Personally I don't think double concave "feels" much better than a single concave. I don't think I've ever seen a real explanation of what the concaves are doing on a kite board other than the simplistic"they give you grip on the water" type line.
Concaves on kiteboards have to operate in very different flow conditions to concaves on sailboards etc because of the hard edging. I'd like to see some flow analysis done of concaves when a board is being edged. What the hell is going on down there - does the water really follow the edge of the board across the concaves or does it separate at the middle ridge on a double concave board?

what about the cabrinha custom 08 board It has 4 little concaves..
Isnt the comcave there so it helps rail keep the edge???

Sunseeker is on the money with regard to fluid dynamics;

Concaves add stiffness to the board. This works by changing the length of the board in different points.eg. in a single concave the edges of the board are slightly longer than the middle. When the board flexes the edges have to "stretch" compared to the middle.
Concaves also change the rocker by making the middle of the board flatter for earlier planning ability and the edges have more rocker to turn on and edge harder.
Concaves are supposed to also make landings softer.
My north prototype has single asymetrical concave on the heel side of the board, what will they think of next..

LouD said...

So hail damage should make my car go faster!

I knew somebody would say that...haha hail damage would improve the flow but only in areas where the surface changes direction like the back of the car where the surface goes downwards.

So if you took your car into a wind tunnel and marked the areas where the smoke starts going away from the surface and localised the hail damage to this area - then you may be in business...

Nick,

I think that is part of it - the board will definitely go faster, the ridge in the middle caused by the double concave separates the water (laminar flow) half way up the board and reduces the friction as any water flowing past the ridge would have turbulence (ie golf ball effect).

But I think it also helps pull the edge down into the water more - sort of like air over an airfoil, the flow of water through the lower concave is a longer path than a flat bottom, therefore faster, creating downward suction due to lower pressure and helps "stick" the rail. The double concave creates this effect over the bottom half of the board only, which is the bit that is in the water.

But like you it is 10 years or more since I did fluid dynamics.

EDIT: Wow - a few posts got in before mine. But Dave is right there are other benefits, softer landings and a stiffer board too.

I thought the dents in a golfball stopped it spinning so much so therefore made it fly straighter... Nothing to do with Kiteboards though. I agree with the suction theory. For example, with malibu riding, getting lift on the nose is largely caused by the rocker at the back of the board sucking onto the water.. More tail rocker = better noseriding...

POP

Bowski said...

I remember someone bought out a surfboard in the '80s with dimples on the bottom following the laws of fluid motion, not sure what impact it had but only ever saw it once.

I remember this and saw a few of them, never rode one though - the dimples were at the back where the channels generally were. Supposed to be faster and looser boards. Obviously were a great success.....

This is a good link for those who just neeeeeed to know all about laminar flow, drag, turbulent boundary layers, Reynolds number, d'Alembert's Paradox (no relation) and golf balls.

www.aerospaceweb.org/question/aerodynamics/q0215.shtml

Hi,
Here is my take on concaves. Our first FLX boards were flat bottomed (train track graphics), when we tried a single concave we tried 19 different versions. Some were better than the flat base some worse. Anything that had a concave out thru the tail was too slow and did not ride upwind well. The one we liked the best had about a 5mm concave in the middle with flats on the rail tapering out to flat in the tips. Compared to the flat base they tracked better (less skately) and rode upwind better (robot graphics). The test boards were much more powerful and hard to bleed off power so we also softened the tip flex. We were pretty happy with the end result except for carving where the single concave lost its hold when flat and it got very slippery. The next years board (crazy fish) we introduced a spine down the centre. The board was still overall a single concave. The spine helped a lot with caving and also smoothed the ride in choppy water as the spine acted like the Vee on a speedboat hull. It also smoothed hard landings.
The next season (current boards) we kept the base shape and changed to a wood core. This made the flex different and offered a smoother ride and landings. We have again over our summer been trying a lot of different concave setups. We have found it difficult to find a base shape that rides better than the one we have now.
The dimples from wakeboarding and windsurfing were intended to break up the waterflow and stop the board from holding, quite a different effect from concaves.
This info is from our own testing and as such is just our opinion.
TA Adrian

Bowski said...

I remember someone bought out a surfboard in the '80s with dimples on the bottom following the laws of fluid motion, not sure what impact it had but only ever saw it once.

Stoney (Mark Stone) used to make them in Gero. I had one - bloody fast it was too!! (At least that's what he told me at the time)

All I know is that when I demoed a mates FLX that had single concave, it felt smoother, and a damn sight nicer to ride than my home made board! It's what has enticed me to save a few coins and get my self a board with concave!!

my view on concave is from a wakeskate/ wakeboard background.
Apart from giving the board a bit more edge hold power, the other difference is the board will sit higher in the water. Double concave is better still in making the board sit higher in the water.
This is sorta not good for a wakeskate; but for kitein its good, as the board wont have as much drag in the water.

The dimples on golfballs make a kind of air barrier in front of the ball. Reducing the balls air resistance (air on air, against golfball on air) making it fly farther. Therefore concave is importnat.

Adrian, to clarify - are the current Underground boards single or double concave?

Matt h here from Auswind

board shape consistently comes up as an area where customers are confused (nearly as much as whats a bow kite and what isnt - more on that in another post)
Yet again The industry as a whole has done a terrible job of educating the customer on what they need - this post is here to try and "clear the fog."

Here is the basic run down on shapes and their applications for styles and ability of riders from the Airush board range perspective:

If you ride in chop the double concave / vee set up is gold - See the popularity of the Airush Pro Toy - the first board to market (3 years ago) with double concave to a pronounced vee - it smooths out chop - feels like suspension in a rally car (working in tandem with the flex, outline and the particlular edge of that board) - makes a choppy day feel flat. many shapers copied this the following year - although many continue to only offer flat bottom (read cheap / economic to build) options in their ENTIRE range.

Vee on the Airush hammer has a similar effect of insane chop smoothing- but for a wave application (ie teamed with narrower tips, more rocker and foiled surf rails) Vee aslo aloows you to "roll" from rail to rail with more ease - critical when transitioning from bottom to top turn (mid face) to smack the lip - timing is critical - this "roll" makes timing the moment easier. Hence the strong cult following of the very unique Airush hammer.

If you ride butter flat water and freestyle you may actually be happier with a basic flat or concave bottom - like the Airush Xpact in airush's range - which is optimised for just these conditions. Double concs can be a little stickier on butter flat water than a flat bottom (there is no free lunch)

the super popular Airush switch (cant get them second hand- when they do come up they are gone in a flash) - has an oval shaped concave (eye concave) that runs throught the already shallow rocker - to deliver a board that flies upwind (thanks to the wide tips and efficent rocker) on the "phaser" effect - this is the same effect as the dimples on a golf ball - basically you get less fluid resistance if you can move the laminar flow off the kiteboard. Feels a bit like riding a hovercraft - super slippery! These boards are insane for getting beginners

a. up and planing
b. upwind and feeling like they are masters of their destiny.
c. control in chop is still good - this time using some extra flex in the board

keep in mind that you are looking at variables in a 7 dimensional matrix of flex, flex response (big influece on "pop"), edge shaping and foil, outline shape, bottom shape -throw rider style preference and conditions into the mix and you have a very complex system to analyse!
If you understand what each variable does you have a much better chance of "dodging the hype"
So choose your bottom shape for the conditions that you ride and your skills - hopefully this post makes it easier for you to do just that!

Robbo

You are a bum, pimp, kook and all the above boards are crap except mine :)

Feel better now......

I agree, super informative. Someone want to explain water flow over a surf shaped edge as opposed to your standard twin tip square edge and why this is better for surfing?