Prevent Buckling without changing Flex

A lot of people want to know how to make boards that don’t snap in half, buckle or crease.  The solution is the same for both scenarios because most boards (non-sandwich) snap in half immediately following a buckle.  The buckle initiates the ultimate failure.  If you can prevent buckling, you will often prevent the board from snapping in half.  There was an in-depth discussion about this in a different thread.

Next I will post the solution with pictures.  I learned not to post anything important in the first post because then it can’t be edited!

Solution #1
Forget about making single-density foam boards.  Make sandwich surfboards.  They are better, period.  However, it does take a little skill to make sandwich boards flexible…I’ll talk about this another time.

Solution #2
If you don’t want to change the way you make boards, but you want to make boards that last a lot longer, this solution was designed for you.

  1. shape your board like normal
  2. take a router and route 1/8" deep channels in the longitudinal direction, anywhere that you are concerned about buckling.  It doesn’t matter how wide your bit is.  All that matters is the edge to edge distance between channels.  For 3lb pu foam with 1 layer 4oz, to completely prevent buckling your channels need to be 1" separated edge-to-edge.  This will allow your board to take about 70% more load before breaking in tension.  At 2" separation it will increase your load capacity by about 30% before buckling.  If you are making a board for a really good surfer that creases/buckles their board when smashing a lip or landing an air, you will only need to make channels that go from the rear fin to about 12" in front of the front fins.  If you want to prevent your board from breaking under a crashing wave, you should make channels for the middle two-thirds of the board.
    3)get your board ready for glassing, like normal
    4)pour resin in channels, then push pre-cut high density foam inserts into the channels.  All the resin will squeeze out of the channel.  Lay you cloth down and glass like normal.  I use 12lb pu foam for the inserts.  Since you are probably concerned with aesthetics, you could use high density tow-in foam.  Then you might not even be able to see the inserts.  I think some people make it in 10lb density.

The routing takes me 5 mins, and placing the inserts takes me another 5 mins.  I am sure any skilled craftsman could do it in half the time. The inserts cost about $1 and add a negligible amount of weight because they are so thin.   The board becomes less than a half% stiffer.  So the board is going to ride the exact same, but it will require a lot more load to break it.  Doubling load capacity does not double the life span…it is more like a cubic function; double capacity=8 times longer life span.  So 70% increase in capacity = 5 times longer life span… 30% increase = 2 times longer.  So you can choose what is ideal for you.  Btw, 1" separation is optimum, but going less than 1" is pointless.

If you have a stringer, I recommend doing two channel/inserts on both sides(4 channels total).  That should give you about a 30% increase in buckling resistance and no change in flex.

The images show test specimens that were broken under a 4-point bending test.  These are single 4oz on both sides, pu foam, poly resin.  You can see that the specimen with the inserts takes 130 lbs load, while the classic layup takes only 75lbs.  Furthermore, the deflection at 75lbs load for both specimens is the same.  Therefore they have the same flex, but the new construction will continue to flex all the way up to 130lbs.

I also included a picture of a board I made with this method.  You may notice that I am a terrible craftsman.  Don’t let it deter you.  I’m sure you expert craftsmen can make this look good.  The engineering is sound and structural theory backs it up.

Good idea, i like the rigorous approach you bring to this forum.
But, in this case,  i’m not convinced your test pieces are representative.

Firstly the test pieces do not have a rounded, glassed  rail which is where a great deal of break strength is gained.
While this gain would be equal in both pieces, it would decrease the % gain in the modified piece
eg from your figures
Unmodified=75lb  modified=130lb  = 73% increase in load (approx)
But if a rounded glassed rail added  say a 100lb extra load capacity to both then,
Unmodified=175lb  modified=230lb  = 31% increase in load (approx)
   (note:the 100lb gain is totally made up, it’s just to demonstrate the point)
Same argument (but with less significance) goes for a stringer.

Also, most buckles & creases initiate at the rail, the board in your picture would crease at the rail at exactly the same time a normal board would (the rails being unmodified)
In your test piece the high density insert goes right to the edge(at the central breaking point), making it much stronger at this critical point.
Also the percentage both in volume and surface area of high density foam is much higher in the test piece than it would be in a board.

Would you be interested in trying two more test pieces with rounded glassed rails, one with smaller inserts, with the inserts away from the edge at the breaking point?
I think the percentage gain in strength would be much,much less.



Bingo!  I agree completely.  Percentages must be exagerated.  Experiment needs appropriate scaling with reasonably analogous structures used.  Significantly increasing buckling strength without stiffening can be done by other means that dont add steps to production…

Hmm, interesting solution. I'd be interested if you or anyone else has done this on the deck side where there is compound curvature.   


IMHO multiaxial glass should be of help.

multiaxial glass



I really hate giving short, unexplained answers, but Stability is a very difficult subject and it is not even taught to undergrad engineers.  I wish I had time to give rigorous explanations, but that would take a few months.  So unfortunately, you will either have to take my word, or do grad level engineering to prove what I’m going to say…truly sorry.

Here are some key words, if you want to learn by googling…though probably your best bet is to go into a university library:

plate buckling, beam with elastic foundation, plate with elastic foundation, plate Navier solution, plate Levy solution, Ritz buckling, Rayleigh-Ritz buckling model********

I know it is commonly accepted that buckling starts at the rail (some say stringer), but this idea is simply wrong…in every board that I have seen built or broken.  Please read

Generally, buckles initiate at the point that is least constrained in the transverse direction.  A rail or stringer are transverse stabilizers that prevent buckling locally.  That’s why the rail fibers aren’t buckled!  rail fibers are crushed and there is NO WAY that fibers will crush before the other fibers will buckle.

What matters is the unconstrained distance between transverse constraints (stringers, rails, and inserts).  This is the way stability works.  This is why I didn’t specify a channel width; I only specified a channel separation.

That does not matter.  If the skin is stable, then the skin is stable.  If you have a stabilized skin on a 4" thick board or 1/2" thick board, neither board will buckle.  Granted the 4" board will take a lot more load, but both boards will fail by fiber crushing in the compression skin or fiber tearing in the tensile skin.  Preventing buckling is just a way to optimize the use of your materials.  You are using your materials well when the materials are taken to their full capacity.  If a material is destabilized (buckles) that means it wasn’t taken to its capacity.

The samples are representative of the region that buckles…that’s what matters.

Before I did anything,I did a series of tests to verify my theory.  I cut out rail sections of real boards and mid sections, then buckled them.  I don’t have time to go into details, but in short, YES the model I have proposed is good.

Percentages are exagerated, but not very much , even when considering the stringer and rail.

Think of it like the “weakest link in the chain.”  The strength of the other parts does not matter if the critical part fails.

That’s true and my test meets that criteria.  Maybe you could explain why it does not.

Please share how to increase buckling strength without adding steps to production!  There is always someone who has a better idea.

It won’t help.  It will make it worse in terms of buckling…assuming you use the same weight and weave of cloth, but just rotate the fibers 45 deg.

However, +45/-45 cloth will improve impact and dent resistance, and make the board more flexible, while stiffening the board in torsion.

Thanks for the info Ben,

I have been doing a similar thing and have headed in that direction.



This one - "The Grim Reaper" has two full depth rail stringers and a 1/4" inlayed center stringer.

I have one with 5,  1/4" deep inlayed HD foam stringers - works a treat.

Its nice to have you back up what I have been doing,

Is it not better to have the inlays on the deck where buckling occurs under compression?

Awesome! 1/4" deep is also fine, but 1/8" is the minimum needed.

I can’t tell if the “grim reaper” has wood or HD foam stringers, but wood will make the board stiffer, and foam won’t…just depends what you want.

You can also have 3,4,5, or 6 HD foam stringers, but the weight starts to accumulate.  Each HD stringer (depending on type) adds about 1-2% in stiffness to the board compared to wood adding about 6-12% per stringer (depending on type).  So HD stringers are better for people that want flexible boards.  However, my goal was to figure out the minimum weight that could be added to the board without changing flex.


Just depends how you usually break your boards.  Sponsored surfers often buckle the bottom of the board smashing a lip or landing an air, so in that case you could just do the bottom side.  The inserts should extend about 6" beyond the region that usually buckles or snaps.  If your glass schedule is the same on both sides, and you are trying to prevent boards from snapping under a crashing wave. Then you should put inserts on both sides for the middle 2/3rds of the board just like in your picture.


I like it…

Just to be clear - the inserts go on the side that buckles/compresses: Correct?

One more point to be clear on: It’s the heavier foams resistance to compression which provides the benefit: Correct?

Sorry if I sound slow, but I want to understand the basic functions of what is going on here.

The Grim reaper was an experimental board (all my boards are but thats a different story). The Idea was to stiffen the rails and allow the board to have some 'twist' so to speak. works well and has a very positive feel especially when weighting and unweighting the rails.

Its a 6'0" x 18 3/8 x 2 1/8  1.9 EPS with 5oz bottom and 14oz deck. Total weight is 2.63kg The stringers are 1/8" 3 ply I got from a cabinetmaker - Dont know what wood it is bit its very light stuff, but I agree foam is probably a better choice. Way easier to work with on those deck curves anyway!

I have about 7 boards with the same template as this, all with different stringer and glass layups. I have found that flex is not a huge issue for me in the designs performance. The advantages of flex change for me as the surf conditions change. Still working on that!

As for buckling, I always seem to buckle the deckside, so hence the inlays in the deck. The only exception is when the board is hit by the lip fins up, then its the bottom that goes under compression. This is rare for me so I don't generally design for it.

Would you agree that staggering the end points of the inserts is a good idea? It just seems that it is the logical thing to do to prevent a 'stress riser" for want of the correct term. 



Interesting but T-Flex is easier and better.  I've used methods like this in the past ... not bad, but not as easy as it looks. Running some glass under the foam pieces would increase strength significantly but it would change flex as well. For sure no production shop will fool with this idea .... too much extra work.  Rail channels give essentially the same results and are much easier.  Nice idea though.

Another option as shown by Bufo Surfboards.... maybe less of a breaking point at the end of the reinforcements.


I agree greg, Lots of work, not an idea for production but for personal and experimental boards great fun. (If you are a bit crazy like me) Plus they look pretty I think!

Primal here in NZ are doing a Carbon roving insert thing. Not sure if its a stiffner or to prevent buckling or both. But carbon is a great marketing tool! 

Im still keen on shipping some RR to New Zealand, just have to sort out the time and $. keen to get some of the Kwick Kick too.... Will email you in the New Year once I have more time. Just got hold of another 45 kg of RR so that will keep me going for a little while longer.



I really hate being patronised.

Whenever someone tries to validate their argument in this way i always think of the Einstein quote.

“If you can’t explain it simply, you don’t understand
it well enough


You don’t have time to show the results you got with real boards?

And you expect people to just believe what you say.





Right! but, if you are trying to prevent the board from snapping when a wave crashes on it, that could be either side depending how you kicked out your board.  Sponsored surfers usually buckle their board when smashing the lip or landing an air, in which case they only need to do the bottom.


Right, but only because heavier correlates with stiffer.  You need stiff sections to stabilize the skin.  If you used pvc, you could get away with 6-8 lb density.


You’re superiority is truly humbling.

I wouldnt be so quick to dismiss the use of cloth that isnt a [0°/90°] weave.

If the inserts are HD foam, it won’t matter if you stagger them because they aren’t really taking any longitudinal load.  If you use wood or bamboo, then you might want to do something to prevent stress concentrations.