When applying concave to the board what is the principle theory behind it? Allows the rail to bite more therefore less slip out?
Where does it end and flatten out…or is it basically from the tail up to where the front foot is planted? from bottom to top?
How deep is to deep when applying concave?
single to double? where does the single concave end and the double begin? Between the designated sweet spots for the feet on the board?
What is the function behind single to double?
I did a search.
didn’t find anything on general placement of the concave or got a strong impression on the overall purpose of the concave.
Another questions…Does the concave create lift allowing the board to plain sooner?
Hi Larry ,
Welcome to Swaylocks.
I did a search for “concave” and came up with 134 pages! Wow! It would take me all week to read through all that.
I’m a backyard repair guy and I check the concave on almost every board I fix. The’re all different. Interesting how many boards change bottom shape around the back fin and go to V or flat or back to single concave or… I’m sure one of the guys here can answer your questions better than me
Here’s a link http://www.swaylocks.com/forum/gforum.cgi?post=141619;search_string=concave;#141619
Have fun and take some time to scan through the Archives. Lots of good info there
There are a few guidelines you can use to help you, but a wise man once told me, “feel free”… words to live by.
The big, single concave should begin between a foot and two feet from the nose and should reach a maximum depth " somewhere between the surfer’s feet." I like to have the max depth half way between the wide point (where ever that is on the board) and the leading edge of the front fins. I use this general rule for both thrusters and twins. If you’re doing a single concave, it should fade out toward the tail. I like my last 3 inches of tail to be flat to keep it loose, but you could carry it out the tail for more drive (same with double concaves).
The double concave should begin where the single concave reaches it’s maximum depth. Doubles should reach their max depth just in front of the leading edge of the front fins. However, I believe where the double begins has a great deal to do with the ride of the board - further back makes it looser, further forward makes it drive better. But that’s just been my personal experience. There’s a lot more to it, I’m sure. Fade the doubles out to flat, vee, or carry them out the tail as you choose.
Again, from my own personal experience, I think the purpose of concaves is to be able to generate pulses of speed when you need them. This is done by pumping. When a surfer pumps a board, he/she exerts pressure downward, essentially squeezing water through the concaves faster than they do when just planing. This temporary increase in velocity of the water under and out the back of the board translates into forward motion of the board. (That’s just high school physics - Newton’s thrid law of motion.) Watch guys who are able to generate a lot of speed on triple concave boards…they’re constantly pumping. And when they hit a turn hard and just right, they come out of the turn going faster than when they went into it.
The down side is, concaves tend to be less user friendly in small, crappy surf, since deep bottom contours tend to be slower overall than flatter bottoms. So…for small wave boards, keep concaves shallow. For bigger wave boards, go a little deeper - I’ve gone to a max depth of 3/8 with good results. For fish and small wave twinnies, I stick to 1/4 inch or less.
Again… just my experience. I’m sure rails, planshape, width, thickness, tail design… even cloth schedule (stiffness) have something to do with it, too.
Good luck.
By the way… there’s a lot of bad info on those links above… I teach physics and I can tell you this:
Newton’s Third Law: Exerting a force upon an object creates an equal and opposite force back on the second. In surfing, forcing the board downward (pumping) makes the water push back. But since it’s a curved surface, ie. concave, the water has nowhere to go but out the back through the concave, which causes it to accelerate. This is proven using…
Bernoulli’s Principle: As the speed of a fluid increases, the pressure in the fluid decreases. (Hence the term “lift,” which is often used when talking about concaves. Speeding up the water under the board reduces pressure - creates “lift.”)
The transition of water from a single concave to a double concave also increases the velocity of the water, because you’re taking a given volume of water in the single, and forcing it into a smaller volume - the double. The double has less volume than the single because the space where the stringer is raised is taken up by board, not water. This is called the…
Venturi Effect: When a fluid is forced through a narrowing channel, it’s velocity increases and it’s pressure decreases (Again, this relates to Bernoulii’s Principle.)
This explanation should be made “sticky.” Perfectly put.
NJ I agree.
Venturi Effect: When a fluid is forced through a narrowing channel, it's velocity increases and it's pressure decreases (Again, this relates to Bernoulii's Principle.)
So to continue with a less scientificly backed explanation, mechanically speaking I think the concave facilitates both the 3rd Law and the Venturi effect by redirecting the water.
A concave bottom is essentialy a channel. When you pump the board you are displacing the water. The water has to go somewhere, with the concave the water tends to be redirected towards the middle of the board (the stringer) instead of flowing outward and off the rails. That combined with the already forward movement of the board, directs the increased flow out the back, water jet propulsion.
Similarly for the Venturi effect as the board moves forward and the entry rocker makes contact the board displaces water. Some of the displacement force will escape sideways of the rails while the captured force creates lift. The concaves again act as a directing tunnel directing the water from flowing off the rail to flowing through the channel. As the water crosses the widepoint of the board now the channel width is getting narrower and the same amount (not counting what escapes off the rail) of water must pass through a narrower tunnel thus the water is accelerated.
Lastly and my least scientific claim, as the water hits the narrowing edges of the concave the water is directed down slightly thus lifting the tail a little bit further.
On the flip side all this creates drag and thus makes a concave board slower when not worked correctly.
Being from Middletown… nice to see a fellow monmouth county surfer tell it like it is… thanks for the concave advice!
“Bernoulli’s Principle: As the speed of a fluid increases, the pressure in the fluid decreases. (Hence the term “lift,” which is often used when talking about concaves. Speeding up the water under the board reduces pressure - creates “lift.”)”
Wouldn’t that create the opposite of lift though? If the pressure under the board is decreased it should tend to suck the board further down into the water?
This has to be the most clear and concise description of concaves that I have ever come across. Thank you for posting.
Larry, You can do one thing that will help you to see with your own eyes what a concave will do:
The Spoon Illustration: Turn on your sink faucet letting water flow normally. Hold a spoon by the handle loosely between your thumb and forefinger. With the convex side of the eating part of the spoon toward the water, move it slowly into the stream of water. The spoon will be sucked into the water and held there as long as the water is moving.
Now, turn the spoon around and move the concave side into the moving water. Bounce, bounce, bounce. It won’t stay. You have to force it to stay in the water.
Translate that to surfboard bottoms. I’m sure the physicists can explain why it happens, I’m just a head-scratcher who knows it does happen.
I’ve made several boards lately with full length single concaves morphing into doubles in the back and one thing they do is plane on top of the water. Lots of lift. Lots of speed. Doug
Yes. That’s why I put “lift” in quotes. It’s only “lift” in a sense that it creates less pressure and a flat bottom, not a true lifting force. A common mistake people make, that you’ve picked up on. Good on ya.
That reduced pressure beneath the board does reduce drag, though, and all disturbances create drag.
That is true lift. water exiting the board’s tail, assuming the last bit of tail is flat, lifts the tail as it is deflected forward, helping to “level” the plane of the board as the entry rocker does the same thing… lifts the nose of the board.
Riddle me not…
What does the single to Double accomplish that a single concave cannot neccesarily then?
Translate that to surfboard bottoms. I'm sure the physicists can explain why it happens, I'm just a head-scratcher who knows it does happen.
Doug,
IMHO this is actually not an illustartion of concave bottoms, but the illustartion for rail shape. Water will flow around/over a soft round surface but will release off a hard/sharp surface.
What does the single to Double accomplish that a single concave cannot neccesarily then?
As NJ stated above in describing the venturi effect…
The transition of water from a single concave to a double concave also increases the velocity of the water, because you're taking a given volume of water in the single, and forcing it into a smaller volume - the double. The double has less volume than the single because the space where the stringer is raised is taken up by board, not water.
Essentially the double magnifies the effect of the single.
Whoops I missed it.
I’ve been looking through hundreds of posts. I appolgize.
Thanks everyone. I appreciate all the help w/ my conprehension of hydro dynamics!
I’ve seen concave done a few different ways. One way has the concave going from rail to rail, and the other way has a flat transition area before it gets to the rail.
What are the different characteristics of these two different ways of doing concaves.
Here’s an illustration to show what I’m talking about (I exaggerated the depth of the conacave just for illustration purposes).
Nice thing about making you own boards is that you can custom where you like the concaves etc. It’s all very personal and very subjective. Me I like a flat nose going to a 3/16 concave under my front foot. I like the double deep 1/4 concave under my back foot. So for me it pretty easy, Single concave under front foot, double concave under my back foot. But then you got the tail area. On the triple bottom, I like a flat exit area. If you start to put a vee in the tail then you start to make the board drivey. And in my scientific opinion you working against yourself giving “lifty to a drivey”. It’s easier to exit a double concave into a vee than a flat tail. But shaping a flat tail is a lot more difficult than a vee from a double. So if it’s you first board, then? If you don’t know what I mean, you’ll see what I mean when you look at the tail rocker profile once your done. Bottom contour is important, but not as important as rocker and rail design. My suggestion would be to make a completely flat bottom contour, and spend all the time getting the rocker and rails right. Flat bottom boards are fast, and they work fine for most wave situations. Once you got that mastered, start with a big single running from the front foot out the tail. Check the difference on those two, and start adding or subtracting design elements. Oh yeah, fin placement…that’s a whole nother deal.
-Jay