Dynamics - The Trim Equation

General Discussion
141
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okay roy

you seem to be on a similar wavelenght.

you gotta get to the top of the waves somehow to utilize that gravity

so how do you get up there against gravity?

its the water flow right!

bouyancy and horizontal wave movement cant be the only factors

Yes, water flow moves board and rider to the top of the wave .

Buoyancy isn’t a source of propulsion though. . . buoyancy is just the interface, like the towbar between car and trailer.

Sources of propulsion:

  1. Vertical flow up the wave, converted to gravitational potential energy and then to kinetic energy.

  2. Horizontal flow, converted to kinetic energy.

  3. Muscular input from the rider.

  4. Vertical flow of air up the face of the wave.

According to my simple calculations, no.1 is typically at least 10 times as powerful as (2) & (3), not sure about the magnitude of (4)

I am grateful to Kevin for this discussion, previously I beleived what I had been told by Dr Kerry Black of artificial reef infamy. … . that the circular movement of water particles in a wave is on a microscopic level. . . . so it was an eye opener to see those circular flow simulations. … . however Kevin is definitely mistaken regarding the way horizontal flow drives a board, it is not only much less of a factor than he maintains, it doesn’t work in the way he has described. . . his desccription is closer to what happens with vertical flow against gravity than it is to do with horizontal flow against inertia.

:slight_smile:

142
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yachts can surf deep water waves!

Yes, but can a surfboard?

And is the yacht using the upflow of the water particles, or the gravity invoked by the slope of the wave?

143

TomBloke,

Quote:
Yes, water flow moves board and rider to the top of the wave .
Quote:
1) Vertical flow up the wave, converted to gravitational potential energy and then to kinetic energy.

I think we agree here, but I don’t think that water flow moves the board to the top of the wave. I think that we would float to the top of the wave regardless if there was any water flow. The water is shaped by the wave. We float and move the same as the water surface below us. Returning to this animation:

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

Mouse over the red water molecule and you will see that it’s the wave that moves the water molecule and carries it from the trough to the crest. If you picture yourself as the red water molecule, you will see that as far as being carried from the trough to the crest, it’s the wave that carries you, not the water molecules. You are just along for the ride just like the water particles. I agree with everything else.

I like this discussion also because it has opened my mind to the fact that concaves might be more than just a means to reduce rocker. Pushing off the water is a big part of surfing and the edge a concave bottom makes could contribute to a little bit more grip to push off from.

144

It’s shadow-boxing. It’s pre-Galilean cosmology. It’s the tree you stare at while trying to see the forest. It’s a child’s eye view of the moon on a nighttime drive. It’s a new variety of relativistic quasi-pseudo hydromechanics. It’s a new religion on the planet.

Hooray and huzzah, Roy–you have a new lease on internet surf-crankery. You should write a book, is what YOU should do. You’re the anti-Greenough. How’d that GPS thing work out?

145
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It's shadow-boxing. It's pre-Galilean cosmology. It's the tree you stare at while trying to see the forest. It's a child's eye view of the moon on a nighttime drive. It's a new variety of relativistic quasi-pseudo hydromechanics. It's a new religion on the planet.

It’s a little bit country. It’s a little bit rock and roll.

146

is this water cooler chatter or is there some real world purpose to all this?

You can’t just take one factor and develop a discourse on it for real world applications.

what about surface chop?

what about 1-20 mph offshore or sideshore winds that often blow up or along the side of the wave face?

what about the pitching lip as it throws 20 yards down the line or the spray blowing out of the collapsing tube behind you?

what about the foamball encroaching on you from behind and swallowing water beneath the board?

where are these factors in the calculations and drawings and at what point do they begin to nullify all the fluid dynamic theories being toss about here like fools gold?

Wave, man, surfboard and nothing else just wish it was that simple…

Again the purpose?

A better board?

A better technique?

A bigger ego?

what will the results of these 6 pages of discussion lead to regarding board building other than a faded memory days from now?

I guess maybe it’s jus doing a walkabout in the billabong of sways…

For me its this new antifouling nanoparticle undercoating that mimics dolphin skin flaking to alter and dynamically manage the affected boundry layer underneath said object that interests me…works of Carpenter, Max O Kramer, and Gray peak my interest.

147

Hi Oneula,

In answer to your question, personally I am finding this discussion to be a fascinating pastime, and it is therefore useful in itself… . . and it’s also practical (at least for myself) because for the first time I am getting some rough idea of the relative magnitude of the forces which drive a board. . . and how they behave. . . I now know more than I did when I started the conversation. . … . you raise many relevant further topics for discussion, and (if I read you correctly) do so in order to make the present discussion look futile . . . . . but it is a valuable discussion. . . .

roy

148

All,

Please not response to multiple parties.

I don’t think the water flow push you up to the top of the wave. Think about it. Does it feel like you are getting pushed or sucked to the top of a wave? Like on a river. or does it feel like you are floating up there.

Isn’t the upflow just the manifestation of the travelling wave energy? Doesn’t that energy make water molecules oscillate? And that cause the wave to rise up higher.

Since water doesn’t compress or expand, where does the wave shape come from?

Yes, if you’ve read any of my previous posts you will see that it’s the wave that’s important, not the water particles. The wave itself is nearly all of it.

Sure, but aren’t the water particles an integral part of the wave taking it’s shape? Seems to me that the particle oscillation sort of causes the wave shape. Then the load (board plus rider) float up that wave and then harness potential energy via gravity.

This is too obvious. Of course it’s about using the wave to allow us to use gravity.

OK. The wave motion and buoyancy like us up into the air, giving us a potential energy load. If the wave were to vanish we would fall, which is where the the Ug is released. Accounting mechanism? Perhaps. But keeping track of it allows us to see the transfer more clearly.

I think here you are talking about surfers exceeding wave speed, not exceeding the speed of gravity. Isn’t terminal velocity over 200 mph. How fast can you go down a hill on your skateboard? This is the sort of gravity I’m talking about.

Nope. Vt of a falling human is maxed out at something like 200km/h. But different objects have a different maximum Vt. The Terminal Velocity equation is:

Vt = sqrt ( (2 * m * g) / (Cd * r * A) )

This tells us when a given object will stop accelerating and maintain its current groundward velocity. Objects will accelerate downwards at 9.8m/s (less the effects of drag) until they reach Vt. This is simplest when dealing with and air drop. Boards in the 6’ range with an 80kg rider will have a Vt somewhere around 87m/s (approx 24km/h) and a 9’er will be around 66m/s (approx 18 km/h). If you aren’t doing an air drop (i.e., somewhat in contact with water) drag will increase and those speeds reduce.

In brief, that’s what I was referring to. That’s the gravity I am referring to.

It’s more about using the wave to tap into the energy of gravity.

Consider: “In general relativity, gravitation arises out of spacetime being curved by the presence of mass, and is not a force.” and “The gravitational attraction of the earth endows objects with weight and causes them to fall to the ground when dropped (the earth also moves toward the object, but only by an infinitesimal amount).”

So - does gravity, in and of itself, have some kind of “energy”? No. it’s treated that way prior to general relativity because it’s the only way to fill holes in certain equations (note that the results are generally accurate enoughg in macroscopic equations).

The idea of potential energy is that the amount of force it takes to lift something sort of “charges” that “potential energy”. This is released when the lifted object is no longer supported. The higher you lift something the more force (energy) it takes to do so. And the more force it will have when it completes it’s fall. There are other versions of this (because I have been talking about gravitic potential energy). I suggest reading the link I posted above for a deeper understanding of potential energy - it’s a good starting point.

I have, necessarily, simplified things a little.

I agree. That’s what I was trying to do too.

Good man! Me too. And hope my discussion above doesn’t come across as nit-picking. That isn’t my intention.

Note that surfers commonly exceed 24km/h. I attribute a portion of that to wave forward force, as I listed previously.

Further regarding upflow vs buoyancy. If you submerge a surfboard in a swimming pool and then release it what happens? There is no upflow in the pool.

For me this is really interesting discussion! I’m really enjoying it (snide comments from certain individuals aside)! Anyone who doesn’t like it should feel free to ignore it.

Then again, maybe they are scared of knowing the truth? :wink:

149

Janklow,

Vertical water movement:

I am saying what I have been saying all along: that the wave lifts the board up and then the board slides back down due to gravity. … . . … . the gravitational potential energy is gained via the wave lifting the board. . … . I thought that you agreed with this ? Perhaps you are reacting to the word ‘flow’ . . . would ‘the water particles travel upwards’ suit you better?

Horizontal water movement:

  There is horizontal water movement, and clearly it can exert force on a surfboard, if and only if that surfboard resists the flow. . . . the only means the surfboard has to resist the flow is inertia. . .  not gravity, because gravity is at right angles to horizontal flow. . . . . thus horizontal water movement cannot exert antywhere near as much force on a board as vertical movement can.

I am not subscribing to any of Kevin’s theory at all, other than by admitting that horizontal water movement IS a force. . . .

:slight_smile:

150
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And is the yacht using the upflow of the water particles, or the gravity invoked by the slope of the wave?

The two are part of the same process. . . the yacht is lifted up by the water and then it slides down again using gravity. … . where’s the disagreement, I don’t see any.

:slight_smile:

151

And Roy

I bet there’s more to your thin flexing dragon (anti-draggin) boundry layer manipulating design than you lead on to us about…

Manipulating the frequencies of turbulance to whitenoise them out is something I don’t think anyone sees in those narrow and long flexing experiments of yours… the only ones doing anything similar is the mat guys… That’s where you’re getting your “I need a GPS to track this” feeling from I bet…

And Bert and Jim are tackling it from a performance aspect

But the concept of give and release (i won’t call it flex) and even avoiding the conflict (air) will prove to be very important in future generations of watercraft.

152

"In the middle of the Outback, there is a new amusement ride for the indigenous people of Australia, given by the fantastically wealthy Mr __________,who made his millions with his book of archaic, arcane and metaphysical surfboard design theory. Having long ago perceived the misdirection of several trains of his thought, he decided to give back to the world in this way.

A flexible sheet of galvanized steel, textured slightly for traction, 500 yards wide and 1 mile long sits on a loose gravel medium. A fleet of tractors with 3 foot diameter rollers mounted on top is arrayed in a 500 yd.-wide phalanx underneath. A rider begins by lying prone on a 45 inch long skateboard of the standard equipage. In unison, the hot-air-powered tractors begin forward, and quickly reach their cruising speed of 12 miles per hour, the sheet of metal conforming over their rollers, causing a wave of sheet metal to process toward the rider’s feet. Shortly, they reach the point where the rider is “paddling” his rolling surfcraft. He pops up on the “wave”. Thenceforth, he rides the wave of undulating sheet metal laterally, trimming, running, sliding, carving, laybacks, cutbacks, etc, just as a skateboarder would on any other bank in the world, or as a surfer, would with the advantage of their waves’ forward motion.

‘The rider’s speed is of course a function of the “wave’s” forward speed in combination with his facility at harnessing the gravitational energy the renewing grade gives him. His acceleration “runs” are overwhelmingly a function of the acceleration component of gravity,’ Mr ______________ said.

'It’s fun. Try it."

153
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Nope. Vt of a falling human is maxed out at something like 200km/h. But different objects have a different maximum Vt. The Terminal Velocity equation is:

Vt = sqrt ( (2 * m * g) / (Cd * r * A) )

This tells us when a given object will stop accelerating and maintain its current groundward velocity. Objects will accelerate downwards at 9.8m/s (less the effects of drag) until they reach Vt. This is simplest when dealing with and air drop. Boards in the 6’ range with an 80kg rider will have a Vt somewhere around 87m/s (approx 24km/h) and a 9’er will be around 66m/s (approx 18 km/h). If you aren’t doing an air drop (i.e., somewhat in contact with water) drag will increase and those speeds reduce.

Hi Doug, I recall that those calculations were done (at Magic Seaweed) using the assumption that the boards were falling with their bottoms at right angles to the air flow. . … I didn’t mention it at the time but in reality terminal velocities are going to be a lot higher because surfboards almost never present their entire bottoms to the air flow at 90 degrees . . . for a start nearly all drop are done with a nose down attitude, bringing the airflow to a more frontal angle, and also if air is flowing up the wave it will do so parallel to the wave face (not vertically) bringing the airflow even closer to a front on angle. … this will reduce air drag enormously. . . and it will mean that air drag is predominantly due to frontal area. . . the rider mainly. . . . the frontal area of the surfboard being relatively small and making board length and bottom area irrelevant for the purposes of the calculation.




Can you please recalculate the speeds using frontal area with the surfer crouching?

I am in agreement with you with respect to the idea, or fact, that horizontal flow does move the surfboard, but according to my calculations it is only about 10 percent or less of the total force involved. . … I see how you are calculating that terminal velocity shows that other forces are involved, but I think that your terminal velocities are way too low and that the portion of force due to horizontal flow is thus much lower, and is in approximate agreement with my calculations (which, I believe, are less prone to error than drag calculations)




Best regards 



Roy
154

Hi Oneula, yes I think it’s mostly about drag reduction for sure, by whatever means. . . re. gps I have had three sessions, I lost a GPS in the drink on the first one, the other two were in very poor waves, best speed so far 21.5 mph 34.4kmh, looking forward to getting a better result soon ( lots of homeschooling, midwinter, not much swell, not much surfing)

:slight_smile:

155

Hi Roy,

You may be right. If I halve the surface area a 9’ comes out at 94m/s (around 26 km/h) and a 6’ is 123m/s (34km/h).

That said we are still leaving out drag due to contact with water… Which is around one thousand times greater than drag due to air. In any case that isn’t really going to affect the end result.

Regardless of Vt a board/rider combo dropping at maximum gravitational acceleration (9.8m/s) for two meters is only going to result in a velocity of 19.6m/s (about 5.5km/h). The 9’er above would need to drop for over 9 meters to reach Vt.

I think the original discussion was on the Surfing Waves website.

Sorry about that post. I editted to clarify.

Regards,

156

Thanks Doug, yes, the Surfing waves website was the one (sorry).

Not sure that a 6 footer and rider will have 33% or so higher terminal velocity than a 6 foot board and rider if we are talking about the frontal area opposing the airflow. . . . the frontal area of a 9 footer and rider is likely to be nearly the same as a 6 footer and rider. . . and the extra length of board in line with the airflow shouldn’t add much I would have thought.

Nevertheless the actual terminal velocity is a bit of a red herring at this point because as you say, a 2 metre drop isn’t going to achieve much more than about 25% of terminal velocity.

If I am reading you correctly your theory is that a board and rider can’t travel faster (under the influence of gravity) than it can achieve in free fall from the top of the wave to the bottom. . . . … . . . . if that’s what you are saying it’s a mistake, because a board angling acros the face of the wave on a glide path which is shallower than vertical can fall for much longer than a board in vertical free fall. . … . . and even though the terminal velocity of a board on a shallower than vertical glide path will be less than terminal velocity in free fall (probably) it is able to accelerate to terminal velocity because it has more time to do so. . . . . … in fact in trim it can keep falling as long as there is a wave. . . … . I hope you are not thinking that a board in trim isn’t falling. . . . .quite clearly it is, it’s just that it doesn’t appear to be falling relative to the earth’s surface (or the wave). . . . just as a glider which is soaring on an updraft from a slope is still falling through the air under the influence of gravity even though it might be gaining altitude relative to the earth’s surface, so a surfboard is falling even though it might not be losing altitude. . . . and of course terminal velocity calculations still apply to objects on shallow glide paths.

So to sum up, the velocity attained through free fall from the top of the wave to the bottom is not a measure of the maximum speed attainable on that wave due to gravity. … . the maximum speed attainable due to gravity is much higher, and is likely to be at least 80 percent of the actual speed attained, the rest being due to horizontal flow and muscular input from the rider.

Hoping that you get my drift !

Regards, Roy

157
Quote:

"In the middle of the Outback, there is a new amusement ride for the indigenous people of Australia, given by the fantastically wealthy Mr __________,who made his millions with his book of archaic, arcane and metaphysical surfboard design theory. Having long ago perceived the misdirection of several trains of his thought, he decided to give back to the world in this way.

A flexible sheet of galvanized steel, textured slightly for traction, 500 yards wide and 1 mile long sits on a loose gravel medium. A fleet of tractors with 3 foot diameter rollers mounted on top is arrayed in a 500 yd.-wide phalanx underneath. A rider begins by lying prone on a 45 inch long skateboard of the standard equipage. In unison, the hot-air-powered tractors begin forward, and quickly reach their cruising speed of 12 miles per hour, the sheet of metal conforming over their rollers, causing a wave of sheet metal to process toward the rider’s feet. Shortly, they reach the point where the rider is “paddling” his rolling surfcraft. He pops up on the “wave”. Thenceforth, he rides the wave of undulating sheet metal laterally, trimming, running, sliding, carving, laybacks, cutbacks, etc, just as a skateboarder would on any other bank in the world, or as a surfer, would with the advantage of their waves’ forward motion.

‘The rider’s speed is of course a function of the “wave’s” forward speed in combination with his facility at harnessing the gravitational energy the renewing grade gives him. His acceleration “runs” are overwhelmingly a function of the acceleration component of gravity,’ Mr ______________ said. "It almost feels and looks as if the sheet of metal is flowing towards you when you’re riding the thing, but of course, that’s ridiculous–that’s simply the vertical component of the wave combining with its forward motion happening under you/in front of you. But relative to a simpleminded rider, it looks as if the thing is actually flowing toward you/under you. Of course, that subjectificated appearientialism and intellectualized theoretification was the problem.

It’s fun. Try it."

Oh good show–bravo, if I do say so myself. Class dismissed.

158

Ahem, point of order Mr Janklow. . … .

Although your moving sheet of steel example simulates the lifting up and sliding down due to gravity effect, it is not an accurate ocean wave simulation overall because unlike an ocean wave, there is no rotational flow to the steel particles. So although one might agree that the overwhelmingly large component of the boards propulsion is due to gravity and the lifting power of the wave . . (and that your example shows this) , it doesn’t follow from your example that there is zero propulsion due to horizontal water particle movement, because you have eliminated that movement from your example . . . . to argue thus would be a circular argument (no pun intended)

:slight_smile:

159
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it is not an accurate ocean wave simulation overall because unlike an ocean wave, there is no rotational flow to the steel particles.

Yes

there is. It’s just bloody incidental. And there’s the rub. (Special

HINT for Roy: That’s why I made the medium it sits on a loose gravel.)

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If I am reading you correctly your theory is that a board and rider can’t travel faster (under the influence of gravity) than it can achieve in free fall from the top of the wave to the bottom. . . . … . . . . if that’s what you are saying it’s a mistake, because a board angling acros the face of the wave on a glide path which is shallower than vertical can fall for much longer than a board in vertical free fall. . … . . and even though the terminal velocity of a board on a shallower than vertical glide path will be less than terminal velocity in free fall (probably) it is able to accelerate to terminal velocity because it has more time to do so. .

Oh well that’s just fu…I mean bloody ridiculous. (And even if–the difference there would be wetted surface straight off and drag…) You’ll say anything…