I thought the I beam theory that we talk about is the laminate , not the boards inner core or shape of the board. Sandwich construction vs just glass or carbon layers.For example a layer of glass then pvc then glass. It's the thin 3mm pvc between the layers of glass that is acting like an I beam. So if you build a board with pvc directly on the blank and then cover with glass , that's not I beam. I'm thinking people are getting confused , (for example ),thinking a stringer in the blank is like a I beam ,and it is, but that's not what were talking about. Or have I got this all wrong ???

An I beam can't handle torsional loads. A tube can. A windsurfer is a tube because the glass and in some cases the sandwich are wrapped to form a tube tube and as such the board resists torsion too. Adding sandwich to one layer or 2 layers doesn't make it an ibeam it's just a tube with stronger walls. To make an ibeam we would need to cut the rails off.

As an example. Is the Patrik air inside board a tube or an I beam.

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Imax1 said..
I thought the I beam theory that we talk about is the laminate , not the boards inner core or shape of the board. Sandwich construction vs just glass or carbon layers.For example a layer of glass then pvc then glass. It's the thin 3mm pvc between the layers of glass that is acting like an I beam. So if you build a board with pvc directly on the blank and then cover with glass , that's not I beam. I'm thinking people are getting confused , (for example ),thinking a stringer in the blank is like a I beam ,and it is, but that's not what were talking about. Or have I got this all wrong ???

Yep. No, not wrong.

i don't know why people try to relate a windsurf board to a piece of ibeam. There is no stringer, its a very poor comparison to make. It doesn't act structurally like an ibeam section at all.


A windsurf board is more like a piece of box section with thicker walls on its length (or without if your talking about no sandwich in the core material) Closest comparison you could get with a piece of ibeam would be to lay it on its side. But even that would be wrong.

Well it's just vocabulary and theory adaptation, so everyone use vocabulary according to their background. From my mechanical engineering background when i read I beam i first see a metallic structure made of beam that work under bending stress. Beam cross section is a I : a vertical web that take shear stress connected horizontal flange that take tensil/compression.
When i teach materials strengh i speak of beam theory to my students whatever cross section shape of beam. This shape is taken into account by calculating the bending moment.
For me Patrick air inside is a tube with vertical web that work more or less like i beam. Lenghtwise UD fiber strips over web can play a lot in this construction.
I make longboard for a really heavy guys. He break them a lot because he surf in powerfull hollow waves without care. We try all kind of build up to the right one: tough skin, 10oz biax glass- 2mm cork- 4oz sglass, around an 20kg/m3 eps core with a center i beam stringer. Web is 3mm hard foam sandwiched with biax glass, flanges are each 3 layers of 12oz 1inch wide UD HR carbon. This one resist for long time, ultra stiff a bit heavy, most surfer should hate.

It's just that the first time I heard it here was in response to a question. What is the benefit of putting a layer of 3 mm pvc between layers of glass , when the pvc has almost no structural properties ?. The answer was that it separates the glass layers giving more rigidity like an I beam. That's the way I see it and makes total sense to me.
I make my boards with thin stringers because it's easier to shape. I understand there is no structural I beam action going on here because the glass will split before the stringer takes any load. I do think a stringer is a good support for the mast box , stopping delam around the box. Works good for me. Next board will just have a little stringer action around the mast box and a super thin one ply stringer through the board for shaping.

You are right in an i beam more distance between flange=more bending stiffness. Web take mostly shear. Same effect with sandwich skin.

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Imax1 said..
I thought the I beam theory that we talk about is the laminate , not the boards inner core or shape of the board. Sandwich construction vs just glass or carbon layers.For example a layer of glass then pvc then glass. It's the thin 3mm pvc between the layers of glass that is acting like an I beam. So if you build a board with pvc directly on the blank and then cover with glass , that's not I beam. I'm thinking people are getting confused , (for example ),thinking a stringer in the blank is like a I beam ,and it is, but that's not what were talking about. Or have I got this all wrong ???

you are getting to the point! What are we actually talking about. I beam or tube, or PVC sandwich, or constructional stability itself....? But mixing everything together is just messy....

for me the discussion splits into:

1. there is the sectional shape of a board. Which looks more like rectangle than a tube. Anyway. The load of the board creates compression and tensile forces.... They create bending and torque deformation. Shear forces I do not see as a topic for this discussion. Shear forces act mainly inside, one of the reasons why the core gets "weak" over time.

2. we use sandwich and wrap it around this shape. Why do we use sandwich? 1 reason is, because we run around the board with our feet. This force is perpendicular to the shape and a sandwich construction is very good in taking this forces. But why else do we need sandwich??
Especially in the bottom? The reason is the buckeling. In the bottom there are compression-forces mainly and buckeling is the problem there. This can be avoided with a sandwich construction... Stringers in the bottom also create a more ridig structure which ensures even less buckeling problems. But in the end, it is about buckeling why the construction breaks down.
The sandwich in the top of the board doesn't help at all with constructional stability other than the perpendicular forces of the feet running around the board.

But one neglected topic of how to avoid buckeling in the bottom is to make the top of the board softer, tensile modulus smaller,i.e. not use carbon.

So boards are a bit of RHS, with walls that are i-beams.

Easy.

Technically it's a flat tube

OK I preface mine with "stepped on....."

Thinking out loud. To make a very strong board it would be worth cutting the blank in half, along the stringer.
then glass each half completely, then glue back together, fully wrap in sandwich core then glass whole board.

kinda similar to tack welding shs tubes together to improve strength. May not even need sandwich core.

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Gestalt said..
Thinking out loud. To make a very strong board it would be worth cutting the blank in half, along the stringer.
then glass each half completely, then glue back together, fully wrap in sandwich core then glass whole board.

kinda similar to tack welding shs tubes together to improve strength. May not even need sandwich core.

didn't you mention the torsional load. (because of jumping and the feet not beeing in the middle) The stringer in the middle doesn't help much.

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SchobiHH said..

Gestalt said..
Thinking out loud. To make a very strong board it would be worth cutting the blank in half, along the stringer.
then glass each half completely, then glue back together, fully wrap in sandwich core then glass whole board.

kinda similar to tack welding shs tubes together to improve strength. May not even need sandwich core.

didn't you mention the torsional load. (because of jumping and the feet not beeing in the middle) The stringer in the middle doesn't help much.

2 tubes welded together are stronger than one tube of the same overall size because the side walls on the welded tubes are supported. The support reduces ability for the tube wall to buckle which helps resist torsion. In our context the tube wall is the sandwich.

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Gestalt said..

SchobiHH said..

Gestalt said..
Thinking out loud. To make a very strong board it would be worth cutting the blank in half, along the stringer.
then glass each half completely, then glue back together, fully wrap in sandwich core then glass whole board.

kinda similar to tack welding shs tubes together to improve strength. May not even need sandwich core.

didn't you mention the torsional load. (because of jumping and the feet not beeing in the middle) The stringer in the middle doesn't help much.

2 tubes welded together are stronger than one tube of the same overall size because the side walls on the welded tubes are supported. The support reduces ability for the tube wall to buckle which helps resist torsion. In our context the tube wall is the sandwich.

the board breaks mostly from the rail on. Its the torque. Of course you can put weight in the middle, but it is not very efficient use of the material. And 2nd stringer from top to bottom are not making sense at all. Whereas a stringer at the bottom can increase compression strength, indeed . Also this stringer on the top reduces the flex when landing which you get because of compression of the core which also reduces the compression of the bottom.... Mechanical relations are very difficult to predict in this complex structure, and these are my assumptions because of my science background but I have never heard anybody yet coming up with proper arguments. But I am happy to hear ideas that are based on physical inference because of cause and effect. I am helping a friend, a very good shaper with a long experience to create better constructions of boards, while still being practical in production Simple solutions like stringers in the middle have been tried already many times.... it is very hard to make this "egg" construction better without increasing massively the weight or increasing the working hours or expensive technical equipment to build... But one can easily spot wrong constructions... Whereas making a construction right is very hard.

What about making the rails out of ABS? Like leaving the core square and laminating on a stringer around the outside if that makes sense?
The hollow wood surfboard guys do it that way.

ABS is very heavy.
I've thought about Gestalts theory of glassing between blank halves. I buy my foam slabs in two halves so it's easy to glue in my stringer. They are perfectly cut square and smooth. That should definitely work well as a structural I beam . I made a 6 mm balsa wood stringer covered both sides in 4 oz glass. The board hasn't snapped in half yet.

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SchobiHH said..

Gestalt said..

SchobiHH said..

Gestalt said..
Thinking out loud. To make a very strong board it would be worth cutting the blank in half, along the stringer.
then glass each half completely, then glue back together, fully wrap in sandwich core then glass whole board.

kinda similar to tack welding shs tubes together to improve strength. May not even need sandwich core.

didn't you mention the torsional load. (because of jumping and the feet not beeing in the middle) The stringer in the middle doesn't help much.

2 tubes welded together are stronger than one tube of the same overall size because the side walls on the welded tubes are supported. The support reduces ability for the tube wall to buckle which helps resist torsion. In our context the tube wall is the sandwich.

the board breaks mostly from the rail on. Its the torque. Of course you can put weight in the middle, but it is not very efficient use of the material. And 2nd stringer from top to bottom are not making sense at all. Whereas a stringer at the bottom can increase compression strength, indeed . Also this stringer on the top reduces the flex when landing which you get because of compression of the core which also reduces the compression of the bottom.... Mechanical relations are very difficult to predict in this complex structure, and these are my assumptions because of my science background but I have never heard anybody yet coming up with proper arguments. But I am happy to hear ideas that are based on physical inference because of cause and effect. I am helping a friend, a very good shaper with a long experience to create better constructions of boards, while still being practical in production Simple solutions like stringers in the middle have been tried already many times.... it is very hard to make this "egg" construction better without increasing massively the weight or increasing the working hours or expensive technical equipment to build... But one can easily spot wrong constructions... Whereas making a construction right is very hard.

i'm not proposing a stringer. i'm proposing 2 tubes glued together.

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Grantmac said..
What about making the rails out of ABS? Like leaving the core square and laminating on a stringer around the outside if that makes sense?
The hollow wood surfboard guys do it that way.

I use parabolic stringers or parabolic rails on my builds. same idea but simpler to build and stronger because of the shape.

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Imax1 said..
ABS is very heavy.
I've thought about Gestalts theory of glassing between blank halves. I buy my foam slabs in two halves so it's easy to glue in my stringer. They are perfectly cut square and smooth. That should definitely work well as a structural I beam . I made a 6 mm balsa wood stringer covered both sides in 4 oz glass. The board hasn't snapped in half yet.

There is no doubt in my mind that glueing two halves (tubes) together is stronger than one tube. It's not a stringer, more like a full wrap and as such 2 tubes. A traditional stringer isn't connected to the top and bottom lams well enough. most likely why t stringers get used in windsurfer these days. so ideally you would need to fully shape the board then cut it in half along the centre, fully glass both pieces then glue it back together.

fwiw, i've started including folds into the sandwich layer to add stiffness on my builds. have also noticed in the surfboard industry some shapers are adding grooves into their blanks to improve stiffness.

with all that said, I'm not convinced stiffer is always better for wave boards.

When board bend lenghtwise you are mostly in beam simple flexural theory ( beam 3 forces flex). one face take mostly tensil and other side mostly compression. More the board bend more rails take shear, then suddenly face under compression became instable start to buckle, shear forces increase drastically an rails break. Delays buckling appearance and board bending strengh increase.
You can play on board overall stiffness by increase "core stiffness": denser foam, crossing stringers (in foam and/ or around (rails))
You can also increase skin buckling stress = increase skin flexural stiffness: thicker skin, sandwich skin, rib (T, V stringer, etc) eventualy play with tiling shape effect (concave vs flat, channels) according to board shape needs.
Then you need to work on torsion stress, dents and dings stengh...
Everything add costs : building time, materials, and more often weight. All modify dynamic behavior of board. Some like stiff, hard feeling others no, etc... Shape is compromise, build tech is compromise. There isn't only one "good way", that's what is interesting ?
Sorry for my frenglish.

I love board geek threads!
Once had a home-made mid length surfboard that i absolutely loved, it was wide, thick, and unsophisticated (except for being a 5 fin Bonzer) I didn't know why it felt so good. A few years on, i realised the deck was completely de-lammed, so stripped it off, reshaped to get rid of the rotten foam, added rail channels like stretch does on his boards and re-lammed. It felt horrible by comparison, way too stiff.
Rail channels (and bottom concave) undoubtably stiffen a blank, and could be used locally, perhaps the mid section of the deck, where boards normally snap, and leave the nose and tail area to allow more flex? cheaper and easier (and lighter) than carbon/stringers etc.
On another note, i made awave board with a 15mm pvc stringer glassed in, (to widen the blank mainly) i haven't managed to destroy that board yet, so perhaps it added strength....Shaping it was ok, as i cut the stringer to the exact shape i needed, so planed the foam down to it.

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lemat said..
When board bend lenghtwise you are mostly in beam simple flexural theory ( beam 3 forces flex). one face take mostly tensil and other side mostly compression. More the board bend more rails take shear, then suddenly face under compression became instable start to buckle, shear forces increase drastically an rails break. Delays buckling appearance and board bending strengh increase.
You can play on board overall stiffness by increase "core stiffness": denser foam, crossing stringers (in foam and/ or around (rails))
You can also increase skin buckling stress = increase skin flexural stiffness: thicker skin, sandwich skin, rib (T, V stringer, etc) eventualy play with tiling shape effect (concave vs flat, channels) according to board shape needs.
Then you need to work on torsion stress, dents and dings stengh...
Everything add costs : building time, materials, and more often weight. All modify dynamic behavior of board. Some like stiff, hard feeling others no, etc... Shape is compromise, build tech is compromise. There isn't only one "good way", that's what is interesting ?
Sorry for my frenglish.

Your frenglish is great.

one element we don't talk much about is board thickness. Obviously Making the board thicker will increase strength. A great book to read is "the surfboard book"

Of course board thickness increase bending stiffness. You are right with two tubes stronger than one, but heavier. Some air plane wings (boxed) are made like that.
You can build same with crossing stringer well connected to skin. Starboard made surfboards with a carbon i beam stringer (air rush tech) but connected to skin with 10mm flexible epp foam so skin can move allowing board flex before i beam take load. Those boards feel good strong and expensive.

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Gestalt said..

SchobiHH said..

Gestalt said..

SchobiHH said..

Gestalt said..
Thinking out loud. To make a very strong board it would be worth cutting the blank in half, along the stringer.
then glass each half completely, then glue back together, fully wrap in sandwich core then glass whole board.

kinda similar to tack welding shs tubes together to improve strength. May not even need sandwich core.

didn't you mention the torsional load. (because of jumping and the feet not beeing in the middle) The stringer in the middle doesn't help much.

2 tubes welded together are stronger than one tube of the same overall size because the side walls on the welded tubes are supported. The support reduces ability for the tube wall to buckle which helps resist torsion. In our context the tube wall is the sandwich.

the board breaks mostly from the rail on. Its the torque. Of course you can put weight in the middle, but it is not very efficient use of the material. And 2nd stringer from top to bottom are not making sense at all. Whereas a stringer at the bottom can increase compression strength, indeed . Also this stringer on the top reduces the flex when landing which you get because of compression of the core which also reduces the compression of the bottom.... Mechanical relations are very difficult to predict in this complex structure, and these are my assumptions because of my science background but I have never heard anybody yet coming up with proper arguments. But I am happy to hear ideas that are based on physical inference because of cause and effect. I am helping a friend, a very good shaper with a long experience to create better constructions of boards, while still being practical in production Simple solutions like stringers in the middle have been tried already many times.... it is very hard to make this "egg" construction better without increasing massively the weight or increasing the working hours or expensive technical equipment to build... But one can easily spot wrong constructions... Whereas making a construction right is very hard.

i'm not proposing a stringer. i'm proposing 2 tubes glued together.

what is physically the difference? please explain?

Select to expand quote

Gestalt said..

lemat said..
When board bend lenghtwise you are mostly in beam simple flexural theory ( beam 3 forces flex). one face take mostly tensil and other side mostly compression. More the board bend more rails take shear, then suddenly face under compression became instable start to buckle, shear forces increase drastically an rails break. Delays buckling appearance and board bending strengh increase.
You can play on board overall stiffness by increase "core stiffness": denser foam, crossing stringers (in foam and/ or around (rails))
You can also increase skin buckling stress = increase skin flexural stiffness: thicker skin, sandwich skin, rib (T, V stringer, etc) eventualy play with tiling shape effect (concave vs flat, channels) according to board shape needs.
Then you need to work on torsion stress, dents and dings stengh...
Everything add costs : building time, materials, and more often weight. All modify dynamic behavior of board. Some like stiff, hard feeling others no, etc... Shape is compromise, build tech is compromise. There isn't only one "good way", that's what is interesting ?
Sorry for my frenglish.

Your frenglish is great.

one element we don't talk much about is board thickness. Obviously Making the board thicker will increase strength. A great book to read is "the surfboard book"

this is why SDM mast break and RDM not... Same physical argumentation here

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Gestalt said..

Grantmac said..
What about making the rails out of ABS? Like leaving the core square and laminating on a stringer around the outside if that makes sense?
The hollow wood surfboard guys do it that way.

I use parabolic stringers or parabolic rails on my builds. same idea but simpler to build and stronger because of the shape.

I think I understand, but do you have an illustration or picture?

this explains it well.

?si=6hJBCf7IhXWZAw8Q

Those carbon rails boards buckle a lot in hollow waves even more when dents appear under front foot. Add rib at top of crown deck increase there lifetime greatly while keeping some damping (how flex play in surfboard for me...). To me JR TRIFLEX is a far better surfboard build concept for strengh+flex.

Tri flex looks great