At the most basic level, the bonzer doubles work in concert with the fins, which are highly canted… essentially becoming an extension of the concave wall. The theory is that the water escaping the concave over its ouside edge flows into the inside surface of the elongated outside fin(s) when on a rail, harnessing it’s energy, and adding speed.
Standard doubles do not have this effect, as they are not intended to be extensions of the concave. Double concaves capture and redirect more flow of water moving up the face of the wave than a flat or convex bottom would, providing more lift and speed. On a rail, the concave helps keep more water under the board, redirecting the flow out the back, rather than over the rail… again, providing more drive and speed.
When concave redirecting flow out the back pressure of water on surface increase (=lift) so there is more “frottement”, force opposit to depacement, so speed deacrease. Concave give lift and drive but deacrease speed.
I politely disagree. Concave increases speed when going down the line, and sensitivity. The drawback of concave is less stability in less than glassy conditions, and a slight tendency to hang at the top during a steep take-off.
Last month I wanted to see how a concaves would chang the ride quality on two of my boards. After making two off the same template one with a single and the other being flat my ideas of what it would do were pretty close. The board with the single road more directional and the flat felt less directional. By difectional I don’t mean it was stiff or hard to turn it just created more stability from what I noticed. It was pretty easy to tell the differences. I was able to notice the differences riding and paddling into waves. I almost want to say the flat felt a tad faster but it may have been due to it feeling more free.
mind clarifying this portion-- it stalls? or the bottom “clings” (a la velcro ?) at takeoff when it’s steep? would appreciate more details to have a clearer idea, thanks
By hanging at the top, I mean on a really late take-off, when you are going to catch air on the way down, a concave tends to stay at the lip just a tad longer before letting you in. This can spell disaster! For my boards for critical take-offs, I like slight Vee in the nose, flat under my chest, a slight concave under my hips, and Vee through the fins. This gives just a slight advantage in keeping from pearling (submarining the nose).
was just wondering whether the same thing would happen in a HWS (with bonzer concaves), which clearly would be heavier than a foamie. i’m thinking the added weight would keep this from happening. the one i’m finishing currently weighs about 18kgs already-- pre-lamination! hehe
Weight is good for getting below the lip. Quads and bonzers - anything that provides extra tail lift can put you out of balance. Did you see Yorky’s bonzer thread? Did you see how pulled in the tail was? Less planing area counters extra fin lift. I’ll bet that balance has a lot to do with how good he said it rides. I’d like to see the rocker though.
I have 2 boards, identical except for the concave (both have dual concaves between the fins). The flat is definitely the one for bumpy waves because the concave gets to be a handful. The concave does seem to hang in the lip a bit more.
To go back to the original post. You can also shape the dual concave from the stringer or leave a flat strip down the stringer and then put the connies in.
There’s also the decision - do you want the deepest part of the dual concave symmetrical in the middle or teardrop shape (deeper towards the rail or stringer).
Interested to hear what others do. Mine always have a 2" flat panel along the stringer (on my vee and concave bottom boards). and symmetrical.
but whats with the different between this two double concave. will they perform the same way? or will one of them be faster, better to control… ??
i repaired a campbell with a bonzer bottom but just with a standart thruster setup and i just ask me how it will perform compared to an “normal” double…