Fin Foils

Ive always wondered what would be the result of having fins foiled on both sides for the side fins as well as the rear fin?ive heard ppl talking about lift but i dont get how that applys to surfboard fins?can anyone explain in laymans terms what the advantage is of the single foil for each side fin?and how it is designed to work?im gonna have to try this i think to see for my self…but id love to hear the theory behind how we ended up with single foils for side fins if anyone has the time to explain it.

There are two possible reasons that i can see, one is a huge performance difference(id have to assume that when they first started making thruster configurations that at least some ppl made double foils for the sides)until the break through.

And the other is very little performance difference and the side fins went to single foil for pure economic reasons(as well as/or for marketing reasons…new tech).

ok talk about “missed the bus”, broke the cardinal sin and didn’t do a proper search…my humble appologies.

Hey Hunty, that was a nice short thread.

You’re pretty lucky you didn’t get inundated with ‘check the archives’ posts.

Actually lets keep this poor little thread alive for a bit longer.

The reason you don’t see a bunch of tri fins with two sided foils (foils like the trailing fin) is…


The pressure diferential each side of the fin experiences has a purpose. I’m not gonna get into the super involved conversion covered in other threads but I can tell you from prsonal experience that when I decided to build a thruster way bac when and put foiled fins in the side boxes, I was the spinout king at Rincon that day…no one was even close.

But we also learned how different elipticals worked and it wasn’t the problem of the symmetrical foil. 60/40, 70/30, 80/20…they all do their thing. You will note that modern day fins are being offered with a little bit of foil on the inside of fins these days with the research claiming more speed as a result. It is also easier to go rail to rail with this type of foil.

So everything has its place…let’s say you have a board that feels too loose…maybe try a higher area fin with a flat foil on the inside…this will provide you more leverage and stability. If the board is still a spin out king think about sticking a canard in front of the fin(s) to feed water to the light side.

yeah, like :

read the why is it important that fins have lift thread…

Where is Otis anyway…?

Why aren’t trailing fins concaved? Water always flows at some angle into the leading edge, so why not?

This recent thread has some very good info.

From what I understood, singlefoil sidefins help you generate more speed while pumping your board (short conclusion).

Yes… that’s a great thread.

My idea is that the trailing fin is primarily for drive and directional stability. If you took a double foiled fin with convex surfaces, and sort of hollowed them out to create a subtle concave on both sides, wouldn’t it still provide directional stability, while increasing drive? Water rarely flows directly head on into the leading edge of the trailing fin, so the concave would always be working on one side or the other. On the opposite side, you’d create some drag, but not much.

…back fin is for stability, hold and is like a rudder/helm

main fins are for direction, drive

A term like drive is used a lot and often its meaning is clear… well kind of. Over the years, for me at least, it has become less clear. How are you using the term?

Also, how is the ‘direction’ function different from ‘rudder/helm’ function? Also, by stability do you mean ‘ability to stay on course’; and is it ‘stability and hold’, as in hold a course, or is hold referring to something else?

I’m not disagreeing with you, (at the moment I not sure I can) I’m just suggesting that you haven’t gone far enough.

The role of foil can be broadly divided into two applications, one structural and the other hydrodynamical. Regarding its structural role, in general a foil enhances the rigidity of a structure, that’s true before any hydrodynamic considerations are taken into account. Then there are the hydrodynamical effects, which run the spectrum from merely streamlining (reducing eddy production) to some funky lift/drag effects.

But it really doesn’t stop there, for over the last half of century we’ve seen a evolution from a predominately almost flat streamlined center fin to much smaller often highly foiled multiple fins, which are generally now toed, as well as canted, and they’re spread out over the bottom of the board – that is, they’ve become smaller, there’s more of them, they’re being placed at funky angles and they’re spread out.

But I’d be happy if you could help me with what you mean by ‘drive’.


IMHO, drive is the force that the rider imparts on the board, and through the fins, to increase the pressure differential between the inside and outside foils of the fin.

Kind of like how a bird flaps its wings to keep flying.

The surfboard designer, and fin designer can help to optimized this transfer of energy, from rider to fin/board surfaces by manipulating area, placement, orientation, and foil of the surfaces (fin and board).

I’m talking about drive as in holding speed through a turn, and speed down the line…

My experience has been as you move all fins back, both side and trailing, the more drive you get, but at the expense of sensitivity, or “looseness.” So it stands to reason, at least to me, that if you concaved the sides of the trailer, less water would slip off the fin during a turn, and an increase in drive would result.

I think I understand the first part of your post- it’s close to my impression as to what is often being referred to. (Being the penis that I appear to be, technically, it’s the displaced water that’s imparting the force – your changing the momentum of the water, and it in turn is changing yours.) I do have some difficultly with the rest of your post however, which is not to imply that it’s incorrect.

… the flow decreases?

Consider a big bottom turn. At the bottom, though you don’t come to a full stop, you do dramatically slow your forward motion. And though you might be inclined to speak or think of heading back up the wave, what is actually happening is that the wave is coming towards you, and once it gets to you, the flow up the face lifts you back up, or you re-engage the flow in some other way. Usually you’ll have some lateral kinetic energy left after the bottom turn, but that’s not going to send you back up the face. Mind you, it feels like ‘drive’ or a some kind of propulsion, especially if the timing and execution of the turn is done well, but such impressions can be deceptive.

I guess my point is that as you complete the turn the flow under your board is actually reducing (that’s near the end of the maneuver.) I know that sounds crazy, but if you watch some video’s you’ll see the transition period and occasionally see all the spray and foam stop from coming out from under the board – it’s as if the surfer/surfboard is waiting for something to start up again, or even better, perhaps you’ve experienced it yourself. If you get the timing and execution right the ‘gap’ or transition period virtually disappears – it looks and feels as though the turn, from start to finish, and on to the next maneuver, is seamless.

… so fins help

I don’t doubt that fins help in pulling off such maneuvers, and that particularly goes for multiple fin setups with toe, etc. -i.e. they allow you to make more efficient maneuvers with out losing the wave, or getting to far out in front of it - its just that the term drive tends to be more of an ‘impression’ rather than an accurate physical description of what is happening. And in order to sort out the role of the design elements, it’s the more actuate physical description we want.

… stepping back

There is actually a lot going on here, especially if you stand back and look at the evolution of fins from single plates to multiple small fins with some pretty interesting orientations and placements. That is fin systems have evolved, in particular by becoming smaller, by increasing in number, by changes in orientation, and by becoming more spread out. There have been, are and will continue to be exceptions, but that has been the general trend.

… what’s the governing this?

What’s governing the evolution? What’s driving (… oops, no pun) their evolution? A combination of enhanced structural properties (foil as a structural element) and the strategic and balanced application of drag (the lift being of no of little consequence), or is it something else? At present I believe that their evolution is dominated by the former – that is, aside from structural and streamlining (reduction of eddy formation) the role or importance of foil is limited.


Actually, see my comments regarding the example of the bottom turn in the above reply to obproud.

… I like it, but

The idea of concaving both sides of the fin is an interesting one. Though my reasoning might not be the same as yours. If you screw with the water you will pay a price, and that price always includes increasing drag. If done subtly enough you might be able to adjust the drag - if increasing a bit of drag is called for. It’s likely that the effect will become more pronounced at higher flows, so that might be a good deal for say single fins (longboard or short.) In the end fins are passive, you get to play with what you bring to the water and those curves which interact with the water, have a way of forcing you to surf a given way, or your forever fighting the flow to get it to do something other than following the curve it’s being presented with. Then again, this is kind of general problem in surfboard design, so perhaps there might be a compromise to be had, as has been the case with most curves on surfboards. How to scale the curve in relation to fin size etc. would be interesting if the effect you’ve described is the desired one.

… looseness

Your point about looseness and the loss of sensitivity is interesting. Fin placement relative to the center of gravity of the surfer/surfboard is critical. The farther away from it you place the fins, the more of an impact any interaction with the flow will have on the surfer/surfboard. Think of it like a lever arm, the longer the arm, the less applied force is required to move some mass. This of course is a two way street. The farther back from the center of gravity your fins are, the more force you’ll have to apply to get the desired effect. I can see how this would be interpreted as the loss of looseness, or loss of sensitivity.

... the rule for viscous mediums, or now I'm just being picky.

All that said, your take on drive is a bit different from what I've managed to get from others over the years, but it's interesting. I think I know what you mean, but technically, maintaining speed in a viscous medium can only be done at a cost. The problem is that the presence of fins in general contribute to that cost. I'm more inclined to believe that fins allow you to maneuver more efficiently then compared to just using your rails alone or concaves as 'fin planes' for example, thereby reducing the loss of speed, allowing for a better timed and better executed turn. So perhaps, this is what you meant by 'holding speed', its just that you don't, maintain speed speed that is, you always loose speed, unless something is propelling you and is doing so in a variable manner. Again see my comment regarding a bottom turn in my prior post to obproud.