Fin theory,tom,halycon and others

General Discussion
161

Hi Bert

God is this a test ? rhetorical question

I guess to your first two questions I would first say, that I don’t claim to be an expert in these matters, I have ideas and theory’s that when aired in a forum like this change all the time after being influenced by other ideas and theory’s that every body else posts.

This is what is so good about this as you can constantly reevaluate

Right the questions

There are many variables from the rider,the board to the waves and conditions

No one thing will be perfect all the time

For myself average joe surfing ability ok but no Kelly Slater and surf conditions again average waves not pulling into sick padang padang over head pits

For me it would be a fin system that gives good lift through a medium to generous range of AOAs

As for the links please enlighten me, there are lots of areas of a surfboard that contribute to performance the more I can find or be told the better.

For me fins are there to give me tight control in turns and then speed in sections

After all try either of those with out them, it’s possible but pitiful

1)speed range I have always guessed around a minimum of 10 knots to a max around 23 knots .This is based on board feeling while skurfing where you can have a speed read out from the boat

2)Range of attack Good question im not too sure,only I think it may be bigger than we guess Have you got any ideas on how we can find out with accuracy?

3) Load on fin areas not a huge load ,once again If you go skurfing you can load up and break fins real easy,This is not so easy to do riding a wave

Yes I know differing energies etc but the load on your ankles is a lot less

4)Cater for turbulence Phew why, how much, where, I guess it all counts the cutaways seem to show that .Where it starts and finishes im not sure ,what are your and theory’s ?

5) ventilation I would say rake and LE radius play the role in this one Your thoughts?

6) I still think further back than current fins In turns its all about lift into the turn and down the line its like a yacht sail/keel relation ,lift one way wave energy the other you get squirted out the middle

I know what you mean about finding the answers low speed foil theories are still very vague and incomplete

Many years ago I was designing and building hang gliders at the time we developed them from single surface wings with small nose angles to wide-angle high aspect double surface precambered foils

It took a lot of ground towed test bed testing and more than a few rather hairy test flights to confirm various theories

The comment on nature’s wings and fins, the only bad thing is that most fish ,birds are lucky enough to have variable geometry

Its interesting that on hang gliders you can alter the wing geometry whilst in flight

As well as being able to increase the speed and Ld this also allows their wings to be asymmetrical when turning allowing for differing AOA from inside to outside wing tip

Developing this was a major breakthrough in hang glider performance and handling

May be VG has a place with us too?

PS I decided to print out this thread to reread some of it while writing did you know its 110 pages

As you say lets work towards what we do and do not need from the foils then refine it

Cheers Mike

162

of coarse its a test…and just to prepare you for tomorows subject…whats the best way to deal with high AOA ???

but seriously , i do agree about the point that you can constantly re-evaluate theories and findings…

fins are the same as boards , different boards for different people for different waves…whenever i go for a surf i have a quiver of boards and a quiver of fins…and i can stretch the range of a board with different fins…

i suppose my question about what would be better ,a board with good lift but poor ability to deal with AOA ? or average lift but the ability to deal with massive AOA and still be providing lift ??

its a little bit one sided coz im looking at it from what would suit my surfing needs better…

i want control even when im out of control…

thats where i looked at stunt planes coz they need to regain control once theyve pushed passed the normal operating limits…

i like a fin that can be pushed passed extreme AOA to the point where your going backwards and still have control…

thats why i liked your biplane fin …it would handle extreme loads to…and still be providing lift even if you were going backwards…

theres nothing really better , just different…

thats why fin systems are not really suited to me ( as in the conventional range of fins) coz for every change of circumstance a different foil will suit…

i think the best any of us could do is have an understanding of fins to the point that we knew exactly what fins to use in any given conditions to suit the board were on ,and know why they function…

look how much boards have changed in 40 years…

fin development has been going on the whole time ,but in the last 6 or 7 years its taken leaps and bounds …

and theres way more crew now looking at the fin package more carefully than ever…

when surfing first started most boards looked the same …but as time went on they changed into many different forms to suit a range of condititions…and now in our time just about everything that has been and gone is acceptable as a surfcraft, depending on the condititions…

fins will go the same way…a larger range of different foils built for a wider variety of conditions…

slowly but surely the current fin system companies are broadening there range…

which is what they will have to do to satisfy the market…

im astounded at some companies who feel they can continue to run with a narrow range of fins …without looking to expand into new areas …especially areas that have proven test results…(comp results)which ever has more signifigance…

im such a firm believer in having positive bouyancy in your fins …after throwing the subject out there and presenting it to a few fin companies …

i now believe theres not many people out there who appreciate the value of a fuller super bouyant foil …

once you experience bouyant foils in small waves up to double over head …you can never go back…it adds a whole new dimension to surfing…

im not just talking about wooden fins either …but they are what got me working in this direction…

since 97 ive been making vac/autoclaved carbon areo fins …but the process involved meant i could only do one set at a time …(not economical) …i can make them 1/3 the weight of a same size wooden fin…

so whether you do them that way or a foam sandwich version…

either way its well worth pursuing…especially from a performance perspective…

but it looks like more people will have to try it before it catches on…

but thats just another fin in the quiver…

its all about taking any given conditions and having a board set up to maximise your enjoyment and get the most out of whatever conditions you encounter…

hey sabs …you pass, we all do …were all looking for something to help us enjoy surfing more…

regards

BERT

163

Hey Bert

Yippee a pass

The bouyant fin,I totally agree my usual fin is H100 foam with either carbon or kevlar skins

Recently, Since we stopped cruising round the world and have settled in Auckland for a while I have been doing solids.

The reason has just been conveniance,while trying new shapes, I manage a luxury yacht building factory here and its just too easy to grab either a off cut of glass sheet or do a quick solid carbon layup,shape on the large belt sanding machine

I can then go from idea to surf in a day I then go back and reproduce the ones I like in foam later .

All this was the point of my little fin boxs that can take either one or two fins setups in the one box

Its made comparision between Standard thruster,cutaways and my slateds easy

As I said before the slateds are still the fins I like the best, so I shall now go and try to apply some of the cutaway benifets to them

Certinally for max AOA stuff they are the ducks guts

Did you see what I meant about the fins of nature being variable?

There must be a way that we can replicate this to some degree!!

Regards Mike

164

Thread is going very well.

Here’s the reading assignment for the holiday weekend. It’s required reading for this thread and will be on the final. jk

http://aerodyn.org/aero.html

The site answers most questions.

I particularly like the chapter on Canards and Strakes because someone else explains how the Multi-Vortex Generator (strake) effects control.

165

Mark, I was glancing through “Aerodynamics for Engineering Students; 5th ed.” (Butterworth/Heinman publisher) at the MIT bookstore during lunch yesterday. There is a section called “flow around slender bodies.” It mentioned that at low speeds the “conformal transformation methods are a good choice” as opposed to the “thin air foil theory” which assumes: “max airfoil thickness is small compared to cord length” and “camber-line shape only deviated slightly from the cord line” which restricts the thin-airfoil to “low angles of incidence”

So my questions are: are surfboard fins modeled on “thin-airfoil theory;” what’s this “conformal transformational method;” does it really work better for low-speeds and high- or greater- AOAs; reasoning for thicker fins? reasoning for foils that have twist or can change shape? ???

So much to learn…

166

Rob, those are precisely the points I made earlier, with different terminology like “low aspect ratio” and “vortex mode”.

The lift:drag relations from NACA foils and the “lift coefficient is maximal at 12% thickness relative to chord length” are derived and apply only to high aspect ratio foils in which length is 5 times chord length, or greater. Almost all surfboard fins are low aspect ratio - length to chord length ratios of 2 or less.

Low aspect ratio foils develop a vortex on the low pressure side, near the leading edge. This increases lift relative to predictions from high aspect ratio foil theory. Very nearly all foils for highly acrobatic fliers operate in this mode. Rake is used to avoid stalls at higher AOAs, and it shifts the lift coefficent:AOA plot to higher AOAs - meaning - to get the same lift from a more raked foil, you need a higher AOA.

Some fighter jets, like the F18, have a “vortex generator” in front of the main raked wing which is placed and presumably operates a lot like the MVG - except that it is placed there to initiate the vortex over the low aspect ratio wing. Micro-vortex generators on airplane wings are placed along the main foil. I just rode a A320 jet and counted over a dozen placed in a line along the main wing.

Also, nearly all surfboard fins are quite thin - 5% of chord length or so. This is not unusual for low aspect ratio foils. There are some here making thicker fins - Bert posted a fin close to 12% thickness the other day. It will certainly help the flow around the foil at low AOAs, but I at least am doubtful there will be much change, and quite possibly a detrimental one, at higher AOAs.

Of course, unlike airplane foils, surfboard foils are typically operated at negative AOAs quite a lot.

167

Mr. Stah,

Check out handglider wings; almost always double-foiled.

Now go back 25 years or so to Pleskunas and Greenough: double-foiled windsurfer sails.

Jump forward to 2003: windsurfer speed record set: double-foiled sails.

Dizzy?

168

Here’s something to think about. As we form inside concavity and convexity on side fins, we are affectively removing camber from the out side foil. We recently made a new soft surfboard fin with a contemporary thruster template and foil. We made the side fin molds first with the inside surface flat and an .025" edge radius and the outer surface max girth 8-1/2% of cord at 32% back from the leading edge. After thinking about our discussions regarding AOA, we combined the two side fin outer cavities to create our new center fin. Now, we have side fins with the same surface contours as our trailing on the working side of the fins. The only difference between the working sides of the side fin to the center fin is toe. So theoretically, the trailing fin should stall closer to the time that the side fin does. Whereas, with traditional thinner center fin foils they should stall earlier.

Now we’re starting to get into the question of whether you want the trailing fin to stall earlier or later. There is a range in thruster fin setups that is from a twin fin with a small trailing fin to a 2+1 setup where the drive is from the back fins. Some people prefer bigger side fin templates and smaller trailing fins. A fuller max girth farther forward would not compliment fin setups on this side of the spectrum. At it’s extreme this side of the range is getting all of you drive out of the side fins and rail with the trailing fin just dampening out (or dragging around). As you approach the opposite extreme, where the center fin develops most of the drive it would be benifitial for the center fin’s max girth be wider and further forward than the side fins.

When I showed our new fins to Bobby Szabad, he said the trailing fin is too fat and people won’t buy it. When I showed them to Tom Morey he wanted all center fins ( and fillets at the base?!). When I rode them, I only had the original keel fins to compare them to. But, the was much less drag and a huge (night & day) drive difference.

So, getting back to my original statement. When you consider the toe that side fins are set with, there is an induced positive angle of attack on the side fins relative to the center fin. The lift vector is dependent upon the difference in length of the path that the water must travel one side of the fin relative to the other. So, is it really a good thing to add length via contours to the inside surface of side fins? First for any given girth of foil the more contour you put on the inside surface, the more you have to take away from the camber of the outside surface. Second with concavity not only do you equalize the differential between the two flow paths. But, you also induce a torque vector that wants to twist the foil and create drag. For lower AOAs these fins will feel more flowing. But, they are going to stall earlier and create less lift when you really need it.

I’m not saying there is no merit in contouring the inside surfaces of side fins. I’m just saying the performance envelope will become narrower where they work well.

169

Hey Rob,

It’s all there in the site I posted. No need glancing through books.

I was on the phone yesterday for a long time with Greg Loehr. We talked about a lot of things including fins. I had just received the day before a copy of a report by a very smart group of UCSD engineering students of a fin test study they conducted. Greg and I discussed the results as they compared to a friend of his results 15 years ago in his masters thesis. Conclusions were very similar.

Otherwise, the general points we agreed upon and have agreed upon for some time is that most fins today are purely visual. That most people dont change fins. That most kids dont know what physics means because they havnt had it in school. And that till now most fin companies didnt consider anything about hydrodynamic theory as evidenced by so many thin fins with sharp leading edges and flat surfaces. Some of us with the help of the Internet are trying to change that. [http://aerodyn.org/aero.html] You know who you are.

Also, I believe Greg’s Theory of Balance is very important. This seminal work is valuable in that it can help anyone understand and explain why a board may or may not work and suggests ways to improve a boards balance by simply changing out fins. Even suggests which fins to use. It’s comprehensive and holds up well. He’s far ahead of all of us being one of the original 5 or 6 deep barrel riders at Pipe, thoughtful(chip board and epoxy), and productive(30K boards shaped and Resin Research). What’s best is he wrote it down and had it published for us to use as reference.

I will except all fin companies from bashing. I have no use for bashing. It’s not a fault it’s just not been perceived as top of the list critical. It’s all a matter of first things first. Getting established as a viable fin attachment system was for everyone the first order of business. That takes a lot of time, money, organization and energy. Lawsuits set the process back several years, too. Now that the field has shaken out it’s the fins themselves that will be changed. A lot of people have put a lot of hard work into fins and we will all benefit.

You gotta love it. Honestly, it’s full on right now. REdX, Locbox, and Futures are all doing something. FCS has their new cambered fin which looks pretty good. And OAM because they have intuitively and otherwise got some nice design features. One even by luck.

I’ve met and or talked to nearly everyone involved in fins and I know I helped nudge the industry in the right direction. Inner camber and inner foil DNA goes Me to R. to Curtis. All others are following the lead. NO one has it all and so there is plenty of room for improvement. Anyone can win.

All I can say now is the thread is going in the many of the right directions. Follow it out look around you and follow it out etc. It’s a thought and observation process and this thread, having so many good clear thinkers, is exactly what the industry needs to help get the education disseminated throughout the surfing population. Advertising alone wont do it and pros, forgetaboutit, they have a lot on their minds already. Knowledge is the key. Talk is the way. There is a lot of learning to do. Use this site as a base. [http://aerodyn.org/aero.html] Good work everyone. Keep it rolling.

170

blakestah id reread your post…maybe make some changes…

fuller foil camber forward ,can handle the largest range in angle of attack and still be producing lift /hold/contol,at a way broader range of speeds…

i dont know if you guys get strong winds over there where you have sand blowing along the beach like we get in our seabreezes every afternoon…

take some fins down the beach half bury them in the sand at varying angles of attack relative to the wind and watch how the grains of sand wrap around each fin…

it all becomes very clear …

regards

BERT

171

Bert, very nice experiment/technique. You can bury them deeper to isolate the tip and add all sorts of cant to see what that does.

Anyone going to Kill Devil Hills, bring your fins and be sure to check out the THICK double foiled handglider wings.

172

Lift from concavity is not from the Bernoulli effect.

In fact, almost all lift from surfboard fins is not explained by the Bernoulli effect.

The Bernoulli effect describes how lift depends on changes in path length. If you go through the calculations, as we did in basic fluids, you see that airplanes cannot generate enough lift from the Bernoulli effect to fly. They need more. They need the additional pressure differential from changes in AOA.

The concavity changes the pressure differential between the two sides of the fin. The high pressure side becomes higher pressure. Also, as the water leaves the fin, the water from the two sides of the fin flows more parallel, and is redirected more efficiently towards the high pressure side, with concavity. The changes in the fluid flow behind thefoil are a perfect “equal and opposite” change from the lift and drag forces places on the foil. If the plane is staying in the air, it has to re-direct air downward.

I’m almost motivated enough to try a fin twice as thick as the ones I ride now…the proof is in the pudding.

173

I understand you’re redirecting the flow with concavity. I refered to it as an induced torque vector. When you’re taking off with an air plane you induce flap into your wings which gives the wing that concavity. As you gain airspeed you reduce the amount of flaps to decrease drag. But, the major point I’m trying to get across is that for a given girth say .3", the more contour you put on the inside surface say .07" the more camber you give up on the outside surface .23". So, do the positives out weigh the negatives. I say they may at lower AOAs. But, they are limited more in their range of AOA.

When racing 12 meter yachts, we have induced trim tab in for short periods of time to gain height to weather. But, we need to take it out as boat speed drops due to extra drag.

174

tom well done…

thats it exactly…

putting concave in your side fins effectivley reduces the pressure difference…so then they have less hold

they respond later …the only way you can get them to work a little ealier is toe them out a little …but then your reducing again the overall angle of attack they can handle once you go deeper and harder into your turn…so exactly like tom said your reducing there range…

im going to quote a guy who owns a fin company and see what you make of this comment …i wont name him for obvious reasons…

its no one whos contributed to this thread so far…

i was having this discussion about fuller foils with him…

he said " i realise they work better but i dont wanna spend money on re-educating the public, we make the fins the public percieve as working the best"

so its got nothing to do with improving the peformance of surfboards ??

its all about profit ,how many fins you can sell…

and you wanna hear the official futures statement to me today…

“we dont want you using our boxes,as were about to launch our australian marketing campaign and we dont need anyone saying things that are contrary to our marketing features”

what do they know that i dont???

are they expecting me to find out things that would be contrary to there marketing blurb???

what there saying is ,i can buy boxes if i buy fins as well …

i would gladly buy fins anyday…making fins is a total drama …but until someone comes up with some decent foils i will continue to make my own…

plus you have to take into consideration the variety of fins people need for different circumstances…

what if futures dont make those fins???

ends up that customer just cant have a fin system,full stop…

i can honestly say futures are backing themselves into a corner making fins for the narrowest range of conditions and the narrowest range of surfers…

just so they can tap into an unsuspecting all spending grommet market fueled by hype…

ladies and gentlemen …

thats the futures of the future…

id love to throw in a few glowing comments about mike and vince right now ,

but hey it wont help…

so it ends up that all my initial thoughts on why futures banned me from using there boxes…ends up being the case …they dont want people who actually have a clue, exposing there insanely inferior foils…

i cant believe they see a guy who makes 4 boards a week as a serious threat , and would try and hinder him from making any more progress towards developing better foils for everybody…

as much as knew this was the real scenario…for the last few weeks i was hoping it was just a misunderstanding…

not so ,futures have now confirmed they have no interest in meaningful r&d .

regards

BERT

175
Quote:
If the plane is staying in the air, it has to re-direct air downward.

This is another effect of increased camber in an airfoil. Not only are you increasing the pressure differential, you are also redirecting the thrust vectors off the trailing edge. With a flat fin the water on the high pressure side is going straight off the back whereas with a cambered fin it will be directed more towards the stringer. In theory this will give the fin more of a forward thrust vector.

tom, your example of the wing flaps at take off is a good example of more camber providing more lift, especially at slower speeds.

In reference to fins often operating at negative AOA, most competitive aerobatic planes have symmetrical airfoils that provide lift by their relative AOA. This is why they can fly inverted straight and level. Most of them tend to have very thick airfoils as well. They are designed to operate at extremely wide range of AOA, with very quick direction changes. Maybe something to look at.

176

O ye gods of fin theory! O illuminati! I quake at the base of your mountain of knowledge! I tremble and grovel and struggle to breathe in your rareified air! I cannot climb Mt. Skeg, for I lack the tools to understand the way!

Ye speak a mighty and thunderous language! Words and concepts I hardly understand! For I am of the base and profane! My Basic Fluids is not something that should be seen in class! My lacrosse coach told me cant is not a word! Yet I see in the gathering storm, a promise of much needed rain!

When, O gods, will fins of funny shapes come raining down from on high?

Those who live on the plain, need not the rain, as they buy their fins and boards from shops (chains), and will not change.

The pros ride what they know will go.

We at the base of mt. Skeg, in the village swaylocks, while we are of the profane, hear the rumbling from on high, we see glimpses of brilliant light! Perhaps we could help the gods, in our humble way, by riding the funny shapes, and posting our crude, subjective observations! If a fin came from on high to strike me in the forehead, and it’s base was lokbox, I would ride the fin, then toss it back towards the heavens! or to another swaylockian! Rape our fertile ground, pillage our village! We will give you the feedback you need!

177

Bert says, "half bury them in the sand at varying angles of attack relative to the wind and watch how the grains of sand wrap around each fin…

it all becomes very clear …"

Now this is pure genius, IMHO.

As center fins become thicker and are foiled with the vertical cord more forward the become more sensitive and release more quickly release from side to side. This type of foil works very well for helping longboard performance. It also will make rail fins engage more abruptly because rail change is more abrupt with this type of foil in the center fin.

If you want aburpt lift for lip bashing and vertical surfing puting this type of foil in the center on a full based fin with a generous tip area, down sizing it a little to keep the board real free along with rail fins with the cord up forward with a simlar template that are on the thick side will give a board a tremendous amount of abrupt lift. Some guys like to surf this way. The fins in the attachment won a WCT event. They are foiled that way. They don’t excel when it comes to trim speed but for vertical top to bottom surfing they excel.

By the way the rails are double foiled and the high pressure side is slightly concaved. Fins like these encourage constant change in the AOA.

Blakestad’s going fast going really fast in that jet but when it comes to changing directions you can’t beat a Sopwith Camel!

Tight loops aren’t fast loops it’s just the way things are; you gotta open up the radius if you want to go fast ~ go figure, Rich

178

wells that was so seriously funny …just for that you deserve to be struck down with a set of fins…

im still laughing and i read it 5 minutes ago…

regards

BERT

179

My college English professor suggested the American Heritage Dictionary.

cant (1)

PRONUNCIATION: AUDIO: k http://us.i1.yimg.com/us.yimg.com/i/edu/ref/ahd/s/abreve.gif nt KEY

NOUN: 1. Angular deviation from a vertical or horizontal plane or surface; an inclination or slope.

  1. A slanted or oblique surface.

3a. A thrust or motion that tilts something. b. The tilt caused by such a thrust or motion.

  1. An outer corner, as of a building.

VERB: Inflected forms: cant·ed, cant·ing, cants

TRANSITIVE VERB: 1. To set at an oblique angle; tilt.

  1. To give a slanting edge to; bevel.

  2. To change the direction of suddenly.

INTRANSITIVE VERB: 1. To lean to one side; slant.

  1. To take an oblique direction or course; swing around, as a ship.

ETYMOLOGY: Middle English, side, from Old North French, from Vulgar Latin *cantus, corner, from Latin canthus, rim of wheel, tire, of Celtic origin.

180

Bad Joke:

My mom always said cant means wont but when now I that get a little tipsey occassionally I have a better sense of the word.

~ slidin’, tipin’ & flyin’ along the wide sea shore ~ Rich