so that is what the stringer is for...fascinating

Surf Science: One of the big design aspects you have worked with is the parabolic stringer.  What do you see as the major benefits of this change?

Bert Burger: Projection, Strength & Higher Performance.  The stringer is the magic bit.  Your stringer is the bit that makes your board spring back to its original position after its been flexed. It also transfers rider energy through the length of the board.

It is really an interesting feeling riding a PU stringerless board.  Its just dead and floppy. At one point in time conventional surfboards were built stringerless. Even though most people would conclude that a surfboard would be stronger with a stringer. That wasn't the case.  A stringerless board could bend further before it snapped, therefore absorbing more force over a longer period.

The stringer was needed for performance.  Quote Len Dibben "Those old stringerless didn't break, we put stringers in because they didn't go." The boards needed to have something that stiffened them up and make them spring back to the shape quicker. That gives a basic picture of what the stringer does.

Surf Science: How does the stringer come into play when we’re engaging the rail on a turn?

Bert Burger: As we get further away from our stringer we are relying on materials that don't transfer energy as quickly or as efficiently.  The stringer will transfer energy the length of the board but not out to the rails. The harder we surf off our rails the harder we need to work.

This means when we put a board hard on rail so the stringer is out of the water we are then relying purely on foam and resin to hold us in, as well as project us into the next turn.

There is a stringer line and a rail line as we load into a turn. The rail line on a conventional board will bend easier & twist off with more curve relative to the stringer line. Picture a flat bottom square tail loading into a bottom turn. The rail that is in the water will load, bend a create roll & V off one side. As surfboards have become thinner, lighter, more flexible, there comes the need for a noticeable concave. This offsets the flex so the bottom of the board is in a more favorable position when being loaded though a turn.

If we place our stringer on the rail line it stiffens up the rail. It allows a greater range of flex in the whole board.  When the rail is loaded hard into a turn and is made to flex, the string back occurs right there at the rail. The rail that is engaged with the water already, you can push right off the loaded rail and feel it want to project or spring you into the next turn.  It makes the board respond quicker so you can leave tight turns until the last second.  I remember when Munga stepped on one back in 2005. First surf and feed back was - difficult  to time. I made the comment "Just leave it as late as possible and push hard, the more you put in the more it gives back". Munga comes back after the next surf raving. He passes similar comments on to Taj, who went on to say "you put 100% in and it gives you back 120%.

Source: http://www.surfscience.com/topics/surfing-lifestyle/shaper-interviews/bert-burger-parabolic-stringers/