Composite Surfboard Cooperative

Here are some observations i've made whilst browsing various threads on different foums regarding composite construction.
Please let me know if i'm on track or slap me and send down the right path!!

Sandwich strength - The strength of a composite sandwich is determined by the thickness of the core (ie the distance between two skins) PLUS the strength of the skins themselves.
Flex - A thinner core will flex more than a thicker core due to the above.
flex is a complex issue in compsand surfboard construction and can be controlled in a variety of ways.

A core that is the same thickness all the way through the length of the board (even though it would look aesthetically horrible) would provide an even flex throughout the whole length of the board.
But due to wanting to be able to paddle the thing we require some thickness in the centre-ish. So flex would only be apparent where the core thins out ie nose and tail and rail edges.

Analysing Flex regarding Bert's construction in the infamous sways thread -
The Concave Deck - i was wondering why? there had to be a reason, so i tried an experiment - got some corrugated cardboard and ripped the top layer of card off and cut out a mini board which had roughly the same principles two skins filled with air having a concave deck and a flat bottom. When i flexed this little model, the concave deck would compress and the underside would stretch, but what got me thinking was when flexed it the wrong way it was really quite rigid and seemed even stronger. A real board couldn't flex past flat in the wrong direction when surfing, only if you stood it upsidedown on bricks and did the trampoline.

I understand that putting a concave in the deck creates three parabolas from rail to rail rather than one in traditional surfboard design, therefore, three being stronger than one and one being stronger than none (flat deck as i mentioned above). These parabolas would stiffen up the deck when flexing the board length ways but the underside could still stretch.

But when i was flexing my little cardboard model it seemed stronger and as if it would recoil better out of a turn if the construction was upside down. Just wondering whether anyone has ever tried this?? ie flat deck with rail pinched near the deck side and concave underneath. If you try my little experiment you'll see what i mean. The board would look aesthetically wrong but may prove to flex with more recoil.

Bert said he likes a low pinched rail, i'm wondering if this is related to my comment regarding even thickness of the core. If the rails are low there is less difference between the thickest part and the tips which would allow for a more even constant flex throughout the rail along the whole length of the board and not just in the thinner parts of the rail.

Anyway these things have probably been brought up somewhere but my head has been ticking over trying to analyse certain things related to flex.
cheers

I guess i didn't or forgot to address that point.
In regards to width, i'd say a wider board would definitely flex more as there is more surface area being forced into the wave during a turn.
A thinner board would tend to pierce the water surface. ie drag something flat thru some water and feel it flex, then turn it on its side and it will cut through the surface.

As for the mid, so many setups - never any swell!
West is best.

Yes I was provoking Sparrow to think about how width impacts flex, all else being equal.

Here's a timber physics tid-bit that may be useful for folks... Here "strength" refers to bend resistance and, equally, to recovery.

"Timber that is twice as wide is twice as strong.  Timber that is twice as thick is eight times as strong."

When the "pyramid bow" was first introduced many people felt it was quite ugly.  But it is actually rather beautiful.  The working parts of the bow are exactly the same thickness throughout, it has a perfectly flat back and belly, the bow tapers perfectly in width from 2"+ next to the handle to 1/4" or less at the nocks and the handle (which doesn't flex) is narrower but around 1/4 again thicker than the rest of the bow.

Interesting repsonse Roy!  Insightful.

thanks for the response guys,
Previously to reading your post, I quickly worked out last night that a thinner board would flex more, i cut up a few flat pieces of eps (ikea packing foam - it's good for something!) and did a few simple tests. As you say, the thinner piece definitely flexed a lot easier. I guess i should have thought about it a bit more before stabbing an answer out on the keyboard.

Hi Doug,

 Thanks for the pyramid bow info, it's right up my street.

 Looking at your statement about the stiffening effects of ading width vs adding thickness,  and seeing these as a way of adding volume, I noticed that one way of adding volume is missing i.e adding length, so with all three ways of adding volume ( keeping the doubling of volume as a convenient place to start ) we have:

1) Doubling the thickness makes the board 8 times stiffer

2) Doubling the width  makes the board 2 times stiffer

3) Doubling the length makes the board  (??) many times LESS  stiff ??  Essentially we are doubling the length of the lever or a bit less taking the width of  the stance into account, is that twice or 8 times less stiff ?


What I'm imagining is that say we have a known board which we want to make with more volume while keeping the flex the same ( if more volume is used for a heavier surfer then one might not want to do this as a heavier surfer will need a stiffer board, but just a place to start ) then we have to make it longer to add volume otherwise it will stiffen up, but if we only add length it's going to lose stiffness so some width or thickness must be added as well .  In order to juggle these three ways of adding ( or subtracting) volume while knowing the effect on flex all we need to know are the three effects above and then it's easy.

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Aloha Roy!
And welcome..

Just wondering if you ever built a standard Al Mimic thruster design using your parallel profile techique and what was the results. It seems like certain design methods lean towards certain physical characteristics liek how composite sandwich foam core designs need some variation in profile and thickness while a hollow wooden board may require something else. It seesm the PP designs have a thickness limitation which requires greater length or width to support the same volume but the the other effects of flex might create a negative effect of added length with thinness.

You've probably done more PP designs recently than anyone else so it would be intersting to see if that strategy could actually be used to build a high performance shortie. Mike Sheldrake is fooling around with CNC cut jigsaw puzzled pieces of cardboard to build surfboard cores and some folks on Tree2Sea where you also hang out have done extreme torsion boax designs which is opposite of your multilayer PP approach.

I have seen that shorty you've published before but it's more of a funboard design and a classic Al Mimic flyer style board most of the Compsanders have focused on.

Be interested in some feedback

And again welcome

You are blessed to have all that Paulownia at your bidding...

Thanks for the good wishes Bernie.

Regarding the 'high perf' thruster and parallel profile, it's most likely easily done I just haven't been concentrating on that sort of board and thought I'd leave it others to do, but am getting back into some shortboards now so am more interested in the possibilities.

The place to start is simply to take a planshape and rocker from a good thruster which you are familiar with, and make a PP blank of the same volume.   Running a quick fairly rough calculation for a typical 6 foot by 18 inch thruster  planshape and a volume of 30 litres I got a parallel profile thickness of an inch and 7/8ths .    The pp  board will be thinner than a tapered board of similar volume in terms of max thickness, and will have a more even flex pattern. . . . .  it will also  carry the thickness all the way to the rail area.  Carrying the thickness to the rail will affect the way the board looks, but it doesn't limit the rail shape at least in terms of the bottom or underneath of the rail.  The trick to getting a similar rail to your thruster using PP could be to facet the rail where it meets the deck as some SUP's are doing. . .  the PP doesn't sit well with lots of deck dome or rail taper coming far inboard, the idea is to do all the rail shaping within a fairly narrow band measured  inboard from the edge of the board.

If  you guys are interested in having a go the idea naturally suggests bending  unprofiled sheets of foam, some compsanders bend sheets of foam  already so it could be a  possibility .  Undoubtedly the main drawback  will not be in terms of peformance  but rather suffering the disdain which most surfboard experts have for the pp concept. . . . in other words anyone who does it will have to be prepared to be seen as not shaping 'properly'

 The only catch with me doing a PP thruster is that I don't have any way of testing the results compared with other similar boards.

Right now I'm working on superwide    6 footer with one big tunnel as mentioned, here's the planshape. . . .  Lol it's going to use  EPS with a weight of 0.0 pounds per cubic foot, it's great stuff as it doesn't require cutting  although I am game for some denser stuff in the near future



PS Bernie regarding your comment that the PP designs need more length or width to support the same volume, that's not really true, it's kind of yes and no as it's  just a natural tendency with PP to go as thin as possible due to the effects of the pp on rail volume.  Compsanders have tended to go to a more even profile for flex anyway, just not the whole hog.
 



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