Bottom Contours - Less Is More

General Discussion
61

hi roy

i agree about the gravity thing in takeoff/trim etc

but when i exit a bottom turn on a shortboard that is when im travelling the fastest

assuming i compress/release my legs

now if gravity was the main force thoeretically i would be loosing speed while exiting the bottom turn

but thats not happening

its funny when people talk about sweets spots with regards to trim

I know that good surfer DO get more speed in trim

however to me the “real” sweet spot is the one that allows you to lay the rail so hard on a bottom turn

that when you release it, your almost left behind

Wish i could find it more often.

hey Kevin

so in your opinion is there any standard design aspects

(besides concaves cuz i dont use them as i dont understand them)

that i should be directing my though process to??

i think that to reduce all the little bits and bobs in shape design

ie keep it simple and functional

and concentrate on materials and appying them using sandwhich panel thoery

is a good thing for me in my journey to make a nice board for myself

i can say,

hmm i think on this board ill make the rail with quarter sawn so it is stiffer with more pop

and ill make the bottom skin flexible and the deck from the mid point to the nose stiff

the evolution of shortboards doesnt have to stop at what pros are riding thats for sure

cuz we are talking one kind of style of riding

i agree its dynamic

but so is drop knee bodyboarding (in certain conditions anyway)

and pro snowboarders and skaters could really bring something outstanding to the sport of surfing

now lets shape a board to suit Rodney Mullen(skater)

if he ever takes up the sport

what would that look like i wonder

no fins? hard rails? uni directional? concave deck? pop?

62

My apologies Tom (I will try not to make the same mistake again. It was not deliberate.)

I believe that propulsion comes from both the flow, and occasionally from what you might call slieghing. If you’re interested in an expanded explanation please see the thread “Surfing the Force.” I have already posted some explanations and diagrams, and intend on posting more.

If you are convince that surfing is falling, that’s fine with me. If you believe that holding a line during trimming is falling without falling, that’s fine too. I do not? I have stated (very roughly) that propulsion for the most part comes from the flow of water in a wave, that its that flow which is impacting the bottom of the board, producing a force. And it is through bottom presentation (controlled by the surfer), directing that force that forward motion is achieved. If you believe this is wrong that’s fine. I have no lock on the truth, its all argument, and in the end, it’s whatever works for you.

Kevin

63

also

im pretty much a kook even after surfing for twenty years

so i need a design that works for kooks like me!!

i think a lot of shapers assume that there client is actually a good surfer

and design for a good surfer

64

I’m still sorting out the general principles. I ain’t ready for prime time.

Kevin

65

too many questions for this tired old skull…

besides NONE of this stuff matters anyway…

what’s REALLY IMPORTANT is that you ride a COOL LOOKING STICK, so you can pick up HOT BEACH CHICKS

66

…and another thing!

67

show off

but your right!

that board would get even me laid

speaking of boo

did you do some peel tests?

i really wasnt happy with the peel strength on the bamboo woven mat

hey kevin

it sounds like your ready for something BIG! :slight_smile:

youve certainly got me thinking

all the way to work i was hitting my head over roys gravity thing

of coarse its the most fundamental principle.!

the surfer attemps to keep gravity and force of flow from the wave in equilibrium!

i keep imagining a ping pong ball suspended by air jets

thats basically surfing

68
Quote:
Quote:

Also, I agree with you about the ‘gravity’ sports. . . . The dynamics of all these sports do overlap with surfing to a degree, but surfing is unique among them in that the power source -i.e. the means of propulsion, comes from the interaction of the bottom of the board with the flow of water in the wave. In surfing, role of gravity is quite different than in ‘gravity’ sports, and the other sports you mention. Surfing can be, but usually isn’t a sleigh ride.

Kevin

There is very little difference between a sleigh ride and a wave ride. . . a wave is a sloping surface and so is a mountain. … . they both interact with the bottom of the vehicle, and both are ‘gravity’ sports . . . the means of propulsion in both cases is gravitational potential energy, the difference between the two is in the way that gravitational potential energy is imparted to the vehicle. . . in surfing the ‘mountain’ is constantly moving upwards relative to the surface of the earth, but the direct means of propulsion remains the same as if the mountain was not moving.

:slight_smile:

Just thought that bore repeating, Roy.

Although I don’t know why you guys are talking about sleighs. Don’t you mean snowboards for a better analogy? Snow just kinda sittin’ there–you falling and presenting your board at different angles to redirect over the mountainside.

And wakeboards–if you follow Kevin’s argument, the “redirection of the waterflow” by you the wakeboarder is the means of propulsion. Of course, in reality, you’re moving over the water pulled by a motorized vehicle burning fossil fuels, so, as with surfing, you’re picking up relatively stationary water by your forward energy (motor/gravity), combined with lateral rail penetration made possible by gravity and your muscles, redirecting the flow’s force at and over lateral control surfaces which results in lateral motion. The lateral progress is entirely secondary to the primary motive force though.

I think this thread was all along the lines of the difference between passive and active drive too, BTW!

69

My Brothers,

The physics of surfing is no longer theory, this is basic oceanography 101, so there is no need for further debate.

First, check out this illustration:

http://www.crocodile-clips.com/absorb/AP4/sample/media/DJFPh063waterwave2.swf

What you are looking at is the path of a water molecule. In a wave, water molecules move in a stationery circular motion. And it’s true that water molecules travel up the face of the wave.

But the movements of these water molecules alone is not enough to allow us to surf. This can be witnessed by watching a piece of drift wood float up and over the steepest part of the unbroken part of a wave.

The real energy is in the wave, not the water. The wave itself is the energy. Another misconception is a lot of people think that waves have forward push. But as you can see there is no forward push in the water molecules (until, of course, the wave breaks). It is however, important that the wave moves forward, but there is no energy pushing forward. Look at the illustration again. The push is up, from the trough to the crest. Watch the drift wood again. It basically stays in the same place, but the wave lifts it to a higher elevation.

In surfing, the wave lifts you to the crest, and then gravity allows you to move down to the trough. Look at the illustration and pretend you are in the same position as one of the water molecules. It is clear to see that once you get to the crest of the wave you will either go over the back of the wave, or you will position the board in a certain way, at a steep enough part of the wave, where gravity can allow you to move down towards the trough.

When surfing, the wave is constantly lifting you and you are constantly moving downward with gravity. This is why it is important that the wave moves forward. When you move down to the trough, the wave must move towards where you are and lift you again. Trim is the balance between the lifting of the wave and the gravity pulling you down. Watch anyone surfing in trim and you will see the board is at least slightly pointing downward.

So what happens in a bottom turn? A lot of people think that the water at the trough is pushing you up the face of the wave. It’s true that the water molecules do move up the face of the wave, but certainly not with the amount of speed that people think. This can be evidenced by trying to do a bottom turn without much speed to begin with. A bottom turn is a combination of using speed gained from gravity to move from the trough backward to where the wave crest is, with the wave moving forward catching up to where you end up. So in essence, instead of the crest coming to you, you are meeting the crest of the wave half way.

In conclusion, waves do not have forward push, and they do not have high water velocity moving up the face of the wave. That’s not to say that you can’t surf a situation of high water velocity moving up the face of a wave, as can be evidenced by the Flowrider. This is the situation I think KC is talking about. Here, the water molecules are moving up the wave formation, which lifts the rider to the crest, and gravity moves him to the trough.

So no matter how you look at it, we owe it all to gravity. Roy is correct on this one.

70

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

If you follow a particle over a cycle, say starting in a trough, it moves in almost a circle - up and forward, then down a back. (In open ocean conditions, there is always a little bit of net forward movement, but it’s usually very small and often treated as negligible.) The point is however, water particles in the leading part of the wave form, do move up and forward, and flow is mass in motion -i.e. the water in a wave flows, and on the face, the flow is up and forward.

A flow doesn’t have to be moving all that fast to produce a lot of pressure. Consider, from a purely dimensional analysis, density which is (mass/volume) times speed (length/time) squared gives pressure, or

pressure = density X velocity X velocity

We can pop some numbers into this and see what kind of pressures we could be dealing with.

Let

d = 1000 kilogram/cubic meters (the density of water, sea water is a bit more)

v = 1 meter/second (about one yard per second, or about how fast you walk)

So,

dvv = 1000 (kilogram meter/second-squared)/ meter-squared

        = 1000 Newton/meter-squared

That’s a lot of pressure for water, in which the particles are moving at speed of 1 meter per second.

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.

That’s all pretty crude, but its only meant to provide some sense of the magnitude of force that can be generated by flowing fluids.

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. 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. (Consider the difference when riding high vs low on a wave.)

By the way, a sleigh is just my way of collectively referring to sports that use gravity as a means of propulsion -i.e. downhill.

Wakeboarding and water skiing have an non-gravity independent source of propulsion. They likely involve Stavitsky’s planing, but they do not use the forces involved as a means of propulsion, surfing is unique in this respect.

If you disagree, you disagree and that’s fine, actually its great, design for what you think is happening.

Kevin

71

Thanks, Tom… I was waiting for somebody to challenge that.

Great thread.

To add… concaves reduce bottom rocker relative to rail rocker. Vee reduces rail rocker relative to bottom rocker. So in terms of “spent flow,” vee allows it to flow off the rail while concaves allow more water to flow toward the tail. And, the more water flows off the tail, and the faster it is forced to do so, the more propulsion you’ll get. (Newton, again.)

72

Since a picture is worth a thousand words and I don’t have time to rehash a thousand words…Here you go.

73

Silly is the only one that’s mentioned this extremely important part of the equation, muscles. The transfer of energy from legs to board when turning is how one generates the most speed. The best surfers are the ones that can most effectively perform this energy transfer. Skateboarding is a gravity sport, but just how do those guys get so high in the half-pipe? They are using their legs to compress as they descend and to push up at the bottom of the ramp. There’s no possible way they can go higher than their starting point unless they are adding additional energy to the system. Can one simply straight line through every section? No, usually one has to “pump” the board to generate additional speed. Lol, that Bloke fellow is probably the exception to this.

On to the next topic. I personally ride a 9 foot board with 2x4 top and 1x4 bottom, with an 1/8" stringer. At best I have to say my boards are flimsy. I’ve always thought the flex is one thing that makes these boards work so well. That said, an overly morphic board will cause what I call phase lag. This is where the morphing of the board takes time and your manouver misses due to being late. This is extremely noticeable when the fins are loose in the box. It’s also very visible on the softie boards, half your turn time is spent merely twisting the board. Now if the springback action is greater than the morphing action, I could probably live with the phase lag issue, if not, forget it.

Bottom contours. There’s many schools of thought on this and no ultimate answer, just lots of design tradoffs. One idea is to consider the water being shed in terms of mass transfer. In physics, F=ma. However, this is only half the equation, the real equation is Force = “the change in momentum”, F = dm/dtv + mdv/dt. The use of concave changes the mass flow by redirecting some of the flow that is orthogonal to the direction of motion to be more inline with the velocity vector, thereby increasing the force which increases the kinetic energy (mvv/2). However, there’s a lot more to surfing than just raw speed. To increase control and turning ability, surfboards typically use various amounts of concave, roll, and vee. A completely flat bottom (rail to rail), will be fast, but may experience phase lag when trying to get it on the rail for turning. I believe the flat contour will experience stiction. This is where the surface tension of the water resists the outer rail from releasing. In the past, I’ve built some boards that had no roll at the rail and they were very tough to turn. I have no clue how the morph bottom factors into this, but these concepts always need to be considered in the design process.

Friction. Everything we do has friction. To maximize speed we need to minimize friction. Think of friction as an energy drain that we need to minimize. Friction has an important property in that the energy loss due to friction is proportional to the surface area. In surfing terms, the less board touching the water the less friction losses we experience. A curved surface may actually have less contact area than a flat plane. Hmm, morph bottom?

I hope this wasn’t too scientific, In the end, I think the rider is the most important part of the entire equation.

74

Interesting posts all.

For me, my major interest lies in the fundamental means of propulsion in surfing. Please consider visiting “Dynamic - Surfing the Force” if you are too.

Thanks,

Kevin

75

the most elusive dynamic in the world -and universe beyond

un containable

overwhelming all attempts to isolate and profit

are for the most part short of the real value.

any litical attempts noble and crass still leave the essential essence untouched.

the number of waves expending them selves world wide as I write and as you may read are boggling.

the seas bear us up to heights

beyond expectations

and only the familiar

can access the weightless freefall of the drop

''Gravity-Sports?

a marketing tool term for ‘’ hanger’s on ‘’

capital investment in surfing

by sports who wish to be related and equated

spread far and wide

the guy with the plywood tied to thetree

next to the creek

on his dude rench in indiana-colorado-nebraska:

river surfing

and now sleighing as a term to deliniate

wave power source measurability

truly rich.

dora would not roll in his grave

he just looks over at simmons and smiles

and slowly shakes his head

simmons frowns a little then just has to laugh

and the infectious quality of his laugh

sets off all the other immortals in the

in the heavenly line-up

into gales of cacophonic laughter

as good as any flock of birds

on our material plane…

why do we surf

because we can

and because its there.

…ambrose…

log on and shop

for ASR gravity sports logo wear

76
Quote:

Silly is the only one that’s mentioned this extremely important part of the equation, muscles.

. . I am interested in how large or small a factor this is in the overall equation.

If, for example we know what the maximum sustained horsepower which a person can produce by repeatedly doing squats is (the ‘pumping’ motion), then, if we assume that this energy is transferred with an efficiency approaching that of an immersed propeller ( probably unlikely IMO except if driving an underwater foil like a tunnel fin, but a starting point anyway). …then we should be able to calculate how much this will increase ‘thrust’ for any board and rider combination of a given mass. . . and how much it will increase acceleration and top speed. … .

I am particularly interested in (1) The efficiency of energy transfer, and (2) how much percentage wise the ‘thrust’ of the board and rider is increased by muscular pumping action. … . . and I realise that all else being equal, a rider with a heavier board will be increasing the ‘thrust’ by a lower percentage than a rider with a lighter board. … . however the rider is theoretically adding a fixed amount of thrust in both cases. . . . wondering how this might pan out.

Regards,

Roy

Quote:

Lol, that Bloke fellow is probably the exception to this.

This Bloke fellow would like to point out that weighting and unweighting by squatting down and then up is adding muscular input to the thrust equation, and that I do exactly that when I am surfing, so get off my case please… . . . this “Tom Bloke just stands there going straight” stuff which i am hearing is all BS. :slight_smile:

77
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.

Kevin

Hello Kevin

Most of your posts assert (as above) that there are two kinds of surfboard propulsion. . . the ‘sleigh ride’ due to gravity and propulsiob from flow interacting with the bottom of the board.

However you have not shown that there are two different means of propulsion going on, probably because there are not two different means of propulsion. . . . the flow interacting with the bottom is just part of the sleigh ride which occurs whenever a surfboard is riding on a sloping face. . . . . obviously gravity is constantly working, and so is the flow on the bottom of the board. . . . . this occurs when trimming in the tube, and also during the ‘sleigh ride’ (I assume that you mean dropping down the face here). … . . . you have said that the means of propulsion is different in these two cases, but it isn’t… . . .

Until this matter is cleared up therest of your analysis is built on shaky ground.

Regards

Roy

78

Primary propulsion in surfing is caused by the moving forward of the crest (if you go straight/do nothing at the bottom of the face pointed toward the beach, the primacy of the wave’s foward motion as propulsive force is obvious) and that provides you (secondarily) with a downhill slope to take advantage of, if you turn up the face and turn back down it, and yes, you can split the difference by “trimming.” But the water flowing up the face is NOT THE PRIMARY PROPULSIVE FORCE.

“Trimming” WHEREIN, you redirect yourself by presenting the bottom of your board obliquely to the water which is front of you in terms of the gravitational fall line. See skiing, snowboarding, sky-surfing, etc. Wherein you keep your nose lower than your tail becuase you’re falling, albeit in a controlled way…

You may also redirect off the water in front of you in terms of your inertial aiming point, as in a top turn or the latter arc of a bottom turn. But let us note especially that out on the flats or on the face, velocity decay is quick–turning out in the flats is actually braking, as anyone can prove to themselves very simply, or if you rip, by gouging a rail-deep powerslide on the face. The wave is moving toward you in the latter half of a bottom turn’s arc, if you time it right, otherwise you’d sink. Gravity gets you moving again after the face gouge, otherwise you’d sink. The sinking, that’s gravity too.

Now you were saying that the flow of water up the face is what’s propelling me? So what enables me to paddle onto a wave? Because I should be able to just stand up if that’s the case, and go. Oh, wave motion–oh, gravity.

So if I stick my hand out the window while I’m driving my car, and it goes up and down on the flow, that’s what is propelling me forward? Or that’s what is driving my hand up and down? Not the fact of my forward motion engendering that airflow in first place?

Surfing even as passively as just trimming takes advantage of both the wave’s (1) forward motion and (2) gravity. You need to define your term “propulsion” better. The act of obliquely presenting your board to water in front of you causes a decidedly tertiary (3) lateral motion product.

You want to go skateboarding. Turning (trimming) more or less perpendicularly away from the fall line does not constitute a primary propulsive force. Look up “fall line” in some downhill skiing stuff EDIT: saved you the trouble:

Alpine skiing fall line

In alpine skiing, a fall line refers to the line down a mountain or hill which is most directly downwards. This can be visualized as the route a ball would take if it were started rolling at the summit, and rolled to the bottom.

79

As Craftee said, a picture is worth a thousand words,

The following web page has some average pics which may put the discussion in context, if they are referred to.

Interestingly enough, all 3 fins are engaged at all times

http://www.geocities.com/cjbowdish/Jalama090203.htm

80

to get say

25 percent higher on the other side of a half pipe while skateboarding

you have to compress and release

period

everyone use gravity to surf

good surfers add the other variables effeciently

to add to it!

combine muscle control

with eficient board presentation to “flow”

and “gravity” surfers are eating your spray

ive seen standout surfers at ridiculous speeds

on pretty old dodgy equipment no less