i LOVE THIS THREAD…just the kind of discussion that makes this place worthwhile…
I follow your less is more argument…why then do all pro riders choose some kind of contour…you’d have thought that some kind of Darwinian mechanism would come into play here and self select the best shape wouldn’t you?
Fantastic thread, I’m really looking forward to how it develops. At the start of the thread, The view of how water flows across was discussed. If I understand right, we should really visualise the surfboard on a rail & Tail, as it taps into the power source of the water moving up the face, which is really an incredible force if we can tap into it correctly. However… In trying to understand the dynamics in a real world application, I turned to www.youtube.com for clips of average surfers on average waves. What struck me was that Joe Average really does not engage his fins like Smelly Skater.
This left me wondering how efficient those double concaves Fins and Rails are for mr average in average waves.
Particularly to top turns. Now top turns have really bugged me. Bill Barnfield in the fins thread re-inforced the angle of attack and water moving up the face theory for me. It clicked. But in the top turn, the toe in is limiting the turn and your dropping speed until you get the board around enough that it picks up speed with gravity. But in a poor wave you may not have enough speed from a bottom turn to project you through the top turn until gravity takes over.
So I guess what I’m saying is that we can postulate on the perfect surfer on the perfect wave, but most of us are not that surfer and 90% of the waves we surf are not perfect. The real challence is finding a contour that works for the average surfer in the average wave. Which may be the fastest bottom & fin configuration with the least rider input.
Perhaps this is why fish and Bonzers are popular, they work well if you couldn’t be arsed dialling in a thruster.
I didn’t actually, and as for dropping the brainy ball, hell, I dribble it.
Variable rocker? You mean flex? A sort of controllable flex, in the control of the surfer, not a passive flex, one that was purely dictated by the force it was subject too? Yes, a controllable flex, like that Greenough was able to achieve with his famous kneeboard, but no, like the limited of controllable flex in snowboards, or a better example might be one of those soft surfboards. Also see Herb’s post ‘Surfing the Air…’ regarding his surf mats for a successful application of flex.
Of course with both Greenough’s kneeboard and Herb’s mats the surfer can use most, if not all of his body to control the flex. In surfing you’ve just got two feet and that includes fancy toe work.
With passive flex comes loss of power. Like a weak piece of cardboard held up to the wind, it will bend until the net force from the wind pressure (stress) on it is minimized. Same for passive flex in a surfboard. In general, you want all the available energy going into motion, rather then into changing the shape of your board. Especially when the shape it wants to change into minimizes the net force from the pressure produced by the flow. In fact, you want to change the flow, or at least its momentum in a controllable way, which is usually best achieved with something rigid.
Surfboards do flex however, but as a design feature? Please let me know who is doing such things, I’d be very interested.
Kevin
No, I disagree.
Surfing can occasionally be a sleigh ride, especially on big drops, but for the most part propulsion is derived from the flow interacting with the bottom of the board.
You state that “In surfing the ‘mountain’ is constantly moving upwards relative to the surface of the earth” that’s a flow by another name. It’s a mass flow, just like anything else that moves (and has mass.) Here we have a fluid flowing, and an impacting fluid flow amounts to a pressure, and that pressure experienced by the bottom of the board, revealing itself as a force, a resultant force (pressure times involved surface area) which is directed up perpendicularly through the bottom of the board - which is the means of propulsion in surfing. Like a fire hose knocking down a protester, the flow of water in the wave impacts the bottom of the board and sends it off, in which direction determined by bottom presentation (which with good design is controlled by the surfer) and any counter forces, like weight, that are present.
Surfing is not a gravity sport, though at times it can be, sleighing maneuvers are apart of the sport, but trimming in the tube is not a sleigh ride. You don’t fall when trimming, you to hold a line. You interact with the water through the flow -i.e. through pressure or stress.
Kevin
Kevin, I’m not talking about rocker, I’m talking about belly. Rocker goes nose to tail, belly goes rail to rail. You can have a flat bottom, a concave bottom or a bellied bottom. What I’m saying is that a bellied bottom fits into the wave better then either a flat, or a concaved bottom.
no I meant variable rocker contained withing the design itself.
properly implemented concave varies the boards rocker from stringer to rail. IOW, most people look at concave as a lift and squirt feature, and that is correct. But I think it affects rocker much more…and IMO, rocker is still King. Im sure you already know all this.
BTW, there’s heaps of board flex & spring discussion in the archive…some believe its the Holy Grail…I cant disagree one way or another but one thing’s certain, as a backyarder making 2-3 boards each year, it aint gonna be easy to figure it out.
Also I agree with you're analysis of skimboards as they were originally used, but the new breed of skimboarders have taken skimboarding a lot farther. They now, literally slip right into the shore break and surf it - as in, on the face of the wave. Watching their 3 to 4 sec rides is a real lesson in minimalist surfing - it appears that all you need is a bottom.
It’s true that you can learn a lot from how skimboards work. But as a skimboarder for more than 25 years, I can tell you that it’s not that simple.
As you witnessed, you can surf with a skimboard. The greatest skimboarder of all time, Bill Bryan has even surfed a skimboard at Teahupoo. But if you really watch a skimboard surfing you will see that the lack of rocker is not ideal. The turning is stiff and with marginal control. In fact, some skimboarders choose to add some tail rocker to improve wave performance with the compromise of less speed on the sand. I’m 100% sure that skimboards would have contours like vee and concave (and fins) on the bottoms if it didn’t make them stick on the sand.
Also, skimboards are not all about the bottom (in fact the bottom is a drawback once the board is on the wave). They have very important components in regards to the surfing aspect. They have to be thin with sharp rails, because the rail acts as the fin. Surfboards are the same way, but to a lesser extent. And the outline of a skimboard is also important to allowing it to turn.
So a skimboard is almost completely designed for getting out to the wave. The fact that it can marginally be surfed is pure luck.
Why?
Why does concave provide lift?
And could you explain the squirt feature to this fool? … Please?
As for rocker being the king, king of what? Lift? How’s that work?
Kevin
Referring to those skimboarders who do slip into waves, would you explain to this non-skimboarder why skimboarders want to put concave or vee in their boards?
Kevin
I’ve never heard anyone call that belly. I’m curious as to what you think it does, or might do? (A little bit of an explanation regarding the dynamics would also be nice.)
The only boards that I have surfed with the kind of belly that you are referring to, have been old (and bad) longboards. Aside from mechanically stiffening the shape, they all suffered a real loss of power generation, not on the drop but when trimming, unless you constantly surfed with a bias to one side or the other, and even then its was sketchy. On face, when you could actually get you out in front (going down the line), it would track like you had way too much fin (again stiffen, but now with respect to turning.)
Kevin
Referring to those skimboarders who do slip into waves, would you explain to this non-skimboarder why skimboarders want to put concave or vee in their boards?
They don’t want to put concave or vee in their boards, because they know it won’t work for skimming on the sand. You have to start on the sand.
But hypothetically, if they could, it would be for the same reasons you would put it into a surfboard. Skimboards would have more rocker if they could. Skimboards need wide tails for planing purposes. A little vee would help with turning.
If skimming on the sand didn’t matter, skimboards would look like surfboards.
No, I disagree.
Surfing can occasionally be a sleigh ride, especially on big drops, but for the most part propulsion is derived from the flow interacting with the bottom of the board.
Quote:Of course the propulsion is derived from flow interacting with the bottom… . . so is a sleigh ride. . . whether or not the surface is moving upwards or not is irrelevant to the transfer of force. . . it happens the same way whether or not the board is losing altitude or gaining altitude. . . just like a glider functions the same way whether or not it is gaining or losing altitude
You state that “In surfing the ‘mountain’ is constantly moving upwards relative to the surface of the earth” that’s a flow by another name.
Quote:Yes, and it is effectively flow even if the ‘mountain’ isn’t being moved upwards. . . it is the same process in both cases. . . the only difference being the gaining or loss of altitude relative to the earth’s surface. . .
Surfing is not a gravity sport, though at times it can be, sleighing maneuvers are apart of the sport, but trimming in the tube is not a sleigh ride. You don’t fall when trimming, you to hold a line. You interact with the water through the flow -i.e. through pressure or stress.
Kevin
Trimming is falling, except that no altitude is lost. . . there is no change in the process of ‘propulsion’ when a trimming surfboard starts to lose altitude by taking a steeper glide path
Surfing is most definitely a gravity sport. . . however the gravitational potential energy acquired by the surfer is gained by wave lifting action instead of a chair lift or the act of walking up the mountain.
Regards,
Roy
hi kevin
yeah i meant getting rid of spent flow
as you were discussing initialy…
you are saying the quickest way to get rid of it is not to have anthing in the way
like a concave or a fin?
meecrafties point is that having the concave alters the rocker through the centre line of the board
and that the rocker is flater through the centre line then on the rails!!
if flow comes off the board at an angle at the back 1/3 of the board
i can see why concaves and channels and fins would slow this flow down
or perhaps rechannel it out through the tail
maybe the concaves could run out to the rail diagonally!
thicker,full foiled fins are more effecient at lower speeds
with less turbulence
promoting a smoother flow of water around the fin
curse, snarl - first discussion I have seen in a bit that really needs my time and thought…and this week my work schedule hit 100 hours.
However, looks like this 'un will be around a bit.
Kevin, by chance are you in the Northeast?
best regards
doc…
Geez and I thought I was cynical.
Why does concave provide lift?
Same way trim tabs do on power boats. Except there’s hell lots more trim tab area on a properly designed concaved board, so the trim tab affect is much greater on a surfboard.
And could you explain the squirt feature to this fool? … Please?
A very mild venturi affect, combined with affects described above.
But like before, is less about lift and squirt and more about…
…rocker being the king, king of what?
King of surboard design, amongst other kings.
Do you ride surfboards?
How many times, if any, have you built a board with a particular bottom design, rode it for some time, then re-worked it to a significantly different bottom design, and re rode it?
Isnt it interesting. The most powerfull computers in the world have a tough time accurately depicting flow schemes under a surfboard…and yet this inexpensive little test tells quite a good story
http://www.swaylocks.com/forum/gforum.cgi?post=157886#157886
ps - I know ‘fools’…and let me tell you, you aint no fool
and for a Silly person, man that dude know his sh_t
Thanks.
What do trim tabs do? How do they do what they do? How does it apply to concaves in surfboards? Also, please, if you’re inclined to, explain where propulsion for the surfboard comes from, how it might related to trim tabs?
Venturi effect - an increase in flow reduces hydrostatic pressure. If that’s the Venturi effect you’re referring to, how is it achieved under a surfboard? What is the reduced hydrostatic pressure suppose to do? How is the Venturi effect related to trim tabs?
How does rocker work? What is it suppose to do?
I ride surfboards.
Please, feel free to send up a few diagrams. There’s no need to go mathematical in your explanations, though an attempt to account for the forces involved would be nice.
Kevin
cool test dave
interesting to note greg Loehr s point that if we are disscussing flow from a “trim” perspective
we may be barking up the wrong tree
top to bottom shortboard surfing gets most of speed from bottom turn
and the flow dynamics there would be very different to flow in trim
i want to understand it!
this is how i see it now
for arguments sake ill call kevins line of reasoning the “waterhose effect”
bottom turn . burried rail - harness energy and release -waterhose effect(maintaining initial speed from gravity as well)
heading up the face. board is flat- so your planning now
top turns . still planning.board is flat and your loosing speed .fins release =less drag at slower speed
comming down . mostly gravity and the waterhose effect
in the barrel . all trim baybee
Also, please, if you’re inclined to, explain where propulsion for the surfboard comes from. Kevin
Kevin, my name isn’t Craftee but I have to point out that you haven’t yet properly understood how surfboards are propelled. . … . . on this thread you have stated that surfboards in trim are not falling objects. . . this is incorrect. . . and have implied that surfboards in trim are propelled by different means than surfboards which are ‘sleigh riding’. . . . this is clearly untrue. . . . . the principle of propulsion is identical in both cases, the only difference being the steepness of the glide path.
.
Yeah, that’s the idea all right.
I have never been able to come up with a bottom design (excluding rocker and template) that would be sufficiently general in its effect to make sense. That is, anything that you place on the bottom is a sort of commitment - its going to impact the way you surf, forcing you to surf a certain way. So any design feature that might remove exhausted flow in one maneuver, is likely to create more of it in another -things change too much and too fast under the board. It’s a real balancing act. (And a lot of shapers do pull it off, and well, whether they understand exactly what they’re doing or not.) But that’s my opinion, and it speaks to my ignorance and lack of creativity. (Doesn’t mean I’ve stopped thinking about the problem, of course.)
By the way, thanks.
Kevin
In your icon, see the spray? That’s the spray root, that’s flow you’re (its you right?) turning around, literally as in completely reversing its momentum. “Cool Test” you are the protester, the wave the firehose.
And you’re right, there’s a lot of sleighing going on, particularly when you want to pick up that extra bit energy for that big air.
So, design for trimming, or sleighing? Are the requirements for either different? Whats the best compromise?
Kevin