bottom contours 101 ?

General Discussion
1

hey swaylockians!

long time no see.

i made my first 6 boards and was able to surf them in july in portugal which was really nice. they actually do work quite well for me so im really stoked.

to be honest, im not a good surfer though and i im not too sure if i would feel a lot of difference, especially when it comes to bottom contours.

i read a lot about different bottoms and all but i am not really sure if i understood everything right and if my assumptions and theories are right. i dont wanna put something into my next bottoms that doesnt really belong there.so i would kindly ask all of you experienced foam sculptors to share your bottom contours knowledge with us newbies.

i know it really depends on a lot of other things and i know that theres a lot of this in the archives already, but i thought it could be useful to start a discussion about different bottoms, why you do them the way you do them, and what you like about it…

a little general rule, which seems to make sense for me would be:

a single concave makes a board faster because you straighten the rocker line at the stringer, the rocker line at the rail stays curvier, which helps with turning.

a double concave at the tail, like in most modern performance shortboards, is put in to create lift to get the tail out of the water a bit and therefore make the board looser.

vee is put in to make wide tailed boards turn easier because its easier to bury the rail into the water.

if this is comlete bullpoo please say so. :wink:

things i dont really get yet:

why put in a deep double concave into a keel fish for example. they have a pretty wide tail most of the times and therefore should generate a lot of lift anyway. what can a double concave do for you in wide tailed boards? i really like the fish platform, in all its variations, so for me it would be really interesting what you guys do with your keel, quad, sword, performance - fishes bottoms and why.

thanks a lot everyone

take care.

trim chris

2

My $.02 -

concave in the mid section = speed; lift/flat - speed

in tail = same

you could have too much in the tail, esp. when waves offer more power.

I’ve gone to serious kick @ 1/4 length from tail, - flat rocker w/Vee.

I’m still happy w/big concave, for smaller waves, just past entry rocker - I recon @ 16% of length from nose - through “true” apex - @ 37% from nose - fading to flat at kick.

But of course - all this is just my own trip… for what it’s worth.

3

The assumptions you posted are mostly valid. Additionally, vee under the fin area is helpful in narrow-tailed boards which are designed for larger, faster waves to give control in a situation where speed comes from the steepness of the wave itself, and especially where surface chop is an issue (i.e. Sunset Beach, HI). A slight vee in the forward entry area also can help calm bump, by displacing water while also freeing up the rail edges. Double concaves in the tail can also be integrated into a slight vee in wider tailed boards to great effect. Good luck with your shaping and happy surfing!

4

I like flat bottoms. Fast, predictable and , most importantly for me, easy to shape.

5

Man I wish Tom Morey would add to this discussion! Two references that have stuck in my head for years are by him.

One magazine article mentioned concaves and introduced readers to the ‘dangled spoon under the faucet’ experiment. Just turn on your water faucet and loosely dangle a spoon between two fingers. Let the belly side of the spoon touch the water stream and see what happens. Flip it around and touch the scooped out side to the water stream. The difference will tell you a lot about how two different shapes interact with flowing water.

The other article was about speed beads. He used silicone sealant to add water deflection strakes to the bottom of his board. Modern designs like a triple concave (dual concaves within a single deeper concave) may well have their beginnings in Tom Morey’s experiments. The peaked stringer may act something like a speed bead inside the deeper single concave and deflect water away from the bottom of the board rather than allowing it to flow all the way across.

If you’ve never done the spoon trick, go try it.

6

That is know as the Bernoulli principle or effect. Here is a link that explains it.

http://home.earthlink.net/~mmc1919/venturi.html

I actually got to talk to TM once a long time ago. He is an interesting chap. I incorporated some of his published ideas into several boards in the 70’s when I was still shaping for myself and friends.

Picture this a board with a sculpted deck and concave rail edges, and a concave bottom with an airfoil and vents to the top deck. It worked in really big hollow surf, but failed in slow waves. Another attempt was much softer and floatier and worked much better. It had a feel of skating once it got up to speed, the airfoil affect I guess. Since then I stick to simple classic shapes, maybe a slight V in the bottom. The bottom line is that water bends, and that causes pressure differences but they are not always intuitive.

7

hey!

thanks a lot everybody!

very interesting.

keep them coming.

some one can talk a bit more on what they put into their fish bottoms?

this shouldnt become just another aquadynamics theory thread, although this is a very very interesting topic, but there are enough in the archives already.

talk about your feelings actually while riding or paddling a board, i think bottom contours start to get in the game as sonn you lay the board into water, so i think paddling speed should be a big part of this too.

what actually works for you and what not?

how does it feel while top turning, bottom turning, down the line, stuff like that.

keep em coming!

cheers,

chris

8

I’ve only done about 10 boards or so, but ove the years my bottoms have been getting flatter to where they are now almost totally flat through the middle 1/3 (rolled in nose and spiral vee onder the fins) (the archives have a long debate about spiral vee).

In doing so I’ve reduced the amount of vee and kept a 3" wide flat panel along the rocker. I’ve found that I’ve reached a point where the boards are almost too skippy in windchop - partly because of how fast they go. My rails and tails are thin so I can overcome the skip by burying the rail (and don’t need vee for this).

9

Hello again -

At least two fish designs, a “Kingfish” and one of the Al Merrick’s both have dual concaves within a single deeper concave with the deepest part ahead of the fins.

A casual examination only reveals dual concaves. It wasn’t until I actually laid a straight edge across the bottom that it became apparent that the whole shebang was inside a deeper single.

It was in Big Sur where Herb Spitzer clued me in to the term “Triple concave” in that sense.

Sorry, I don’t have any personal experience riding them so I can’t give feedback on that.

10

Manny Caro (Mandala) does or did the double inside a single concave on fish. Had one as kneelo though it was a bit too narrow worked great. Very fast, caught waves easily (pretty big, floaty fish it was) turned well. All around wish I’d kept it…but long gone sold

11

You can smooth out the ride in you boards by using a small amount of roll through the first 2/3 of your board rather than the flat. I’ve done a similar design and it’s my go to board in windy conditions.

12

Flatter bottoms, especially with wide tails, paddle faster. A wide, flat fish with a wide tail paddles faster than “modern fish,” which is basically a flatter, shorter, wider modern shortboard. The difference in how they ride are obvious… one rides more like a shortboard, but can perform in smaller waves better. The other catches anything and goes faster down the line and over the flats, but isn’t as snappy as a it’s more modern counterpart.

Traditional fishes that are wide in the mid section and tail, have vee bottoms in the back third or so to allow the tail to sink more into the water (all displacement features in the tail do this). This helps compensate for the wide tails, which, when at top speed, can get skippy and hard to put on a rail. The vee adds some control, as does the deep swallow, which is basically a relief feature that allows water to release between the tips, allowing greater penetration to the rail on a turn, and greater hold in steep parts of the wave. Without the swallow (relief) feature, you’d be skipping like a stone or getting sucked up the face in the tube.

“Modern fish” have variations of concave for lift, as has been stated already. Their primary purpose is to be able to perform like a shortboard in less than ideal surf. Combine that with different fin setups and you can have a lot of fun in sub-standard conditions. My favorite right now for small summer surf is a quad version with shallow single to double concave and relaxed rocker. I wanted a shortboard feel with the ability to ride small surf, but not a twin keel fish.

13

identical boards (cnc machine) different bottom contours. world of difference. that is what my friend and his shaper worked on this winter around santa barbara.

14

Think of concave acting like a parasail or a hang glider. Water moving up the face gets trapped under the board, so the energy of the moving water is contained, and released out the tail, generating lift and speed.

If you put the concave toward the nose, you will pearl less, but add a little drag. I tend to surf more off my front leg, so on my average board, I like about 1/8" deep starting about 18" down from the nose. I then put the deepest concave at about the midpoint (about 3/8" under my front foot, but I weigh 210 pounds). If the board is for head high and above surf, I end the concave just in front of the fins, and put a V under my back foot (about 1/8"). If the board is for waist high and under, keep the concave all the way through.

V does just the opposite of concave; it allows the waters energy to escape. With V through the fins, and out through the tail, it lets the back sit a little deeper in the water. Sitting deeper in the water isn’t really all that important, but by removing the lift from the tail, you can carve a turn easier, and on a late drop, pearl less.

Don’t think of concave adding speed by removing rocker. You can still have any rocker you want. Concave adds speed and looseness by lifting the board higher out of the water. Imagine what you would surf like if you weighed 20 pounds less.

Lastly, Quad fins do the same thing as concave, but only in the tail, and to a greater amount. Generally, a quad will keep up speed through a cut back by catching the water pressing up againns the bottom of the board. Sadly, a quad will to a hard bottom turn in the pocket worse, because it doesn’t want to let you push the tail down, and the nose up.

15

Just as a point of clarification…the only concave is rail to rail. There is no concave nose to tail… it’s still convex there, so no water pushes up in the tail of the board creating lift. That’s a common misconception. You can generate actual lift in the tail using fin cant, however. In which case, a quad creates tons of lift if all four fins are canted and single foiled.

Also… vee adds tail rocker along the rail, while concave in the tail does not. That’s why vee helps with flatter shapes, like traditional fishes. You keep the stringer rocker flatter, but the rail rocker curvier. So when you put the board on the rail, the turn you make tends to follow the rail rocker line.

16
Quote:
Man I wish Tom Morey would add to this discussion! Two references that have stuck in my head for years are by him.

One magazine article mentioned concaves and introduced readers to the ‘dangled spoon under the faucet’ experiment. Just turn on your water faucet and loosely dangle a spoon between two fingers. Let the belly side of the spoon touch the water stream and see what happens. Flip it around and touch the scooped out side to the water stream. The difference will tell you a lot about how two different shapes interact with flowing water.

The other article was about speed beads. He used silicone sealant to add water deflection strakes to the bottom of his board. Modern designs like a triple concave (dual concaves within a single deeper concave) may well have their beginnings in Tom Morey’s experiments. The peaked stringer may act something like a speed bead inside the deeper single concave and deflect water away from the bottom of the board rather than allowing it to flow all the way across.

If you’ve never done the spoon trick, go try it.

I agree with NJ surfer. I don’t think Bernoulli’s Theory (of lift) really applies to the concave on the bottom of a surfboard because the water is NEVER travelling at 90 degrees to the stringer which is the only way that bernoulli’s theory would match a surfboard’s concave. Since water is running more parallel to the stringer, the concave is actually just reducing the CONVEX of the rocker at the center of the board- straightening the rocker. So the theory is that a concave will make the board faster down the line when going straight (less rocker) and looser when turing on the rail where the rocker remains. Or at least that’s how I understand it.

17

If you need something to smooth out chop and allow the rails to release, stay away from roll (too much drag) and shape in a defined tri-plane instead. You can keep the tucked- under edges crisper through the middle of the board with the tri-plane and they will not catch.

18

It’d be nice if someone could do an experiment on this cos it seems like everyone has a different opinion…

Search archives for Bert burgers ideas on this and they say that

  • flat is fastest

and

  • concaves slow you down
  • .........as does V.........
  • However.........he said that he thinks that PU boards flatten the concave when loaded in a turn.

Hmmmmmmmmmm…not sure 'bout the last point - sounds wrong to me.

It’d be nice to know the truth based on experimentation…or maybe this has been done?

19

I guess the point I’m making regarding rocker is that you can create any rocker, regardless of the concave. I can make a 4 1/2" nose and 2 1/2" tail rocker with or without the concave. Yes, the stringer would be straighter than the rail, but concave doesn’t necessarily mean a flatter board.

Regarding water pushing up under the tail, look at a picture of a rider in trim on a steep face. The front half of the board isn’t even in contact with the water. My comments were accurate after your first turn and you are heading down the line. Go back to the picture I was talking about. Which direction does the surfers wake travel? It goes up the face. The more you grab into the water moving up the face, (think thicker rails, concave, larger fin) The more the force of the wave is going to try to lift you up with it. The more you lift, the faster you go. The trick is giving as much lift as possible before you loose control and get sucked up the face!

20

“Just as a point of clarification…the only concave is rail to rail.”

Hi -

I’ve seen localized teardrop concaves in noseriders that were scooped out lengthwise.

I’ve personally incorporated a reverse end stage tail rocker combined with a rail to rail concave and it really seemed to help lift the tail. A straight edge balanced at the midpoint showed some tail rocker but a straight edge placed on the very end of the tail revealed a slight lengthwise scoop behind the fin. FWIW