Go with flat...it planes the fastest. There's a lot of hype out there when it comes to bottoms just to sell more boards. Water flows at angles to the stringer most of the time (except when paddling and going straight towards the beach) so the bennies of fancy concave schemes are minimal. Yet there are diffs between a concave and a Vee (angle flowing water may actually release off the apex of the vee during turns reducing drag). Most concaves are not much deeper than 1/16. As you get better shaping then you can experiment (if you can afford it). Comparing boat and water craft bottoms to surfboards is interesting...as an engineer I can use mathematical relationships (aspect ratios and percentages) to help validate or dispute technical arguments. Just one example: calculate the aspect ratio of the width and vee of a boat and compare this to whats common in surfboard design today...negligible when it comes to boards...actuall riding results will be validated by your own mind.
The best way to make a fast board is to keep the trim angle of the board as low as possible. A consequence of this is that more wetted planing surface area exists under the board. Minimising the trim or angle-of-attack of the board as it planes on the water will minimise the so called drag-due-to-lift. This makes the board go faster as less drag exists to slow the board down. Very simply put, the component of the board lifting force which is directed in the backward planing direction is the drag-due-to-lift, also known as the "induced drag". It does not take much tilting of the board toward the rearward direction to produce a sizable drag force. This result can be derived from a simple geometric vector force diagram. Interestingly, in a turn the "induced drag" produced by the board is correspondingly increased because the "cornering force" produced by the board is greater than in a straight path. (g-force effects) As an aside, the surfboard is a "simple machine" in the classic sense. It is closely related to an inclined plane but you won't see that in school textbooks. So long.
you want the most amount of planeing area for the least amount of surface area simple... regards BERT
I agree with Lee D's comment about the slowing effect of a concave on turning. I find in my experience that concaves DO slow a board down when turning, particulary if you turn before reaching the bottom of a wave, ie. before gaining a huge amount of speed. The board sometimes even stalls. I have an old 70's shortboard with an absolutely flat bottom that can turn the instant I drop in on a wave and it keeps it's speed - it even accelerates. But there's nothing worse than being on a board with too much concave, catching a large storm swell that might close, you turn soon because you might not be able to make it to the bottom before closeout, and then the ride simply dies due to concave, the very thing that was supposed to give the board lift and some punch from extra area and volume. I guess it's important to be going fast or that you are in a powerful section before turning when there is too much concave. It makes sense when you think about it - the board needs water to lift it a little but when it's moving slowly or when it is in a less-powerful part of the wave, it simply has too little lift and the rails dig too much. I think a storm wave - ie. punchy for a second or two, then less punchy when the backwash inihibits the top-to-bottom breakage is a good way to test the effect of concaves. Fortunately, I only have one board that has TOO much concave, but I can literally feel the effect since it only occurs with that board. But it's critical that I separate out other effects, such as the thickness flow, and other factors so that I'm sure that concave is solely to blame. But my intuition tells me that concave is what makes that particular board stall if I turn too soon. The board does ok on a fast beach-break inside wave that's pretty punchy - like a 3-4 ft. puncher (because it has enough lift). But frankly, in that case, a flat board would probably also do ok.
Good question of the guy who started this thread - concave or flat. The commenters are right - worry about the concaves last. I have boards in my quiver that are absolutely without concave that can squirt all over the place and they get plenty of speed on take-off even with less surface area and less channeled water. I think of it this way: a board is more maneuvarable from the moment of take-off (when it's moving slow) if it does not have concave. Also, a lot of times, when we take off on a wave, we already have a huge amount of speed an instant after take-off. A concave isn't really needed as much in that case. One things for sure: your board will surf even if you don't put concaves in it.
But It's obvious that the reason for concaves is to try to lift the board up and make it slide better and faster. It's just that we have to lift with the trade-off that it might catch a little when it's moving slower and there isn't much power to lift the board. In this case, it can help to kick-turn the board by jamming your foot on the rear of the board or perhaps remove the center fin for that session (twinzer), to decrease turning radius. Removing the fin might give it less drive but it's only a slight effect.
Cool topic though as nearly every board one reads about has some sort of concaves for lift or displacment for rail-to-rail, and in big waves when lift isn't the issue. So we should build some of our boards to have two or three different bottoms - flat, various concaves (and various depths as depth matters a lot), and various displacments such as vee, leaving all other factors the same. Then compare the rides and see where each board has advantages but in the end, pick something that works in wide range of conditions. ALL ONE HAS TO DO is build each model and take them out and test them! It's just that you have to make 6 copies of the same board so it'll really cost ya unless people love to buy your boards!
Well, I don't really know that much, just thought I'd chime a little two cents on what my experience tells me. But honestly, I DO plan to shape boards with vary amounts of concave and without to see what the difference is, so I really know with my own body on the board. And I'll try different methods of turning the board, and at different moments on the wave.
I also wonder what effect concave has on a real steep section of a wave where the board is almost in free fall. Will the concave board always beat the flatter board to the bottom, and how much of this effect is due to lift and how much due to more/less surface area? I don't think the concaves really give us less surface area as one might expect, it gives more unless we are so vertical that the board is almost 90 degrees. But there are times when surfers get on real steep sections of waves. This is something else to test - and well, a good reason to buy a ticket to G-Land!