Project snap

I have a sub 5lb stringerless EPS epoxy I snapped a while back. Hauled the bits down from the rafters for a spring project. Even with the repair it should come out reasonable weight.

Having snapped about 4 of these stringerless things (even when they are compsands) I’m thinking of routing out a stringer slot from the top deck and then dropping a stringer in. It will run to 6" from nose and tail.

I’ll CNC cut the stringer to fit. I did this on my last construction and glued the stringer into the shaped blank after wire cutting it in half. Worked out pretty well.

I’ll use a rocker table to get the rocker right on the repair, then glass a layer of tape over the stringer and bandage the snap.

Suggestions: do I go all the way to the bottom glass or should I leave a little layer of EPS?

Any other thoughts?

Don’t ask me why because it’s just a gut feeling, but I would think a stringer >50% of the thickness of the board inserted from the bottom would work better. I can’t tell if your board snapped top to bottom or bottom to top.

I’d skip the stringer and use carbon fiber top and bottom.

@ tkelly:
Your above observation, is correct. However, it is so much easier to simply put a through and through stringer in the board. The simple answer, is most often, the best answer.

A routed stringer will not result in any gain in strength. It must make contact with both the top and bottom skins in a lamination to achieve the purpose of creating an I-beam structure, which is the most snap resistant configuration. A wood stringer’s advantage is more a factor of preventing compression when flexing and keeping the two skins at a fixed distance. When a board bends it puts tensile force on one side and compression force on the other. Prevention of over-compression is what resists snapping.

This one had a stringer that didn’t go all the way through, wrapped in carbon. Never broke a board that had a full stringer. Lemat would tell you that the stringer won’t keep the board from snapping. I’m using Innegra on my new board with no stringer to see if that works better. I like the feel of no stringer boards.

Plenty of boards with stringers snap, Even the older, heavy longboards of the 60s snapped some times. But, if you build two identical boards with the same foam, glass, and resin but make one of them a stringerless, it will always snap first under the same load.
Find the book “Essential Surfing” by George Orbelian. He gives detailed info about tests he did using different construction methods.

The way a composite sandwich structure (= a surfboard) break under flexural strains is well known : under strain the overall structure flex, skin that take compression load elongate negatively so it buckle, shear load core that try to keep it in shape. When core is overload and break, skin debond, and break. this is the end…
Every thing that increase flexural stiffness increase flexural strengh: stiffer foam (or foam with stiff stringer), stiffer skin, etc…

Édit: … but a thin wood stringer that cross middel of board is not the most effective way. For sure it increase overall stiffness but it’s not so effective to prevent local buckling, and because glass contact surface is reduced it’s not effective to keep thin skin to buckle.

Usual way. Compressed top glass. sheared bottom clean.

I’m a great fan of horizontal stringers (veneers, carbon and exotic cloth patches, etc.), stringerless compsands, parabolic stringers, and so on, but after a number of years and way too many snapped boards I don’t think I can make it work in my kneeboards.
Maybe when I stop launching over the lip and landing hard in the flats…lol

Thanks, your reply points to the question of going to the cloth or not.
I clean all the way to the glass and then use cloth and resin to glue the stringer to the bottom skin.
I’ll glass a strip of resin over the top of the stringer to bond it to the lam.

Always a bummer to snap a board.
I’d suggest that stringerless is not going to reduce likelihood of another snap.
I’ve repaired and snapped at least one stringerless veneered boards, and snapped every version of stringerless (veneers, compsands, parabolic stringers, etc) I’ve made over the past 8 years or so.
Yes, I am working with boards in the sub 5 to 6lb range, but the glass has been good brands and the laminator (mostly not me) is very experienced.

Great book. A real classic.

Hi lemat,
If I “get” your last comment you suggest multiple stringers?

You omit to mention the part the rider plays in the deal. Some guys put their boards in harms way because of the way they surf. I snap most boards landing free fall take offs or late re-entries in punchy beachbreak conditions, but I’ve blown the bottom out of a few boards tuberiding.
In particular kneeboards are a different animal because of the fulcrum effect of the knees parallel at the half way point of the board and the relative flatness across the deck (because of the width)

I have come to the conclusion that, for me, a decent stringer is a good option. Just off the back of 6 months travel in west Aus (Granites, Streaky Bay, Cactus, Red Bluff, Tombstones, etc.) and the stringer board is still holding together.

Great comments, thanks.
I’ll post some pics as work progresses.
As always getting the pads and glue off is a major part of the job.

Just because I can I’ll put the board on a rocker bed and use the CNC to route it. Could be a disaster, lol.

For sure kneeboarding is a little différent. I would suggest you two stringers, one under each knee. Wide stringers in balsa or hard “strucural” foam. Cap them both top end bottom with ud glass end use multi layers in multiaxis of glass under your stance to spread load to those stringers. Finish laminate as usual. An alternative build can be the “old” Classic windsurfing pvc sandwich build with T or Omega stringers on top, it’s still the best weight/strengh build when well make, but far to be the easiest to do well, and still expensive. As a cheaper alternative you can use balsa for sandwich core, but at least 2mm.