons are:
a. the occasional optimum foil lift force vector is acting perpendicular to the plane of the fin, thus the force is acting sideways towards the rail (this is good to hold the rail in place on turns (probably why we don’t spin out) but has little effect on tail lift which is what “I think” we are after…toein drag delivers this…just my opinion)
b. Lift is very dependent on AOA and in rail to rail surfing the AOA is all over the place, thus optimal foil lift is not common while surfing.
I suspect the thicker foils do a better handling large and variable AOAs and minimize seperation and turbulence (at higher speeds) and hence why one group preferred Bert’s customs over the futures…still like to know any other correlations within the groups…???
Regarding Re…the 20MPH in water and 100k just didnt jive.
K.I.S.S. is my preferred M.O.
When I went through this before, about a month ago, I diverted into talks on leading edge vortices and delta wings. The basics, however, are simple. And I think it is correct.
When a low aspect ratio foil (aspect ratio under 5, or ratio of depth to chord length), turns, it often creates a leading edge vortex on the low pressure side. This leading edge vortex forms a complete swirl, and does not cause flow separation. It does, however, invalidate two dimensional linear flow theory.
By this, I mean all those nice graphs posted in this thread.
Foils working with vortex generation have much more lift than that predicted by those lift drag curves. And, the foil thickness becomes irrelevant at larger angles of attack.
It is a bit murky when you play with thickness and rail fins, because if you make an assymetric fin, and keep one side flat, and thicken only the other, the foil directionality changes. We know changes in toe-in cause substantial changes in performance.
A better test would be to use doubly foiled rail fins, and then alter thickness, while keeping the fin doubly foiled.
You can also look at fish/marine mammal pectoral fins, which have a reasonably similar role to a surfboard fin. They are also thin.
Or, use a single fin setup. I can tell you THOUSANDS have tried thicker foils on single fins, and yet all the major fiberglass fin makers, who all foil fins by hand one by one, use the same range of thicknesses - 5-6% of chord length. This is not an accident. You can even call Bill or Shod or Chuck and ask them directly about foil thickness. They will tell you they’ve been there, done that. Many an engineer has tried to apply flow theory and tried thicker fins, but the revolution is not there.
The most convincing result, however, is one you do with your own hands. I sincerely hope you do the testing and get a clean result.
Arright, I for one am a bit twisted around, and I hope I don’t inadvertently derail this train into a semantics swamp, but I think the term “lift” is being used a few different ways, and maybe someone ought to clarify things. here is the picture I’m getting, please correct / clarify:
Lift with respect to fin foils in a turn points roughly 90deg. from the stringer, nearly level to the deck, aka going out of the low pressure side of a fin. it helps pull the board into the wave face, and pull the board around in a turn, no?
“Lift” with respect to concave bottom contours actually acts downward, pulling or sticking the board to the water, as in hydroplane hulls? too many people think concaves “Lift” the board up onto the water because they cause “lift”, but the opposite is true, no?
Finally, lift with respect to fin cant is the small vertical (with respect to board) component of the fin foil’s largely horizontal lift, which does serve to push the tail up out of the water, no?
I get the impression that the lift caused by the foil, pointing out the side of a fin, is the one that’s relevant to the discussion about foils?
wells
As a fluids term, lift refers to forces acting perpendicular to flow. Drag refes to forcces parallel to flow.
Wells,
Blakestah is correct. Foil lift is just that, perpendicular to flow but in most commonly applied and discussed foil theory, flow is near parallel to the fin chord so mostly, foil lift is a force that acts sideways/perpendicular to the fin plane. This by itself cannot cause the board to lift, IMO. Adding fin cant helps but more on this below…
“I get the impression that the lift caused by the foil, pointing out the side of a fin, is the one that’s relevant to the discussion about foils?”
Yes but my core question going back months and months is “how does foil lift (a side force) benefit the board and rider?” How much more emphasis is placed on foils over other critical fin factors like thickness or placement, for example. I think the reason foils get so much attention is because to most, including myself, it is a mystery…toe-in and cant is boring and played. Foils are one of the last frontiers. Enter a new thread asking about thruster fin placement (which I would love to understand better) and compare the response.
Everyone has their opinion…here’s my take:
There is a foil lift present but it is very occasional in rail to rail surfing because of the ever changing and large AOAs
Fin Cant helps to produce some board lift from foil lift but is a very small contributer to board lift when compared to other factors. Toe-in has a much more pronounced effect on board lift but it is simply caused by frictional and pressure drag. Btw, fin cant only helps the rider when the board is going straight or nearly so…once your on rail sharp AOA negates the effect. The question still stands…how much tail lift will you get from fin foil lift if fin cant is zero?
When you turn a board, water (actually the board is moving faster) is flowing at sharp angles towards the inside face of the rail fin and is deflected towards the rear of the fin/board…this redirection adds a little “thrust” (I cant believe I just used that term) so IMO, a fin acts a lot like a sailboard sail. From these lateral forces, pressure in this case, are likely much greater than any side/lift force from the foil…again this holds the board/rail in place on hard turns. But since AOA is so large foil lift cannot be a factor on hard turns…perhaps some on low even moderate turns like smooth bottom turns.
While I think foil lift and its effects are overated, I still enjoy discussing it because I have learned much from the regular contributors to these threads…I’ve changed my mind about certain things along the way in my research and learning about fins and its effects is very interesting.
Im still interested in thickness (well not too thick Blakestah) and position, leading edges, foils that reduce drag, etc. Good useful infotainment. Still waiting for Bert and his additional test group feedback, if any.
Thanks, Meecrafty, for explaining your viewpoint for me…I am thoroughly enjoying wrapping my noggin around these concepts, it’s fun when you get little breakthroughs in understanding, the lightbulb going off, etc… I realize that lift is perpendicular to flow, but that still leaves you with 360 degrees of possibilities…
Great, just when I get real interested in experimenting, nor-cal gets a godawful flat spell. when’s it ever gonna end?
wells, I ordered some swell from 290 degrees for tomorrow.
Enjoy.
Hello Wells,
You are correct in saying that thruster fin cant introduces a vertical lift component to a fin setup due to the small horizontal control surface component that fin cant entails. In my opinion this horizontal control surface component accounts in part for the remarkable performance of the thruster, and can also at times cause a few problems at high speed.
Regards, Roy
PS Your comment that as ‘lift is perpendicular to flow that leaves 360 degrees of possibilities’ is exactly correct. The tunnel fin takes advantage of this fact, as it is a control surface which acts through 360 degrees.
ok guys sorry to post a subject and then disappear…
ive been competing so i was crashing early and didnt want my thoughts distracted ,
meecrafty the answer to your question…
its toe in, the 50% that felt more drag from the fuller foils had less toe in set into there boxes, because there boxes were set more parallel(not completly straight)
that was one of the main points that if you want to take advantage of a fuller foil more toe needs to be added …
or else the problem is , a thicker fin gives more pull/lift perpendicular 90 degrees to the flow, because if you have no toe in and thick foils you get massive opposing forces…those opposing forces cause drag , not the thickness of the foil itself…
those thick fins i used in the drawings was my favourite foil traced around…
there the fastest fins ive ever used to date,
the main reason there so fast is they handle high AOA without losing speed …
the extra hold and bite means you can push them harder into the power zone …
thin fins lose speed to quickly ,surfing with thin fins means the drift or the slide has to be incorporated into your style , thats lost recovery time…
bite, hold , no loss of speed at high angles of attack , means tight fast power surfing…
blakestah…
why arent the pros using them???
is it only pros who can steer the progression of surfing???
most pros ( i say most) under 30 think just like any other guy off the street…then the problem is they figure it out but by then there opinion doesnt count anymore…
coz there not pros…
primitive thinking would say thin is fastest??? thats true if your not turning…
what is the fastest creature in the ocean…
how fat is it???(i want you to give me the answer blakestah)
i think you will find that some surfers have already blown the pack away…
proof is in the results right???
but once your up in the elite level , its more about pocket size and marketing and whats percieved by the public as being functional , and not about what really does work…
pros are just brought …
the amount of team riders ive had , who have dominated and attracted attention , then been poached by labels with bigger offers, only to disappear again coz they went to crap equipment…
i dont give my work away , thats a blow to the ego of an up and coming pro or an existing one for that matter…
if i was a pro and won comps with these fins , only then would you believe they work??? blakestah ! youve always got alot of facts and figures so im surprised you cant pull the logic out of the original post…
tubedog can see it …and hes working more from just what he feels in his board…
sometimes our university degrees dont give us all the answers…
halycon… yep i agree with your observations about moving camber for and aft relative to speed …
tom youll be happy to hear that appart from the fins i customise for my riders yours rated highest among some of the generic brands …
i have 2 team guys whove kept me hell busy remoulding ofishll,red x,fcs, to fit the futures boxes…
so we were able also to compare 5 brands against each other…
ive always liked your foils more anyway , but they recomfirmed it from trial and error in true comparison…
as usual theres more points , but my posts always seem to drag out…
so ill leave it there…for now
regards
BERT
Bert, I was more making the point that, at least with single fins, width has been played with ad nauseum. It is not something that is unknown. I can, without even leaving San Francisco, grab a half dozen people who’ve made fins of varying thickness, and none of them found substantial benefit in it. Further, if you talk to any shaper of more than 30 years (when all boards were singles), they will all tell you they made their own fins, and many of them did width series as well. This includes people like George Greenough, who sells his fins through Ames and also uses relatively thin fins.
Of course, a rail fin is not a single fin, you’d have to believe, truly, that they generate lift differently. I’d rather be convinced if you did the experiment with double foiled fins, because your assymmetric series will change the “effective” toe-in along with changes in camber max width, and this we all know has a pronounced impact on surfability.
Another approach would be using the Edge fin system and playing with the toe-in and cant on each fin set, then evaluating each at the best toe-in and cant.
As it is now, the experiment is horribly confounded. A change in width from 6% to 13% will change the exit angle of the water by more than the toe-in angle!!! Of COURSE they are going to ride substantially differently. But I think you may be drawing the wrong conclusions from your experiment, because the variable “exit angle from fin” is confounded with “fin width”.
Yet another approach would be to try to account for this some other way. But, I think experimenting with width of assymmetric fins, or concaves, on fixed fin boxes will lead to potentially erroneous conclusions.
The Edge fin system is a tremendous aid in such an effort (or the Swivelfins), that lets you fine-tune toe-in after installation, and still change fins. I can only imagine how difficult it was to study fin template/camber/depth/rake before fin boxes…
Again, please note that I’m a rank newbie here, but…
Could side fin toe-in be ideally set at the angle at which lift is zero, as the board goes straight? (for an assymetric foil, or wing section, the lift curve crosses zero somewhere in the shallow negative AOA’s, right? and the thicker a foil, the further the AOA’s go negative before the “wing” stops lifting and starts pushing down…I would assume that as lift approaches zero on side fins, the net water deflection also approaches zero…it’s pushed one way as it’s engaged by the fin, then pushed an equal amount the other way as it leaves the back end…Also, for a wing, is net drag (induced + parasitic) least at this zero-lift point, or at Zero AOA, where the profile is minimized? or somewhere else?
If set so that straight forward = zero lift, then the fins would no longer be pushing against each other going straight… Is this what you guys are already saying? It would fit with Bert’s observations that fatter fins need more toe…“shifting” the steeper lift curve into a useable range across the AOA’s…of course turning is a whole nother matter entirely, I would think that if a foil had more lift and pulled the rail down harder, you could use thicker rails and still sink it to the hilt…
Again, please excuse the ramble and iffy fiszziks, I’m trying to get a handle on it all…please tell me to cheez it if it’s disruptive…
Blakestah!!! the swell came in as promised, thanks a million! now I can get on with some esperimints, this morning’s OB sesh was all about the “control group” hee hee…
wells