bottom design and flow

General Discussion
1

 

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Lately I’ve been thinking about water flow and velocity as it relates to
curves and contours… Hopefully I can spark more discussion in relation to
surfboard bottoms design.

I’ve spent countless hours porting and measuring airflow though cylinder
heads. With my flow bench I can measure flow rates and change port contours to
increase flow. If you want to speed up and direct flow up you create a concave shape.
If you want to slow down and relax a flow profile you create a flat shape. By probing these two
shapes while pulling air though a port you can see the verry different velocity profiles
they produce. While this is airflow some of these principals seem like they can relate to surfboard bottom
design.

Wow… There are lots of threads on here talking about concaves and how they affect
performance. It’s pretty interesting reading all the different perspectives and ideas behind them. Many
people feel concave is producing lift while others feel it’s for control.

If you run water down a concave surface flow velocities are higher towards
the center. Even if you change the angle of attack flow is still directed down
the concave and out the back of the board to a point. This seems like it would
be more of a controlling / drive factor?

On a flat surface flow velocities are much more even over the entire
surface. 

Perhaps some feel concaved bottom boards are faster because they
help gain control resulting in more speed? maybe somone who surfs more off the front foot than the fins benifits from the controlling factor in concaves?

What do you guys think?

2

I think water does not flow along the bottom of the board only. It also flows up into the bottom, and that complicates things significantly. Add to that, that a concave is only rail to rail, not nose to tail, and the degree of rocker and and depth of concave only make the scenerio more confusing.

3

Water and air are both dynamic they tend to stick surfaces in motion. Bernoulli’s principle illustrates this:

http://en.wikipedia.org/wiki/Bernoulli's_principle

It is the reason that properly foiled wings can fly. In short the pressure differential between the top and bottom of a wing actually sucks the wing up countering gravity.

4

"Add to that, that a concave is only rail to rail, not nose to tail"

FWIW, I've shaped boards with a reverse tail rocker in the last 18" or so... a straight edge along the stringer revealed that in at least a portion of the rocker, a concave existed fore and aft.  I think some of the edged tear drop nose concaves have some concave parallel to the stringer.

5

How did it ride?

My nose concaves tend to be nearly flat along the stringer. But I like mine comparitively subtle and blended.

6

Concaves redirect water to the tail, that means a force straight forward is generated.

The water flows from the rail in the face to the rail hanging over the water, concaves redirecting water more downward than a flat board thus generating upward force.

If you see the rail as a foil redirecting water to gain force concaves increase the redirection of water thus generating more force.

This could make the teardrop concave a bit more understandable too.

7

I think that some of this may be counter-intuitive. Bernoulli’s principle is well established in fluid dynamics and states that the speed of a fluid over a surface is inversely proprotional to the pressure on the surface. Thus given a constant velocity thru the medium, the pressure on a flat surface would be higher than the pressure on a curved surface. That said, this assumes that the entire surface is in the medium, which is often not the case with a surfboard (visualize a turn for instance). The rider (obviously) is a vital part of this “equation”.

So, all else being equal, a concave should generate a low pressure region which if severe enough could lead to cavitation (disruption in the laminar flow  of the medium).The faster the concave moves thru the water (or the deeper the concave) the more the pressure is lowered.

 

8

"'How did it ride?

My nose concaves tend to be nearly flat along the stringer. But I like mine comparitively subtle and blended."

 

Hi -

That type of concave was shaped on a couple of bellyboards I did.  Since most of the rider's weight is aft on that type of craft I wanted to maximize lift in the tail.  The concave was subtle to avoid inducing too much drag.... maybe 3/16" or so.  It (fore and aft concave) was combined with a side to side concave. 

After imagining the lift created by airplane wings with flaps down I figured it might work. 

The boards seem to ride OK.