Stringers

or "the steel cable and the rubber band" 

 

Most traditional PU surfboards sport wooden stringers – one, two, three, many. While decks seem to perpetually collapse on either side of them, causing fractured gloss coats and eventually fractured glass as well, they are nonetheless considered helpful.

Most single-skin EPS and XTR-cored boards faithfully replicate the stringers that were, after all, so very “successful” in PU boards.

When newly converted sandwich Epoxy owners bring me their busted boards, they will often complain that “this would not have broken if only it had had a stringer in there!”

O really??

 

How much sense does it make to reinforce a fiberglass-based board with wood? Just about as much sense as to reinforce a steel cable with a rubber band!

 

Picture the fiberglass skin (the steel cable): it is immensely strong (approx 25,000psi) and will not stretch appreciably (elongation approx 1%).

Let our stringer (the rubber band) be made of Cedar, with a tensile strength of around 450psi (and that’s being charitable), and an elongation of approx 5%, i.e. about 5 times stretchy-er than fiberglass.

As a load is applied, the fiberglass skin will remain virtually unchanged, until finally, as its yield strength is approached, it will stretch a minute bit, and then snap. The Cedar stringer will initially not see any load whatever, because it is happily bending along, not even close to its elastic limit. By the time the fiberglass gives out at its 25,000psi tensile strength, and all the load is transferred  to the Cedar stringer, it – with its puny tensile strength of 450psi – is hopelessly outclassed and will fall over dead immediately.

 

Worse yet - not only is a stringer not helpful as far as strength of the board, it is downright harmful, since it introduces a hard edge: while relatively weak, a wooden stringer – being a “plank-on-edge” – is very stiff vis-à-vis the predominant bending mode of a board, i.e. up & down. On the other hand, a single skin of fiberglass, being very strong in tension, is very flexible. Put the two together, with a surfer hopping around on the deck, and the stringer will remain unchanged, while the foam core on either side slowly collapses, allowing the fiberglass to follow suit, forming depressions, with a sharp crease on top of the stringer. Given enough time, this will fracture, leak, and kill the board.

Without a stringer, the deck would have been able to collapse into gentle depressions without hard edges, and, while not pretty, would have lived infinitely longer than its stringered cousin.

 

Foul you cry – there are many examples of successful stringered structures. Indeed! But when you look closely, you will find stringers and plating made of the same or very similar material, so they will react alike under load. One fine example are Aluminum boat hulls.

Look closer still and see that over the past 30 years, many traditionally stringered structures (such as Aluminum boats, airplane fuselages etc) are increasingly built from composites instead.

One of the basic engineering truths is that the lightest possible structure is the thinnest shell supported by the most closely spaced framing.

Take a traditional 14” stringer spacing on a 50ft Aluminum racing boat, reduce support spacing to ¼” and you arrive at a honeycomb structure, shedding about 20% of weight in the bargain (and acquiring all manner of bonding problems – but that’s a different story…). Go more extreme yet and arrive at a sheet-foam based sandwich, with the support spacing reduced to the cell size of the foam - just like the Epoxy/Divinycell sandwich boards Cobra builds.

Amazing!

 

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