I shaped a 5’10 x 19 3/4 x 2 7/16 twin fin some 10 years ago.
I was aiming for a single to double and vee out the back as that was what all the other good boards had and what everyone was talking about. The finished board ended up with a single concave from the nose that goes out to flat about 4" behind board center, transitiions to a vee that runs all the way back out the tail. The bottom contours are shallow, less than 1/4 deep and the boards just goes! At the time I had around 10 boards but this one took over the role for all except maybe the 6’4" step up.
I made the 5’10" to work in a wide variety of conditions on our pointbreaks. Sometimes it crumbles and is more of a longboard wave and sometimes it is very powerful, hollow and tubing. It’s usually breaking quite fast so you need to hustle down the line.
In good conditions the board holds very well high up on the face screaming down the line. It carves deep speed arcs both of the bottom and of the top.
In lesser conditions it is still very responsive, lively and loose. It needs very little wave face in between sections to burst speed to go around to next section.
The fins are glass on laminated wood shaped pretty much like a beefed up thruster side fin. Flat foil inside. Base, height: 5 1/4 x 5 1/4
I don’t know what the magic ingredient in this board is but I suspect it is mostly thanks to a lucky blend of rocker and bottom contours. When it comes to bottom contours I think they are important but should be kept quite shallow (as long as you are not looking for something specific, like in a hull or a noserider).
My understanding is that a venturi effect can be attained in a closed passage like a tube, but not on an open surface like a surfboard bottom - the fluid must be contained to be pressurized.
See “Venturi Channel.” The fluid does not need to be enclosed. Venturi channels are used to measure flow rates.
I mention the Venturi concave because of past discussions at Sways — seems to be some confusion. If a VC improves performance, it does not increase water speed flowing past the rear fin(s). Low pressure and higher water speed would be in the narrow section, not before or after.
venturi effect is modelize by bernoulli équation which are right for linear water flux, mostly not what happen under a surfboards. Turbulences, « smaller » shape change, lower speed all this make most hydraudynamic theory out of game so…
I used to see an add for large Triamans being built in Hawaii that used the Venturi bottom. Just like your drawing. I have wanted to try one for years and may yet.
I’ve got an appreciation of double concaves in the back half of a board.
I recently gained some insight on another argument for double concave in the tail.
I’ve been playing with a new rocker that kinks the rocker upward just a few inches behind the center thus adding a bit of rocker to the center of the board.
A flat bottom would never work with that rocker because you would notice a very obvious kink at the rail line, upsetting the otherwise graceful flow.
Using vee to smooth out the curve of the rail line corrects that problem.
Here’s an unpopular theory for you all: The majority of surfers would surf fine on a flat bottom surfboard. In other words if you’re under 45 and can’t do an air reverse then riding a concave bottom won’t make much difference to you. How did I come upon this thoery? About 15 years ago I made a board and forgot to put a concave in it. I rode the thing and I honestly couldn’t tell the difference. I made boards for people I know and didn’t tell them they were flat bottoms. They didn’t notice either. I’ve copied a couple of well known designs by respected shapers as an experiment. Couldn’t tell the difference between the my flat and their concave. Try it sometime. You might be surprised by the results.
The latest phase of my shaping / board construction adventure is to try to get back performance that I lost when I started glassing heavier, for longevity.
I started working on boards doing ding repair, then glassing and finally shaping. As a ding repair guy, I was just getting into business at about the same time longboards were making a comeback, and I’d be flooded with broken longboards the first good sized swell of the winter. These were off the shelf longboards glassed in SoCal. There was a local shaper who would glass boards heavier and he has some custom features that help longboards work better with a heavier winter glass job, and I tried following in his footsteps. But to this day, the board that I surfed best was one that had a very plain bottom and a very light glass job.
The point that I’m trying to make is something that is in agreement with the point made by spuddups ; I have been thinking that fancied up bottoms have been something that i have used in my effort to replace performance that is lost when a heavier glass job is used for purposes of longevity.
I don’t think that there is any replacement for flexibility in terms of performance
But something that occured to me a while back was; Eliminating sideslip is critical to producing speed through a turn.
This has been where channels and concaves and the principles of channeling water can be applied
Think of times when you had a good skateboard that you can pump for speed on a level surface or even maybe up a slope - Then think of how that goes when there’s some rain on the pavement. It’s impossible - That’s how critical it is to minimize sideslip when you want to create spee3d by pumping.
Seems to me that this is the kind of thing that you need to try and see what you like.
I have been building iterations of the same board for the last 5 years and have tried a few different bottom contours… Flat, single throughout, single to double with V off the tail, Single to double, etc.
I know what I like and there is ZERO theory involved. 100% seat of the pants.
In fact, some things that I fully expected to like surfed like a dog.
Theory is fine but real-world testing is KING. Period.
imo, you should be wary of advice from folks who don’t surf boards similar to what you ride and/or don’t make those kinds of boards for other (hopefully competent) surfers.
Some things that work or don’t on logs are not gonna work on a standard shorty or groveler and vice versa.
Single is good too but I feel that the double has a little more control in the upper range with no real loss to performance on the low end.
Flat was “meh”. Flavorless. To be fair though, I think flat bottom is more rocker sensitive. Obviously GG has his stuff DIALED.
V sucked. The board was looser but I felt like I had nothing to push off of when pumping. Like I would push and it wouldn’t push back. And, this was a TINY amount of V. It just wasn’t for me.
Again, this is all under my feet and in my opinion for the waves I surf.
Somebody else might be totally opposite.
I have too many irons in the fire right now and am not making fins for other folks.
I started making flat bottoms after reading what Greg Griffin had to say about them about 8 years ago.
I’m light (75kg/165lb) and my boards have thin rails, so that has an effect.
The flat bottoms give control in chop and bump where concaves tend to skip. Flat bottoms also don’t seem to have the limit on top-end speed that I felt in concave bottoms.
I think it (single to double) makes a difference for the Pros / top 1% of surfers. Their boards are usually at the cutting edge of design.
As for me, I just hack them out and then ride them. at the moment I have 17 different boards from 9’6 to 5’0. Quads, thrusters, twins and singles. Fat boards, thin boards. Wide ones and narrow ones. Hard rails, soft rails. Lots of rocker, flat as a barn door. I ride them all. Unless I’m replacing a board I’ve broken I’ll generally try to make something different. Sometimes I’ll get a board that’s a bit tired and chop six inches off the tail and change the fins round. Sometimes I’ll strip the glass of a couple of old boards, slice them up, glue them together and make an entirely new shape. So yeah, I like experimenting, but as I mentioned I tried concaves (and also V for that matter) and couldn’t notice any difference, so I just started making flat bottoms only.
When you consider that aircraft, designed and built using the most state of the atr formulas and programs, are always tested and then modified after flight testing, it gives credibility to the idea that you just don’t really know til you try it.
“During this competition, the Pukas Dark was the fastest board reaching a top-speed of 38.4km/h. Pukas shaper Axel Lorentz wanted the board to be super fast: “I lowered the exit rocker and made the rails more parallel. The more parallel they are, the fastest the board goes.” Following this approach, Axel also acknowledged it would have a negative impact on maneuverability hadn’t he deepened the single concave; “deepening the concave in the Pukas Dark eases the maneuverability, which I also enhanced by adding a classic kick tail.””
Forever trying to wrap my head around board design. So I thought that i KNEW single concaves were for lift and speed (whether by lowering stringer rocker or redirecting water is another argument), but their negative attribute would be “trackiness”. Here Alex Lorenz is talking about using deep single concave for maneuverability to counter parallel rails.
For me concaves are like the matrix. Everything in this life that i know to be, is not.
