30 Years of Testing and Tinkering #4: The Law of Dynamic Positives: Part 1

 

 

Engineering professors love to say, with cynical glee, “If something can go wrong, it will.”

“Murphy’s Law” is the generally accepted name for this assumption. But, in fact, the above quote is properly referred to as “Finagle’s Law of Dynamic Negatives.”  (Google it!)

So what does Finagle’s Law have to do with us, the elite mat surfing community?

Well, nothing…because there are a good number of random events in the history of surfmat engineering that fall into the category of, “If anything can go right, it will.”

Mat design, as it turns out, is “right” most of the time. While it’s hard to build a really great mat, making a serviceable one is pretty darn easy. And that's why mat surfing was able to flourish as smashing good fun for the first half-century of its existence….say from the mid 1930’s to mid 1980’s.

Like most delightful things in life, the reason behind this is simple.

The ideal configuration to move through the water is a parallel-shaped object with an approximate length-to-width aspect ratio of 2:1. And, lo and behold, inflated tubes lashed together tend to be parallel. Further, the original mat dimensions  were designed to fit the length and width of the average kid's torso…roughly a 2:1 aspect ratio. So, without even knowing why, mat design was on the right track from Day One. The first mats ever made, way back in the 30’s, would be perfectly fun to ride today.

Bondi Beach, Mid 1930's

 

The downside of all this cosmic bliss is that, because other forms of surf craft have evolved so dramatically over the years, it’s natural for surfers to imagine how those same “advancements” might be applied to surfmats. And so, about twice a week, someone sends me an email and asks about trying a surf mat with an outline shape. Some think they should be curved like a surfboard…with a narrower nose and tail. Others go the opposite way, and want to use a concave outline shape, with the outline template generating a wider nose and tail, with a skinnier mid-section.

Please don’t misunderstand me, these are perfectly reasonable ideas. George and I have tried them all…or enough of them to realize that tweaking the outline shape of a surfmat not only doesn’t improve the performance, but actually degrades it.

But why?

There are two main reasons, from I’ve observed. One is that when you cut into the straight outer perimeter weld with either a concave or convex curve, not only is the outline changed, but the thickness flow changes as well. Pull the tail in with an outline curve, and the thickness flow of the outer pontoon in the tail thins out at the same time. So you lose the “hold” of the thick rail line. And that’s a serious drawback.

The “undercut” mats (narrower in the middle) work much better than the surfboard-shaped mats…but a mid-section that’s narrower is also thinner, so the noses tend to catch a lot. And the thinner middle drifts around at odd times.

 Example of a severely undercut mat.

 

 

The other downside of a surfboard shape on mats is that the outline curve tends to kick the mat out of its track. Mats need a straight outline and straight thickness flow to keep them moving in a straight line. Essentially, outline curves want to destabilize the mat. And since mats have no fin to rely on, straightness wins the prize when it comes to forward projection. 

Here’s a surfboard design anecdote from the 90’s that relates …

Back in the 90’s, Spencer Kellogg and I started shaping full length Simmons boards (9’ plus). We both assumed that the original Simmons boards, with their enormous tail width, were woefully underfinned. So we dropped a fin box into the first one and stuffed a longboard fin into it, confident we knew what we were doing.  Well, that first Simmons board, with a big fin, tracked in a straight line so badly we even couldn't paddle it without falling off the side of the board! It turned out that a small fin was the right call after all…which was what Simmons had figured out in the late 40s.

One of our Simmons boards -- a 10'4'' x 24" for Roger Kelly -- with a fin box allowing for experimentation. A relatively small Greenough Stage 4 fin worked best, even in Island surf!

 

The conclusion Spence and I drew was that outline curve, not tail width, determined how much fin a conventional surfboard needs. A straight outlined board, like a Simmons, needs less fin area, even though it has a wide tail. A curvy outlined board, like a thruster, needs a lot of fin area, even though it has a relatively narrow tail.

To further make that point, here's a quick evolution of outline shapes, relative to fin area...

 30's era plank. Very wide tail, dead straight tail outline, surfs OK with no fin.

 

  

 Restored early 50's Simmons board. Wide tail, slight outline curve, works best with a small fin.

 

 

Mid-50's era Velzy Pig.  Narrower tail with lots of outline curve, much bigger fin.

Early 80's Simon Anderson Thruster. Narrow, very curvy, needs lots of fin area.
 

To review:

 

• The ideal surf craft, in terms of moving efficiently through the water, is a parallel shape with an aspect ratio of around 2:1.

• A surf craft with a perfectly straight, parallel outline needs little or no fin area to surf well.

• Mats naturally fit both criteria.

What makes this favorable outcome so cool is that it came about by engineering circumstance, not design intention! 

Maybe we should call that phenomenon,  "Mat Surfing's Law of Dynamic Positives."