Thanks NJ Surfer.
Mmmmm.
Anyone else agree? Has anyone else experienced this 'slippery feeling' at high speed on a single concave? Does anyone else see the role of the central ridge found towards the tail end of a single to double concave as an effective convergence diffuser?
This cuts to the heart of the single vs double concave debate so lets push on!!!!!
I still think the double, which is often with in a vee, does some of what NJ suggests, and, my personal hypothesis is: the veeishness helps w/rail to rail, and the concave on the side that is down in the turn works to produce a similar assist in directional flow on that side, as in extreme turn some/much of the other side is out of the water.
Thanks TaylorO
Yup. I've had plenty of single to double concaves with a hint of vee right out the back, normally starting at about the rear fin.
Anyone else? I'm that close to being convinced.
Regards all
What I alluded to earlier… the vee helps balance out the concave. It provides some stability under the back foot, while giving you the opportunity to use the concave induced lift under the front foot for speed, drive, and responsiveness.
yep. The terminology of “flatter concave rocker” is misleading. It sounds as though the board is finish shaped and then has concave cut into it. Of course, that’s often the reality, but the trick is in getting the central concave rocker to match a proven rocker. In my world it’s a normal rocker (the same rocker for my concaves and my flat or vee bottoms), and increased rail rocker for the concave bottoms, so we agree.
Of course, increased rail rocker lets you straighten the outline, so there is quite likely a formula out there somewhere for depth of rocker and amount of rail curvature that can be lost. I might have to play with this idea.
Interesting what you and TaylorO suggest about vee in concaves. Even though I have twin concaves inside the single concaves through the fins on my concave and flat bottomed boards, I carry a 2" flat strip down the length of the stringer, so I never have a true vee. I notice my boards are a fair bit quicker than others with a “pointy” vee in the concave. Don’t know if this is just a better rocker or a water channeling effect from the concaves.
I thought that spiral vee was meant to loosen up the tail of the board? Or is the stability you refer to relative to a single concave through the fins?
heya T,
the confusion apparently stems from everything that’s been published about bonzers since the 70s. most infidels today still refer to the venturi principle when describing bonzer bottoms without realizing that it was merely the inspiration for the bonzer bottom configuration-- not the explanation on how it works.
hmmm how about energy redirection…
cheers,
first off, i think NJ is on the money...
what about surface area? has anyone considered that? same outline, moresurface area with a single concave vs. flat...???
This is awesome reading! Thanks everyone!
Wow! A dynamics thread without a lot of disagreement. I guess the ''concaves for control'' thread served a purpose after all.
Mike Daniels had been telling us for a while that it is a pretty insignificant difference in surface area, less than 1%, so we made a rough calculation in another thread using triangles, and he was right…
well, there you go... ha ha, thanks for the info
Go off for the weekend and all the answers are already here… Ha!
Yeah - I’ll say a big thanks to Mike D. for all the good stuff.
I’m gonna put rear “edge fins” on my older daily rider - much flatter bottom than my newest - Should be some good feedback. I’ll post.
lol: in the surfers path bonzer link: "It’s all about reducing entropy, which needs to become the emerging paradigm of the 21st century. Sorry about that last bit, but with us it is never just about surfboards."
I completely agree! eventhough its absolute bs in relation to his bonzer bottom theory...
i imagine the concave to do three things
1: it locally flattens your rocker making your board faster
2: they work like channels that (re)direct flow, in a conventional double concave i see little of this happening, in the deep bonzer channels i do. This offers resistance to water flowing at an angle (be it right or slight) to the channel ridge: for the double concave the stringer and for the bonzer all the channel edges
3: !this is the big one!: in relation to a flat bottom you are pressing two or three very elongated very shallow 'fins' into the water...in the middle of your board and at the edges for the double concave and just your edges for the single concave...your board will want to come up and ride on those 'fins'...(like windsurfers with 50 cm skegs...they hardly ride the board...its just the fin thats in the water) and then the same thing happens as with a catamaran: you get (two) longer/slimmer hulls which are faster than a shorter/fatter one. (increasing a ships length will almost always increase its speed...eventhough wet area and thus surface drag increases, the being more slender makes a (big) bigger difference)
about the venturi effect: if you press those two or three elongated fins (read: sides of the concaves) into the water then water pressure locally will increase...but you still weigh the same...so it can decrease eslewhere: in the centre of your concave...this decrease in pressure over a large area reduces overall drag...this seems like the venturi and lift effect where an increase in substance speed reduces pressure....but i doubt this to be applicable on our concave boards...as you can't turn the effect around...decreased pressure does not result in increased speed
imagine this one...and then i'm going to bed: you're a tri-maran...three hulls...your chest and both hands...skimming over...sorry: rather ploughing through the water. your chest is wide and shallow your hands hold slender hulls....now press your hands into the water...those slender hulls will carry more of your weight...which will take the pressure off your wide shallow chest...thus decreasing the overall drag because those two slender hulls plough alot easier than that fat one
sleep tight
just think of yourself as a bit of water and then as a board and then alternate rapidly untill they blurrrrr
It would be great if some big money like Rusty or Merrick could fund research that is specific to concaves/venturi as applied to specifically to surfing as opposed to all of the info we transpose over from aeronautics/ aerospace/ marine etc. Seems that there is a lot we could be missing. That way we could elevate from a post surf-stoners around the camp fire hyper-theorizing discussion to some real science. My current stoner around the campfire theory, after many exhausting discussions with my high level surf philosopher colleagues, concludes that if there is any lift benefit from concaves it is miniscule at best but likely unnoticeable. Instead I think the benefit is simply a faster board because of a flatter rocker.
It is difficult for my pee brain to see how a concave 1/8" deep, 5" wide, narrowing to a point at maybe 20" long, totaling roughly 50 square inches of area moving over water at maybe 20 mph could generate any lift recognizable to something other than some incredibly sensitive lab instrument. Same scenario applied to an airplane skin at 200mph- again reaching to aeronautics- we know some things happen there.
However, a proper study might tarnish some big names if concave generated lift proved to be snake oil. A lot of marketing sure is hinged on it. But Im not a scientist and just puttin’ my 1 cent out there- not trying to offend anyone and previous theory.
Put a spoon in the faucet.
… and look up the “concave for control” thread…
Just did…
I may be wrong, but it seemed to me the Bonzer bros. talked about venturi effect at first (back in the day when bonzers were something new), but not anymore. I think they found the design worked, but now seem to focus on the channels and the sidebites capturing energy from water off the rails during a turn as the reason.
The results of the spoon/faucet test are in: Inconclusive!
Put away the bong!