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#2908292 - 02/04/19 06:41 AM Re: Do concaves work? [Re: Spinafex]
Greg Griffin Offline
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Registered: 10/31/04
Posts: 18317
Loc: Palm Coast , Florida
A flat bottom board, or even a board with a conventional shallow concave, allows increased flow to escape off the rails resulting in lower pressure under the board. The goal is to increase the pressure on the bottom of the board to increase lift. More lift allows the board to carry the weight of the rider with less drag. Less drag lets you go faster

Flat bottom allows increased flow to escape off the rails resulting in lower pressure under the board.

This is release enabling everything you could want - lower drag too

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#2908357 - 02/04/19 11:33 AM Re: Do concaves work? [Re: Greg Griffin]
patrolman Online   content
Miki Dora status
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Registered: 07/28/05
Posts: 4334
Originally Posted By: Greg Griffin
A flat bottom board, or even a board with a conventional shallow concave, allows increased flow to escape off the rails resulting in lower pressure under the board. The goal is to increase the pressure on the bottom of the board to increase lift. More lift allows the board to carry the weight of the rider with less drag. Less drag lets you go faster

Flat bottom allows increased flow to escape off the rails resulting in lower pressure under the board.

This is release enabling everything you could want - lower drag too



Isn't the purpose of concaves to subtly redirect water, forcing more water into/over a certain area of the bottom thus increasing the speed of the water, thus creating lift? Good ol' Bernoulli?

Well, don't ask me. I don't pretend to know. All the study I've done of hydrodynamics came from reading books on yacht design (truly scientific volumes on surfboard hydrodynamics do not exist as far as I know). Sailboats are working in a much more steady state environment. Surfboards work in such a dynamic environment, with the speed and direction of water changing so rapidly and displacement (weight) constantly shifting,at times weightelss, at times loaded up, sometimes forward, sometimes further back, that it seems impossible to design with truly applied hydrodynamic theories in mind.

I didn't buy it when Tomo was spouting hydrodynamic theories. He was using Bernoulli to defend his use of concaves and his outlines. OK. No doubt his boards work and he a phenomenal test rider, but to say you understand exactly how those hydrodynamic forces are working on a surfboard and you can tweak your design in response is a stretch IMHO.

Wouldn't it be much more honest to say that test rider feedback and years of iterative design has shown that flat bottoms (for Griffin) or concaves (for Tomo) work?

I like flat bottoms. I consider them neutral. However in certain situations bonzers (single to double) have blown my mind. Once in a while, when the stars are aligned just right, I'll do a cutback on my bonzer that feels like you hit the throttle halfway through and you gain speed in the turn (the legendary "fifth gear"). But even the Campbells used the term "venturi effect" to describe their boards. But Bernoulli's classic experiment was with water flowing through a closed tube. Wouldn't water flowing over the bottom of a surfboard, encountering a concave will seek the path of least resistance which, seems to me, is to move downward away from the board. Or am I missing something here?

Anyways, let's hear your explanations. Always fascinating stuff.

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#2908384 - 02/04/19 12:10 PM Re: Do concaves work? [Re: patrolman]
DublAK2 Offline
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Registered: 06/18/03
Posts: 207
Loc: NJ
GG. Sorry man didn't understand your post. Do flat bottoms or concave bottoms create less drag? I was confused about which one u were speaking to.


I'm going to attempt a groveler. I'm thinking about splitting the difference and making a super shallow single concave. something less than 1/8". Of course, execution of the plan will be a crapshoot.

What feeling am I trying to achieve with a super shallow single? Not an effin clue! If I can stand on it I'll consider it a success.


Always fun to talk surfboard design though and act like my DIY project will work

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#2908428 - 02/04/19 01:03 PM Re: Do concaves work? [Re: DublAK2]
Greg Griffin Offline
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Registered: 10/31/04
Posts: 18317
Loc: Palm Coast , Florida
I copied the article .
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#2908441 - 02/04/19 01:33 PM Re: Do concaves work? [Re: laidback]
oakfoils Offline
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Registered: 10/01/04
Posts: 820
Originally Posted By: laidback
Originally Posted By: oakfoils


calling grandpa griff to the suction cup.


Nobody likes suction


oh contraire, hint; all majority of the modern hpsb commmon design element is the control and release of suction.

everybody riding one likes one even if they realize it or not.

all things work to some degree, knowing when to stop is the key.
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#2908587 - 02/04/19 05:03 PM Re: Do concaves work? [Re: oakfoils]
Greg Griffin Offline
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Registered: 10/31/04
Posts: 18317
Loc: Palm Coast , Florida
Some of these articles say the downturned lip on the rail created by the concave is similar to a long fin .

Fins are toed in , this lip is toed outward by following the tail template .

Imagine what happens when you do this with your fins .
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#2908594 - 02/04/19 05:19 PM Re: Do concaves work? [Re: DublAK2]
NinjaPete Offline
Grom

Registered: 07/11/18
Posts: 63
Tomo's designs are based on real math. Maybe initially he came up with his designs based on experiments and theories in the past done by others, but later Firewire worked with him and did some hard math. There is an article about it on their website. You can clearly see the effect of concaves in the heat maps, they describe in great detail the effect of hard vs. soft rails in the video, the importance of rocker et... It would be awesome if board designers gave this kind of detail to the customers for all of their board designs.

https://firewiresurfboards.com/the-wire/...ard-performance

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#2908745 - 02/04/19 09:57 PM Re: Do concaves work? [Re: NinjaPete]
Greg Griffin Offline
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Registered: 10/31/04
Posts: 18317
Loc: Palm Coast , Florida
Several ass backwards statements in that Video .

I like Tomo's quad concave bottoms , just not inset in a concave , but like those done in the 70's .
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#2908804 - 02/05/19 12:54 AM Re: Do concaves work? [Re: DublAK2]
NinjaPete Offline
Grom

Registered: 07/11/18
Posts: 63
The video is a hasty summary of what is going on. I wish they would have went into more details as to why the Omni is faster than the Vader in more critical wave conditions. I like the sequoia website how they give details on drag numbers and lift for a given model. That way the customer can weigh the pro and cons of wanting more lift/drag for mushy waves vs flat out speed in critical waves. https://www.sequoiasurfboards.com/en/cfd

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#2908831 - 02/05/19 06:31 AM Re: Do concaves work? [Re: NinjaPete]
Greg Griffin Offline
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Registered: 10/31/04
Posts: 18317
Loc: Palm Coast , Florida
If the turned down rail creates lift to the point you adjust the size of the board due to the additional support , it would fly out of the water at higher speeds and larger waves .

Pipeline , Off the Wall , J Bay etc .

I have no problems with concaves - big problems with the descriptions of what they do and how they do it .
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#2911377 - 02/10/19 01:16 AM Re: Do concaves work? [Re: NinjaPete]
patrolman Online   content
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Posts: 4334
Originally Posted By: NinjaPete
Tomo's designs are based on real math. Maybe initially he came up with his designs based on experiments and theories in the past done by others, but later Firewire worked with him and did some hard math. There is an article about it on their website. You can clearly see the effect of concaves in the heat maps, they describe in great detail the effect of hard vs. soft rails in the video, the importance of rocker et... It would be awesome if board designers gave this kind of detail to the customers for all of their board designs.

https://firewiresurfboards.com/the-wire/...ard-performance


I'm still not buying it. Real math? Heat maps?

I could be totally wrong but someone will have to convince me.



In this photo he is indicating a path around the rail that would be perpendicular to the board. Water is never wrapping around the rail in the direction he indicates. It is moving nose to tail and so the profile it would be wrapping around would be a slice parallel to or maybe 20-30 degrees off the stringer. And for sure, no water would be moving in the direction of the little arrows, perpendicular to the bottom of the board. Overlooking this fact, and taking about water movng the way he indicates, to me indicates BS. It is ignoring how water is really moving.

In yacht design you have "waterlines" which are horizontal sections, "buttocks" (yes I said buttocks) which are vertical sections, and "diagonals" which are, obviously, diagonal sections. Those diagonal sections are really important because that is the path water will moving when the boat is heeled which is almost always. It's good to keep in mind how the water is moving.






My first reaction to this photo was "How do they get pressure readings from a photograph?" Heat maps? shrug Also, what we are looking at is a backside top turn. I find it hard to believe that the rail that is being buried is the low pressure zone and that the separation zone, where water is leaving the bottom of the board (the beginning of the spray) is the high pressure zone.






In this part of the video it keeps showing "splashes" of water directed forward. When has anyone ever seen water being thrown forward while surfing? This leads me to doubt the validity of the whole model.






Finally, in this photo it shows standard rocker having low pressure across the length of the bottom, whereas the lower rocker board has higher pressure across the bottom (the two different boards are combined in a composite in this image). The presumption here is that low pressure is release and therefore faster, whereas the lower rockered board with all that high pressure would be slower. Since when has board with less tail rocker (all other things being equal) been slower?


So again, when I'm confronted by someone or some company trying to use "real math" to hype their boards I just don't buy it. I know they use CFD extensively in yacht design but the models the use are backed up with a lot of tank testing data and even then it doesn't always work. I don't think any tank testing (or wind tunnel testing, which is probably what this car designer uses to get his numbers) would accurately mimic the dynamic environment a surfboard operates in, so no, at this point in time I personally do not think CFD has any real practical application for surfboard design. Maybe as a marketing tactic. Maybein the future.

For now design,shape, glass, REAL WORLD testing and refine.

And in the end anything will surf, even a lunch tray or kitchen table. wave2



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#2911397 - 02/10/19 07:42 AM Re: Do concaves work? [Re: patrolman]
Greg Griffin Offline
Duke status
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Registered: 10/31/04
Posts: 18317
Loc: Palm Coast , Florida
I personally do not think CFD has any real practical application for surfboard design. Maybe as a marketing tactic. Maybein the future.

For now design,shape, glass, REAL WORLD testing and refine.

Excellent cheers
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#2911493 - 02/10/19 01:19 PM Re: Do concaves work? [Re: DublAK2]
000 Offline
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Registered: 02/20/03
Posts: 20486
my surfing improved when i went from flat bottoms to concaves

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#2911508 - 02/10/19 01:46 PM Re: Do concaves work? [Re: 000]
Maz Offline
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Registered: 05/18/04
Posts: 983
Loc: Innzid
Originally Posted By: 000
my surfing improved when i went from flat bottoms to concaves


Bingo!

Or, my board feels spicy and alive, and you can lay into a turn with five times the power, ooweee!

Funny that 99.267% of expert surfers don't feel that way. Oh wait...

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