# The Decelerating Wave-Form

A shoaling wave is constantly slowing down.

All shoaling waves decelerate.  This deceleration is often imperceptible to an observer, but it can be quite dramatic - as if the wave seemingly just stopped and “jacked”.

The deceleration is the result of a frictional interaction of the water particles currently experiencing the wave action in the very lowest portion of the wave-form with the bottom topography. This slowing down will continue to increase as long as the wave-form continues to move into shallower and shallower water.

Communication between the upper and lower regions of the wave-form is not instantaneous.

The deceleration currently experienced by the lower portions of a shoaling wave is not instantaneously transmitted to the upper regions.  This is why waves break they way they do. The uppermost portions, attempting to continue move forward, slide up and over the lower portions which continue to decelerate.  When the difference in deceleration between the upper and lower portions of the wave-form is sufficiently great - the wave “breaks”.

As the wave shoals the height of the wave-form increases.

As a wave starts to shoal, a large portion of the kinetic energy (energy by virtue of relative motion) of the participating water particles is transformed into potential energy (energy by virtue of relative position, here, position in the gravitational field of the Earth) as the upper-most particles continue to slide up and over the lower portions. (Some energy will be lost due to frictional interaction.)  This increase in height results in another acceleration which is of paramount importance to surfers – an accelerating upward flow of water, which on the leading or forward face of the wave-form, manifests itself as a flow up the ‘wave-face’.

Drag keeps a surfer connected to the deceleration wave-form.

But if a shoaling  wave-form is constantly slowing down, what exactly keeps surfers from being seemingly launched forward, that is leaving the forward wave-face in the direction in which the wave-form is progressing?

Newton’s First Law states,

“Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.”

(If you like you may insert ‘constant velocity’ for uniform motion. During Newton’s time, he and other scientists of his day, saw them as one in the same thing.)  Therefore, if the surfer is to remain connected to a decelerating wave-face, there must be some force or set of forces that similarly slow down the surfer, or similarly decelerate him, at least in the direction in which the wave-form is moving.

There are a number of forces that can be at play here, but the dominant force that keeps a surfer on the wave-face is drag. In particular, the drag which is generated from the interaction of the surfboards wetted-surface (which  might include fins, etc.) and the flow of water past that surface.

Some of the other forces might include wind-resistance (another source of drag), or the surface tension, or that from material interaction (surfboard surface and water in this case). But by far, the largest contributor is the drag generated by the wetted-surface and the flow of the water up the wave-face.

The curl region is decelerating faster than the shoulder region.

The ‘curl’ (the region where the wave appears to be breaking or where breaking appears to be imminent) is an important region to surfers. Curiously, this region however, relative to the unbroken portions of the wave, is decelerating faster. That is, the ‘shoulder’ region, that part of the wave-form which has yet to break, is actually moving faster than the curl.  The transition between the two regions generally appears quite smooth and continuous. at least for most of the breaks that surfers frequent. So smooth, as to be almost imperceptible in terms of the difference in deceleration as you move from shoulder to curl.

The flow up the wave-face is greater in the curl region.

And in a manner similar to that described above, the degree of upward acceleration of the wave-form is different for these different regions too. Slowest for the shoulder regions, becoming increasingly more rapid as you move towards the curl.

So surfers have to contend with at least two kinds of acceleration; a deceleration which is greatest in the curl region, and an upward acceleration which is also greatest in the curl region. Both of these accelerations becoming progressively lesser magnitude as you move out onto the shoulder.

As mentioned above, the dominant forces that keeps a surfer on the wave-face is drag, and that drag is generated by the interaction of the surfboards wetted-surfaces with the flow of water up the wave-face. So the acceleration of the flow up the wave-face is used in part as a way for the surfer to stay connected to the wave-form - allowing him to decelerate in kind as the wave continues to shoal.

Not mentioned, gravitational acceleration/force exerted on the surfer/board.

EDIT:  Also, without centripetal acceleration/force, there can be no turning**…**

Welcome back kcasey !

Or have I just missed your recent posts ?

Thx.

without a breaking wave face

how do open ocean foil boards work?

gravity and pressure against the wings of the foil generating lift?

or are there circular forces within the wave form that are ongoing until it hits the shoals to slow down?

interesting thought

The Unbroken Wave

The water waveform has no mass.  Only the waveform itself travels toward the shore.  Watch the yellow dots.  I believe the wave does not break if the water is deeper than the wavelength.

http://www.acs.psu.edu/drussell/Demos/waves/Water-v8.gif
http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html

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that’s what I was taught oceanography 101

that the motion in open ocean is circular but stable (that’s what will be generating electricity for some coastal communities soon)

only the energy is passing through

like a sonic wave passing through air

things change once the bottom becomes a factor though

the cresting creates another phenomena

but the circular motion is still there it just get’s pushed up further

that’s why when you duck dive

you need to penetrate deep enough to get past the core circular motion energy

or you are going over the falls upside down

something i’ve been very good at

I disagree with the idea that it is drag which keeps the surfer on the decelerating wave form. It still has velocity even though it is declerating. Furthermore, lift is also a force keeping the board connected to the wave.

I’m don’t follow your argument.  If the wave is decelerating, what is decelerting the surfer in kind?

Say you are being pushed at a relatively constant speed while on a skateboard. But at some point the person pushing you tires and begins to slow down, unless he holds on to you, or friction, air resistance, or you’ve somehow managed to get your friend to push you up an incline, that is, if there isn’t something else present to decelerate you, you will keep going, putting distance between you and your friend -i.e. you will lose contact with your friend, or you will fail to stay physically connected to him.

As for lift keeping the surfer connected to the wave, perhaps you could expand on the too.

As you stated different parts of the wave are accelerating and decelerating at the same time. The main objective in surfing is maneuver in and out of those parts of the wave. You just can’t think of the wave as a whole system.

Let me clarify my statement though… I am not saying that drag doesn’t play a role, and am saying that it isn’t “the reason”.

Yes, we don’t surf on flat water. In surfing you are being pushed up and incline (quite often).

Fins and rails also create lift, and they are often times countering the forward motion of the waveform.

I’m still not following your argument.

Drag, in engineering terms is taken to be in a direction perpendicular to lift and parallel to that of the a given direction of motion. The direction of motion is usually taken to be a ‘desired’ direction of motion. For example, say the direction in which someone is walking, and in that case wind-resistance might be generating some drag. But the term is often used in more general terms, as in to hold, push or pull back, which I have no problem with.

Newtons Second Law states that an object traveling at a given velocity will simply continue to travel at that velocity unless acted upon by a force or some component of a force that is parallel in direction to that velociy. If at some point you are moving forward at the same velocity as a wave-form, and at some later time that wave-form starts to decelerate, you will just kept moving forward, unless you also decelerate in kind. Therefore some force must exist that similarly decelerates a surfer on a decelerating wave-form. That force is a drag force, a component of the force generated by planing. (See prior post “The Decelerating Wave-Form”.)

As for the role of lift, in particular how it relates to this, I still don’t follow you argument here either.

Again, in engineering terms lift and drag are perpendicular. If an object is traveling is one direction, say forward (whatever direction that might be in this case) and you apply a perpendicular force, the object will begin to change its direction, but no matter how much or how long that perpendicular force is applied, it will never reverse or even decrease the velocity in that initial direction.

Some force, or component of some force, has to be acting in a direction parallel and opposite to the forward motion of the wave-form to keep a surfer ‘connected’ to the decelerating wave-face, and as stated, that force is the drag component of the forces of planing.

not disagreeing, but you also have to factor in that the surfer can turn the surfboard back into the wave if he begins to get out ahead of it

Lift and drag are perpendicular to net flow (from the moving object and moving fluid). e.g. you can create lift and drag without moving the object and ONLY moving the fluid.

I agree, except in surfing waves different sections are decelerating a different rates, also there are sections of some waves, where the flow is opposite (really the flow of a wave can counter the waveform direction at many angles) of the direction of the wave form. Surfers can move in and out of these sections to stay on the wave.

On most surfboards there are multiple (lift and drag creating) surfaces aligned at countering angles. The hull can be creating drag in a similar direction that the fins are creating lift. To put it more simply, one of the main functions of fins (specifically side fins) is to “pull” the board into the wave face.