Ok, I’ve been seaching the forum for a few hours and I can’t find anything on my question. My question is this: What shape should a concave (single or double) be if viewed from the bottom of the board. I have a good grasp of what they look like when viewed from a cut through but I have no idea what their shape should look like from the bottom.
Just so you don’t fall off the page without at least one reply, and because I think it’s a natural question, the “shape” of the concave is dependent on the outline of the board, and it’s a blended thing, from the entry to the release(s).
Picture a plank-flat board. If you have concave ahead of the wide point, suck, stuck. If you put some entry rocker in, not such a problem.
But there’s no real answer to your question…
I think what he is trying to say is … what plane shape should it have. Tear drop, oval, eliptical or so on. I don’t have the slighest idea either.
There is an endless number of answers to this question, because you can do anything you want really. Just remember how the water will be running along the bottom of the board, you want it to transition smoothly between your concaves and your flat spots. A single concave that starts ahead of the WP and ends near the fins will probably look something like an oval, because at it’s widest point it will extend out toward the rails but as it fades out and becomes shallower it comes closer and closer to the stringer. This is because you want more rocker at the rails and less at the stringer to increase speed. In fact it’s much easier to think of it that way; a flatter rocker at the stringer for speed when you’re not on rail, and more rocker when you put your board on rail for ease of turning. To begin to understand what concaves look like you should try and figure out what it is they do for you, then apply your own designs to make a board that works for how you surf. This info and so much more can be found in the archives.
Thing about this too: making water change direction costs energy. The only place you get energy is from going “downhill” on the wave. If you’re surfing small mushy waves, at low speed, I don’t believe concave will help you at all.
But you can always say to the bros, hay mon check dis radical concave, it surfs soooo gooooooodd…
Here are some pics. Hope it helps you out. Single concave through tail of a longboard. Let me know if you need more.
schro: good pics – shows how subtle concaves can be. I love putting a straight-edge across my boards to see and compare – start up by the nose perpendicular to the stringer and slowly pull in back toward the tail to see how the concaves ( or belly, in the case of one of my boards) flow.
Here’s a tip for measuring concaves: runs a line of masking tape down the stringer and mark on the tape every 1-2" (x axis)new take a shapers square or a ruller and measure parrallel to the stringer and make a dot at the outer limit of the concave were it meets the rule (y axis), do this for each 1-2" incrament then join the dots. You can also measure the depth at the centre of each of each incrament and then every 1-2" from the stringer to the outer limit (z axsis)
Now you have the numbers fro a perfect 3D reprisentation of that concave.
An easier way is to simply have the rule parallel to the stinger and run it along the length of the board and watch how that outer limit moves.
So, you guys do your concaves sort of like one big channel?
Meaning that in some places the curve does not reach all the way out to the rail?
That seems strange to me…
My concaves (a single for simplicities sake) always extend rail to rail.
For example, travelling from nose to tail on the bottom of the board, using the straight edge trick:
A little bit before you reach the widest point, the stringer is in the same plane as the rail, meaning flat bottom.
Move towards the tail a bit more, and the stringer starts moving away from the straight-edge ruler that you have across the board, perpendicular to the stringer.
So the shape that is being made between the board and the ruler is a shallow semi-circle. (but not a true circle arc of course, some kind of parabola or something).
The curve starts where the ruler meets the rail, and ends where the ruler meets the other rail.
So if you layed the board on the ground bottom up and looked down on it, the “shape” of the concave would not be an oval, or anything, there would just be a straight line across the board somewhere just forward of the widest point, where the minute transition from being flat bottomed to very slightly concave starts.
But even there, the curve extends all the way from rail to rail, just a very shallow curve.
The deepest point of the concave tends to be forward of the fins, as from there width of the board starts decreasing, so the semi circle is much smaller.
Moving on, the rail line and the stringer line get closer and closer to the same plane, as the tail is approached.
Quite often i’ll actually come off the end of the tail as a flat bottom again, but usually with a single it’s still slightly concave, but very shallow, as the tail is so narrow.
With a double concave, the rail line even passes the stringer line, coming out of the tail as a vee sometimes.
Maybe i’m reading the previous posts incorrectly, are you guys doing something different?
Hope this helps for anyone trying to visualise what a concave does as you travel from nose to tail.
Kit