since there was sooo much great feedback on the single concave thread, i’ve following up on the last concave thread - i’ve been wondering about a few similar issues: -why single concave -vs- single to double ? what are the changes in feel/performance, given the same concave depths for both designs in a given board. -also, what have you guys found as the practical limit to concave depth in a typical performance shortboard. in my experiments, i have yet to go over 1/4" deep in either a single or double. i really appreciate the feedback from you guys that really know your stuff. thanks for you help guys ! brian
The following are just generalizations and final results are affected by lots more variables, but: single concaves especially deep ones to give maximum lift and speed in small waves, and are most often found on shortboards up to the mid six foot range. Singles might give too much lift on larger waves and could lead to loss of some control esp if some surface chop is added. On boards over the mid six range, mini-guns and such, doubles are more common, to give a little more lift than a flat bottom. Guns over 7’ generally have little concave, often going flat to a little vee to help manage speed more reliably.
I think another benifit of the double concave is that you can shape some Vee into it - and the tail. And actually I think that you shape the concave into the Vee. Still providing some lift, but also the turning ease associated with tail Vee. Eager to experiment with different bottom contours, I tried a concave on my second board. Unaware of the subtleness of a good concave, I probably put close to 3/8" in the bottom. It was a 6" Mod Fish. It was probably too much, but on clean small, steep Newport days, the thing flys down the line. The slightest bit of chop and I’m heading home to switch boards. My more recent boards have used a much more mellow approach, creating less lift but a much more well-rounded board… still though, I use my Fish all the time.
I might have been a little more specific on the double description, but it is many times an increasing spectrum as board length increases. For example, often I see on short short boards 5’8- 6’2, a simple single is used. Then moving up on slightly longer boards (maybe around 6’3 to 6’6 a little rise in the stringer is often used, making a faint double, but the stringer is not yet raised to the level of the rails so I don’t call it a true double yet. Then moving up to miniguns about 6’7 to 6’10 what I call a true double is used on either side of sometimes a level stringer, or a sometimes a slightly raised stringer (then called “vee”). At some point on guns most often the concaves fade out and the bottoms are mostly flat with maybe faintly raised stringers (vee).
I dont understand the term ‘lift’ in the context of concaves. In what direction does this lift force apply ? Intuitively I can see how concaves would improve the hold when going in a forward direction similar to a channel or a fin.
Lift? Im not sure that is the “proper term” for what it does but as i understand it what it should do is create a straigter area of rocker where water can flow in a straighter line. Therefore creating faster waterflow through the concave area. Water and air does not want to bend or go around something. When it does it slows down.
when we talk about double concaves, are we talking about double cancaves throughout most of the bottom or just in the tail? How would double concaves throughout the entire bottom work,and what kind of waves would they be better suited for?
Anyway you want to conceptualize it concave helps direct water flow, lowering resistance and thereby increasing speed. Problem comes when you want to be able to put on the brakes a little (turn) and have a hard time struggling against inertia from difficulty of penetrating the wave face to purposely increase drag (turn). So large-wave boards have features (thinner rails with no concave plus maybe a little vee)to help increase drag when needed. A “double barrel all the way” really just adds a ridge in the concave all the way, which breaks the flow and adds a tiny bit of drag when board is flat but channels water nicely when on a rail or slotted. Think “curves and mounds add drag.” Drag can be desirable. I’ve found variations of double barrels good for for decent sized juicy point waves.
Wow tubedog, that really explains it clearly! Thats the best breakdown of the whole concave vs. flat theory I’ve heard! How do you think this double barrel thing is going to work on flat mushy waves, and or choppy ones as well? Thanks man!
tubedog…well said. the “double concave” principle is indeed based on a “way out” for water from the point of entry through the tail. this is the concept behind the brilliance of the bonzer shape(thanx malcolm and duncan campbell). if you give the water a place to go…it will and you will have greater drive…MUCH greater…this is most easily felt on waves that you can really let it run on(perfect points) but you will get the benefit whenever you surf. i spent the better part of two years on some of the best waves around on a bonzer bottom and honestly…flew across the wave face. at first, it may feel a little stiff or awkward…this will fade as you learn the sweetness and really power through your turns…i recommend checking out the campbell bros. web age at least… …truly 30 years ahead of their time…anyway…take care…jmo http://www.bonzer5.com/home.htm
Peter Costello had a Kingfish at Big Sur. The board had a double concave ahead of the fins but the raised stringer between the concaves was still below the rail edge line (board was bottom up)… Herb Spitzer called it a “triple concave.”
The slower the wave, the more a deep single concave helps. Doubles better for faster waves. Add chop, then best to decrease the concave and go flat.
I realize that this response is a bit late… but I really couldn’t let this one slip by. “Water and air does not want to bend or go around something. When it does it slows down” Water and air does bend quite well… that’s actually part of the definition of a fluid. Slows down? …That is not always true… if it was, airplanes wouldn’t fly.
Perhaps someone with aero or hydrodynamics backgorund can chime in but as i understand it air and water want to follow a straight line. an example is the air intakes on those hotrod hondas and acuras, its as straight as possible and when i ask why they told me straighter = better airflow. Another exaple would be a wing foil, more curve on top slows the air down =lower pressure straighter on the bottom = higher pressure which in turn = lift.
Jason
'Another exaple would be a wing foil, more curve on top slows the air down =lower pressure straighter on the bottom = higher pressure which in turn = lift.'
No. More curve on the top means the air has to travel further over the wing than the air which passes under it. The static pressure of the air passing over the wing is therefore less than that which passes under it (the Venturi effect). The difference between the pressure on top and the greater pressure below produces the lift.
My understanding is that a single concave works (if indeed it does work) by allowing a straighter path for the water to pass under the board (reducing the dispalcement of water by the surfboard's rocker) and thereby reducing the amount of turbulence (which creates drag).
There are plenty more theories about such as the ability of a channel to produce thrust by squirting water out of the back of the board when downward pressure is applied by extending one's legs. I think the jury is out on this one.