With this lockdown I’ve had chance to carry out a finite element analysis of a longboard surfboard.
Please kindly see the attachments showing the stress on the board. As expected the results show the highest stress under the foot placement.
To make life simple I made the board 3m long, 30 inches wide with max width at the front of the board and assumed the whole board is on a soft foundation.
I made the placement of feet at 1.8m and 0.9m from rear of board
two runs completed a) one with 50 /50 weight dstribution b) 70% to front 30% to rear, to stmulate the action when trimming
I believe you are bored. Ha. Interesting work. What does this tell us?
I know what FEA is. But I have people for that (Because I’m an old fart).
Maybe focus instead on the torsion around the fin area to see where the loads are and where to reinforce? Being an average surfer, I’ve always considered fin/tail patches more eye candy than anything else. Could FEA tell us something about that?
-there is a big difference in torsion, resistence; etc between 22 1/2 and those 30 that you put (may be you are a Sup per?)
-enormous difference between a performace longboard at 2 3/4 thickness than a 3 1/2 multi stringered shape.
-great difference between gloss coat with hot coat resin and gloss resin. Those stress marks that you can see in some longboards.
-in most longboards you are not standing up in only one place doing all the supposed force there. By the way in the boards you do not need to make “force” you need to put some pressure in diferent parts aided changing your gravity center if not the board actually do not work for you. You do not need to fight with the board; you need technique.
-with a longer board should be a difference between a mushy large wave and to ride a clean condition.
-also pretty different between a hollow powerful wave than a clean mellow 1ft.
-the type of glassing schedule makes a difference.
-the type of density of the foam.
-PU is softer at the center core so overshaping changes the values too.
I don’t know whose foam you are shaping; but the two Poly foams I am using are consistent density throughout. Not softer in the middle. Maybe you should switch to something other than Elova or the Brazilian foam you’re buying. Just a suggestion.
This is all geting good now, and thank you for all your comments ( no matter how crazy they are).
I modelled a fin ( thank you for the idea Sir) I checked , very simply with the froce of the water acting on the front face, assume as flat 3mm plate.
Basically the red is the areas of highest stress. If the maximum stress is great ter than the permissible stress the material can take cracking wil occur. From experiience the fin will snap at this location in the fin box.
Previously modelling the board, the maximum stress is shown and these are the areas which require reinforcement.
…hi McDing; I used most of the brands that are in the World (I still do not used couple of new ones from Spain and S Africa)
I use Arctic. in fact they came with my name because are ALL custom. I spend a fortune in foam. I not buy couple of blanks. Container is what arrive here…
All the PU foam is softer in the middle; a simple test is to grab one of the bones and do the thumb test and see. Overshaping (the machine and wrong plug for the intended board) is really what is destroying the small HPSB under feet of radical surfers.
—by the way; Surfblanks Brazil have the best chemistry; US foam is way behind Aussie foam chemistry (now I am testing new Arctic chemistry but still on the heavy side)
but not so good plugs in certain sizes. Also not so good customization.
The “bones” are all off the outside rails. How can you use those to judge the center of a blank. Besides, I can press my thumb thru any foam on the market. I doubt that you see more or know more about foam than I do. I have sold 200–300 blanks per year up here in the NW for the last ten years. And that doesn’t count the many many blanks that I shaped over ten or so years before I decided to resale blanks. I have stated what I know about the three U.S. Poly Foam suppliers and that is from first hand knowledge as a shaper and a blank supplier to other shapers. You can talk shit all you want, you’re not impressing me. Lowel
We seem to have gone off topic here, but i take on board both differing views of the foam of PU boards.
The following marked up photograph shows the stress iin a fin when planing (please be aware these are not the forces when turning, just triming). Anyway, what it shows is that fin manufactures could possibly reduce the weight of their fins by reducing the glassing at the areas shown in pink. If theres anyone here who makes their own fins please try it and let us know the results. Fin manufactures please take note in you R&D departments.
Highest stresses on a fin are when laterally loaded, not frontal. The thickness is also much less than the chord, so even on frontal impact the stresses (in the fin) won’t be significant.
Also note that your highest compression force in your picture is there because of the angle in the outline (bad model). With an accurate outline, that spot will disappear.
I think you have quite some work to do
You’ll also need a 3D model and a good estimate of your pressure distribution to calculate anything relevant.
If that’s the case; What are your “bones”? Chicken bones? Baby back ribs? O’ What? . Rail cut offs on the Arctic’s I sell and shape are Dense and consistent. Not porous. The center is the same. I don’t know if you ever shaped any Clark Foam, but those days of soft centers are over.
