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

  …or lucky you, it’s raining.

There's a kind of rule-of-thumb when consider the force developed by moving fluids impacting solid objects – the direction of the force developed is perpendicular to the surface upon which the fluid is impacting. It's not a hard and fast rule, but it's unlikely to steer you wrong when dealing with water. Admittedly, factors such as viscosity act to make the net force somewhat less than perpendicular, but for water, it's 'more perpendicular' than not, so to speak.

An Occam's razor approach

Put aside turns, for that matter put aside most of the more interesting radical maneuvers in modern surfing, and consider a case where the direction of the incident flow across a toed/canted multi-fin system is close to parallel to the center line of the board.

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.

Neither of these contributions is perfectly back or down, but they are sufficiently so, so referring to them using these terms is not unreasonable here. The direction of the incident flow will vary of course, but the proposed case covers a large range, plus of minus a few degrees kind of thing. The point being this is not an unreasonable departure point, it kind of sets the stage for consideration of other flow conditions.

… back and down?

“Back” here amounts to resistance to the forward movement of the board. The question is then, “Why would you ever want that?”. The “down” amounts to sinking the tail of the board, which also begs the same question.

… model building

The magnitude of the force by any single fin in the system, is basically a function of three things – the actual area of the interaction with the flow, the speed of the flow and what is commonly referred to as the angle-of-attack of the flow. In fact you could model the magnitude as being directly proportional to the area and the speed squared, for a given angle-of-attack. The relationship between the magnitude and angle-of-attack is not as simple, but we can at least say, with increasing angle-of-attack the magnitude of this force will also increase. Please note: This isn't a lift/drag argument. It's about the net characteristics of the net force developed on an object by a moving fluid, in this case water.

So for a given flow speed and angle of attack, the force will increase linearly with fin area. For a given fin area and angle-of-attack, the force will increase as the square of the speed of the flow. And for a given area and speed of flow, the force will increase with increasing angle-of-attack, not exactly in a linear manner, but nothing too crazy either.

We now have a crude modeling of the direction and magnitude of the net force developed by a given toed/canted multi-fin system. Hence when faced with a given set of conditions for which a board is to be used, the builder can make his adjustments accordingly. It's definitely a simple model – let's call it Occam's model

… back to “back” and “down”.

… “down” first

At a sufficiently high enough speed a surfboard will appear to want to leave the water. In fact as the speed increases the amount of bottom surface generally contributing to the force of planing is decreasing. This generally occurs as a direct result of the increasing force of planing itself – pushing the bottom up and out of the water.

Obviously, in the extreme case of a high flow speed, staying “connected” to the surface would be of some benefit. Then again, at the other end of the spectrum, “down” begins to work against you. But then there's that pesky middle range of conditions, and getting the amount of “down” right is kind of a crap shoot. But luckily for us we've got something called the marketplace, and from it we get information like “what works, most of the time.”

… now “back”

Surely speed is critical, so why slow a board down? I would suggest that speed alone is not critical, being able to remain in the more critical section of the wave is however, and that's at whatever speed that requires. Skill can get and keep you there, but maybe a little design help wouldn't hurt. Think of the small amount of resistance produce by modern fin systems, the “back” to the forward motion of the board, as a “tether” to the wave.

By the time a wave becomes interesting to a surfer, it's virtually all acceleration, or if you prefer acceleration and deceleration. The curl is decelerating faster than the shoulder as it continues to move toward and over the bar or reef. The flow up the face in the curl region is accelerating much faster than that out on the shoulder. It's very easy to get too far out ahead or behind all of this change, and having a little help from good design surely couldn't hurt, even if its just a little tether, helping to inform you and adjust to these changes.

… adjustments

But once you install a fin-system for a given average set of conditions, the deed is done. And though it's functional properties are somewhat fixed, you can usually compensate here or there by bringing on line more or less bottom surface area to adjust the force being developed by planing. This is generally done with adjustments in posture or position on the board. Or by using other slight modifications and adjustments to technique. If you're surfing the board with a given installed fin system in conditions close to those optimal for which it [fin system] was designed, it's likely that these kinds of changes will be made automatically and go virtually unnoticed. If they become too noticeable, then you're likely to want to ride something else because fin-system is well outside the conditions for which it was designed. (We're assuming the builder knew what he was doing, of course.)

I'm inclined to believe that modern toed/canted multi-fin systems are all about allowing the surfer to move beyond the continual preoccupation with the rudiments of the sport -i.e. it makes the boards more 'user friendly' if you like. It allows for greater opportunity to be more creative on a wave. It does this by both enhancing sensitivity and connection to the flow in the wave though this notion of a 'tether', as well as the more traditional application of fins - control and stability - see below.

The amount of force that's being created usually isn't that great. Actually comparing it to a tether is probably pretty actuate. Tethers can offer some resistance, but they can also be broken. Their real value is often one of transmitting 'information', telling you how the motion of the tethered thing is changing. Here, that thing being the flow in the wave face.

You really don't get this kind of helpful information from a single fin. You can get your information about the changing flows from other clues of course.

… note on stability and control

The direction of the net force produced by a modern fin-system is perhaps not the best way to stabilize a craft.  But then again, when you think about it, this choice of less than the maximum probably works for surfing. Too much stability would be simply a burden. But something more stable that mere a single fin, for example, but less than the maximum, kind of makes sense.. It allows for greater ease in control, yet another trade-off, if you like.



Given the  "no edit" on the opening post of thread policy, my apologies for any errors.


Since the industry has referred to attachment mechanisms as fin systems, it might be clearer if you used the tern fin configuration rather than system. Also at the very start you are considering surfing without turning. Multiple fin configurations are optimized for turning. If all you want to do is go straight use a single fin. And finally try a system like 4-Way or Edge if you want to empirically experiment with the affects of changing paramenters such as cant, toe, template, foil and postion.

question ??

or more specifically....

you've covered a lot of ground here.

I am definitely not qualified to critique your theory, overall.

but if there is a specific question to which an answer may fill in the blanks, i'd be glad to contribute.


I think what Mr. Head is expressing is a modicum of confusion over your cant-to-pressure-differential vector…perhaps one could interpret his query with regard to the possibility that cant produces lift, not the opposite of lift…or is it a query with regard to one’s own meaning of “down”?  It might be the tether analogy… your musings regarding stability…or, maybe he just likes typing question marks???


Fin configurations it is.

After reading Dr Carswell’s work, which is great, and thank you (very much!) again by the way, I can see the argument, at least your point. As for a 4-way or Edge, sure that would definitely be a way to find out what works for me personally, but I seem to be doing that, if not via a 4-way or Edge system(s), in other ways.  What I’m more interested in is the kind of data generated by Carswell, which tends to address larger trends.

Thanks again for the paper - nothing like real data to slap the nonsense out of a fool.


I get it – I agree with it.  If you watch the 06 Bells videos – I think you can see Kelly slowing in maneuvers near the lip and it seems to makes Slater’s lip moves more managable. Drag from concave, tip drag, the propagation flow, a combination?  The double pump bottom turn seems to possibly be a big byproduct of that tip drag too.

I’m not sure what’s going on here, … jeez… did Mike hire a guru to stand-by and field questions from the confused? Thanks Mike, I think?.. might take some of the fun out of musing and spewing though.

Anyway, in a nutshell, when trimming, for example, given the general the direction of the force inherently developed by a modern toed/canted multi-fin configuration, which has contributions both in the opposite direction to the forward motion of the board and down into the water, I ‘theorized’, though I would have went with hypothesized, given I’ve got squat in terms of real data to back it up, that perhaps this force operates much like a ‘tether’. That sure, there’s some additional resistance, or energy cost,  but its value may be in providing the rider with a better or enhanced connection to the changes in the flow on the wave face.

And by the way, what tomatdatum suggests in his post is likely correct. (I would have written “… correct too.” but that would have suggested that there is more than squat to back up my hypothesis.)

I’m inclined to believe that the tweaks that we are seeing in design are more geared towards making boards more user-friendly rather than simply ‘faster’ or even efficient for that matter. That is the big work has likely been done with regards to efficiency. Here ‘user-friendly’ is not meant to suggest that any one can just hope to hop on a given board and surf it under the conditions for which it was designed. At least no more than “anyone” could hope to climb into a cockpit and fly a plane, yet modern cockpits and aircraft design in general, are far more ‘user-friendly’  than they were merely a few decades ago.

If you see some blanks, please feel free to fill 'em in?

I hope you don’t plan on charging me for this Mike… If I’m going to pay for therapy, I rather go the full nine yards and check into a sanatorium. (I hear quite a few now are offering broadband.)



Wow.  Huh.  I tend to agree.


I guess it’s a pretty broad statement, but still. Pretty good summation.

Drag cofieient vecktors working in parellel with a venturie effect will inherantly create a kavitation behind the lead fin. Theregfore you must go straight…I don’t go straight in my a surfnboard…unless the end result is to pinch mr Mellville into the pit.

i’m a bit lost.
lots of terms and conjecture being tossed with some pretty fuzzy definitions.
is it still possible to get a copy of the Carswell paper?


in trim in the pocket or turning, as well as in any theoretical straight-off, the outside fin’s tip is at neg AOA, causing drag, which KC is saying is a useful stabilizing influence, and possibly key to thrusters’ usability. I believe.

I’d be happy to send you a copy. Please e-mail me at so that I can e-mail it back to you. The file is too large 4Mb to post on the site.

I guess its true then… it’s kicked… you know, the resin in your head, resinhead.


Thing is, you’re not giving near enough weight to the “ON” portion of the dynamic.  You’re focused on the passive, when everybody knows the pump and carve are where the things come into their own–I thnk that you’re focusing too much on the tip drag for people’s taste, considering the lift in the turn and the pump and the lift that gives drive in the pocket are where it’s at.  Thrusters are not solely about tip drag. 

I see the point about user-friendliness–but then that’s like saying every facet you put into a board adds something and takes something away, in terms of the broad definition of performance.  That tip drag means cruising at higher and higher speed in trim (and cutbacks without pumping in speed) is lost for the most part for the sake of controllability in critical areas like the lip, the pocket etc.


Maybe you actually said some of this but it got kinda lost in the shuffle.


Think about a thruster center fin and it's relative position to the the stringer. Not much to do with that. Has to be on the stringer. Nice piece. It highlights for me the validity of the quad, and cements it's position as a pivotal set-up to not only teach us but prgress the sport forward. Great thread! Thanks Jim




The flow is never close to parallel to the centerline of the board for all three fins.


The flow ALWAYS goes from front to rear, and from centerline to rail.


The angle of this flow relative to the centerline depends on speed, nose/tail angle,

and the wave. More nose-up, and less speed, and the water moves more

towards the rail.


The modern toe was set so that when the rider’s weight on a shortboard is

in one position, the flow is nearly parallel to all three fins at once.


This induces compromises, because the rider MUST increase his fin drag

coefficient if he wants to lower the nose to reduce the hull drag.




I’m no fin guru… in fact, I rarely post on fin threads because I know next to nothing about them. But I think you’re dead on about the validity of quads, and the role they could play in moving the sport forward.

Blakestah said it, too… flow is never perfectly sraight through the tail. It’s always at some angle, no matter what. So on a multifinned board designed for those surfers who are, indeed, pushing the sport forward, a trailer makes less sense to me than a second set of rail fins.

I spent most of my surfing life on thrusters. But I’ve found the benefits of a finely tuned quad set-up over the last few years, and have no desire to go back… at least not in the near future.

I understand your point. And there are no ifs or buts about it, things change with more radical maneuvers.

It was not my intention to suggest otherwise.It's likely that whatever impact modern fin configurations have on trimming, positive or negative, the gain during more radical maneuvers is worth the trade-off. I was just exploring the former, which among other things, I tend to include almost all those moments between those more radical maneuvers, however brief those moments may seem - those forgotten moments when the surfer is re-establishing his link to the flow in the wave - the stuff between.


This kind of musing is always dangerous - it's purely subjective, and therefore, easily contradicted by the same, that is, another person's equally valid subjective take – as it should be.

Whatever is happening, one thing is clear, choice is key. Nothing new about that. And you fellows are doing a nice job of providing it.