carbon fiber wing mast for my cat

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

  1. AndrewK
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    AndrewK Senior Member

    Rallard
    The mast sections look great, both old and new.
    Commercially produced CF masts are single skin, how did you decide on a cored section?
    Out of 250lb how much is CF?

    Cheers
    Andrew
     
  2. rallard
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    rallard Junior Member

    AndrewK,
    everything you see is my own design and decision. After looking around and concluding that there is basically no substantial solid and reliable info to work from when it comes to carbon fiber wingmast building for home builders, I simply decided to make my own design, lamination schedule, etc. Is something goes wrong with the cf mast, at least I'll know whose butt to kick. On another hand, I have a fair bit of experience working with those kind of laminates under vacuum. After running a few tests I choose 3/8" CoreCell A600 as core.
    The total weight of the carbon fiber is 84lbs. The two half mast sections are assembled now and the resulting stiffness exceeds expectations. I will be working on the head, base and exterior reinforcements for tang attachments during the rest of this month and will report on final weight, complete with all tubings, wires, etc. when completed. My cat Jolie Julie will hit the water on Lake Champlain with the new mast only beginning of June when and where testing of the mast will be carried on through the summer.
     
  3. AndrewK
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    AndrewK Senior Member

    rallard,
    how does the compressive stiffness of the cored section compare to single skin section of same weight?
     
  4. rallard
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    rallard Junior Member

    Andrew, I honestly do not have professional qualifications nor the lab facilities to answer questions on comparison of compression, tension strenght, shear strenght and stiffness. My view is that to make valid comparisons you need to test whole sections of the cored mast against non-cored sections, and then, it would not mean much unless the tested sections include a representative part of the complete structure subjected to similar dynamic loads.
    My thinking leading to the choice of a cored section is:1) carbon fiber is strong but very brittle and highly sensitive to sudden impact vibrations;I believe that a core material will considerably provide good absorbtion and damping of such potential shock waves and greatly reduce potential catastrophic failures; 2) creating an I-BEAM using a quality light core with good adhesion of the skins should maximize the mechanical efficiency of the whole structure; I believe the theory is that by using such a core material and doubling the thickness, the resulting flexural stiffness is increased 8 times,quadrupling the thickness increases 32 times,etc...
    If I am right, the real important question then is not comparative mechanical strenght but rather damage tolerance of the choosen materials, resin and core and resistance to delamination. The choice of the core in such a case is therefore of prime importance: good elasticity of the resin cells and elasticity of the foam cells is an absolute must to resist the dynamic loads. I think that the use a tough linear SAN foam with good elasticity is the best choice for this application....Time will tell!
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    In my experience....

    In the work that I have done on stayed and free-standing rigs alike, putting a core into a mast laminate is not worthwhile. In stayed rigs with all the load in compression, the load is carried by the skins and not the core. The core certainly adds thickness which makes the overall shape more resistant to buckling, but then you run the risk of inner or outer skin buckling which is just as bad. In a free-standing rig case, one side of the mast is in tension, the other in compression, and you end up with the same problem--better overall buckling stiffness with a cored section where the core carries no load, but increased skin buckling susceptibility.

    If engineered properly, the size of the mast section will have a solid wall thickness of carbon that is of sufficient thickness to resist overall section buckling. Much of this characteristic is dependent on the layup sequence, ply by ply, of the axial layers and off-axis layers. This type of construction is the lightest, cheapest, and easiest to build. To add a core to such designs only adds weight, cost, building complication, and the potential for core/skin voids and delaminations.

    I do not wish to criticize Rallard's effort for he may end up with a very nice mast. But usually, it is best to consult a designer who has been around the block on such rigs in order to get the best use out of the materials for the lowest cost and easiest construction.

    Eric
     
  6. sailor2
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    sailor2 Senior Member

    Not quite so :

    Cored laminate with laminate thickness on both sides = a
    Core thickness = b, total thickness b+2*a, w=dimension perpendicular to thickness.
    Stifness =I*E , where E = modulus for material and I = geometrical part of stiffness
    E < 120GPa for autoclaved standard modulus carbon laminate.
    Assuming around 70GPA for yours or even much less by following estimate, for aluminium E=70GPa
    Fibre content by weight for your mast is 84lbs / 250lbs= 33.6% by weight and carbon is the heaviest density material used in layup so even less than that by volume. But then perhaps your bare tube weight was significantly less than 250lbs ?
    Compare to 70% fibre volume content mentioned for top class laminates.

    Anyway here's the I calcs :
    I = w*((b+2*a)^3 - (b^3) ) / 12 , examples :
    1) a=2mm, b=0mm, b+2a=4mm, I=64w/12 = 16w/3
    2) a=1mm, b=2mm, b+2a=4mm, I=56w/12 = 14w/3
    3) a=2mm, b=4mm, b+2a=8mm, I=448/12 = 112w/3
    4) a=1mm, b=6mm, b+2a=8mm, I=296/12 = 74w/3
    Moving from case 2) into case 3) by doubling all dimensions lead to 8 times stifness.
    But moving from 3) into 4) reduce stiffness, therefore from 2) into 4) only increases stifness by 5.286 times when doubling thickness by core only keeping laminates the same.
    Your original statement was between 1) and 3) doubling thickness by adding core while keeping same skins. So 112/16 = 7 times stiffer, not 8 times.
     
  7. Jimbo1490
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    Jimbo1490 Senior Member

    This is one of the cool things about building a mast of CF sleeve. With biaxial sleeve, the angle of the fiber is function of diameter, so you select sleeving for each layer to give the desired fiber angle. The uni sleeve always orients the fiber parallel to the mast length, but the biaxial can do anything from about 15* from parallel all the way to 45*, depending on the diameter selected. The manufacturers offer a little cardboard hand 'calculator' (a nomograph, really) which shows the fiber angle achieved by a given product in their line used at a given diameter (section) to aid in sleeve size selection.

    Jimbo
     
  8. rallard
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    rallard Junior Member

    Sailor2,
    250lbs is the total weight of the mast with the complete structure. cross members, reinforcements, etc. The fiber/resin ratio was more like 65%.The empty tube is indeed substantially lighter.
    Thanks a lot for the calculation basis provided, I can live with 7 times stiffer instead of 8; at the very least it shows that I was not completely out of my mind.

    Eric, I agree with your position with regards to costs and added work. The additional cost for the core was about $700.00. Additional work preparing the foams with desired cuts to fit exact form desired was ± 35 hours, i.e. quite a bit of work but not too difficult to do. I do understand that I must also live with added risks of delamination and hopefully I did the job right. You are right also for the additional weight for the core which is ±30 lbs., but on another hand, I do not intent to win the next Vendée Globe. My goal is to have a mast that will be strong and not too fragile and will last for many decades, so it would be appreciated if you or other composite experts could also offer comments on my goal number 1) with a view to reduce fragility and brittleness and vibration damping.
     
  9. catsketcher
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    catsketcher Senior Member

    Url

    Hello Jimbo and Rallard

    Jimbo do you have the URL for a supplier of Carbon sleeve? I would like to look into it some more. Do you pull it over a mould and tie one end off and heave the other end huskily befor wetting it out? It seems like a good way to avoid problems I have laying up cylindrical objects where I have gotten ripples in the interior laminates as the outer ones went on.

    cheers

    Phil Thompson
     
  10. Eric Sponberg
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    Eric Sponberg Senior Member

    I agree 100% that a mast should be strong and stiff and last for decades. And above all, you should be comfortable with your own work. And you are to be commended for actually embarking on your mast build and working through the complexities of its construction and outfit. It is not a simple process. My comments were to stress the point that carbon is expensive, and you don't want to waste it, either in cost or weight. Laminating is not easy for the inexperienced builder--usually the amateur builder will be overwhelmed by the cost, control, and equipment requirements needed to build a quality mast. There are very good engineering reasons why each and every piece of carbon tape or cloth is placed precisely where it is in my mast laminate designs. And this information is thoroughly described in the laminate schedules for every mast, tailored to the skills of the particular builder. That is, good design and engineering decisions made early in the process will, in the long run, lead to a better, longer lasting mast with the minimum amount of weight and cost, but not to the detriment of durability and long life. And I am not saying that your mast is not going to be optimum, for other measures of merit are in play. I just wanted to use this opportunity to express these points, that mast design and construction is complex and requires care to do well. I am sure you are doing well.

    I have long held a critical eye of offshore racers who seek to eliminate every conceivable ounce of weight in their boats in the quest for speed and first place. Weight reduction is not the answer, durability is. I think too many people lose sight of the fact that if you want to finish first, first you have to finish. I once heard a story that an experienced round-the-world racing sailor was quite pleased with himself for removing one through-bolt out of four from each base of the lifeline stanchions in his quest for weight reduction on his new open class 60. The handful of unused bolts was surely going to give him a competitive advantage, so he said. One need only look at the current Vendee Globe to see that this lesson still is not learned--not with respect to stanchions, but simply because the boats are not holding together. There are failures occurring that should not be happening.

    I'd also like to comment on the braided carbon tubes raised by Jimbo1490. I have had some experience with these, and most of it is not good. They are terrible to work with in multiple layers. They don't slip easily over one another, the fibers snag and pull. Also, I never considered them for axial fiber because the minimum bias angle of 15 degrees off axis is simply too great. Typically, if you want 0 degree fiber for maximum effect, anything more than about 2-3 degrees off axis is going to be completely worthless. In laminate design, strength and stiffness diminish rapidly with off-axis deviations, and this is extremely important in tubular structures such as masts, booms, and poles. I have given up on them and rely on unidirectional tapes, knitted fabrics, and occasionally woven broadgoods.

    Eric
     
  11. Boston

    Boston Previous Member

    if Im following this right
    you guys are talking about woven tubes as apposed to rolled

    just a thought on production and what Eric said about the dificulties of working with the woven tubes
    could you use a inflatable central tube in a female mold to ease the process
    if slipping one over the normal solid mold and the other over that is a pain
    maybe its the mold that need change rather than the seamless nature of the tube
    ( assuming Im following you guys accurately )
    maybe you could lather up the pieces
    slide one over the inflatable and with the whole mess flexible slide the next one over that
    then place the top on the outer portion of a mold and inflate the inner compressing the laminates together
    kinda the reverse of a vacuum bag system
    just an idea
    never tried it mind you
    actually am just avidly reading the thread trying to keep up
    but
    seems like if there is any merit to the use of a woven tube as apposed to a rolled one then a solution could be and should be found to facilitate the production end
     
  12. Eric Sponberg
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    Eric Sponberg Senior Member

    The way I have used them is over a male former for rudder stocks, as an example. The first braided sock goes on over the former just fine. The second sock has to slip over the first, only it does not slip so easily--it catches on the fibers of the first sock and fibers snag and pull in both socks. The absolute last thing you need in any laminate is snagged fibers. To wet out the fibers first only makes matters worse.

    As far as I know, there is only one supplier of carbon braided socks or tubes in this country, Atkins and Pearce:

    http://www.atkinsandpearce.com/

    I think they supply their braided socks to builders who are highly automated and handle the socks with the minimum of hand labor. They are probably used by sophisticated machinery for making drive shafts which rely heavily on off-axis properties for the best torsional properties.

    In my opinion, the braided socks are not well suited to hand layup for shafts that require a lot of axial strength and stiffness.

    Eric
     
  13. Jimbo1490
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    Jimbo1490 Senior Member

    THE supplier for braided tubes and socks is A & P Technology

    Boston,

    Your idea will work and is called 'balloon molding'. The Warp Drive propeller blades are made this way. You can use pressure in the central balloon, or evacuate the air between the mold and the central balloon (this amounts to traditional vacuum bagging) or even do both and get very high compaction pressures, a sort of 'poor man's autoclave'.

    I did none of this, as the bugs were not yet worked out of the process enough to risk $4K worth of CF to give it a try, and the project had run out of time to make the 2002 boat shows if we'd waited any longer. Instead, I got expanded polystyrene cores of the appropriate density hot wired for the project, then layed them end to end and passed a aluminum square tube through them. This was covered in shrink wrap and the mast was layed up over this, then a traditional vacuum bag put over this. A cable passed through the aluminum tubes and was stretched tight to about 5000Kg tension, which (mostly) kept the mast pretty straight. One little support in the middle got it perfectly straight. An enclosure dropped down from the ceiling which functioned as an oven. The heating and air circulating functions were built into the 15M' long work table; the drop down box just held the heat in. A standard process controller with data logging controlled the temperature. The cores were later melted out with toluene, though after the project, I found an easier way to do this task as well.

    I had a lot of experience with this method, and though it's not perfect, I understood it's quirks so I could make it work. Subsequent masts were going to be balloon molded after I had worked the bugs out of that process.

    There are many ways to successfully skin this cat. Which method you settle on will depend on your personal resources in terms of skills, money, time and space constraints, and long term intentions (whether you just want to make one or go into the mast business).

    Test layups are crucial to success, whatever the method you choose. When you do tests layups, try to make the test as realistic as possible, especially for 'time to complete'. If you do a test layup of say 1M long, you could probl'y complete this in a half hour if you rushed it. You might then conclude that a 10M mast would take 10 times as long to lay up. But it will probably take 20 times as long or more, and that difference just might spoil a bunch of CF and make you very unhappy. Or worse, it might make a mast that seems OK but has some big delams built into it. Get in touch with an NDT specialist to get the layup assessed. I found a guy that usually checked aircraft windows with an ultrasonic tester. No delams in either mast. This was $300 well spent for peace of mind.


    Selecting the right resin system will help a lot here. When I did my project, resin infusion was in its infancy. Nobody had off-the-shelf systems with anything like the open time needed to do a 15M mast in one wet layup without running out of work life. So I had to learn all about resins and make my own. The curing agents that might do the trick were (at that time) called 'experimental', so you could not actually buy them. You could get a small amount for free, but nothing was for sale.

    Anyway, now you just call up and say you need a resin system with 24 or 48 hours work life and they just send it to you. No big deal. All these will require the application of some heat. Of course the heat needs to be applied evenly and not get too hot or too cold or heat up too quickly or slowly, in other words 'well-controlled'. There are many ways to do this part as well; the way I selected made the most sense for me at the time. With balloon molding, you might circulate hot water (heated with a modified water heater) through the central balloon. Many ways to skin that cat too!

    Jimbo
     
  14. Jimbo1490
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    Jimbo1490 Senior Member

    Just found these at the 'Wayback' Machine from my old website

    BraidApplication.jpg Braid.jpg FinalWrap.jpg ApplyingBiDirectional.jpg



    You can see from the pics that I had the bugs (including the snagging problem Eric mentioned) worked out of the process. The snagging turned out to be pretty easy to beat: you apply the first layer dry, then wet it out, then slide a plastic sleeve over the first layer, then slide the second dry layer over the plastic sleeve, then slide the plastic sleeve out. The sleeve comes out fairly easily and does not snag the wet layer underneath.

    Note also the 'interesting' tool used to smoothly drag the sleeving over the layup in the first and last pics.

    Jimbo
     

  15. Jimbo1490
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    Jimbo1490 Senior Member

    Eric:

    Have a look at picture #2 above and estimate the angle off axis of the fibers in that sleeve. Yeah, it's 0*. Only A & P makes that product (uni sleeve) as far as I know.

    Jimbo
     
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