Bottom Contours - Less Is More

General Discussion
81
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The URL of the wave is a great illustration of a water particle in a deep-water wave.

The only difference in shallow water is the water molecules below the level of the trough are eliptical. However, the water molecules from the trough to the crest still move in a circular motion. You can see this illustrated here:

http://www.seafriends.org.nz/oceano/ocean04.gif

There is no forward motion other than the forward part of the circle. The only time the water molecules move forward with any significance is when the wave breaks, and of course the white water. But since it’s possible to surf on the part of a wave before it breaks, and we don’t actually surf on the part that breaks forward (except for floaters), I find it irrelevant.

The part I don’t think you are realizing, is the water molecules don’t have any significant power. A water molecule on the surface of the wave is no different than a surfer or a piece of driftwood. Look at the animation again. Pretend you are the red water molecule. You would move in exactly the same way. And you wouldn’t really go anywhere, just like the water molecule doesn’t. I still think you are think that the water is operating like a Flowrider, but it’s clearly not.

Quote:
Lets consider an area of it (the moving water) of approximately 2 foot (.6 meter) by 3 foot (.9 meter), or 6 foot square (.54 meter-squared) not exactly the area of the rear half of a shortboard, but in the ball park. A pressure of 1000 Newton per meter-squared impacting on an area of .5 meter-squared (pressure times area gives force) could potentially produce something on the order of 500 Newton, or about the same amount of force required to lift something or someone that weighs about 112 pounds.

But it’s not the water that’s doing the lifting, it’s the wave. The water is what’s being lifted.

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That's a lot of pressure for water, in which the particles are moving at speed of 1 meter per second.
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Surfing can be a gravity sport as in a sleigh ride, downhill ski, or snowboard, but most of the time propulsion is derived from the interaction of flow and the bottom of the board. The flows to do it are there.

It’s irrelevant, because that’s not what’s happening, but it’s not even close to enough speed. Take a look at the Flowrider, which is a good simulation of what you are talking about. The flowrider requires the water move at a speed of 30 mph versus the 2 to 3 mph you are talking about.

Quote:
In fact the velocity of the water particles vary over the whole cycle of the wave form, of course the concern in surfing is only with the face, or leading part of the wave form.

Are you suggesting that the water molecules are moving faster on the front of the wave than the back?

82
Quote:

Surfing can be a gravity sport as in a sleigh ride, downhill ski, or snowboard, but most of the time propulsion is derived from the interaction of flow and the bottom of the board. The flows to do it are there.

It’s irrelevant, because that’s not what’s happening, but it’s not even close to enough speed. Take a look at the Flowrider, which is a good simulation of what you are talking about. The flowrider requires the water move at a speed of 30 mph versus the 2 to 3 mph you are talking about. "

Now we’re getting somewhere… if one was looking for multipliers to explain the lateral velocity of a good surfer going fast, one would look at the wave’s forward speed, steepness times the height of the wave times standard gravitational acceleration, the rider’s pump inputs to add speed (where concave helps),

the coeffecient of drag of the board’s concave bottom versus a flat one in a static attitude “trim” is a distant distant almost infinitesimal, meaningless . Unless that’s all you’re interested in doing.

If you want to trim on Stavitsky’s plank, go ahead, but don’t spend this much time and verbal acuity selling it.

Have a bite of this red herring here? It may not be much to eat, but I’ve got a hell of story about it.

83

Interesting interpretation.

Let me summarize the points I had hoped to make, it may help.

Water particles in a wave in deep water have roughly a circular path. The particles in the leading part of the wave form move up and forward, in the trailing part of the form they move down and back.

A water particle in motion is but one particle in a flow of particles. (This was not explicitly stated.)

That 1 m/s, a relatively low paticle velocity could potentially produce such a pressure, suggests that a greater particle velocity could produce an even greater pressure. (Pressure, at least potentially, increasing with the square of particle velocity.) Or, for even for small flows, the pressures appear to be in the ballpark for making surfboards move.

Surfers are concerned with the leading part of the wave form, where the particles move up and forward.

In the end, it comes down to this.

The water in a wave form flows. It flows up and forward in the leading part of the form, and back and down in the trailing part of the form. The flow in leading part of the form, the up and forward flow, is a source of propulsion in surfing.

If you don’t think that a flow is basically a lot of particles in motion, that’s fine. If you don’t think the water in a wave flows, that’s fine. Or maybe you do see the water in a wave as a flow, but don’t see it as a source of propulsion - that’s fine too. I’m guessing that you’ll design for whatever you do believe is going on.

Thanks,

Kevin

84

water is obviously flowing on a breaking wave at high pressure

the area in the pocket has way more power to harness than out on a shoulder

gravity is still doing it thang

but your travelling way slower on the shoulder than in the pocket

its becuse the water has more pressure in the pocket

sort of a flow vortex if you will

that intensifies as you get closer to the Curl

moving upward and then

is spent

and moves in a downward forward motion toward the beach

85

I’m going to go out on the limb here, but… IT’S ALL A MATTER OF PERSPCTIVE. What would your ride on a wave look like IF you had a bird’s eye view? My thought’s are that you would see the surfer accelerate from the crest of the wave to the trough. Then it would look like the surfer stalled as the wave catches back up to him as he performs his bottom turn. Do this turn to late and what happens? You stall and sink from lack of speed/energy?

As a surfer you’re very involved with what is happening from YOUR perspective. You fall down the face of the wave (gravity) as you perform a bottom turn there is a vector directed towards the board. You FEEL that pressure as acceleration. Then as the wave catches back up to you, you unweight and with the use of muscle redirect your vector back up the wave and do a top turn or snap turn (stall from the birds eye view) back down the face of the wave (gravity) and accelerate away from the crest. Watch the body position of a surfer or skate boarder as he goes to vert. Why does that work?

My dad as an engineer calculated that I could only make a ramp 6’ tall and reach the top at a given speed that I could achieve pushing the board on flat ground. I was adding two feet to the top of that ramp within 2 days and still wheeling out the top with in hours. Because my weight goes to 0 as I approached vert. A very similar situation that occurs as you turn back up and into the wave. It had him mystified for some time.

Have a look at the Jay Bay competition from surfline.com. You can see the competitors body position remain perpendicular the surface of the wave. Accept during the bottom turn as he is trying to get back to the energy/slope of the wave. Then he starts all over again.

Its getting late so ignore if you please. I just see this all from a different perspective.

Hafte

P.S. Silly, what you’re saying is due to the differance in steepness of the wave from the pocket to the shoulder (gravity again).

86

Yes, the water in the pocket is lifting faster. . . and is thus able to replace the gravitational potential energy of the board and rider faster, enabling the board to take a faster and steeper glide path. . . also because the water in the pocket is steeper it is easier to reduce drag by reducing wetted surface area than it is in flatter water. . . …

.

87

I remember about a year after I started surfing, I was out for a dawnie in glassy little waist high waves. I fluffed up the pop-up and landed with all the weight on my back foot. As I straightened up, the board and I (200lbs incl wettie) were shot violently and quickly to the top of the wave. Where did that power come from I thought? Well certainly not gravity, which may have made a small contribution.

Whichever way I presented the tail and fins to the board unleashed a power far greater than gravity.

Now fins have a consistent surface area, and If was just related to that then I would have experienced it more often. It has to be something more variable than that.

Why do wider tails work for small waves and pin tails work for big waves. We all know the answer to the latter, better control - but why ?

Obviously because they present less tail area and so there is less board in the wave for the wave control, its the surfer who does the controlling. Almost all the speed is coming from gravity and its a matter of the surfer controling how much power and when by straightening or trimming as appropriate.

In a smaller wave, riding a wide tail board allows to you bury the tail, which presents a greater area than the fins to the rising energy, which allows greater wave energy to be harnessed, albeit with reduced control as you have to lighten up on the front foot to harness it.

In a small wave (knee-waist high) gravity does not contribute a whole lot, and yet good surfers are generating enough speed to launch themselves.

This seemed strange to me. But thinking about it, that same little wave can raise and drop a large boat.

I think that the whole point about the water molecules is that they only move in a circular motion because they are displaced by wave energy. Its the wave energy thats moving. When the wave comes into shallow water the wave energy is moving forward AND Vertically.

Giving an effective direction related to its forward and upward movement. I believe that by presenting a particular surface orientation ti the wave energy, which is possibly an angle between the fins and the board bottom we are giving the wave something to focus its energy to push against.

Concaves therefore allow a greater surface area to be presented to the wave energy, with a reduced amount of rider needed to bury the tail.

Why do hard rails in the last 18" work ? It stops the water releasing when the rail is buried.

My finger does not have enough power push this computer monitor off the table. But if the the whole of my bodyweight is focussed through my finger then its no problem.

I hope this empirical view has contributed something other than mark me as a looney.!

88
Quote:
Or maybe you do see the water in a wave as a flow, but don't see it as a source of propulsion - that's fine too.

Exactly. I’ve tried to explain it the best I can, so I’ll leave it at that.

89

Ditto

90

Hey Burnsie - i was fortunate enough to persuade my family to go on a camping holiday to Dingle last year…I experienced Brandon Bay — the North end… at 6 foot with light off shores and killer whales travelling North…

Shit…i can’t explain any of it…but i like your attempts…

I too had a similar event to the one you describe - as to the forces…

oh dear I’m drunk and rambling and that dolphin’s a boy racer

91

Hi Silverback,

Hoping to score Brandon Bay on Sunday morning at overhead and offshores (12 knot). Its normally a 4 hr round trip for me, but its a bank hol here so double that!

92

When a wave breaks doesn’t the water suck up from in front of it? And is then sucked up the face and then goes over in the lip.

I’d say that the flow of water up a wave can be quite fast. Also the steeper and more hollow the wave breaks the faster it expends its energy.

So the faster a wave expends its energy the faster the water has to move up the face of the wave and over per time unit.

Therefore a more hollow wave has higher flow with more pressure up the wave face. Another effect of this is that the wall of the wave gets vertical in a hollow wave.

Steep slope gives more speed, just like in skiing. Flow up the face also translate into forward speed. I agree with Kevin on this.

If You don’t believe that the water flow gives speed then try a bonzer or channel bottom board in sucky waves. To an extent this works even in small waves if they suck hard enough.

I was out surfing my bonzer the other day and on two of the better waves it went into “fifth gear”. I really enjoy those sucky waves on a bonzer, just gives so much speed its amazing.

Another thing I’ve noticed is when You get those bursts of speed and accelerate on a bonzer it makes some really weird noises. I’d say its the water flow being redirected and compressed out the back that makes those weird and beautiful noises.

Anyone else experienced this?

93

Of course the water flow up the wave produces the power which gives the board speed, I don’t think that anyone is arguing about that. … at least I’m not. … . the point is that it produces speed by lifting the board and giving the board gravitational potential energy. . . which the board then expends by falling. … .steeper wave sections lift faster than flatter ones and so they do more lifting work per second, which equals more power. . … . more power to lift board and rider, giving them gravitational potential energy. . . which can be used as fast as it is gained without altering the process of energy transfer.

:slight_smile:

94

yeah Roy

i see we all agree on the same thing really

but surfboard “design” relates not to how we utilize the gravity

but how to we utilize the “flow of water”

a sqaure end plank would utilize the potential energy of gravity exactly the same way as a surfboard as a teapotwith a moose in it.

its the way the board directs and interfaces with the flow of water on its bottom.

that makes it fast/loose fun etc.

95

Umm, yes no maybe . . . . utilising the flow of water and utilizing the gravity are the same thing, they happen constantly and simultaneously, and neither can happen without the other… . .

not too many square ended planks out there on the mountains these days either. . . . .Please post teapot with moose in it pictures

Cheers,

Roy

96

ahh yes

both together

btw im utilizing gravity when im falling off

97

Agreed Summary ?: The gravitational force is operating downwards the wave energy is moving upwards and forwards

So how are we moving diagonally accross the medium(water)?

Is it the fins and rails which use the tension between the two to translate into diagonal movement, like squishing a bar of soap?

98

http://www.bennetttrimtabs.com/tabsdo.htm

Water-force on the trim tab surface creates upward pressure, thereby raising the stern and lowering the bow. The principle is simple. The results are impressive.

Properly sized trim tabs can significantly reduce the time needed to get up on plane. They also allow a boat to keep its bow down and stay on plane at lower speeds.

Performance

Increase Speed • Reduce Pounding • Correct Listing • Eliminate Porpoising • Offset Prop Torque

Efficiency

Reduce Fuel Consumption • Reduce Engine Laboring • Eliminate Squatting

99

Thanks.

Motorboats have engines (independent power sources) surfboards don’t. What creates the flow that the trim tab, in this case ‘trim tab’ being a concave on a surfboard interacts with?

Kevin

100
Quote:

Water-force on the trim tab surface creates upward pressure, thereby raising the stern and lowering the bow. The principle is simple. The results are impressive.

Properly sized trim tabs can significantly reduce the time needed to get up on plane. They also allow a boat to keep its bow down and stay on plane at lower speeds.

Performance

Increase Speed • Reduce Pounding • Correct Listing • Eliminate Porpoising • Offset Prop Torque

Efficiency

Reduce Fuel Consumption • Reduce Engine Laboring • Eliminate Squatting

Hmmmm. … . . exactly what we are doing over here . . . .

these are submerged trim tabs. … . even more efficient than surface trim tabs. . .

:slight_smile: