Mast choice Carbon or Aluminum

Build, Repair & Mod
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If you asked for opinions and someone (even if it’s a manufacturer) gives you his opinions, there’s no real justification to get triggered by it. He’s simply giving you what you asked for… Even if he redirects you to his brand page because that’s where his information is, so what… in fact the link he sent is to the progression project which is completely independent where multiple brands are spoken to.
At no stage did he tell you to buy his product and no one else’s…

The bold was used because it’s a very important point that a huge number of “engineers” in this forum just don’t seem to understand. The foam core in a carbon mast adds zero strength. It’s simply used for laminating so that the carbon does not collapse on itself during manufacturing. All the strength comes from the carbon itself.

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Actually you’re only halfway correct, the core has the purpose of supporting the laminate in buckling, if it wasn’t there or had insufficient stiffness then it would allow the carbon to deform locally, buckle and break, therefore it adds both strength and stiffness to the laminate (but not in the direct way that you might mean)

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That would be completely negligible stiffness. The cores are low density. Hence why some DIY bulders even use hollow 3D prints.
There are more than enough lift the foils with hollow masts to very clearly see the masts are not buckling in use. The cores are chiefly used for laminating…

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Google ”composite core function” or something similar (or take my word for it, i know what i am talking about, both from a theoretical and practical perspective)

You are right that you can build a composite that is thick enough to not buckle, like a bike frame, but if you’re looking at max strength/weight then you use the core to separate composite shells. If you want max strength per area then solid fiber will do it best, it’s just a balance in desired properties what core is used.

Steel masts would give higher strength per area and higher stiffness per area than carbon fibre, though with a weight penalty. If low drag is the desired property then steel would rule both carbon and alu.

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I’m not sure what you are trying to achieve?
Are you trying to be right by using Google?

I can suggest maybe speaking to some of the existing carbon foil manufacturers about the construction techniques used and why they’ve made those choices… I have…

The existing carbon masts with foam cores are all designed so that the foam is only there to aid in the manufacturing process and if closed cell is used it also decreases any water infiltration. From a strength perspective the foam adds absolutely nothing. It’s simply not designed to.

Could they use true composite core function, sure, but it would further increase the cost of an already costly carbon mast and then there’s the further debate on whether it would be worth it… Add in massive complexity in repairs etc…

Not sure why you mentioned steel at all. No one would ever dream of using steel for a mast. Carbon fiber is up to 5 times stronger than steel and twice as stiff…

If you going to choose another metal other than aluminium it’s going to be titanium which is already being used on some foil fuselages for their large pump wings…

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I mention steel since a lot of people think carbon is stiffer, but it’s not. It’s stiffer per weight. I don’t need to google to know that.

I get your point though, at least some masts are optimised so the profile thickness is enough to not buckle. Gong carbon mast might not since they have been reported as flexy after core removal, i can’t really know, and i don’t think you can speak for all manufacturers designs, right? This is why neither of us can say that core removal has no influence on stiffness.

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I said I was done but I just can’t listen to @Larsb continue to spread incorrect data. I have literally been designing composite structures for 20 years professionally. Airplanes, cars, foils, bike wheels, and so much more. I have 15 patents related to mostly composite structures. I literally make foil masts for a living, and you are arguing with me and telling me to Google things? Now I’ll probably get accused of being arrogant or “aggressive” again but do you argue with doctors or lawyers or other professionals the same way you argue with me?

The core on hydrofoil masts is not there for stiffness OR strength. You seem to mix the two terms often, which is one sign you really don’t know what you are talking about. You say the core is there to add stiffness but that it stops buckling which is a really a strength issue. So what exactly are you trying to say? The core in hydrofoil masts is there entirely due to the manufacturing process of closed mold compression molding. It is there to achieve adequate pressures within the laminate during cure. Instead of a core you can use a bladder, which is what NoLimitz and Cabrinha do. But this is more expensive. Cedrus was the first hollow carbon mast, made in two halves, and bonded together. Core is used in stiffness-critical structures that have very low strength requirements and therefore very thin face sheets. I’m talking 3-4 plies of material. For example, control surfaces for aircraft (ailerons, flaps) because the pressure loads are quite low and result in primarily in-plane stresses. Cored structures are much more prone to damage due to hail, impact, abuse, and never used in structural components like heavily stressed skins or spars. When the core delaminates or fails due to shear, you lose significant strength and stiffness of the structure because the face sheets are so thin. There are no hydrofoil masts with 3-4 ply laminates, they are all much thicker than this, and therefore will not buckle without the core. Trust me, I’ve benchmarked nearly every foil mast and in some cases cut them apart! I know what I am talking about.

As for other materials, you are wrong on steel. The average young’s modulus for steel is about 200GPa. Carbon fiber comes in a range of stiffness (standard, intermediate, high, ultra high modulus) from 150 to 350GPa. So stiffer grades of carbon are much stiffer than steel. Secondly, this is all really irrelevant because stiffness and strength of the mast is a cubic relationship to thickness. So you can use a high modulus carbon, make the mast thinner, but then you become strength critical and it breaks. We are seeing this happen in the industry now. I will not use high modulus material for this reason, and have always used intermediate. So your argument that steel could be better for a mast than standard or intermediate modulus masts is wrong because if you try to make the mast thinner due to higher modulus of elasticity it will actually break because steel is not as strong as carbon fiber fiber.

It’s not as simple as you are trying to make it sound, and if you’d like to educate yourself on the challenges of multi-disciplinary optimization of foil masts I have many blog entries that go into the details:

Of course you are welcome to keep “googling” info and no I am not trying to sell my product or advertise or direct traffic to my site, I am simply offering accurate technical information that I have already gone through the hassle of typing and sharing. You really should take advantage of this opportunity and learn from me, because while you claim to know material properties you do not seem to understand solid mechanics or have a lot of experience with applied materials science.

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I’d be interested to know what has made you think that? Sure the same thickness steel can be stiffer if carbon fibre is laminated incorrectly. But laminated correctly the mathematics will simply prove you wrong every time.
The irony of trying to point me to Google initially…

It would probably be wise not to use one of the cheapest carbon masts available as an example. Also it would be interesting to see what tests were performed on the mast to actually quantify the stiffness after core removal. I’ve seen a massive amount of foilers think their mast stiffness is an issue when actually the interface connections (fuselage to mast or wing/stab to fuselage) were just loose.

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@ProjectCedrus:
I’m sorry to have stepped on your toes. I didn’t know your background and wasn’t careful with my words. I don’t think you’re aggressive either, I like a fact based discussion. I will have to stand corrected if you benchmarked most of the industry carbon masts, then you know what the baseline is. My sincerest apologies!

But, i’ll make an example regarding steel vs carbon for the fun of it:

Take your 200Gpa as reference for steel, assume it’s isotropic so we have same stiffness in all directions. We could probably get better than this with a forged part.

Then carbon fiber comes in a range of stiffness (standard, intermediate, high, ultra high modulus) from 150 to 350GPa.

You use intermediate modulus fibre with what, 200Gpa?

We know that composites have stiffness mostly from the fibre. Lets say you have a high compression molding process and fibre content is 70% (this is a high assumption in my book, what do you achieve in your process?) so we have a youngs modulus of roughly 0.7x200=140Gpa
Already here we are substantially lower than 200Gpa.

Then, you can’t build a part that needs to withstand multidirectional stress with only unidirectional fibres so parts of the fibres are in the right direction and others aren’t. Those that aren’t in the load direction only contribute partially to the stiffness.
Let’s say we have a 50/50 relationship where 50% of the fibre are running in the right direction and the other 50 are only contributing 50% to the stiffness due to their angle towards the loading direction.

That’s 140x(0.5x1+0.5x0.5)=105Gpa

A 100% UHM fibre part with these assumptions is 183Gpa - still lower than 200.

This is what i mean: For the same area, in a functional carbon fibre part that is not only optimised in one direction then steel will have an advantage.

Yes, you can use 100% UHM fibre, the comparable numbers will be more close but the part will lack robustness against strike etc.

It was a while ago since i studied composites (and i’m on vacation so my brain is slow) so if you can show where i am wrong i’d appreciate it, should be easy for you since this is your job since many years.

Mind you, i’m comparing stiffness per area, not stiffness per weight - as i already have stated in my earlier post. You can’t increase the profile area or thickness of the carbon part and balance this against weight. I’m talking ultimate numbers here.

(This is not a useless theoretical exercise since drag is directly proportional to frontal area)

@Jezza: you’re welcome to do your calcs too.

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And i really hope that my question doesn’t seem to be for the sake of winning this argument - I like to be proven wrong, because then i learned something.

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I appreciate your acknowledgement and effort. However I have essentially already answered these questions by providing data for both composite and fiber properties in the links I shared. The “state of the mast” post contains a graph of fiber properties. The “Solid Mechanics” post lists comparable properties to alloys for functional composites. Below is a TDS (technical data sheet) for M40J. You can see the tensile modulus of the fiber is actually 377GPa, but the composite properties (factoring in epoxy resin) list a 226GPa tensile modulus. So while your calculation above is partially correct, you are using the wrong starting point.

And your assumption regarding layup is way to conservative. A properly designed carbon mast should have no fibers not contributing to the loadpath. So saying only 50% of the material is non-structural is completely flawed. Cedrus has no 90 degree fibers. It’s all a combination of zeros and +/-45 to manage torsional loads.
And I just noticed you started talking about drag, and continue to make the standard industry error of associating it with FRONTAL area and not REFERENCE area. The reference area of a mast (or wing) is actually the wetted area, and chord length contributes to drag much more than thickness. Once again, this is discussed in my “state of the mast” post which I really wish you guys would just read instead of investing so much time in proving me wrong.

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Thanks a lot, i’ll read your post. It’s one of the difficulties of forums, the mix of decade long professionals and newbie loudmouths makes it hard knowing who is who. I really appreciate the response.

One thing though: if a 0 degree fibre in bending has unit stiffness 1 then a 45 degree fibre in bending would have 1/sqrt(2) due to the relative length, these are roughly my numbers. I calculate zero percent 90 degree fibre, all are loaded in my calc.

226x(0.5x1+0.5x0.707) =193Gpa which is lower than 200Gpa but a negligible difference in reality.

On the reference vs frontal area: didn’t know that!

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I’m not sure where you 1/sqrt(2) is coming from. It does not have to do with the length of the fiber running in the direction of the load. The composite properties under different states of stress (tensile, compression, shear) will be dependent on other factors like layup, ply angle, and matrix primarily.

A mast is subject to both bending and torsional loads. The zeros contribute most to bending stiffness, but the 45s contribute some as well. The 45s however contribute a lot to torsional stiffness, and again you’re still getting a benefit from the zeros. I can’t really give you a “% contribution”, because it’s a much more complex interaction. I run FEA (finite element analysis) and if I need more torsional stiffness I add a ply of 45s and my bending stiffness also goes up (albeit less so). I don’t know why you’re “knocking down” the composite properties based on fiber length, it’s not correct. All of the material is contributing to stiffness (and strength). At least in a well designed carbon mast (or any structure). That’s my job, align the fibers with the loadpath and not waste material/weight/cost with fibers not doing any work. Not all carbon masts are created equal though. And I’ve seen a lot of poor design decisions by other brands who may not have the technical experience or rely on factories doing the manufacturing to also do the design.

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Yeah, i got that. The 1/sqrt(2) comes from the different directions when placing force on fibres of different angle.

Take a section of composite with two zero degree plies and one +45/-45 ply and load it in zero degree direction. Even though the modulus for each fibre is the same the fibres will be loaded and deform differently as their directions differ. The 45deg fiber having 1/sqrt(2) less force composant in the 0deg direction than the 0deg fibre in the same section and hence it’s force per deformation (which is the same as stiffness) is 1/sqrt(2) of the zero deg fibre in the zero degree direction.

I know in a full layup you’d need FEA to get the whole picture, done simulations way back when but i believe that on a simplified level this calc is correct (even though it was now 20 years since i studied composites and i should’ve done some thinking with pen and paper😄)

So my reasoning is that when optimising stiffness in one direction with composites the composite stiffness will suffer in the others due to the anisotropy (less fibre percent available in the section that can be oriented in the other direction)

Steel being isotropic in the modulus don’t suffer from this so it’s stiffness will be hard to beat in any direction - if you need both bending and torsional stiffness.

Are you using 100% M40j unidirectional carbon in your masts since that’s the datasheet example? (I ask since all fabrics would have a decreased modulus, to compare the stiffness correctly)

Must say i loved the data in your links: clear, informative and true, didn’t think there would be a manufacturer sharing reality based info and not just marketing fluff :+1:

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I’m not about to drop into FEA here and I think ProjectCedrus has more than covered it with links to good information.

I feel like you are dragging this topic way off course to try be right. You made an assumption that I didn’t know what I was talking about wrt the use of foam cores in CF masts, but you were wrong, just accept that.

Also using bad examples of CF laminate to try prove that steel can be stronger is a weak example at best. Properly layed-up CF is going to stiffer, stronger and lighter and has been proven across a multitude of applications in marine, aerospace, motorsport, cycling and host of other applications with examples all over the internet. Steel will still retain its own use cases where CF is not correct for the job.
I’m exiting this convo now as I’m sure you can at least agree that steel definitely is not fit for an efoil mast and I also believe that its been established that CF masts are more a personal choice benefit, if you can afford them, rather than a benefit to the masses.

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I already said i wasn’t correct on mast cores, i assumed they were thin shell composites, but benchmark shows they’re not, right?

I was also quite happy with saying that the carbon/steel comparison didn’t come out favourable enough for steel to be an option. It won, but by too little. If it followed my first calc then it would’ve been an option as almost a doubling of the carbon modulus for the steel could be balanced with a profile size decrease to roughly the third root of 1/2 (that’s a 20% decrease) with same stiffness. That could be enough for a racer to get an advantage.

BUT:
”Examples on the internet and multitude of applications” are hardly numbers or proof in a different application!

Have you designed, simulated or calculated composites and compared to steel in a mast? To be so sure of the results i hope you have, otherwise i think you’re just lucky that the numbers were this close - I was pleasantly surprised by the toray composite properties, they are higher than i expected.

To be clear: It’s not a flat earther situation were there’s a difference of huge numbers in the wrong direction like you make it out: It’s actually that in a realistic layup example with a really high performance fibre the carbon comes up as the loser

You could say ”it was a poor laminate”, shift the numbers and compare a 80% 0 deg and 20%+/-45 layup in zero direction bending - but at the same time this would shift the torsion stiffness even more in favor of the steel!

What do you propose to counteract it? I think you owe an answer since, i qoute, ”laminated correctly the mathematics will simply prove you wrong every time.” It’s simple and every time so show it. I did to the best of my (no paper/pen or google) abilities - and i think you can agree that it wasn’t proven.

Boy do i hope that your response now will be ”i’ve been the principal mast engineer at Axis for 50 years and here’s how it’s done”. That would be slightly awkward but then you could provide insight - get it on the table, please. If you can?

The thread will be better for it, fun filled, fact filled. I’ll buy you a beer in a pub of your choice if it’s like this😄