Help me understand the physics of foam core layup and board strength and rigidity.
1. Thicker boards are usually more rigid. Why? Is it the extra core material providing strength, or the thicker rails giving more skin material that provides stiffness?
Traditional surfboards need to flex, so reliance on the strength and compression properties of core materials is a must. What about for a totally rigid foilboard? Could you conceivably have a foilboard that is all thick skin and essentially hollow except for structure at the mast attachment area? That's basically what cheap EPS core is, just something to fill the space while the skin is made around it, right? My fiberglass canoe doesn't need a foam core to hold it's shape when I stand in it, the skin with its curve is strong enough.
At some point, too thick a skin is too heavy. I guess what I'm asking is: Is there a point where the combination of good foam + less skin is lighter than empty space + thicker skin for the same strength/rigidity? What if you made a board entirely of 1 inch thick carbon fiber tubes banded together in a raft and capped at the ends (assuming you figured out a way to reinforce the mast plate area)? Not really serious about doing that in particular but for a thought experiment.
It just seems to me that traditional surfboard construction doesn't necessarily need to be used for foilboarding where a totally rigid board is desired. I used to make rc planes out of balsa structure and heat-shrink fabric, so that's another thought experiment. I have seen some hollow cedar-strip surfboards with similar construction.
I'm preparing to make an ultra short strapless pocket board for honkin high wind days and am concerned about flotation. I ride a glassed plywood board I made that works fine but you can only go so small with heavy wood before the weight of the aluminum foil makes it sink.
Stiffness is mostly determined by the thickness of the sandwich, but the elastic modulus of the reinforcement (carbon/glass) plays a large role too.
Generally the core needs to withstand compression and shear forces.
Typically the more dense the core is, the more compression it can handle without deforming permanently.
If you build a surfboard, the rail volume and shape is important, so builders tend to use a low density foam to keep the weight down.
Stiffness can then be adjusted by your layup schedule and reinforcement choice.
With twintips, flex is more important than rail volume, so you can have a thin core.
Because the core doesn't take up a lot of volume, you can go higher density (wood, corecell) than a surfboard without too much weight penalty.
You do get 'empty space' cores, like your 'raft' idea, i.e. honeycomb cores. Material and processing costs with these are much higher, but you do get some high end surfboards that use this.
With 'empty space' cores, you need to manage the empty air inside the board, i.e. leaving your board in a hot car can make it warp/explode because the air expands, so these have some air vents of sorts.
If you want to make a short, stiff, light pocket board, use a corecell/pvc/airex foam core that's about 2cm thick, and use 200gsm biax carbon top and bottom. That should be plenty stiff.
My 100cm x 40cm x 2cm wakeskates use corecell and they are about 2kg, and quite stiff.
61da054972bb5246a4884d582145fd5c.jpg (78.34 KiB) Viewed 552 times
Yep. Ryhardt sums it up. With increased core thickness you have an exponential increase in stiffness. But you can lower ultimate tensile strength. IE a surfboard is stiffer and lighter than a TT bit will faill and lower loads!
A typical surfboard blank has foam that is 25-30 kg/m3. A short 120cm foil board has 10-15 liters volume. That means: 30gx15= 450g.
In practice my blanks weigh about 900g after I add 80kg foam to plate area and through hole reinforcement, plus a stringer. It's easy to shape and if you live in the right place easy to get.
There's a lot of waste.
If you make a board out of corecell or divinycell 80KG/M3, 2cm thick it will be about 700g.
No need for any extra foam reinforcement and skins can be thinner during to higher core compression strength and sheer strength. If you get 2x 10mm sheets you can glue together for rocker. Less waste, but hard to find them. U have factory 70km from shop, but still more expensive than getting a blank shipped from the UK.
Xps i get åt builders store is about 15kg/m3. Softer. Has to have thicker skins (€€) and reinforcement. It's a hassle to shape.
I've looked at hollow wood board construction too. Thinking of designing it in CAD, cutting parts for frame in laser cutter and putting it together. Problem i see is that kite Hydrofoil board gets a lot of foot pressure, way more than surfboard. How many minutes is a surfer standing on a board in its lifetime??? A promille of what a kitesurfer does. And kiter is putting high forces on feet. So skins will have to be thick under feet...Will be beautiful, fun to build, but not lighter or easier to build...
I wonder what the best strength to weight use of carbon cloth is for a foilboard. Is it better to lay it flat on the deck or would it be more efficient to make I-beam stringers out of it between lateral halves or thirds?
we've had this discussion on another thread. Once you put enough skin on the foam to be enough to resist heel dents the board will be stiff and strong enough longitudinally, if foam is 25mm+ thick.
A foam core with skins is extremely efficient structure.