Engineering flex.

[Surfdad]

When you look at a board like the Shred Stixx Chase Hazen signature board, the plan shape is such that over the 15 years that Jerry has been working on boards he builds it into the thickness - a thinner tail offers a bit more flex and more importantly the flex return...all the folks that have been building boards for ages have their own secret recipes

While I am working on my painting masterpiece, I also have another board under construction. This board will have balsa rails, rather than the baswwod stringers that are inboard a bit from the rail.

I'll talk briefly about the stringers and strength and flex from a composite engineering perspective tomorrow.

The board and HD bottom skin in the bag. I've used 1.4 oz glass between the sandwich.

[Surfdad]

In conventional surfboard construction, the stiffness of the stringer cannot be substantially greater than the stiffness of the glass, or else the board will delam easily and break. The stringer is there to provide I-beam support for the glass on either side of it. The stringer/glass stiffness is a cubic function of the stringer thickness (height), and the strength is a square function of the thickness.

For example, a board with a stringer 2.25" thick will be 80% as strong as a board with a 2.5" stringer, and 73% as stiff. If you take it to a 2" stringer (compared to 2.5), the stiffness is halved, and the strength is 65% of the thicker board.

The formula is odd, but:

2.25^3 / 2.5^3 = .73
2.25^2 / 2.5^2 = .8

Using the composite I-beam model, if you want to double the strength, you can increase stringer thickness (height) by 40% (ie: 2.5" to 3.5"), or double the stringer width.

The arguments for perimeter stringers are that the load is highest on the rail, the rail fails before the center fails, and you should put the strongest design element where the load is highest to maximize efficiency. You still need to consider the thickness when you do this, and its relation to flex and strength.

There are so many variables in conventional surfboard construction, that it's hard to distill them into meaningful info.

Anyway...the board inthis build will be eliminating the inboard stringers, and making the rails the stringers. It moves the strongest element to the furthest spot on the board.

This image demonstrates the location of the two methodologies, they won't be used in the same board, it's just a visual aid.

[Surfdad]

I didn't explain one section very well. You'll notice that I trimmed the bottom skin even with the outline of the core. I could have attached the rails and then attached the bottom skin OVER the rails, but that would make for a stiffer board.

I attached 1/2" of balsa for the rails, then shaped those to the final dimensions. I also used 5 pound pour foam for the fin box inserts and made sure that the foam connected the top and bottom skins.

[Surfdad]

H45 bottom skin, A400 top skin and full balsa rails. I wish that I had taken better care of the rails as that is a good look.

[Surfdad]

Bagging the top layer of glass, I'm using 3 oz. I was able to wet this out at a 60/40 ratio, so I'm really working the resin out. Resin is the majority of the weight in a board, plus it causes a board to be stiff.

[Surfdad]

2.75 pounds with fins and traction! I'm keeping my fingers crossed it doesn't snap.

We're also going to test a full on C-5 fin pod tomorrow.

[Surfdad]

It worked! Seriously light and while it flexed well, the return wasn't snappy enough...Dennis suggested a different rail material rather than Balsa.

[Surfdad]

oops I forgot the pictures!