wow, lots of good things going here. I was reading about the tire hydroplaning on the water. The key aspect to see here is aspect ratio. Aspect ratio is the span of a wing divided by its chord length. So a wing 20 feet across that is 5 feet from leading to trailing edge (direction of flow) has an aspect ratio of 4 to 1 (20 feet across the flow / 5 feet with the flow). A couple of Aspect Ratio facts: higher aspect ratio lifting bodies (a sailplane wing) are very efficient, it produces very little drag for the lift created. The slender wing provides little time or distance for the passing air to go turbulent (turbulence equals drag). The drawback is that higher aspect ratio lifting bodies are usually unstable; sailplane wings are very susceptable to stall. Back to the tire: it might be hard to explain this w/o pics but imagine the footprint of the tire on the water as having an aspect ratio. I am envisioning a footprint kind of oval shaped. A long slender 10-speed tire has a chord length in the direction of motion that is greater than the width of the footprint (from left to right). This is not efficient so it does not lift well but it is stable. If we go with a wider tire, or even flip the ratio to something like a steam roller wheel (pretend it weighs the same as the bike) we get a high aspect footprint. If the footprint is the SAME AREA as the tenspeed footprint, it will lift with more efficiency BUT it is inherently unstable. Flow could break down while in motion and the hydroplaning would momentarily cease and start back up again. Kind of a dipping motion. (The bike tire could also be inflated or deflated to change the aspect ratio; keep in mind that the footprint AREA would change as well). NOW, let’s look at a surfboard. Focus just on the wetted area. (Just cruising along at Waikiki to make this easier). The wetted area of a planing surfboard has an aspect ratio. As expected the long slender aspect ratio of a standard longboard is a LOW Aspect footprint (Stable, but susceptable to turbulent flow along its’ length). Theoretically, the most efficient footprint would be a HIGH Aspect ratio. (Remember this is WIDER than it is LONG with respect to flow direction). I see something like a super wide fish, say 28" wide that is incredibly short (in wetted area; the board could be long so you could paddle). At speed, the wet footprint might be only 1 foot long(nose-to-tail) and over 2 feet across (rail-to-rail). Like the sailplane wing this would be very efficient (lots of lift given the area) but very unstable (you’re bound to eat it quickly). With this understanding some key elements appear. First, there is an optimum low drag aspect ratio for a given surfer/situation. Second, there is an aspect ratio that is controllable. Bottom contours mess with the effectiveness of the aspect ratio. “Hot” areas can be created that produce higher lift and “cold” areas that produce drag. The best solution for the next board you make is up to you, the shaper/designer. The bead described is another way to maintain Laminar flow. Mr. Morey was fully aware that devices such as “trips” and “vortex de/generators” can bring smooth flow along the length of a board. I calculated the flow along the bottom of a board has the POTENTIAL to start going turbulent (bad) in as little as 5 inches of flow distance. (Visualize a dam overspilling and the water is smoothly pouring down the concrete surface nice and clear and at some time the water starts to turn into “boiling” water, still falling, and eventually ending up as a froth just before striking the bottom of the spillway; the boiling is turbulent and is dragging, skidding slower over distance) To reduce the bad turbulent flow a crosswise piece of narrow tape can be placed every few inches (I tried built up resin pinlines) along the part of the board that is most often wetted. This works. Instead of crosswise “trips”, patterns such as reverse Chevrons promote Laminar flow. This is fun to try on existing boards. I eventually found that certain bottom contours will promote laminar flow others reduce it. Watch water flowing over a smooth stone in a brook or down a sluice; interesting. Lastly, Vee off the tail will allow a rider to do a “wheelie” or kickstall; that’s easy to see. As the name implies the wheelie eventually results in a stall. Well, take this situation and roll it over on its’ side. Board up on rail, sideways wheelie, takes more time to get to the stall. A skillful rider disguises the stall with a swing of the nose and shifting weight back up on the board… Remember all that was mentioned is ignoring a lot of other elements such as rail shape, rocker, foil, fin lift/downforce, and flex which as you know play a large part in finding good answers for you…
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!
Hah-hah!
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!