Know Your Flow

General Discussion
1

Say you're being pushed by a friend while you're on your skateboard. At some point your friend begins to tire and slows down. Unless he holds on to you, you're likely to pull away from him, if only momentarily until you (on you're skateboard) start to slow down (due to friction and air resistance) and match your friends speed once again. The point being that it takes a force to change an objects velocity.

 

In the open ocean there is generally little net flow of water associated with the passing of a wave. But the waveform itself is associated with a flow. As the wave passes, the water first begins to flow up and forward and then at some point begins to flow down and back. This cyclic motion extends from the crest to a great depth.

 

Shoaling waves, that is waves that are moving into shallower water, at some point start to 'feel' the bottom (the bottom being the sandbar, reef, etc.) The water molecules literally start to rub along the bottom, and in doing so exchange momentum with the bottom and start to slow down. This slowing down is transmitted to the layers directly above, and eventually right to the crest, but not instantaneously. There is a lag in time as it takes some time for the slowing down to propagate up through the waveform. The result is a differential profile of forward velocity – at any instant, a lower layer is moving slower than the layer directly above it. The wave as whole is decelerating, but not all regions of the wave are doing so at the same rate.

 

This is why some shoaling waves break. The differential forward velocity profile exists along the whole face of the wave, from bottom to top – the layer beneath any given layer decelerating, or traveling that much slower than the layer immediately above. As the wave continues to shoal, the upper layers, will literally start to slide up upon the lower layers – the wave increasing in height as a result. When the difference in forward velocity between two layers becomes sufficiently great the upper level begins to shear off in a dramatic fashion – it breaks – the shearing upper layer pulls away from layers beneath – 'you on your skateboard just keep traveling as you tired friend falls behind.'

 

This differential (forward) velocity profile of the wave face is not only important during the breaking process. It plays a role in virtually all aspects of surfing from catching waves to surfing and, I would argue, to surfboard design itself.

 

Remember, an object traveling at a given velocity will continue to travel at that velocity unless acted upon by a force. As a surfer 'drops' down the face of a wave, the wave is slowing down in its direction of propagation, or decelerating, but the surfer is not slowing down as fast -i.e. there's a bit of a lag. Its as if the water on the face is trying to pull away from him, or in this case the bottom of his board. In fact its quit common for a surfer to lose contact the face during a drop – sometimes for just a moment, sometimes more. If you surf, its likely, when it has happened to you in the past, you've just shrugged it off as a hop or a bounce after hitting a little chop, which it might have been, but then again, maybe not. Or perhaps you've been lucky enough to have experienced, on those bigger days, when after taking off, have found yourself momentarily in free-fall.

 

This curious effect works to keep your board from partially submerging – that is you'll ride higher in the water – at least on the dropping maneuver. When you start to climb the situation reverses. As you climb you are counting on the wave to accelerate you both in the forward and up directions. But once again there is a lag – so you appear to sink or ride lower in the water. These effects are in play all the time while you're on the wave.

 

In fact, its why you can catch shoaling waves. As you 'paddle for the wave' it eventually starts to overtake you. Since its shoaling the wave is slowing down, and because the upper layers are starting slide up upon one another, the wave itself is continuously changing shape. When if finally catches up to you its upper layers try and drag you along for the ride – both up and forward. But not instantaneously, as it take time to accelerate you. As a result you appear to sink a little, but you actually don't drop vertically, the water is rising or flowing over you as it tries to move past you. In the meantime the wave continues to shoal and change its profile – walling up even more – the lower layers decelerating even more. You however, continuing to paddle and be accelerated or dragged and pushed along by the wave's upper layers until you begin to feel as if the wave has 'captured' you. But by then the layers beneath you are now moving slower than you are and as you stand, you seemingly 'pop out' in front of the face, or drop onto to a receding face, as it continues to decelerate in the forward direction (faster than you) - 'you on your skateboard just keep traveling as you tired friend falls behind.'

 

The same mechanism is involved when you attempt to score some 'air time'. When you leave the wave you are traveling forward at a speed close to that of crest or lip. The rest of the wave is slowing down. So if your 'time in the air' isn't too great, that is you aren't slowed down by air resistance, when you come back down you land on the face, which was continuing to slow while you where in the air.

 

How might this differential forward flow impact design?

 

I'm inclined to believe it forces a 'rethink' about the role of everything from fin configuration (including toe and cant) to bottom contours to rail profiles, as well as rail curvature, rocker, and even its role in planing, the primary means of propulsion in surfing. And by rethink I mean how they might be functioning. Its not that a given current design is necessarily wrong, just that it might not be working the way you think. If you understand how a given design component is interacting with this aspect of the wave, it might lead to some interesting innovation.

 

This curious aspect of interaction, that of the wetted parts of a surfboard and a constantly decelerating wave face is not easy to get one's head around. Especially if you view surfing a one big toboggan or sleigh ride. For one thing it gives new meaning to all that spray that you can generate when you 'hit' a lip – is it due to you carving off a section of water traveling at a slower speed, or is it you colliding with water traveling at a faster speed (as you enter a faster zone of forward flow.)

 

I'm inclined to believe this much is true – that those design features which assist you in staying in sync with the decelerating wave face are likely to be among the most beneficial, at least they are likely to reveal themselves as such over time - that is, they'll tend to be among those design features that manage to stand the test of time.

 

This last statement doesn't sound all that unreasonable, until you realize the only way you're likely to manage to continually keep connected with a decelerating wave face is by some clever use of 'drag'.

Surfboard design is all about strategically implementing a mix of passive and controllable drag, as opposed to an approach that is driven by 'eliminating drag wherever you can find it'. Well, at least that's my take.

 

kc

2

 Jeez… I sort of messed this one up. Live and learn. Anyway, I hope the following helps.

A hundred words is sort of arbitrary. And a single Tweet just ain't gonna be enough, so let's compromise, I'll try point form. You can think of it as a series of Tweets.

 

The questions (or topics) being addressed here are:

  1. Aspects of how a rail generally interacts with the face of the wave.

  2. Is this notion of 'hold' appropriate, or is it just a 'perception'.?

     

Background:

  • The motion of the water particles in a wave form is cyclic. In the leading face the motion is upwards and forwards, on the backside, downwards and backwards. The net motion, at least in the deep ocean, is relatively zero. This cyclic motion extends to great depth.

     

  • As a wave approaches a reef, sandbar, or just shallower water in general, it begins to 'feel' the bottom and slow down. It does so because the motion of the water particles is impeded (attenuated) by the presence of the bottom.

  • This slowing down (attenuation) however is not instantly transmitted up from the bottom to the crest of the wave. The upper regions lag behind, hence at any point, they are moving faster than regions below.

  • As the wave continues to move into even shallower water, those upper regions will tend to 'slide' up and over the slower layers, causing the height of the waveform to increase and deform – that is it 'walls' up.

  • If this 'lag' becomes great enough the upper portions can literally shear off – that is, the wave breaks.

 

Surfing:

  • Surfboards interact with the flow on the face of the wave. Flow is just water in motion, and here the flow associated with the face of the wave form is both upwards and forwards.

  • As stated above, as the wave continues to shoal (enter shallower water) it continues to slow down. So for example, as a wave heads towards a beach it is always decelerating (slowing down).

  • And as stated above, this deceleration is not the same for all regions of the face. The lower regions are slowing faster than the upper regions. In fact at any point on the wave face, the point immediately below is moving slower than that immediately above. (In my post I called this a differential flow profile.) With respect to the point to be made here, lets consider only the forward component of the flow of the shoaling wave (as it approaches the beach.)

  • For a surfboard traveling transversely across the face, the waveside rail will interact with this 'differential flow'. The upper surface area of the rail that is wetted by the wave, will experience a greater forward flow than that of the lower surface area (of the rail.)

  • Forces are set up when flowing water comes in contact with a rigid object. Here the forces set up on the rail are unbalanced. Unbalanced forces on an object tend to rotate it, and here the unbalanced forces set up on the rail tend to rotate the surfboard. (Its as if somebody grabbed your rail and tried to rotate your board alone its long axis as you were surfing.) The surface geometry presented to the waveface therefore plays a big role in how much of a rotation is produced. A more balanced rail, less rotation, a more biased rail, say a hard rail, more rotation.

 

Perception:

  • A the waveside rail travels along the face, it is subject to not only the flows described above, but a transverse flow by virtue of it motion across the face. Again flows set up forces when they interact with solid objects. In this case the forces are such as to 'push' the rail out of the water, the effect in general is called planing. (Which, has on occasion also been called dynamic buoyancy, as opposed to hydrostatic buoyancy. Hydrostatic buoyancy is what Archimedes experimented with. Its due to hydro”static” pressure exhibited by the fluid.) So the waveside rail if, it presents any surface area to this transverse flow, will be pushed out the face.

  • But the rail is also experiencing the unbalanced forces described above, or a 'torque' causing it to rotate. This added effect, that of rotation, when present to a great degree, say with a relatively unbalanced rail such as a hard rail, will create an even greater sensation of the rail being 'pushed' out of the waveface, or what is using described as an inability of the rail to 'hold' onto the face.

  • The term 'hold' though it may describe the experience as far as the surfer is concerned, does not describe what is actually going on dynamically. Therefore, rails that don't exhibit as much rotation as others are considered to 'hold' the wave better.

  • There are some who believe that 'hold' is the proper mechanism at play here, and see the phenomena, like the apparent stickiness of fluids as key to addressing the issue. Some see it as the fluid literally 'holding' on to the rail, perhaps via some suction effect, possible attributed to the effects addressed in, what is usually called the Bernoulli principle.

  • I do not agree (apparently), but see the effect as predominately do to what I've described above. I don't rule out the 'stickiness' of fluids, nor am I questioning the Bernoulli principle, just that there's a lot to rule out before you even considering such mechanisms, if only from the standpoint of the respective magnitude of the forces involved.

 

That's the meat of it (the post), but its not all of it (the topic). This differential flow impacts a lot more.

 

In fact there is a differential forward flow tranversely along the face of the wave. You can often see this as the curl region lagging behind the shoulder as the wave continues to shoal. Depending where you are on the wave face this will also have an impact on the interaction of your rails with the wave face. (This effect can be particularly pronounce for reef breaks, which, in terms of bottom topography are usually less gradual than bar breaks.)  This transverse differential flow also stands to impact longboard design.

 

With respect to the motion of water in a waveform, none of this is new. However, consciously incorporating it into design, which may have been done by some in the past and some may even be doing so now, does not generally appear to be the case. Unconsciously incorporating it into design, like so many other design features, has been done. That is, you need not know what is causing some effect, only how to either enhance or minimize (or control) that effect to make something that “works' the way you want it to.

 

 

kc

 

Additional...(added later)

The impact of the differential flow should be extended to the whole wetted surface -i.e. the rails and bottom. If you think about, rarely is your board deck, or bottom plane completely horizontal. Because of this torques, or moments are being generated on pretty much all the wetted surfaces.

 

Beside....

Writing is pretty much a linear thing - you make one point, followed by another, etc. You can attempt to be non-linear, or try to incorporate a parallelism by being parenthetical.. but that can be pretty annoying to the reader. (This is in addition to the inadequacies of the writer, e.g. like me.) The point I'd like to make however is that, managing the rail profile may only go so far, you also have to consider the bottom surface, plane shape, etc,  or, perhaps to state the obvious, design features are not isolated in their impact.

 

3

[quote="$1"]   If you understand.....  [/quote]

This is at once, the operative statement, and a tall order.    

4

kc,

One of the most interesting reads for me in along time. I printed it out so I can digest it a little at a time. While it may not change in a big way the shapes and contours I use, it will definitely influence the way I perceive the ride and add to the deal. Come to think of it, it will make shaping more enjoyable just thinking about what's going on between the board and the wave. Thanks so much.

5

KC, That is an interesting synopsis of swell movement. Having made those statments, I'm curious to know what design ideas you've come up with that are radical departures from what we've already seen.Seems to me, there are few options in the shape of moving an object through water.

6

Do you actually think that there are “layers” in the wave form or are you using this as an analogy? Does the “stratified deceleration” reconcile with the well-observed circular motion of current within the wave form?

Isn’t the spray on an lip maneouver just displaced water (root spray)?  It’s just the same root spray as a bottom turn but it appears to be deflected upward because of where the turn is being made. 

7

Kcasey, Thank you for bringing back a fond memory of my father and my Mother’s father. Dad taught me how to surf, though he never as much as once did more than body surf. Grandpa built pleasure boats for Huckins, Cris-Craft, etc when they were of wood construction and he never surfed. Both have long since “crossed the bar”, but they understood the physics of our world in lay terms. I remember as a young man hearing them discuss the physics of surfing in that it was a function of gravity and the surfboard and rider “falling down the face of the wave”. At the time I was sure acceleration was a function of the process and that it came from the board and rider, I was astonished to hear their words and at the time thought they were speaking out of school.

8

 

I had no new designs in mind when I wrote the post.

 

Also I'm inclined to agree, if you meant that any future changes in design that we are likely to see will probably be incremental with respect to whats already out there (excluding new technologies.)

 

In the final analysis, If you've got a board that 'works' for you, or know what kind of board 'works' for you, and don't really care how it does it, great. I seem to differ on that last point – I'm sort of curious. For me, I actually get pleasure out of having some functional notion of whats going on. Not knowing (or being wrong) however, doesn't seem to keep me out of the water. (Also, it has impacted my personal design choices.)

 

But, and I throw this in just to be clear, and its not necessarily directed at you. There are people who frequent this forum that are true master journeymen, as well as just some great designers - I'm not one of them. Use my 'nonsense' or not.

 

kc

 

 

9

You’re right, but only if you take the
analogy too far. I used the term here, perhaps not successfully, to
address the degree of attenuation as you ascend from the bottom to
the crest.

 

If you were to move through a wave, from face to back along along a horizontal straight-line, the velocity changes from forward and up to backward and down. So its not like there are physical horizontal sheets of uniform flow. But if you restrict the analysis to the face, at least you don't extend it to deep into the wavform – thinking of it as layers ready to shear off isn't a bad one – and I mean that functionally. Unless you're surfing a hydrofoil, in general a surfboard's interaction with the flow tends to be pretty superficial – not completely so, but you tell me, what's the deepest fin you have?

 

With regards to the trajectory of cyclic motion, yes its consistent, the lower portions of the trajectory distorts because of the varying degrees of attenuation. 

 

As for the root spray, sure I get your point. But hopefully we can avoid some headaches arguing relative motion. Here the 'extra' spray, can be attributed to the fact that the surfer has moved into a region of higher flow. Sort of like if you were in a canoe and moved into a section of river that was moving a lot faster than the section you just came from.

 

kc

 

10

 

Thanks. But you may not like what follows....

 

The view of what makes 'surfboards' go has (hopefully?) progressed a little since then however. Surfboards can, seemingly toboggan down the face of a wave, but the primary source of propulsion comes from a unique application of planing. You can't trim a toboggan – you're either converting gravitational energy into kinetic energy, or you're burning off kinetic energy that you've just acquired after dropping through a gravitational field. In surfing you can 'trim' -i.e. move without falling through a gravitational field. The energy comes from the flow of water in the wave face -the same forces that are involved in moving a small stone around a driveway or patio with a jet of water from a garden hose are at play.

 

kc

 

11

I’m not disagreeing with you at all, though most of us can’t escape gravity. I hope this string continues to get input from others. Given no two waves are alike and bottom contours can change significantly in a short period of time at least where I am, it seems any shape is destined to be non-optimal with time (argues for a large quiver). Nevertheless, my original post was more a thank you for bringing back a memory of two gentlemen who did not surf, but had a reasonable grasp on some of the physics in the matter at hand and managed to astonish at teenager who had no idea of the forces going on in one of the thing he loves most.

12

Ok, let’s bat this around a bit…

“planing, the primary means of propulsion in surfing”

Perhaps on logs, where peak static speed of glide is limited by the algorithm of planing area, drag and kinetic energy (wave steepness, swell speed, etc).

For high performance boards, however, currently evolved foil/rocker/bottom/fin packages enable the surfer to redirect kinetic energy along with the self-generated surfer energy through the package which acts as a force multiplier creating thrust power far exceeding the limits of static planing, allowing angles of attack that dominate natural wave dynamics and create even more energy.  And that is now the primary propulsion in surfing.

There is a good reason why the changes in surfboard design the last 20 years have been incremental rather then revolutionary. 

The boards are now so well designed and executed, the revolution lies not within the boards, it lies within the surfers, the best of whom are now so supremely conditioned, imaginative, and confident they are exploring new vectors that almost, but thankfully don’t, defy imagination… 

The “Oh My God” wave of Laird’s at Teahupoo many years ago is now an average ride whenever it breaks.

A 50’ bomb Shane Dorian recently paddled into at Jaws was only considered ‘towable’ until just the past couple of years.

And whereas the boards will continue their incremental improvements, an average surfer will never be able to go out and surf Pipeline solely because of some quantum design leap - it will only happen if the surfer improves well beyond average…and that’s exactly how it should be.

 

 

 

 

13

KCasey, I never mentioned that your thoughts on the subject were "nonsense". My viewpoint of surfing is to tailor boards to my style and the different places I surf. I did mean that any further devlopement will be, yes small increments, and in my view in the fin/tail/ rocker areas. I figured with all the thought you have put into it that perhaps you had ideas.The study of  naval architecture is as old as human history. My main comment was, that we've already found what goes through the water easiest. The future is in the refinements. You may not think me to be a true master journeyman, and I'm not great designer but I know what works for me and my friends. I've learned alot and hopefully always will keep learning through exchanges of IDEAS and not simply theories. It's nice to have a conclusion to one's musings. I see you are from the Cape. It so happens my family has been involved in ship building since the late 1700's for the whaling industry. Take a trip to Marion or Mattapoiset and you will see my family name around both towns. There is a private school on the site of my great-great grandfather's shipyard in Marion. The name Blank is only the the start of it. Having rambled all that out, I'm trying to say that boatbuilding has always been a part of my family's life. Some of it has trickled down even to me. I ain't the sharpest in the shed but I know what works.

14

It’s topics like this that make me wish Doc still chimed in, once in a while.

 

kcasey…

Do you have either edition of this book?

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15

My reply was a bit defensive.  My comments about master journeymen and great designers was self-directed - in particular, that I was not to be counted among them. It in no way was it meant to be directed at you.

I drive by Mattapoiset and Marion all the time, I'll make a point of dropping by.

kc  

16

Bascom's book is a classic. I've never read the revised and update version though.

As for Doc, I saw him at the supermarket a while back... so unless he died in shucking mishap (he's an oyster farmer) I suspect he's still around.

kc

17

KC, this is a great thread, I love it! When I surf, I always enjoy trying to tri-dimensionally visualize how the wave height, period, swell angle, tide, ocean floor contours(channels, trenches, sandbars, points and reefs) and wind direction and intensity interact… and I am only talking of the wave alone with no one surfing it, you have taken this to a whole new level and my brain synapses are firing! Reading this makes me feel like on a magic mushroom trip! Thanks!

18

…ironically it also makes me remember one of my favorite comedy routines by the late George Carlin when he talked about airline procedures: -As soon as they close the door to the aircraft, that’s when they begin the safety lecture. I love the safety lecture. This is my favorite part of the airplane ride. I listen very carefully to the safety lecture, especially that part where they teach us how to use the seatbelts. Imagine this, here we are, a plane full of grown human beings, many of us partially educated, and they’re actually taking time out to describe the intricate workings of a belt buckle.

“Place the small metal flap into the buckle.” Well, I asked for clarification at that point. Over here please, over here, yes, thank you very much. Did I hear you correctly? Did you say place the small metal flap into the buckle or place the buckle over and around the small metal flap? I’m a simple man; I do not possess an engineering degree nor am I mechanically inclined. Sorry to have taken up so much of your time. Please continue with the wonderful safety lecture. Seatbelt–high-tech shit! -Sorry about that KC but to keep the discussion relevant, have you surfed a surf mat? I beleieve that it can be a great tool/research vehicle to help underatand the interaction of surf craft in general with the wave dynamics you are talking about as it has variable rocker, planing area, rail shape, flex etc. and you actually get a close feel (literally) to what the wave is doing plus it likes to ride in the pocket naturally seeking to flow with the wave’s movement and energy… Flow, what a wonderful word…

19

kc, good to see you back on here.

The physics of surfing is not ''settled law'', which is a great thing IMO. I really don't want all the mystery ever taken out of it. I just spoke to an oceanographer friend the other day and asked him if science was any closer to modeling what goes on in the upper portion of a plunging wave. His answer? ''Not really''. I smiled.

This picture makes me smile too. Ropes!

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Hey Mike,

Luckily the choice isn’t between ignorance and bliss… it appears you can have 'em both.

Interesting photo.

kc