Fin Theory , vector forces

over the last 6 months or so one of my long time theories have been blown out of the water by experimenting with concave foils.

also im going to present a theory on vectored drag , ive had a lot of feed back from a heap of my team surfers and some longtime customers who surf real well , all up the following is a composite collection of comments and thoughts ,feedback and observations from a solid list of surfers …

the crew who have helped me on this are , the current west coast o/35s shortboard champ,the current west coast o/35s longboard champ,the current west coast o/40s shortboard champ,the current west coast o/40s longboard champ, the current west coast o/45s longboard champ,current open longboard national champion , current west coast and former national open longboard champion, plus a handful of other good surfers , mostly shortboarders with various styles and ages …

since ive been using the futures boxes and making my own foils to fit a few interesting things have happened . most of my team guys and some customers have managed to get hold of futures fins and make some interesting comparisons , plus comparing the concave fins ive been building…

there has been some contradicting feedback but the theory im putting up clearly explains the contradictions…

at this point its only theory , and till its excepted by a wider audience its still a theory ,

so ive posted some diagrams as well, most of the regular fin experts will know already the stuff ive written and drawn in the attachments but ive put some stuff in to help some of the laymen , so a few more crew could see how ive come to this conclusion…

im relying on the regular panel of experts to examine this lot and see whether you guys agree or not…pust it past theory…

templates aside this is just a comparison between concave foils and flatsided plus fuller foils…

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…

what ive personally observed is the futures concaves have less straight line drag , but increased turning drag , less hold , less responsiveness …

the guys who said they could feel the extra drag from fuller foils , had there boards set to take futures fins (future team guys, boxes set with less toe in )…

not one longboarder even rated slightly favourable any concave foil mine or futures …

all these comments start to make sense when we look at each fin as a vector force acting on the board and the magnitude of those vectors is depending on the camber of the foils and the toe in there set at …

so my basic theory is , a fat fin doesnt create extra drag by itself but 2 fat fins going in a straight line will cause drag because of opposing vector forces, as soon as there is the slightest angle of attack the fat foil has way less drag , more drive ,more hold,more turning power… all that enables the surfer to connect tighter turns closer together…

a thin fin has less straightline drag but increased drag everywhere else and prone to fail way ealier…

my previous theory that ive droped now , i use to beleive that as water went around the curved side of a fatter foil it created drag at that point , that if the foil got fatter the drag increased because the water had to go further around the curved side of the foil (like going over a steeper hill)…

but now i say the drag doesnt come from curved side it comes from the larger pressure difference , creating a larger opposing vector force between each side fin, because in reality set up right a fat foil will make your board go faster so it cant be the individual fins creating drag but the collective set if there not set right…

there are way more comments that crew have made … ill post the attachments see if they make sense and i can post the other comments and observations later …ok time to fry the brain…

regards

BERT

Bert,

I found similar results when I compared our 80/20 laminated foils against our original injection molded foils. We do not have any concavity on the inside surfaces. But, with the inside leading edge filleted by 20% and the over all foil thickness thinner, the end result is definitely less differential of flow rates and hence pressure on either side. This characteristic is most pronounce at lower board speeds. On the fins I tested I found that there are two things going on. 1) The toe is slightly less on the 80/20 foils 2) There is less difference between the flow paths of the inside surface vs, the outside surface.

The results I felt were 1) The 80/20 foils had a smoother rail to rail transition and flowed better during shallow turns at higher speeds. 2) The original fin foils gave me more to push off of in down speed situations and felt more positive during more extreme turns. I also found that if I ran a stiffer set of the original foils I could ride them as a smaller template. I have been riding Carbon X-2 Air Cells as opposed to the X-4 80/20 foils and X-4 Composites. In doing so I got even less straight line drag than the 80/20 X-4s and more torque out of the more extreme turns. So, I got the best of both worlds.

Then I started testing Halcyon’s Feather Fins and they’re 5" deep. So, far in the tiny waves we’ve had they have the least straight line drag yet and have been very torquey in radical direction changes. But, I’ve yet to get any thing over head high on these fins. Still testing.

Conclusions: The 80/20 foils seem better suited for bigger wave riding. Fat flatsided fins appear better suited for average size surf. Halcyon’s fins are sick in small surf…awaiting bigger surf testing to expand further.

yep all those observations make perfect sense to me…im out till tommorow…

regards

BERT

I cannot read the attachment - what is it - some sorta proprietary Microsoft Office format?? I’d be great if it could be done in pdf or jpeg or png or html or some other format that web browsers read.

In lotsa ways changes in camber alter effective toe, including changes in leading edge and presence of concavity.

Bigger wave riding…I thought that was mostly about reducing chord lengths…

“Publisher cannot open files from another version.”

Microsoft Publisher. Bert, try saving them as .txt files or copy and paste the info into Word. Haven’t finished reading the post yet, just tried to open the files.

scanned and posted as jpeg’s see how this goes…

if this doesnt work ill try the other suggestions tommorow…

its late now…

regards

BERT

Hi Bert

Yes all makes sense

If i can harp back to hang gliders,we spent a lot of time and money to get rid of concave foils and create true ,what we called double surface foils as opposed to single surface foils

As you have stated its all about pressure difference , hopefully now my earlier comments on pointie and small elipse tips will make more sense

The foils where I have used concaves are my sloted or as you called them biplane ones ,these are way different because the two fins act as one foil ,if you take away the aft one they just dont work very well by themselves

The two foils have to be close together to get the venturi effect working ,this increases the flow rate on the low pressure side which which increases lift

The thicker foils you show are better all round performers allthough I would suggest that as you go thicker you move the max camber point aft and concentrate on a better leading edge shape

As I stated some time ago the foils with the max camber forward are all about foil stability not lift potential

The relitivly slow speeds that we go would I think produce better lift with the camber aft more.

My latest have the camber at about 42 percent aft

regards Mike

PS its good to be back to the main meat of the disscussion again

Hey Sabs,

Wait a minute I’m confused, or at least I think I am. Moving positive camber aft yeilds more lift. Yikes!

Well I guess I better throw away some ideas because things seem, I say seem to be different to me. Maybe they are.

Move positive camber aft and get more speed. This is what seems correct to me.

When I want to make speed fins I put the vertical cord about 45% back off the leading edge. When I want fins to be sensitive and lift more rapidly I move the camber forward accordingly.

Help me out guys.

Gone Fishin’, Rich

Just one point at a time. I don’t want to interject any other variables right now.

Hi Halcyon

Lift is a result of pressure difference,pressure difference is a result of thickness and separation

If all the thickness is bunched up the front of the foil two things can happen a)induced drag can increase b)stalling can occur more offen over a wider speed range and range of angles of attack

If you go from a well shaped leading edge curve into a moderate camber curve throughout the foil it will operate throughout a broader speed and angle of attack range ie it will be more forgiving

I think that our speed range is is quite diverse it goes from very low ,at times sub planing after snaps etc then quickly up higher on bottom turns etc but very rarely to extremly high during turns

Just look at the speed that you go into a full roundhouse cutback then compare that to the speed that you have at the end of the cutback,it is rare for it to remain consistant throughout,

The camber position you discribe for speed if combined with the right leading edge curve as you will have on your sensitive and lifty fins can all be place into one multi fuction fin

These observations are based on years of sail designing,hang glider designing,aeromodeling and surfing all of which are at low and varing speeds

I belive that most common fins would be more suited for faster applications

In general most fin designers seem to think allong the lines that thin and streamlined is the only fast way to go

I tend to think that its more about lift than streamlining

Am I right --god knows it just seems to work for me,I guess in this forum we are narrowing it down to the answer all the time

Cheers Mike

Hey Mike,

““I belive that most common fins would be more suited for faster applications”” Help me out. I don’t follow you on this one.

If it’s more about lift than streamlining for you so be it. F

For me it’s trying to fit everything together. I know it’s impossible but they say adversity builds character.

So what you think about the fact the on a blue and leopard shark’s pectoral fins (these guys swim at an 5 to 25 knots most of the time, which is the same speed range as surfers are in most of the time) the thickest part is about 20% back off the leading edge. Their fins are very thin and the leading edge is nicely rounded? They’ve been working on it for 409 million years. Might they have it right? Well whether they are or not that’s what I’m goin for on my center fins except fot tow-in fins which will be foiled like you suggest because there speed is the issue more than lift is.

My feeling is that as you move the vertical cord of the fin back from about 1/5th of the distance from front to back that you decrease the area of the fin that produces drive. After all you do push off the back of the fin not the front. So everything behind of the apex of the camber is where the drive comes from – or does it. Hell I don’t know – but it sure seems that way to me.

As far as rail fin foils go, fish and sea mammals don’t have undercambered fins because the have now need for them. My take on how the under camber should occur in the fin shape is that it should be a relatively symmetrical part of the fin itself not cut into it – that the under cambered shape should twist the fin in the direction of laminar flow so that’s how I make my rail fins. Maybe someone else is doing the same thing but I haven’t seen it yet.

Anyway I have a hell of a lot to learn. Thats’ why I keep comming back here. You have a lot of really interesting things to say mate.

Mahalo, Rich

interesting so far…

some else had posted this link…very relevant to this discussion

http://www.vacantisw.com/foildesign.htm

I’ll be making a refresher pass on it

Bert, a point to think about.

Flat sided assymetric foils, and concave foils, both have different “minimium drag flow” angles at each end.

If you simply draw a cross-section, you can see the front 25% of the foil ‘points’ in a different direction than the last 25% aft. And, btw, the front edge should be radiused and point in the same direction as the front of the foil or you have conflicting design elements and sub-optimal performance.

Back to the point, because the front and rear edges are not aligned, there is no reason to expect them to be optimized at the same toe-in angle as a double foiled fin. A double foiled fin should be toed in MORE. An assymetric foil should be toed in less for optimal performance. The more assymetric (with respect to leading-to-trailing edge alignment), the less toe.

Keeping a rail fin flat sided, but making the foil thicker, will also increase the assymetry.

One thing to think about is using a system like the Edge fins or Swivelfins which allow substantial changes in toe-in angle. Then, on one board, you could appropiately set angles for thin concave fins, thicker assymetric fins, and double foiled fins. It is a HUGE confound that these are all typically run at the same toe-in (roughly 3 degrees).

HTH.

I’ll post the pics I’m referencing from meecrafty’s link above just to save the hassle of going back and forth between pages.

Figure 2:

Figure 3:

Quote from link:

“Figure 3b plots for each NACA foil type the lift-curve slope versus thickness as percent of chord length. Note that for every NACA series except the 00, the lift-curve slope increases with increasing thickness, But there are practical limits to how thick a foil can be made without incurring large penalties in drag, since friction drag also increases directly with increasing thickness.”

This data seems to suggest that when increasing the thickness as % of chord of a foil that it would be better to move the wide point further back relative to the chord length. Look at how the lift curve for the 00 series drops of with increased thickness while the 63 series increases with increased thickness. As had been discussed in some earlier threads by moving the wide point further back you are also keeping the boundary layer attached longer preventing flow seperation and the induced drag that comes along with it.

What does this mean relative to fins? It looks as if it would be better to move the wide point further back as you increased the thickness relative to the chord to maintain a better lift curve, from what I can gather from the information above.

edit: Blakestah, what you posted above seems about right with my experience with my FCS FG-5 fins. When I first put them in my board I noticed they seemed to have more relative toe-in due to the camber. When I rode them it felt as if the fins had too much toe angle on them, seemed slow in a straight line but turned quite well of the top and bottom. That was on a very weak day though, so that didn’t help the matter any. Curious to see how they go in something with a little more power.

Begging your pardon, but I noticed a bit of a conflict…

meecrafty wrote:

“This data seems to suggest that when increasing the thickness as % of chord of a foil that it would be better to move the wide point further back relative to the chord length. Look at how the lift curve for the 00 series drops of with increased thickness while the 63 series increases with increased thickness. As had been discussed in some earlier threads by moving the wide point further back you are also keeping the boundary layer attached longer preventing flow seperation and the induced drag that comes along with it.”

While the article states

"The most discernible difference among the NACA foil series is where the position of maximum thickness occurs along the foil; it can occur between 30 and 45 percent aft of the leading edge. NACA researchers found that positioning the maximum thickness farther forward results in a higher lift-curve slope. For example, the maximum thickness of the NACA series 63 is 35 percent aft of the leading edge. It has the highest lift-curve slope of the foils examined here (Fig. 3). The maximum thickness of the 66 series is 45 percent aft of the leading edge; it has the lowest lift-curve slope of the G6-plus series foils.

NACA also found that the farther aft the maximum-thickness point occurs, the lower the theoretical minimum friction drag. This fact is highlighted in the preceding table of drag coefficients; the 66 series foil has the lowest friction drag and the maximum thickness point farthest aft.

A designer’s choice of foil type must balance development of lift force per degree of" leeway as expressed in a higher lift-curve slope against the potential for increased theoretical friction drag, which accompanies high lift characteristics."

Carry on, this is intensely interesting, even if I don’t always know what’s going on…

Wells

Ah, S#!t, I told you I didn’t know what was going on… the above post should read “lawless wrote”, not “meecrafty wrote”, sorry, wells

This is good, it follows closely what I have experienced. Lawless: the FG-5 double foils work really well in power. They do feel like they have perhaps a slight bit of added drag in straight-line trim, but in powerful or fast waves the added umph you can put into turns is a blast, and gives a little more squirt/thrust to at least make up for it. To me it’s way more fun to push the G’s in a whole series of hard turns than to just go straight anyways. In keeping with Bert’s thoughts here, the worst fin set I ever tried was the FCS carbon Merricks, which were also the thinnest fins I’ve tried, with very little camber apparent. With Bert’s supporting logic of the thicker foils giving greater lift, AOA and maintained speed through turns making sense to me, I’d love to find a set of thicker foils made for FCS plugs. I’d enjoy giving them a go.

Hey Wells,

The boys over there in the red corner talk quite a game I must say.

May I join you as a trainer in the blue corner. I believe we may have a true contender in the making old boy. If we can get him to bob and weave and counter punch I believe he has the talent to make something of himself.

Genius! “” by moving the wide point further back you are also keeping the boundary layer attached longer preventing flow seperation and the induced drag that comes along with it.“”"

Yes, pure genius!

But we must keep him lean now – no extra girth – it will only impeed his foot work.

We may be on the world stage before you know it!

Spartan lines and fit for every test.

Yes! that should work. Oh my, He want his trunks down to his ankles, what ever shall we do?

Tally ho! Rich

I was specifically comparing the 0010 foil (which to me looks very similar to many “stock” surfboard foils) as opposed to the 63 series.

Quote:

The most discernible difference among the NACA foil series is where the position of maximum thickness occurs along the foil; it can occur between 30 and 45 percent aft of the leading edge…

30-45% seems to be further back than most available foils was the point I was trying to emphasise.

Quote:

…it has the lowest lift-curve slope of the G6-plus series foils.

The paragraph you listed was comparing the 6x-010 series and doesn’t mention the 0010 by comparison. It’s comparing 45% aft thickness relative to the same foil with 30% thickness aft. Seems most surfboard fins are around 25% but I’ll have to measure and see for myself. If you look at the graph you can see the 0010 drops off while all the 6x series increase.

Thanks for pointing it out though, made me go back and re-think it all again.

BTW Wells, you can hit “Edit” and change the name in your post.

It IS much funner to link turns with the FG-5s, but it was a weak day and I was trying to link sections with limited success. That’s where I could really feel the drag of those fins.

LOL Halcyon, have you been hanging out with ambrose?

The ‘Vacantis’ information thus supports Halcyon’s experience that a fin with the maximum thickness closer to the leading edge has high lift, while one with the point of maximum thickness further aft has lower drag. This is the assumption which I have been using also.

I have been using relatively thick single fins to increase their tolerance to high angles of attack. If the ‘Vacantis’ information is correct then I have in some cases been making my fin bases too thick as they sometimes exceed the 13% ratio suggested as a maximum. Some of my fins have had a ratio of more than 15% at the base.

It is also interesting that swept back fins are likely to be less tolerant of high angles of attack in the tip area (according to the 'Vacantis information). I have always used upright elliptical fin planforms for just this reason.

Thankyou to all who have been posting this fascinating material.

R.S.

Okay, at this point in the discussions about how and why we foiled our fins the way we have I usually refer back to my notes on foil theory.

When we first started Red X we thought alot about making our foils from the theoretical formulas such as NACA-4. But, then we figured that if we used the foils from the recognized master shapers, the foils would have tens of thousands of hours of testing and proven performance. So, our first master came from Maurice Cole. Then our second was Al Merrick. For our third we went back to Maurice. Then T&C sent us their template. Timmy Patterson followed shortly thereafter. Matt Biolas gave us our twin fin template. And we’ve kept adding templates in this manner with the same logic.

Now I look back at what may have been produced if we went with say report 586 Variations of the Reynolds Number. We first we need to determine a Reynolds number for our typical template. Where v=speed of vehicle (20MPH= 352 in/sec)L= cord length of foil (4.7")p=density of water (0.577 ounces/cubic inch)u=dynamic viscosity of water (.00002325466 ounces/ square inch) g=acceleration of gravity (386.088 inches/second squared)

RN=vLp/ug or (3524.70.577)/(0.00002325466*386.088) or RN=106,352

Reynolds Numbers of 100,000 or less fall into the definite laminar flow range. Reynolds Numbers greater than 1,000,000 fall into the turbulence range. Everything in between is “in transition”. So, thruster foils fall in the bottom of the transitional range as they approach max speed.

Now, according to the NACA Report No. 586 the accuracy of their airfoil tests:

“Hence airfoil characteristics dependent on the shape of such curves, e.g., the optimum lift coefficient and the aerodynamic center position, are considered unreliable and in most cases are not presented below an effective Reynolds Number of 800,000” (pg. 230)

To interpret NACA foil section code:

First digit equates to the foil series number

Second digit equates to how many 1/10th of cord length the position of minimum pressure is aft of the leading edge.

Third digit indicates how cambered the centerline of the foil is.

The last two digits indicate the foil thickness as a percentage of cord length

Lift-curve slope vs. friction drag

In general as thickness increases lift curve also increases

However, so does friction drag

The closer to the leading edge the position of minimum pressure occurs the steeper the lift curve & the greater the friction drag

Optimum position of min. pres. is between 30 and 45 percent of cord length

Finer leading edge radii create less drag at 0 degrees angle of attack and more drag at angles between 3-8 degrees

Thickness from 6 percent to 13 percent of cord length. Greater percentages = greater stall angle.