Trade-offs: Modern Toed/Canted Multiple-fin Systems

Interesting take.

As to my view of the flow, please consider visiting a recent (relatively unvisited) thread entitled '...Flow Below' by me. It probably hasn't made it into Swaylock's bit-bin yet. But I've been arguing a similar take since my Rocket Science threads, pre 2000? Which have likely made it into the bit-bin. I'm not to sure that was the point of this thread, but you objection is not unreasonable, Thanks.

By the way, it would be interesting to read your take on Carswell's “Hydrodynamics of fins”, and how it relates to your product. In particular, how Carswell's data jives with what your product offers in terms of angle-of-attack.




This is one of those subjective statements – if you grant my point about effortless acceleration, and give it an equal joy value as turning, the argument you’re making starts to elicit “Huh?” reaction, at least from me.  Contests are one thing – you get points for fans of spray – whoopee! points! –  but the moments between turning maneuvers and lip blasts, if those can have the fun left in them…  I think that’s what I take from Lokbox’s comment too – quads’ rears, being angled into flow even in trim, and having a better turn-in moment and better AOA through the maneuver, can give both.  Speed and control at all times.

As a backyarder, thrusters have this appeal for me right now because of the fewer variables, but really I suppose I should tune my quad set-ups and tails toward similar pro shapers’ models with proven performance – the kind I want.  I had thrusters and I never liked them, then I shaped my first as a quad, and got one from Greg Tate, and they immediately blew me away with the speed/glide and lack of rear fin drag.  Having both the drive fins angled properly makes a big difference–the drive side seems to overwhelm the toe drag on the non-drive side --you don’t feel the toe drag on the non-drive side until youre going straight.  whereas with a thruster, you can feel – and push off, to be fair–the drag from the non-drive fin and the rear fin most of the time.  I don’t know–I start to think it’s the rear’s drag actually that sucks.The non-driving fins are still providing the control aspect you talk about.  I supp if I want none of that, I should get into singles, which I very well may do.


Thanks for weighing in Jim


Lucky for us, we at least have “a” possible physical baseline to
assess what benefit some configuration might, at least potentially
offer - Carswell’s paper on the “Hydrodynamics of Fins” for

That’s not to say, personal preference isn’t king, just that if a
point can be supported with a little data, all the better, and a
paper like Carswell’s provides such an opportunity, if your so
inclined. (Wish I had it a long time ago.)

There is truly something to quads, it would be nice to be able to
point one’s finger to it, though. This trend, and it may not be, of
moving towards configurations of more fins, toed and canted, and
generally smaller in size is kind of interesting. I’ve surfed quads,
and I’ve both read and heard the testimony. But to what is actually
happening dynamically remains kind of fuzzy, to me at least.
Carswell’s paper goes a long way towards providing the data to begin
to build a model.

As to my take on the flow, please see my response above. It was
not my intention to imply the flow is parallel to the stringer, nor
to be comprehensive. It was not the intention of the thread, but your
interpretation or reaction, as was that of others, was not
unreasonable. Thanks.



You’re right, it should have read “…trade-off, if any.”


Missing, in this discussion , is the grasp of the ''relative flow'' of water over the fin as it moves THROUGH the water.    In other than a skidding ''radical''  maneuver, the relative flow is always parallel to the centerline of the board, as it moves forward over the water.  The deflection of water toward the rail, is a shallow surface phenomenon, though it can contribute to a ''root stall'' at the base of the fin.   This will, many times, grow into a full stall over the entire fin.    Granted, there are many directions of movement, of the water in a wave (eddies, boils, etc.) but the significant one to pay attention to is the relative flow of water, which is a function of your direction of travel on the wave.    It is that relative flow that most fully engages your fin, or fins.    Much of the rest, is like counting the number of angels on the head of a pin, in my opinion.    Interesting, but of little significance, in the larger picture.  

I think you could possibly disprove this by getting on a surfboard and leaning down to trail a string or a rope in the water as you trim down the line (on something more than a ripple I suppose this would work)

Aloha Kcasey


Assuming the board is attempting to support the riders mass then wouldn’t the direction of flow at zero forward velocity, be in the opposite direction of the board sinking into the water until buoyancy is achieved.  In this sample, case that would be (with good balancing) perpedicular to the bottom and upward against the downward sinking of the rider and board.

As Blakstah pointed out, as the board began to move forward the flow would be upward and backward and out to the rails as the flow “leaked” off to each side of the board. See my drawings below from a past discusson on this subject

[img_assist|nid=1044246|title=Fin Foils|desc=|link=none|align=left|width=640|height=361]

As the forward velocity increased one would no longer be looking to buoyancy for support but rather planing.  At higher velocities the impact force of each molecule of water hitting the board would increase allowing less water to support the riders mass or weight, rising him out of the water.  This allows the angle of attack to go from 90 degees for buoyancy support to maybe 20 degrees (I am just guessing) for planing support at some level of trim as the surfer compenates for his mass which is pulling downward, with an upward force of the water hitting against the bottom of the board. 

While there is much more to all these flows … this one described above is the foundational one that has to be recognized foremost.  Without it, all other flows become confused and meaninless.

Even in a full rail turn, rider mass is the primary force directing the water flow.  Even though we can see in photos, water spray being slung off in different directions, it is important to recognize that somewhere underneith it all, there is a near equal and opposite flow to the riders mass, that has to be there to compensate for the riders mass.

This is why there is value in the “double pump bottom turn” that Janklow mentioned.  The riders mass in increased by centrifugal forces in a bottom turn.  (It is so powerfull that you can increase it to the point of stalling or spinning out.)

In turn, the waters force on the bottom and rail of the board has to increase radically to support this extra and opposite “rider force”.  For most, much of the waters extra force, is lost in the average surfer not knowing how to utilize it well.  BUT… If the rider backs off his growing mass, just slighlty in middle of the turn, he can “capture” so to speak, a big burst of energy that was/is building under the board, before the water realizes it doesn’t need to be that forceful and poops out   A skilfull rider, depending on how good he is can use this for all kinds of cool moves.


Under these conditions the direction of the net force developed by the laterals of a toed/canted mult-fin system, as they are commonly installed, is both backwards towards the tail and down into the wave. The backwards towards the tail is the result of the toe, the down into the wave, a result of the cant.



I don’t think I agree with this.  I think the force is, upward and outward.  Not down and back.

Bill Barnfield,

Don’t you also have to consider the moving water?

Granted the ‘relative flow’ from the board and fins moving through the water is the primary flow to be considered, shouldn’t we also consider the flow that is moving up toward the sky and off the rail?

This flow, IMHO, definately changes the flow vector.



Don't you also have to consider the moving water?

Granted the 'relative flow' from the board and fins moving through the water is the primary flow to be considered, shouldn't we also consider the flow that is moving up toward the sky and off the rail?

This flow, IMHO, definately changes the flow vector.




I'm not answering for Mr. Barnfield, but adding my resounding YES, to your question.     Any decent bottom turn, in a sizable wave, reveals how significant that water movement is in relation to the board/rail/fin combination.

Aloha obproud

Yes of course any water moving in any direction will have to be accounted for when establishing or discussing the “theories” regarding what forces propel a surfboard.  But when considering the flow along a surfboards bottom it is less complicated.  And the flow you are speaking of, for example, is accounted for primarily by the Rider adjusting the boards trim according to any flow necessary to get from point A to point B. That flow you mention, while it surely exists, exists from a surfboards point of view, relative to where the surfer is pointing the board.  In reasonably proper trim, the water flow will generally be as I have drawn above, or shifting from side to side a bit.  

If not, the surfers weight would not be supported and the board would not be moving forward.  The flow will most always have to be, centered and focused such that it counter balances the force of the riders mass.  If the rider is generally centered on the board then the opposing force of the water will have to be pretty much centered and opposite the Rider.  

Fins add another dimension to this because they also gather energy from the waters flow.  If enough force is being gathered by the fins, then the apparent balance of the riders mass can seem to shift out of alignment somewhat.  But it isn’t, or is only momentarily so as the fins do their thing.

Sometimes the waters flow is best used in perfect alignment and harmony with the direction of the board.  Sometimes energy is gained from tapping an opposing flow or at least what seems like an opposing flow.  Altitude gained on the face from the upward flow of water on that face is a perfect example.  But an opposing flow can rarely be maintained for very long before other factors, like maximum trim height will require a more complementary flow.

Certainly in contemporary surfing, riders are all over the wave and often even the air.  Their boards are, for short moments, at all kinds of contrary angles to the water flow.  But to support the riders weight requires high velocities of flow to generate the needed planning force.  And to get that rapid flow, the board has to spend a majority of its time in reasonable harmony with a water flow.  And since surfing is also about traversing through space, that flow also has to allow the rider to get from point A to point B with the greatest amount of lift, else he will be very limited in any maneuvers he might try on the way.

The flow of water up the face at Pipeline can be so extreme at times that it is difficult for the rider, who needs to trim horizontally to make a long tubing section, yet not let this flow suck him up and over the falls.  If he points the board downhill to align the fins to the upward flow, he can lose the high horizontal line he needs to make the wave.  But if he doesn’t release the upward pull on his fins, he will get sucked over the falls.

Balancing these forces takes great skill and can produce powerful forces and speed.  There are sometimes design changes in boards that can assist.  For example, in single fin days when I was surfing Pipeline a lot, I would use fins that didn’t have very full tips.  Too much tip area caused the fin (and tail of board) to be pulled up the face too fast causing one to have to point the nose downhill to release this force generated by the upward flow.  This caused the long parallelish area in the middle of “Pipeliner” board template, to no longer fully engage the face of the wave, allowing less rail and edge to be set in the tubes face. This makes it hard to make those long tube rides where your altitude in the tube needed to be continually adjusted by alternating the tilting of the inner rail down into the waves face to gain altitude and then tilting it up to rapidly release and lose altitude before it was too late!  With less fin profile, especially at the tip, the upward rush of water could be shed by the fin and better utilized by the rail.  Much of this is different now due to multi fins.

If boards had motors and rudders, then to go in any particular direction on a wave we would just steer our board that direction and hit the gas!  But boards don’t and we are pretty much at the mercy of natural forces.  To maneuver then we must harness subtle forces and flows in the waves and wind to push, pull or lift the board around into the directions we want.    As Thrailkill noted the upward flow of the water can be significant and an asset if one is skillful enough to absorb the energy by pressing the board against this contrary flow and then rapidly releasing this energy by directing the board away from that contrary flow and into a more harmonious flow.  Coupled with factors of buoyancy, weighting and unweighting, board flex, fin flex and a few other things.  A tremendous amount of energy can be created.

So… yes all flows matter.  But the general flow of water under a board will be pretty close, I think, to the picture I posted.  Still I am open to contrary opinions, so bring em on! :slight_smile:


Great post! Especially the part about Pipe.


I think that the only time that the flow is similar to the second image down (the one of the bottom of the board) is when the rider is dropping in, or doing an (almost) vertical reentry. When the rider is doing a bottom turn the flow is closer to perpendicular to what you drew, and when the rider is in trim the flow is at a slightly more acute angle than the image.

Considering the last image (“loaded side fin turning”), I think that is more similar to how the flow is working when the board is in trim. The lift created by the AOA of the fin and board bottom, because they are turning the fluid, balance the weight of the rider, similar to your description above.

The rider can direct the board back down the wave, decreasing the AOA of the fins relative to the flow, more similar to your third image (“trimming side fin going straight”). When the rider wants to travel back up the wave, he increases the AOA, and the flow becomes alomst perpendicular to the fin.

So, IMHO, the side fins are toed in and canted so that the maximum AOA occurs when the rider is turning up the wave, and the minimum AOA is when the rider is turning down the wave. If there is no toe then the maximum AOA is achieved at a less than vertical turn angle, but if there is toe the rider can still create maximum lift when turning the board at a more vertical angle.


Hi Bill,

At zero velocity, I'd agree, its all about hydrostatic pressure, or if you like buoyancy, but it should be understood 'static' is implied. I mention this because it's not uncommon to read or hear the term dynamic buoyancy in reference to the forces developed by moving fluids. In this case, hydrostatic pressure is still very much present, but once the water relative to the object starts to move other force are developed.

Regarding the direction of the flow, I agree. I've said as much elsewhere, most recently in my thread “Notes on the flow below”. As I pointed out to blakstah in my reply. Here, in this thread, I was presenting the most general of cases in order to suggest another possible, and somewhat unnoticed benefit of modern toed/canted fin configurations.

While we are on the topic however, the following three diagrams I believe represent the general case regarding the direction of the flow, and the forces developed, see below.







I won't bother with detailed explanations, as they can be found elsewhere, but if you like, I will.

Planing is actually a name given to a very common phenomena, in fact arguably the biggest, if not the most important class of phenomena in fluid mechanics.

All forces developed by moving fluids are about changing momentum and the force that is required to do so. Consider some solid object which is impacted by a fluid flow. The momentum of the fluid particles are changed as a result. Formally, momentum is mass times velocity, conceptually it's just mass in motion. To change mass in motion, a force must be applied. So the object must apply a force to the fluid particles. But the same holds true for the object - the momentum of the object being impacted is also changed, and the fluid is applying the force to do so. Think of it as a collision – the fluid colliding with a solid object, both are going to both subject the other to a force, and of course, both will receive a force from the other in the process. Planing, actually hydroplaning refers to a specific instance of this momentum exchange, where the fluid is water and the object is a watercraft of some sort.

In the most common form of planing encountered, planing itself, is not the means of propulsion. For example, an outboard motorboat which is planing most often has some external source of power, like an outboard , which is used to propel the boat. The planing here is associated with the rise of the hull out of the water. But in surfing planing itself is a means of propulsion.

It's not the only means, you can also simply slide down the face. Oddly enough, during such a slide, you will also likely be planing, but like in the motorboat your “propulsion” is coming from an external source, which is dropping through a gravitational field, and not necessarily from planing itself. By the way, both dropping through a gravitational field and using planing as a means propulsion can also occur simultaneously.

Forgive me for that explanation, but when they coined the term, the only form of planing people had in mind was the hydroplaning exhibited by high speed watercraft. I'm never quite sure if people quite understand its application in surfing.

In a nutshell, it is that upward and forward flow of the wave face impacting the board which gives rise to forces developed -i.e. commonly referred to as planing. And in surfing the forces can resolve in such a manner as to provide a means of moving the surfer/surfboard forward or laterally, depending on the orientation of the board. Of course, the same force will also try and lift the surfer/surfboard up and over the top of the wave, and here's where gravity comes in again. It [gravity] is used by the surfer to literally hold himself and board in the flow, so he can then take advantage of the forward and lateral components of the force of planing.

So here the way the moving fluid is developing a force is referred to as planing but the actual mechanism, is the same during a turn of otherwise. The displace of water requires changes in motion, or momentum, even Bernoulli's principle can be brought back to this fundamental notion.

On turning however...

You can't increase mass by turning. Perhaps you meant weight, or apparent weight, or just something akin to pushing in the right direction, which ever that direction happened to be. Turning is possible when a force makes it possible. You may think of this force as a centripetal force (center seeking force). In surfing you turn by applying a centrifugal force to the the water (center fleeing force), the reaction force from the water is the centripetal force, which makes you turn. The actual mechanism, again being a matter of changing momentum of the fluid particles, and they in return changing your momentum.

Pumping is actually a interesting application of, well, planing. During a power stroke on a pump or pumping cycle, the surfer momentarily forces his board downward at least against the direction of the incident flow, which results in an increase in the flow under the board, and hence the force of planing. The same thing happens at the bottom of a turn, or even before. It's the same dynamic phenomena, being applied, fairly creatively, in different ways.

I don't know why you think the force developed on fins is upward and out. They can be during particular orientations or maneuvers during surfing, but in general unless you are pulling off some particular maneuver, the toed/canted configuration tends to produce a force which is directed roughly downward and backward. It may be small, it depends on the orientation of the configuration, but its there. You may choose to resolve this force into lift and drag components, but the net force or resultant force, is nevertheless the same. Perhaps you might consider explaining why you think the forces resolve the way you've suggested. Perhaps it's just a matter of language and I've simply misunderstood.

In the post, I don't argue that this is always the case, in fact I begin by stating the case that I'm addressing, no turns or maneuvers. The benefits of modern toed/canted multi-fin configurations go well beyond what I've addressed in my post.

Sorry for the lengthy reply, but it seemed important to point out that a lot of the things people seem to be suggesting here and elsewhere, as being unique, are really just the same thing presented in a slightly different form. Planing, in the end, is just another term for 'force developed by a moving fluid'. As are the the way fins operate at the most basic level.

At some point, if you want to get back into pumping I'm game. There has been some suggestion, more explicitly in other threads, that modern fin configurations enhance pumping via another king of mechanism. tomatdatum termed his interpretation as "kinetic propulsion" which is tied to the asymmetrical nature of the paired lateral fins. Sounds interesting and I'd love to read more about it.

But the thread wasn't really about all this,.... but it does seem to have become so.




I'm inclined to open any post on fins with a reference to a video like Bill "Beaker" Byran surfing on his "One" (of Tom Morey's design) - a finless piece of less than 0.5 meter sq. of bottom surface - surfing's own 'quark' if you like, see  ... it's likely to help to give any discussion of fins perspective.. let alone the forces developed between the flow of water on the face of a wave and a surfer/surfboard.... pumping too.


During any reasonably successful turn, the forces on the inside rail fin are upward, towards the rail (out), and forward because the water flow is hitting the inside surface of the fin at an angle < 15 degrees. This is the high pressure side of the fin. Anyone who has mastered the thruster and tried a bunch of different combinations of toe, cant, and fin template knows this (as do many others).


“Particular orientations or maneuvers” seemed clear enough, but I guess not.   At no point do I, or am I now, arguing that the resultant force produced by fins is always downwards and back. Thanks for giving me the opportunity to state that (again?), I had assumed it was pretty clear,… my fault.

By the way, have you given any consideration to providing the forum with a more physical model of what your product may be doing? - now that we have Carwswell’s data. It would seem easy enough to build a model and we’ve got some real numbers and relationships to work with.


What Blakestah said about turns seems to me to be also the case with the inside fin while trimming.  The outside fin whether turning or trimming has some of the force vectors you’re talking about at all times by dint of its cant and toe, but the incident flow isn’t total negative (force to the rear and down) on the outside fin either, I don’t think–the tip vortex off that fin is where this neg incidence congregates, and the down vector on any fin I think is negligible.  I think there’s an overall impact on the foil side of the outside fin but the angle and work isn’t to the extreme degree you suggest. So in a nutshell, I think you have something useful there as far as tip drag on that outside fin (your “tether”) and tip drag off the rear, which you don’t mention–but that’s it.


I wish the gist of this was a sticky somewhere at Welcome to Swaylocks about theory:


I have to inject, for myself, and as others ought to, that what I have here and wherever I post in threads on theory is just an intuitive two cents–if you don’t have a flow tank
or a set of instruments hanging off your surfboard, and if you’re not
analyzing your surfboard’s flows at all times with empirical readings
from these, you’re blowing smoke–and presenting it as fact, especially
with any degree of arrogance, makes you look like a standard issue
Sways tinfoil hat guy–I don’t care who you are or how old you are or
where you used to surf.

Please, spare us any arrogance and just present your case with regard to any theory about surfboard dynamics.  You’re not writing a textbook–of course you’d like to convince people, but please check yourself a little bit. Confidence is one thing, arrogance is another.  (not directed at any one poster) Chapter headings are not necessary esp if theyre just pithy little asides to break it up because you’re realizing how you’re going on and on using words to fill in where proof is lacking, and dropping in to dismiss the entire thread and everyone else’s opinion, without much case made makes you look like an ass, especially if there’s any suspicion adrift that you don’t surf anymore.

Your math (and diagrams) are not much evidentiary if you don’t have the instruments to resolve real flows and their values around a surfboard. 

And your memories and your feet and your eyes don’t count for much in the real world of physical dynamics, at least as far as explaining what’s really  going on.  The earth is not flat and JFK Jr. trusted his eyes and the seat of his pants, and he’s dead.





If I had a PhD in theoretical physics I would be only interested in equations. Instead I have an engineering PhD and all I am interested in is making things work. The theoretical physicists will always follow with their equations. Especially in the case of surfboard fins, there is far too little known. There is a great deal of fluid data on lift and drag, but when you consider differences in low and  high aspect foils, and attaching a planar surface to one end of the foil, everything changes, and there is very little known. My approach was, and is


Get an idea. Make a prototype. Surf it. Repeat. Get others to surf it. Use that to make a better model prototype. When you cannot make it any better, maybe think about making it for others. 


By the way, have you given any consideration to providing the forum with a more physical model of what your product may be doing? - now that we have Carwswell's data. It would seem easy enough to build a model and we've got some real numbers and relationships to work with.

Here's a thought: wouldnt it be easier to try Blakstah's product on an actual physical surfboard? Maybe what it does is provide more fun to certain physical surfers with particular physical tastes?

Can you provide a mathematical model of 'fun'? 



Quote from Mr Thrailkill: “ In other than a skidding ''radical''  maneuver, the relative flow is always parallel to the centerline of the board, as it moves forward over the water.  The deflection of water toward the rail, is a shallow surface phenomenon, though it can contribute to a ''root stall'' at the base of the fin.”


Quote from Mr Barnfield (with credit to Blakestah): “As Blakstah pointed out, as the board began to move forward the flow would be upward and backward and out to the rails as the flow "leaked" off to each side of the board. See my drawings below from a past discusson on this subject”


(Cool pic btw)



At the moment the two Bill’s appear to have contradicting positions regarding the flow over the bottom of ones board ( for straight and steady state)


Personally, I am quite convinced that there is little debate regarding the lift and drag produced by any fin if its inlet flow has been identified.

(really went out on a limb there, huh)

but if you dont know the flow....well, the thread gets like this one.


So any clarification from either Bill would be much appreciated.

As reasonable and rational individuals I suspect there is quite a bit of wiggle room to allow for common ground.


I am very much enjoying Mr Carswell’s thesis. However the bulk of his work has been to confirm the viability of his modeling tool to compute flows empirically quantified by NACA almost 100 yrs ago.

Unless there is a surprise ending or appendix, I suspect his effort will fall short of  modeling the flow field on the bottom of a surfboard.

… and leave all of us still in the dark


So Bill and Bill , we are back to your areas of expertise.



another bill

Both Bills are right. The caveat is that it depends on how deep under the hull you look. 

The flow is very strongly towards the rail close to the hull. And very strongly parallel if you look deep enough. There are even very smart folks who have made series of fins that are twisted in toe-in to take advantage of this change in flow with depth from the hull. I met two of them, and their “twist” fins both varied the same with respect to depth - almost identical in that one specific detail. They independently discovered this at roughly the same time, several thousand miles from each other. Because the devil is in the details, and that requires riding the fins and using the feedback you get. 


Unless I am misunderstanding the quotes I don’t think that either quote is indicative of the flow. In my opinion this image represents the flow the best. With the flow moving from the ‘wet-line’ off of the rail.


I am trying to visualize the difference in flow that you describe, but I don’t see it. If anything, I think that the fluid closer to the hull would be moving more parallel than the fluid further (deeper) from the hull, because the hull is turning the fluid that is closer to the hull more than the fluid that is further from the hull.