Fin Foils - Flat Edges on Rail Fins

There seems to be some disagreement about why the majority of rail fins have flat foils on the inside edge. In this thread:

I think that this discussion warrants it’s own thread. So I will ask my questions here.

  1. Why do the majority of rail fins have a flat inside edge? Is there a performance advantage? If so, what is it?

  2. How do boards surf different when the rail fins have foils on both sides? I noticed that Lokbox has the option to have 80/20 foils on these fins. How do they effect performance?

Most of our front fins still have flat sided foils these days. For quads we do mostly 80/20 foils on the rear fins only, or certain templates like the Stretch set-up having 50/50 or symetrical foils for the rear fins. We do some custom sets with inside foils (mostly thrusters) as per request. I’m sure this will be a good discussion here with tons of info and theory added, and I’m glad you posted it up. For many years the only time you saw inside foils on front fins were on tow boards or possibly guns. They are now being incorporated into shortboards at a fairly fast Rate. FCS has their “F” series, and futures has the “Vector”. I’d be very interested to hear some feedback from people that are using them, or have tried them. Especially if you’ve tried both. After some responses I’ll post up some feedback from our customers and riders, as well as some of our theories. Cheers Jim

I’m a newbie here but have been lurking off and on for awhile…basically when i’m bored at work! Thought I’d give my 2 cents on fins and the difference in ride.

I mainly ride a 5’9" twin fin right now. the fin set up i originally put on it had a very slight to non existent inside foil and a sharper leading edge. Worked ok but didn’t seem to have the bite that it should. so first i sanded out a little inside foil. Then i took out some of the sharpness out in the leading edge. A good friend of mine layed it out like this… think of a fin as the wing of an airplane. wings don’t have sharp front leading edges and they are not flat on the underside. With a smoother leading edge and inside foil, the water breaks better along the fin and produces better thrust (lift). My set up now has a way better bite and a hell of alot more thrust.

How about when you want to make a really tight turn into an oncoming section? If you watch the pros closely you’ll see that they almost powerslide their board through sections and turn basically on a dime. Although I definately need more test time on inside foils, initially I’ve found them wanting to drive around sections rather than up into them. Commercial airplanes are designed to provide a “smooth” ride for the passengers and just don’t turn that radically. The leading wing edge of a T-38 trainer is very sharp, and pretty flat on the underside of the wing. It is also thinly foiled. Designed for manueverabilty at high speeds.

While the leading wing edge of a stearman biplane may be more rounded, it is still relatively flat on the underside. Fuller foil thickness as well. Designed for manueverability at low speeds.

Something to think about…

Here’s a pic of a stunt plane showing a more rounded foil and leading edge close to the fuselage (base) and flatter out towards the tip.

while it’s great to show all these airplane foils, and much of it is valid, seems to me that we ought to be looking to the mammels and fish that live in the ocean and see what they have going on. thousands of years of natural evolution in the ocean has probably produced the best foils for moving through WATER yet.

Touche! but the fin setup today are a hybrid of the two. You’re not going to ride a fin with the characteristics of the biplane. fat and round, it will just take a crap in the water ( no turning point no drive surface), and you’re not going to ride something so angular as a fighter plane, its to jitterie ( take a knife and pull it through a bucket of water, it jumps and kicks and doesn’t want to follow where you want to lead it). I just find that when you buy a new fin set up, the leading edge is to sharp. Lightly sand that edge down (nothing blunt, just mellow the edge very slightly, a few pass of the paper) and add a little concave, nothin harsh, just a very…very slight swoop. But then again why am i trying to battle theories with Lokbox, you’ve applied more theories than i have. Just seems to work for me, could be all mental

Damn images won’t upload from comp. oh well

They are trying to do that. can’t remember who but someone is trying to make a quad setup with the characteristics of a tuna. a tuna’s fins working off each other to produce more streamline and speed.

I am a big fan of inside foils.I have not bought fin with out it in at least a year.

Airplanes stopped using a flat bottom foil in the 1920s.

The fins don’t stall and lose water flow around them.Most of the time on waves with power I don’t want the fins to stall-slide and recover.It feels like in a steep wave I can get higher on the wall.This allows me to make waves that were considered close outs before.Speed!

Some options.

Yes I like big fins.

Just my 2c,


I hope people are clear when they mean inside foil convex, inside foil concave, and inside foil flat.

A rounded front edge rounded to half max camber in diameter, combined with a flat inside foil, and an outside foil with max camber at 25% (rear edge thickness 0.030"), seems to work quite well across a wide wide range of conditions. And if you order a glass fin from a major manufacturer and do not specify the foil, that is likely what the rail fin will come with.

Some people like concave inside foils in smaller waves, or with a heavier rear foot to force the water to flow more into the inside of the fin surface.

Some people like 80/20, esp in bigger faster waves or with a heavier front foot and the water not flowing as hard towards the rail.

Flat inside foil generates more lift when the inside of the fin is the high pressure side, compared to a convex inside foil. Concave inside foil generates even more lift, but it sucks (literally and figuratively) when it is the low pressure side. In smaller waves the water flows more towards the rail and the outside rail fin spends less time at negative AOAs, and both fins are positive AOAs when you go straight.

In faster waves both fins are negative AOAs when you go straight and the outside rail fin is HARD at negative AOAs, so a concave inside foil pays a big penalty.

hth, but if it doesn’t just make a bunch of glass fins and go surfing and see how full of crap I am for yourself…


while it’s great to show all these airplane foils, and much of it is valid, seems to me that we ought to be looking to the mammels and fish that live in the ocean and see what they have going on. thousands of years of natural evolution in the ocean has probably produced the best foils for moving through WATER yet.

Good point. However, most dorsal and pectoral fins act independantly of the body (fuselage) as they are flexible as is the fuselage itself, which means constantly varying AOA to say the least. If you took the dorsal of this shark and put it under his belly you’d basically have a thruster set-up with all dual foils(and three more small stabilizers and a tail) to help maintain direction.

Manueverability (tight turns) I’d guess would come from the flexible body, with all the fins helping maintain direction. Not a good comparison IMO as surfboards have completely fixed (non moving) fins mounted on a ridgid non-moving body(Blakestah will love this lol). Just for laughs here’s an artists rendition of a speedialer set-up on a boat lol.

…so whats your opinion about bonzer side runners?

double foiled or the way that you describe here?

seems that no one tested that

If you archive this subject to reference…I’ve been back foiling my fins for over 6 years now,with Tomatdaum/Red-X right behind me.

It’s works,…you guys are catching up.I must say…lol.Herb

Don’t get me wrong, I feel there is most definately some validity to inside foiled rail fins. My question is more aimed towards what type of surf would you want it in, and what benefit will it provide. Considering water is around 700 times more dense than air, there is limited amount of reference we can use to the aviation industry. Surboards are just their own beast. Thank god…

Manueverability (tight turns) I'd guess would come from the flexible body, with all the fins helping maintain direction. Not a good comparison IMO as surfboards have completely fixed (non moving) fins mounted on a ridgid non-moving body(Blakestah will love this lol).

There are actually interesting differences in best template and foil when you let the fins turn in toe-in…some aspects of the foil and template exist to mitigate the negative performance attributes of fins at times between turns when they are hurting not helping the surfer.

Surfboard rail fin templates have a pronounced U shaped chord length curve. Long chords at the base, then shorter about 2 inches up from the base, and then longer again. If you ask all the people who want to use NACA foils on surfboard fins why this exists they cannot tell you. FCS designers actually took it out in the H2 series. But if you do something else, on a board with fixed fins, performance gets worse in specific ways.

You can also push the max camber further forward, 20% works noticeably better than 25% if the fins can rotate. There is a performance gain on turns and a loss between turns if you do that on fixed fin boards.

After 5-6 years of back foiling my own personal fins/ boards,it is very evident to me Lok, that it works ,

They’re faster,and control the displacement much more efficent=less resistance.

Negatives,ya, sure=less hold,less vacuum.

In bigger,I mean really good,serious size surf and /or steep,sucked out sections(w/ 20+/80 back foiled side fins) the sidefins lose there vacuum=soapy/slides/spinsout.For a bigger wave board or a more sucked out/square wave board,5-7% in the back foil is all you need.Less that 3%= has no felt difference at all.

If You want to talk to me more on this, let me know.Herb

I actually ride both, and just wanted to see some discussion on the subject Herb. I know many people here don’t really care about what the pros are doing, but out of the top 15 or so, the majority are still on glass-ons with flat foils. Surfings progressiveness is at an all time high no denying that. Dane Reynolds isn’t on tour, but highly considered as the best free surfer in the world right now. Glass-on guy with flat foils. However, standard flat foils didn’t work for Jeff clark at huge mavericks. Too catchy I reckon. That’s the point. Where are you surfing, and how would you like to surf while there?..

There’s some great back threads on this subject.I’ll see if I can locate them.

Glassons are consistant and keeps designs simple.

Of those 15 pros you spoke of:

The contour on there outboard fins may indeed have flat inside foils , but I bet if you took a closer look, you’ll find out that the leading is tapered back about 2-4%.

Jeff Clark,to say the least is an exceptional surfer, and I value his judgement w/ high-regard.

I wonder what his back foil ratios are ? And if the foil of the general insides are flat ?

I personal don’t recall them to memory.Herb

Blakestah is there a prevoius post (in the archives?) that you could direct me (and other beginning fin experimenters) too or something in the resouces that will explain the turms that you use so we can follow???

ie… chord length, U shaped chord lenght curve, chords at the base(?),

Is there a beginners fin fabrication resource? That would be GREAT!

I make balsacomp long boards and the thick (1/2") fins i’ve been making, especially the design you posted a few months back, have been working GREAT! Thanks! But I’d like to learn more!?#$%&^*! (it’s fun experimenting!)


chord length is the distance from leading to trailing edge of the fin. The “base width” of a fin is the chord length at its base.

The U-shaped chord length curve is a plot of the length of the fin’s chord at different depths from the base. Because of the rake and tip shape of a standard thruster fin, and the curvature of the trailing edge of the template, the chord lengths decrease from the base to the middle of the fin, then increase towards the tip, then drop off.

Both FCS and Futures made fins that worked off this. Futures adjusted cant at different depths so that the long-chord tip area was much more canted than the base, and made the base foil concave on the inside. FCS got rid of it entirely with the H2 series.

It is an interesting surfboard specific thing. It is not found in nature much, or in airplane wings, or really anyplace except a surfboard fin.

I’ve evolved to doing all my fin design by

A) choosing a leading edge rake, and then

B) choosing chord length at different depths.

C) riding it and going back to step A.

On a thruster fin the base chord contributes a LOT more to drive and thrust than the tip (and drag when the fin is not the inside rail fin - going straight and as the outside rail fin). The tip contributes a LOT more to hold (and drag in turns). You can change these fairly independently and get a fin that goes from good drive/poor hold to good hold/poor thrust.

Camber describes the fin thickness as a function of position on the chord. If I say “max camber at 25%” that means the thickest portion of the fin will be 25% of the distance from the leading to the trailing edge.