# carbon fiber wing mast for my cat

Discussion in 'Multihulls' started by rallard, Dec 28, 2008.

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### Eric SponbergSenior Member

Roger that.

Sounds like you have a workable system there.

Eric

2. ### BostonPrevious Member

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### rallardJunior Member

Sailor2, The actual increase of thickness in this case for my cf mast is closier to 4 times the thickness of the skins; if doubling the thickness by core only increases stifness by 7 times rather than 8 times, my earliest mention of increase of stiffness 32 times when quadrupling the thickness by core only is also wrong!!?? I read somewhere that it would be 37 times. Does this make sense? Using your I-cals, could you please provide the right answer.

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### sailor2Senior Member

5) a=2mm, b=12mm, b+2*a=16mm, I=2368w/12 = 592 w / 3
Comparison between case 1) & case 5) >= 592 / 16 = 37

So in words changing from 4mm thich solid laminate into 16 mm sandwich laminate with 12mm core & 2mm skins is 37 times as stiff as the solid laminate was. If both skins do not have same properties, this result is not valid. Tell what they are, and it's easy to correct for that.

You do realize this is stiffness of the panel, not same of the whole tube.
It's related how the section resists if you try to squeese it narrower & longer after it's finished. The hoop stifness. And as we are talking about composites, it's valid only if orientation of fibres is same in both cases and no delaminations anywhere occures. In practise you would propably reduce hoop fibre content, since there is no need to this high stiffness in that direction.
And it's not strength comparison, just stiffness.
The main stiffness case if of course longitudinal one, and section dimensions determine that. Also with core, some of the fibres are further awy from section surface and that results reduction of longitudinal stiffness for the same amount of fibres. I'm no mast designer, but it seems to me that that favours putting more fibres to the outside skin than inside expect those that are at 90 degs for hoop.
What were the real dimensions for your lay up, what core thickness and what laminate thicknesses, and how much in each direction & ...

Waite I' already gave the answer in my previous post as well. Only needed to multiply the comparison values : 5.286 * 7 = 37.

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### rallardJunior Member

sailor2, thank you for the answer, this discussion and exchange of information is definitely getting most interesting. I do understand that increasing flexural strenght ( by the way, does it cover bending and torsion as well??) by 37 times is only one of many factors that must be considered towards the determination of sufficient and safe strenght of the global structure of a mast.
At this time, rather than getting more specific about the actual construction of the mast, I would prefer to draw on your technical composite expertise and hopefully also that of Eric Sponberg on the more general subject of single solid skin vs. core construction.
In an earlier reply, Eric made the statement that: "...putting a core in a mast laminate is not worthwhile". Although I am not a professional builder, I do have a fairly long time experience in core construction under vacuum pressure. It was therefore a natural trend and bias for me to choose a core construction for my mast over single skin. It is also the reason why, with all due respect for Eric, I cannot agree with the above statement without first seriously considering also the advantages that core construction may bring to this application. My position is based on the following general principles( do not hesitate to correct me where needed):
The general main ways to my knowledge which can lead to the failure of a composite mast structure are: reaching flexural, impact, compression or tension strenght limits, and also exposure to natural harmonic resonance of the material used and uncontrolled vibration, an important factor of delamination. Vibrations travel well through a single skin laminate( and likely better through a stiffer rigid laminate like carbon fiber??) and the use of an adequate elastic foam and resin system substantially dampens the energy.
The "Ultimate guide to core materials" published by Nida Core discusses at a certain point the dissipation of impact loads by the "elastic and damped response of the core under the skin" and further states: "without the damping component, the structure would respond like a spring and have resonance....The damped resilience permits the use of lower safety factors in designing structures because they are less prone to catastrophic failure." I presume that the same line of reasoning will apply generally to vibration resonance control.
Under the general title " Why sandwich construction" the guide also refers to a statement of "Famed Composite Pioneer Mr. Brandl: "The main function of the core material is to distribute local loads and stresses over large areas. Local stresses applied to one side of the sandwich have only a reduced local effect because the exposed skin and the core will distribute the loads to a larger area of sandwich. Because of this fact, a sandwich structure generally exhibits superior behavior under bending, torsion, impact, and compression, parallel or perpendicular to the skins..."
Should we take for granted that such generally accepted principles should be denied or simply judged importantly non-applicable simply because they are more specifically applied to mast construction?

6. ### BostonPrevious Member

Im curious if a material like CF being embedded in an epoxy would gain strength from being placed in slight tension while the epoxy was setting up
kinda like the concept behind pre stressed concrete

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### Jimbo1490Senior Member

Not much to be gained there, I suspect. The Major reason reinforcing steel is pre-stressed in concrete is to make sure that there is never a load reversal in the concrete matrix, ie , the loads becoming tensile in the concrete rather than compressive. As you are aware, concrete cannot handle any loads in tension, unlike the typical thermoset resin. The resin system I used had like (IIRC) 18 KSI UTS, with about 5% elongation at break, so not at all brittle.

Jimbo

8. ### BostonPrevious Member

how about a using little dc electricity to orient those dipole molecules
would that help

works well in steal
or is it Iron ?

just kinda throwing out some ideas

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### rallardJunior Member

Boston. I believe that the subject that you are adressing now is called in french " effet de gauffrage" or " effet de crêpage" which I think would translate in english as "crimping effect" although that phenomenon may well be called differently in english, the pros certainly know what I am talking about and are better able to inform us on the subject. Basically put in layman terms, it means that when a fiber is crimped ( as it is in all woven fabrics, by opposition to stiched fabrics), if you exercice the required tension on it, it can theorically be streched by a lenght equivalent to that originally used to make the crimping, and that, when under compression, the fibers loose most of the compressive strenght or resistance because of the kinks that are already induced with each small curve in the fibers. What happens is that the loads which you would otherwise like to be supported by the fibers are substantially transferred to the resin matrix surrounding the fibers, causing the resin cells to break and in turn provoking destruction of the structure.
No need to explain that it is to be avoided in mast construction and that is why unidirectional fiber or stiched fiber material is used opposite woven fabrics.
That beeing said, if you use non-crimped directional fibers, the fibers are already naturally streched to their limit and further streching becomes useless.

10. ### BostonPrevious Member

beaucoop de mercis que mon ami était juste lui répondent que je recherchaes

now what about electrically orienting the molecules in the resin
does that tend to dissociate the individual molecules or does it tend to make for a denser matrix and a stronger bond

sorry I think I spelled some of that wrong ( nothing new for me )
I havnt written French in ages

best
B

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### Jimbo1490Senior Member

Crimping has mostly been beat in modern fabrics. The various biaxial and triaxial glass fabrics are knitted rather than woven, and the major tows have no crimp at all. Only tiny little cross fibers hold the various uni layers together. The twill, crowfoot and satin weaves address the crimp issue by reducing the number of crossings. If you're using a 'balanced' plain weave fabric in an app where you need the most stiffness, then you're just misapplying the fabric; you should switch to some kind of unidirectional and use alternating layers to get the degree of ortho/isotropy that the design calls for.

Jimbo

12. ### BostonPrevious Member

orth/isotropy ?

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### Jimbo1490Senior Member

Orthotropic materials have differing (in this case strength) properties depending on the direction from which they are tested. Isotropic materials behave the same in any direction that you test them.

Now there is probably no material that is truly 100% isotropic, but for practical purposes, we can treat many of them as such. Many metals, for instance are considered isotropic. A big hunk of cast metal has the same tensile and compressive strength within a couple of % no matter from which direction you stress it.

But now consider wood. Wood is composed of cellulose fibers oriented in a particular direction. If we compress or stretch wood with loads parallel to the lay of the fibers, we'll get one result. The result will be quite different if the load is applied across the grain of these fibers, however. Wood is obviously substantially orthotropic. It does have some strength across the fibers too, so it's not completely orthotropic.

Composites are always orthotropic. A composite made from unidirectional fibers only has any strength to speak of parallel to the lay of the fibers. If we use a balanced bidirectional reinforcement, then we get the 'full' strength of the fibers in the N-S direction and also the E-W direction, with strength falling off rapidly in the NE-SW and NW-SE directions. If we wanted those 45* off-axis directions to be as strong as the major axes, we need to add another layer of our plain weave oriented +/-45*. There will still be weaker point on the compass, but for the most part, we have achieved 'isotropy', at least 'planar isotropy'. Note that the best we can hope for in composites is planar isotropy, as a stress applied crosswise through the laminate will find no real resistance as there are NO fibers at all oriented against such a load.

Jimbo

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### Eric SponbergSenior Member

I should like to clarify my comments. I am not against cores per se. In most instances in boat building, the advantages of cores as Jimbo describes are very real and worthwhile. But in these instances, the loads on the structure, primarily hulls and decks, are perpendicular to the surface. Cores work well in these situations. But this is not true with a mast where the loads run parallel to the surface.

In a mast design, you have either pure compression (stayed rig) or a combination of compression, tension, and shear (unstayed rig). A core in these situations does very little work because loads would run through the core in the same direction. Because the skins are very much stiffer (have higher modulus) than the core, the skins are going to carry the vast majority of the load. There is no reason for the core to be there, except give local buckling stiffness if that is the way you want to handle it. That is, the sum total of the laminates have to be there anyway, and to add a core does nothing--it only adds weight, cost and complication.

The very best mast designs have the right amount of carbon fiber for the loads, and the overall section size is the minimum possible because one always strives for the least amount of windage. There is always an optimum minimum size of mast section shape that will give just the right amount of wall thickness that will also not buckle prematurely on its own. There is no need for a core. Go to any production composite mast manufacturer, like Southern Spars or Hall Spars, and look at their mast sections--no core!

If you are relying on core for your carbon fiber mast sections, then either your skins are too thin to carry the total load, or your mast section size is too big--and too heavy.

In composite mast design, you do not want to have a core in the laminate.

In hull design, that is an entirely different engineering problem and cores are most worthwhile.

Eric

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### sailor2Senior Member

I certainly agree most of what you say, but there are exceptions when minimum section is not used :

If using rotating spar for an all out racing boat, there might be aerodynamic reasons going to longer chord with same section thickness despite of a little more weight compared to the minimum weight design. If that's the case, there can be core to get minimum weight for a given chord solution and no extra windage while sailing. Extra chord also helps while deeply reefed in heavy winds, even if not a structural consern, extra longitudinal stifness due to extra chord helps with keeping suitable mainsail shape in those conditions with rotating spars.
http://forums.sailinganarchy.com/index.php?showtopic=67782&st=25
http://forums.sailinganarchy.com/index.php?act=attach&type=post&id=64282
If you have SA membership go take a look at the thread or the pic from it.
There is clearly a cored broken mast section from Groupama3 tri in that pic.
Increasing chord above optimum for minimum weight both increses area and needed hoop strength at the same time, which leads to cored construction becoming optimal. Do you know something about how mast for Bor90 known as Dogzilla was constructed ? Was it really without core ? I would expect not dispate of being from Hall.

And I agree a minimum weight for more common size masts do not include cored solutions, if resulting shorter chord is acceptable, thickness/chord being less than 0.4. Below that value mast alone might cause lift while left alone docked causing issues. Cored solutions for bigger spars become optimal sooner with increased chord as the needed extra weight from core & glue line joining it become less compared to the weight of rest of the spar.
Also rigid wings for C-class cats propably don't use cores either, as there are no loads on trailing edge from sails. So just a mylar film is enough. But that's propably outside the subject anyway.

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