# Creative Drag: Revisiting the Ancient Hawaiians

Creative Drag: Revisiting the Ancient Hawaiians

The original title of this thread was ‘Creative Drag’. In his post below, JohnMellor points out that the notions presented here are not unique. In particular he references George Downing’s interpretation called ‘Calculated Drag’. The reference given in my post below John’s is highly recommended.

Creative Drag

Assertion: Drag is critical, not as something to be minimized, but actively controlled by the surfer through good design.

WATER FLOW direction test resultsFigure 1 below is a quick and dirty analysis of the forces operating during planing as it applies to the diagram I posted in thread. The model is very simplistic, however it does serve to illustrate the relationship between the forces of drag and lift.

The physics make be a bit thick for some. The important point is the relationship between the angle (greek letter phi in the diagram) and the ratio of the coefficients of drag to lift.

A beside on coefficients

Savitsky diagram Coefficients like those used here - CL for lift and CD for drag, see figure 1 – are very useful in this type of analysis. They can be viewed as ‘lumps’ of all the variables that are likely to have a bearing on determining a given quantity, in this case the forces of lift and drag. Here they take into account parameters like fluid density, template shape, tail shape, rails, board orientation with respect to wave face, wetted surface, etc… For a given body, they are generally determined experimentally. Often in lift and drag analyses the coefficients themselves are given as functions of angle-of-attack, which in this case would be somewhat analogous to the tau angle (see in the WATER FLOW direction test thread mentioned above.)

Please note: The phi angle given here is not the tau angle (i.e. angle-of-attack) but is just used to locate the board on the face of the wave.

As suggested by the analysis in the figure, the position of the plank on the wave face is a function of the lift/drag coefficient ratio. The board will ride higher on the wave face for a greater drag to lift coefficient ratio, or to put it another way, all else remaining the same, increasing the drag coefficient will tend to make the board ride higher on the face.

In surfing the surfer has some control of the value of these coefficients e.g. he can change board orientation. In fact a surfer is constantly adjusting the value of these coefficients, at least with respect to those parameters that lend themselves to adjustment by his actions. It could be argued that surfing is about the creative use of this relationship – that good design provides the surfer with a large degree of creative control of these coefficients.

burnsie’s postConsider ‘shape-drag’ –i.e. the shape of the template, particularly the rear and tail end of the template. Take a look at the picture in the in the thread reference above. The surfer isn’t exactly moving as suggested in the above diagram, he’s got some lateral motion, but it’s close enough – he’s likely finless, and what he’s on is pretty close to a plank. Obviously the shape of his tail and rear rails contribute to drag. Consider what the difference it would make if his rear rail line and tail were parabolic -i.e. a curved template.

Extrapolate this a little further. The surfer in figure 3 (click for the video)is leaning on his inside rail –i.e. dragging it, in order to maintain his position on the face.

Here there will be a large drag component both in the lateral direction and in the direction ‘up the face’, as in figure 1. This latter drag component will be operating in a similar manner as in the simple case presented in figure 1. The formal relationship however will be more complex.

… continued in next post.

continued…

Still, the shape of the template is likely to matter, the curvature determining to a large degree how readily the surfer can bring drag on- and off-line –i.e. manipulate his drag to lift ratio in order to maintain position on the face. Harsher template corners and angles force a sort of commitment in terms of drag, which may be appropriate, it depends on the what the board is designed to do, see figure 4.

Of course template isn’t the only variable here, there’s rocker, rails, etc.

Then of course consider figure 5. You’ve got to wonder where he’s getting the drag, my guess he’s making good use of his fins too – another critical source of drag in modern design.

I guess the point of this thread is to suggest that drag is a critical component of design. In particular that the template, rocker, fins, rails, etc are basically a tool set for the surfer to actively control his drag, and of course, but to a lesser extend control lift. They [lift and drag] are not independent so it’s ultimately a balancing act, by surfer and designer. In terms of control however, drag is far more easier manipulated than ‘lift’.

kc

Very interesting. Your insightful comment that increased drag will cause the floating body to ride higher in the face as it moves opens up speculation about the effect volume (as it results in flotation) will have in combination with drag. So many variables to account for, not the least of which are surface area and its drag coefficient, and planshape. Fin placement and number of fins probably don’t have as much effect in regard to riding higher in the wave, although a fin cluster that produces more drag than lift might.

Hey KC,

Very cool. I see that you have drag varying as the square of velocity. I’m not sure for planing bodies, but is form drag proportional to the square of velocity, as well as induced (lift-producing) drag? For wings/fins, induced drag is proportional to the inverse quare of velocity…but I’m pretty ignorant about planing dynamics…

Very thought provoking, though…

JSS

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continued...

Still, the shape of the template is likely to matter, the curvature determining to a large degree how readily the surfer can bring drag on- and off-line –i.e. manipulate his drag to lift ratio in order to maintain position on the face. Harsher template corners and angles force a sort of commitment in terms of drag, which may be appropriate, it depends on the what the board is designed to do, see figure 4.

Of course template isn’t the only variable here, there’s rocker, rails, etc.

Then of course consider figure 5. You’ve got to wonder where he’s getting the drag, my guess he’s making good use of his fins too – another critical source of drag in modern design.

I guess the point of this thread is to suggest that drag is a critical component of design. In particular that the template, rocker, fins, rails, etc are basically a tool set for the surfer to actively control his drag, and of course, but to a lesser extend control lift. They [lift and drag] are not independent so it’s ultimately a balancing act, by surfer and designer. In terms of control however, drag is far more easier manipulated than ‘lift’.

kc

KC,

Brilliant post in my mind…nail on the head.

Diagram 4…does not take into account vortices of stall created by the drag ratio. My point being that the foil shape and characteristics will greatly determine the induced drag coefficient. A force that an in tune surfer can use to great advantage, not only in the tube but specifically utilized in a board volume X ability X wave type X backhand/forehand attack x desired style x x x…an infinite set of variables. Tip of the iceburg…we know so little!

Rocky

Not the ‘floating’ body, but the ‘planing’ body – there is a difference here. If you check out the reference to mentioned burnsie’s post in the craftee thread ‘Flow…’ you’ll see a fellow surfing on basically a plank – buoyancy, if you wish to use the term, is ‘dynamic’ not static –i.e. he’s planing.

That aside, I disagree, surface area (wet area), planshape, fin placement, number of fins, etc., all have an impact on how the board will ‘like’ to sit on a wave face for a given surfer. The surfer can control a lot, but the design tends to push him towards a given solution e.g. the orientation of the board with respect to some maneuver or position on the wave face. All of which is very heavily influenced by the relationship between drag and lift for his board. As mentioned, for the surfer, controlling drag is far easier that controlling lift – once on the wave his ‘drag tools’ are far more important.

Lots of variables sure, but yet surfers seem to master them all somehow.

kc

You’ve taken it way past were I did. (I’m on the ‘Ten Step’ program to keep my posts short…apparently, I still have a few steps to go.)

It was a ‘sketch’. Figure 4 was an attempt to suggest that template is also about bringing or taking drag on- or off-line - no real detail presented. But taking a step further, a given template will tend to force a given solution. This is not likely to be something most would have a problem with, but my point is that, what is being discussed is basically about controlling ‘drag’ – not just trying to minimize it, but using it creatively in surfing and design.

If you start running the kind of exercise in figure 4 for different templates (also thinking about rocker, rails, tail, etc.) you begin to see that certain shapes tend to lock you into a high drag state with little rotation. Which, by the way, can be good, for a given application. Think about guns, subtle works given the level of forces you’re generally dealing with. Then think about a classic fish with a swallow tail, subtle may still work, but you need the option to be ‘brutal’ when it counts.

kc

It’s a sketch.

In fact, if you peruse the planing literature, you will find that there is little available for high tau numbers (analogous to the classic angle-of-attack). High tau is very much a part of planing in surfing. Of course, so is low tau. Most of the literature is written for hydroplanes or motorboats where relatively low tau angles are expected. You’ve got to rotate Savitsky diagram to turning planing into propulsion. If you keep to the literature, you’re likely to call it stalling, which doesn’t seem to fit the phenomena.

But like I said, it’s a sketch. Maybe I should have peppered the post with more ‘kinda’s and ‘sorta’s – mea culpa, for assuming to much context.

kc

According to George Downing, the ancient Hawaiians had this stuff figured out a long time ago. G.D. refers to it as “Calculated Drag” but as far as I know he didn’t have any mathematical formulas. In some of his early shapes, he seemed to have hit the nail on the head by intuition and a good eye for shaping. His template known as “The Curve” combined with his knowlesdge of rocker and bottom contours has been used repeatedly - even on a relatively recent gun ridden by his son when he won a major contest at Waimea.

Calculated DragExcellent, my ignorance knows no bounds, see .

Thanks,

kc

Well look what you guys were up to while I was at work!

Nice model, KC. After I read it the first time , I thought, well, he hasn’t taken into account the different coeffecients of

lift and drag that come about from the rider’s control of pitch, yaw, and roll. But then I re-read and saw you ‘‘allowed’’

for that. Difficult to measure, perhaps, but included in the model.

Eliminating the fins simplifies things considerably, but limits real-world applications. At times, as you illustrated in that one photo,

the fin(s) and a tiny bit of rail are all that’s in the water. And of course the fins are almost always engaged and producing drag

in other circumstances.

Your basic premise is true; drag is necessary for control. Fins contribute a great deal of the control, and hence drag, in a surfboard.

I guess what I’m getting at is that you need to somehow include fins in a more advanced (but unfortunately more complex) model.

It’s beyond me, I’ll be the first to admit. But I might be able to grasp it if I saw it. It might be easier to do a seperate model for the

fin(s) and then integrate the two.

The limited amount of planing literature I’ve studied has applied to level water. The slope of a wave and the intersection of rail into

face introduces new variables unaccounted for in that literature. A lot of boat design doesn’t take it into account either, ask anyone

who’s piloted the ‘‘wrong’’ planing craft in a following sea! What you’re doing here is surfing-specific, and it’s to be commended.

Mike

Mike, please try and read the url referenced in my post above.

As for a model…

After re-reading some of the papers that I have collected the thought occurred to me that is might be possible to develop a unique vector coordinate system which reflects the wave face, sort of like a cylindrical reference system used in the original post, but one using hyperbolics or some other system (which could be adjusted for different wave parameters.) Perhaps others have tried this? Anyway, while screwing around the relationship expressed in the original post just kept popping up, and even though it’s just a rough sketch, it looked like a fun thread.

You’re absolutely right about all the other variables. For now however, this notion that drag tends to be far more controllable than lift seems fruitful - just another [idea] wave to ride - it will be interesting to see where it takes me, and perhaps others too.

… and if you read the reference, my guess is that the ancient Hawaiians thought it was a valuable paradigm.

kc

“might be possible to develop a unique vector coordinate system which reflects the wave face, sort of like a cylindrical reference system used in the original post, but one using hyperbolics or some other system”

That would be awesome.

My thoughts on lift v drag are pretty much intertwined. For the most part, other than skin friction and form drag, I really think most of the drag inherent in a surfboard is induced; in other words, it is drag that is there because it is creating lift. At typical small-wave surfboard speeds, I think induced drag is the biggest player, but I am really not sure; there are formulas for this (I’m sure), but I haven’t dug into it like you have. For fins, speed and fin size dictate where on the total drag curve you fall (whether induced or skin/form drag is the major player). The faster you go, lift is more in abundance at small AOA, but the drag contribution for that lift is negligible compared to the skin friction and form drag of the fins. Not so sure for planing craft. Do you have an idea?

JSS

I do a lot of lurking here, generally because I don’t feel qualified to comment on much, but; I don’t think you’re being strong enough when you say that there isn’t much you can do about lift, in reality there is nothing you can do about lift. Lift can never equal more than the weight of the rider and board combined. Lift is a force and F=M.a, where a=g +/- acceleration. You can’t do much about gravity, and acceleration will depend on how you turn and how steep you drop, and barring the call of nature mass is pretty well fixed too. With a surf board you can alter the bottom shape and rocker which will effect the relative AoA and hence the coefficient of lift which will be very wave and directional dependent, but I reckon at the end of the day it’s all about drag…

Hey Dunk,

I agree that you need the lift, but I disagree as to how much you need at a given time. If climbing the wave face, the lift force needs to be greater than weight of board+rider, as well as in some turns, as the ‘centrifugal push-back’ the board/fins must generate can sometimes be more than the surfer/board combo weighs (hard bottom turn), and sometimes less (unweighted top turn).

Although I think we are just using different words to describe the same thing. At typical surfboard speeds, you need lift no matter what, like you said. To get that lift, it will cost you drag. How much drag it costs you is dependent on the things you talked about (rocker, bottom shape, AoA, probably mainly rocker/AoA and outline, with bottom and rail shape a close second, but that is a wild-ass guess).

Anyway, I think this is a pretty cool topic, it gets me thinking…

JSS

Quote:

“might be possible to develop a unique vector coordinate system which reflects the wave face, sort of like a cylindrical reference system used in the original post, but one using hyperbolics or some other system”

That would be awesome.

My thoughts on lift v drag are pretty much intertwined. For the most part, other than skin friction and form drag, I really think most of the drag inherent in a surfboard is induced; in other words, it is drag that is there because it is creating lift. At typical small-wave surfboard speeds, I think induced drag is the biggest player, but I am really not sure; there are formulas for this (I’m sure), but I haven’t dug into it like you have. For fins, speed and fin size dictate where on the total drag curve you fall (whether induced or skin/form drag is the major player). The faster you go, lift is more in abundance at small AOA, but the drag contribution for that lift is negligible compared to the skin friction and form drag of the fins. Not so sure for planing craft. Do you have an idea?

JSS

Here’s a graphic from a computer model of surfing first developed in the mid-70’s, then modified several times in the 80’s to include new processes and more detail in simulating the processes included in the basic model. These include: buoyancy, changing shape and planform of the wetted area (with changes in speed, trim angle, and pathline speed relative to wave speed), etc…

Obviously there still remain a goodly number of assumptions involved with regard to the board shape (“natural” rocker, hard rails, rectangular planform, steady-state conditions, etc.). But the simulation does seem to yield reasonable predictions of speed, wetted area, location of center-of-mass of the rider on the board, etc…

In this first graphic, the predicted steady-state speed across the face of the wave (in which the rider stays in the same location on the face of the wave and at the same position relative to the curl of the wave) is shown as a function of where the rider has positioned the board on the face of the wave (the “Wave Slope”) and how he has trimmed the board (e.g. moving for or aft on the board) via the “Board Angle-of-Attack”).

For the particular wave and board conditions used in the simulation that is illustrated, the maximum (steady-state) speed the rider can achieve is a little less than 23 mph. That occurs when the board is positioned where the slope of the face of the wave is about 47 degrees, and the rider is positioned on the board so as to produce an angle-of-attack of about 11 degrees.

If the rider positions the board lower on the wave face (resulting in a smaller wave face slope) the propulsive power is reduced, and hence there is a reduction in speed. If he positions the board higher on the face of the wave, the hydrodynamic properties of the board are altered making the board less efficient (primarily through a decrease in the aspect ratio of the wetted area of the board) and hence there is also a reduction in speed.

If he moves aft from the sweet spot (to reduce the wetted area), the induced drag increases faster than the skin friction drag decreases (note: the fin(s) remain completely wetted, so their drag remains about the same apart from any change associated with the change in the steady state speed). Conversely, if he moves forward from the sweet spot, the angle-of-attack decreases (lowering the induced drag), but that is more than compensated by the increase in skin friction drag associated with the increase in wetted area.

The next graphic shows how the wetted area of the board changes with position on the face of the wave and how the board is trimmed by the rider. When set up for maximum speed, the wetted area is predicted to be about 2.8 sq-ft.

The third graphic shows where the rider needs to position his center-of-mass along the length of the board to achieve a desired angle-of-attack. In this particular simulation, from this graphic and the first one, it can be seen that maximum speed is achieved when the rider’s center-of-mass is about 3 feet forward from the tail block. Moving forward about 6 inches on the board will result in a speed reduction of about 0.3-0.4 mph.

Other factors that are also computed include: the maximum wetted width (necessary within the mode to determine the aspect ratio); the length of the wetted bottom (where the water first strikes the board on the wave side of the board); and the path angle of the motion relative to the crest of the wave.

mtb

Remarkable, thank you MTB.

Am I correct in assuming that wave slope in your figures is roughly phi in my crude, highly simplistic and one dimensional sketch (see first post). I assuming that your slope is a tangent to the surface, see diagram.

As always, very interesting stuff.

kc

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Remarkable, thank you MTB.

Am I correct in assuming that wave slope in your figures is roughly phi in my crude, highly simplistic and one dimensional sketch (see first post). I assuming that your slope is a tangent to the surface, see diagram.

As always, very interesting stuff.

kc

Yes, that is correct. Keep in mind however that in the case of the surfer traveling laterally across the face of the wave, from the standpoint of the hydrodynamics and force vectors, the slope of the wave face along the pathline of the surfer is less than at the same position on the wave when headed straight off. It’s like a skier going diagonally down a ski slope, the more he goes across the slope, the less effective propulsive force.

That’s one reason that it’s difficult to make big improvements in speed. The more you reduce the drag, the faster you would expect to go–and you do go faster–but not as much as one might expect since since as you go faster, “nature” (or, more correctly, geometery) takes away a bit of your propulsive force.

(Also keep in mind that I’m always talking about a steady-state condition, i.e. where the surfer always remains at the same location on the face of the wave, and always at the same lateral position relative to the curl).

mtb

I’m serious…

You guys should write a book on this stuff

I think you have something here a science publisher would be interested in…

If not, I bet some wannabe entrepeneur will gather all these clips and push it off on their own to make some money or get some recognition.

I really mean it

the stuff you and some of the rest have put here

is enough to make a good book on the subject as there hasn’t been a really good one on the topic for many years…

The last one I remember was something called Waves & Beaches(?) from like 30+ years ago

I found it last weekend cleaning out all my college stuff to give to the local library

be neat that a bunch of discussions on swaylocks spawned something like that…

+1 on the book idea…

JSS