| > | I also don't see how fiber reinforcing the "pillars" helps much for bending strength.

| > I don't know what you mean by 'bending strength'.

| The correct term seems to be flexural strength, or bend strength. If it was unclear, I was talking about the case when the sandwhich is being bent, which means that the skins are under either tension or compression, and the core is either a mix of shear/compression or shear/tension.

https://en.m.wikipedia.org/wiki/Flexural_strength
Flexural strength is about ultimate strength before damaging movement, or yeild.

It is my understanding the core only needs to keep the skins apart, like an I beam web. This simplification allows us to think of the core only in terms of compressive strength. The horizontal component of the stress is carried by the skins.

So, to answer your first question, from my understanding, tow in the holes adds to the column stiffness, or compressive strength. This assumes tow running through a column of epoxy is stiffer than just a column of epoxy.

| But foam core panels need to bend somewhat even in super flat friendly HP designs. I am not sure ultimate rigidity is a win condition.

| I suppose that for the parts which are built on a flat table and later bent, a fiber reinforced "truss" wouldn't be a good idea. But for the parts which are in the mould for the hulls, shouldn't be an issue.

Yep, of course. Obviously this would seem to work fine. I think parts of my brain broke for a moment. I apologize for my sillyness.

| And about removing the foam with a solvent - I don't like that idea for a boat. A tiny hole in a skin could fill the whole void between the skins with water. Better to keep the foam in place.

Agreed.
But for ultimate racing conditions it seems a neat technique to add lightness. Interior holes, which you need anyway to feed/drain the solvent, should allow management of leaks.

| > I return to the hexel sheet's suggestion that the skins carry loads, and the cores just keep them apart. So, to my mind, we only need compressive strength in a core, not rigidity. Actually we need flexibility.

| As far as I understand, during bending, the core will see a mix of loads, but shearing is the main part of it.

Then we understand it differently. It is my understanding is, before yield (damaging movement), the skins job is to take the shear (horizontal) loads, and the core the compressive. Just like an I beam. Certainly the core may need to tolerate some shear forces. I am not sure it needs to resist them, generally. Certainly, some specific, high load, heavily engineered parts of a boat may need a higher performance core that can tolerate higher shear stress. But in the general case, say 400gsm bd both sides majority if a boat, the skin should handle it.

Can you show a loading that shear loads a core the skin would not be able to resist?

| > I think epoxy will flow along tow to some extent. I think the desired flow rate and pressure needed for the resin flow in an infusion is too much to allow a simple stitching through virgin foam to replace drilled holes.

| Yes, epoxy seems to flow very slowly through glass weave. But it might work with thin foam.

Perhaps. I imagine one can add the 2mm channels to the 45° foam too, to ensure good pressure and good truss wet out. And if the vertical holes are adequate, the epoxy will feed from back side of the infusion up the truss.

Given that holes are spaced 40 to 50mm apart, perhaps one can guess that epoxy will safely flow 20 to 25mm along fiber?

| > Any prior mention of sewing by me was meant to suggest through the typical drilled holes.

| But as we have agreed, that shouldn't increase the shear strength, so will not help during bending.

We agree it does not help the shear much, if at all..
We disagree how important shear is in the core.

I had a thought that tape might add more shear and compressive strength. But it is way more expensive than tow. Cheaper to stitch to more adjacent holes I think. And this adds more strength to the holes.

I thought, What if my understanding of the shear needs of core is wrong? And then this got me thinking that slanted holes (5 axis cnc needed), (mini trusses?) Would add a lot to shear, and save full length truss weight?

Rob; it would seem tow through the holes would adds a lot to skin to skin adhesion, right? To the point that foam skin interface might not matter at all/do not need to worry about it?