On this site, in discussing surfing and surfboard dynamics people often have disagreements about what forces are acting upon the surfer/surfboard. I think that there are three forces.
Gravity, Water Flow up the Wave, and Forward Wave Motion.
It seems that lots of people think that there are only two forces (pick any two).
If anyone out there thinks that there are more or less or different forces acting upon the surfer I would like to have this discussion.
Uhh oh…
A few simple points to help spark the conversation.
If gravity is not a force acting in surfing then why do you fall to the bottom of the wave if you jump off your board at the top?
If water does not move up the face of the wave then how does it get there?
If the wave is not moving forward then how does it get from out in the ocean to up on the beach?
It gets complicated in a hurry – there’s also friction (why do you slow down after the wave is over)
and buoyancy (why don’t you sink through the wave if there’s all this gravity stuff)
and lift (the whole fin thing)
and etc
Every thread I’ve read on this ends up: The whole enchilada is more complicated than anyone can handle. I don’t care if you’re a physics phd emeritus at Cal Tech, everyone jumps off the boat at some level of assumption and says, “This is enough detail for me to shape the board I want”
the water in a (non breaking) wave is moving in a vertical ellipse; you can represent the motion of the water at any point on the ellipse with one vector, so that would be one force.
Gravity is a second force.
So there’s a first approximation.
Once the wave is breaking and interacting with rails, fins, and the bottom of the board … we’re gonna need a lot more vectors.
It gets complicated in a hurry – there’s also friction (why do you slow down after the wave is over)
and buoyancy (why don’t you sink through the wave if there’s all this gravity stuff)
and lift (the whole fin thing)
and etc
OK so we add friction and buoyancy.
Lift comes from gravity and water flow.
the water in a (non breaking) wave is moving in a vertical ellipse; you can represent the motion of the water at any point on the ellipse with one vector, so that would be one force.
Gravity is a second force.
So there’s a first approximation.
The water on the face of the wave is moving up and back to the crest, and the wave as a whole is moving forward.
Once the wave is breaking and interacting with rails, fins, and the bottom of the board … we’re gonna need a lot more vectors.
Are these other vectors you are speaking of derivatives of the other forces? (gravity, flow, forward motion, buoyancy, and friction)
150 MPH is as fast as anyone can freefall.
You reach that speed in about 30 seconds.
I know…I’ve done it time and time again.
H
Oh that’s with 80 to 120 lbs. of cr#p strapped to your body,of course.
H
There’s some wind.
Anyway… identify your named forces’ vectors, and comparative percentage values.
Then go surfing and forget all about this.
My 2 cents.
The vector forces and their percentages are all variable (this is why it is hard to visualize).
It depends on the wave size, speed, shape; it depends on the surfer/surfboard size, weight and shape; it depends on the riders position on the wave, and current direction of travel.
That’s a pretty good approximation, actually. And yes, I am gonna regret getting into this one, but here we go.
Lets break these forces down into two vectors: those that accellerate the surfcraft in question and those that retard it. Push and drag, if you will.
Accelleration forces for something travelling along a wave in a straight path, not turning? Well, I make that to be MG sin A, where A is the angle taken with the horizontal. At the extremes, you’d be either freefalling down the face of the wave or slowing to a stop on the flat with no accelleration at all. No other forces are really having much effect on accelleration.
That’s Physics 1, ‘motion’ - see http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/VideoLectures/index.htm , with videos. It’s kinda cool.
What’s slowing the thing down, or at any rate defining the terminal velocity? Drag. All surfcraft are planing hulls, not buoyant hulls ( displacement hulls) when in use on a wave. No matter what sort of silliness surf terminology give’s 'em.
Planing hulls can be defined as something travelling greater than 2 times the square root of the waterline length ( in feet ) expressed in miles per hour. Displacement hulls are those that travel at less than 1.34 times the square root, etc.
Doing the math, a displacement hull that’s travelling at 20 MPH ( a not unreasonable speed for a surfcraft) would have to be 225 feet long, which really would give a new meaning to the term ‘longboard’, ya know?
Nope, they’re all planing, the effect of buoyancy is nil. Take buoyancy and save it for paddling. Along with waterline length.
Okay, so how do they plane? They push against water, generating lift. The heavier the surfcraft-rider combination, the more lift needs to be generated. All other things being equal, though, the faster ya go, the more lift per unit area is generated, so that less of the board is in contact with the water. But the drag also goes up with speed, so ya reach a balance where acelleration times mass equals drag - terminal velocity, as Herb says. There’s also small drags from fins, rails and what have you, but pushing against the water to generate lift is prolly the main component of drag.
Waterflow up the face of the wave? Naaah. If you think about it, if there was waterflow up the face of the wave to any real extent then what would happen?
Ah huh. When a wave broke, it wouldn’t throw out a lip in front, it’s throw out a lip in back. But…it doesn’t. Instead, the elliptical form of the wave rolling along hits something shallow, breaks and throws out the front.
What would make you think there was something like this going on? Well now - as the wave is moving, the water actually stays put. So you’re going along in the direction the wave is moving through water that’s pretty much staying put until the wave itself is breaking and actually coming over.
Air drag - not a helluva lot compared to water drag unless you’re really, really moving fast. Like ( again, as Herb says ) 150 miles an hour. It’s not an important component.
Now, that’s simplified, but it’s a start. Also, if you think about surfcraft as things that kinda find their own dynamic equilibrium, it’s helpful.
have fun
doc…
also- have fun with http://ocw.mit.edu/OcwWeb/Mechanical-Engineering/2-20Spring-2005/CourseHome/index.htm MIT’s Marine Hydrodynamics intro course. Also very cool, but no videos…
Doc,
Please reconsider that there IS waterflow UP the face of the wave. Obproud is correct about that.
Hi Bill,
I really have to question that there is waterflow up a non-breaking wave. Couple of reasons for that:
First off, if there was, then somebody just sitting there, not taking the wave, lets say, they would be accellerated up and in fact moved further out. Or, a buoy out there in the wave, if there was it’d be pushed further out to sea when the wave went by. But they stay the same…
Or consider a mush wave, just flopping over at the top. If there is waterflow up the wave to any serious extent, the mush/foam would all be going out the back rather than just kinda mooshing along just below the peak, no?
Second is this: the unbroken wave is moving along at a pretty good clip, as I said. If water is just sitting still on top of it, as the wave goes by, and if you’re on the wave, then water is moving relative to you but it’s not flowing up the wave. Though it’d sure seem like it.
But, there is a part of the wave when I’ll agree it’s maybe moving upwards. And that’s when it’s actually breaking, then it’s all going up and forward and out. Sort of spiralling around, making that nifty hollow area we all like to find…
maybe this’ll do a little better job of illustrating what I mean:
If we use the one at the top as waves in open ocean, and the oceanographer guys have studied them and come up with that, it’s a sort of circular motion, coming back to the origin. A swell at sea , a boat on it doesn’t move one way or another once it’s past. .
Then, we come to where the wave starts to feel the bottom. And the face goes convex. If you think about it, if there was flow upwards then wave faces would be straight or maybe even convex. But we’ve all seen it, concave. Which bears out the circular model. No upward flow yet. Kinda like, say, pushing your hand forward, just a little underwater. You make a bump ahead of your hand, but nothing really moves other than your hand.
And at the bottom, the circle is broken, as it were, ( my apologies to the Carter Family ) , the wave breaks and the upward flow that was at the back has now come forward to the wave face while the top of the circle is going out and over… you have upward flow, to a point, but then you also have flow over…and basicly, the wave energy all goes in that last gasp.
But I don’t think that’s what obproud was referring to… though as always, I could well be wrong on that.
Best regards, as ever
doc…
there’s waterflow up the wave at the base of the wave:
but once you’re near the top you’re right, it’s heading forward again.
wiki has a nice pic of how the movement deforms as the wave approaches a beach.
the entry on breaking waves is a little odd:
Plunging
Purportedly a rather dramatic form of break. The crest of the wave curls over and crashes into the base of the wave, creating a sizable splash.
It tends to happen most often on steep beaches.
“splash” isn’t really the word that comes to mind.
“splash” isn’t really the word that comes to mind.
Right, especially when Teahupoo ( if I didn’t misspell that it’s a miracle ) is taken into consideration.
Actually, looking at that wave, it’s not a bad model for ‘circular motion, tripped up’ or interrupted by a sudden shallow stop.
Now, if I may jump a little further:
from http://en.wikipedia.org/wiki/Shallow_water_equations - italics mine
The equations are derived from depth-integrating the Navier-Stokes equations, in the case where the horizontal length scale is much greater than the vertical length scale. Under this condition, conservation of mass implies that the vertical velocity of the fluid is small.
So, I’ll take it as read that vertical flow is pretty much negligable, except as the wave breaks.
Having had a rotten day and having got on the outside of a fair amount of Liebfraumilch, I am not gonna attempt ‘hyperbolic partial differential equations’…indeed, it’s been a decade and a half since I played with them with malice aforethought. If indeed I ever was able to do it well.
Best
doc…
150 MPH is as fast as anyone can freefall.
You reach that speed in about 30 seconds.
I know…I’ve done it time and time again.
H
This fellow freefell at over 750 mph (but at 12 miles up there was almost zero atmospheric resistance):
But to stick to the topic, its mainly gravity: sliding down a moving hill of water, except when “porpoising”, like a body surfer, body boarder, mat rider, extreme flex rider or some form of sinking into the energy under the surface of the wave. If you put a marble on a blanket, and your hand underneath, you can push the marble around even though the blanket stays more or less stationary. You hand is the energy of the swell, and the blanket’s the water.
Or something like that.
Liebfraumilch is defined by the German Wine Law as one step above the lowest category,
Which pretty much defines my oenophile aspirations…
Nice graphic, isn’t it. Wouldn’t it be neat if you could hook that up to a Theremin?
Meanwhile - allangibbons pretty much has it too. Though I wonder if porpoises ( and seals ) are a slippery enough form to go inclined with a gravity ride at the front of the wave. It’d be useful if we had underwater photos in profile. .
And, when you think about it, it kinda confirms gravity as the operative force and the lack of water going up the face of an unbroken wave. When you think about it, the seal…or porpoise…would have to be amazingly hydrodynamicly slippery to go against an upward flow in addition to plain old water resistance just using gravity.
doc… one step above the lowest category,
Hi Bill,
I really have to question that there is waterflow up a non-breaking wave. Couple of reasons for that:
doc…
With all due respect Doc, and in deference to your excellent explanation, it is only breaking waves that we ride.
Log on now to the Chopes contest and watch it live . if there is no water flow up the face then all our sensory input must be suspect…an that is a whole other philosophical argument.
How do you explain the water drawing off the reef and rushing towards the base of the wave to then become part of the lip. This water is obviously, measurably moving.
Steve
