How long did your existing carbon/printedcore mast cure before you loaded it, and at what temperature?
Calculation at this end of the scale will probably never replace experience; if it breaks, go up; if it doesn't, go down. Best suggestion I have would be to do a birdmouth spar with minimum recommended wall thickness ~15-20% of diameter. Since your mast is a cantilever taper the carbon, three layers the bottom third, two layers the middle third and one layer the top. Use the biaxial, it's easier to apply theoretical gains of using uni are minimal.
Best wishes on the deal, hope your mast falls in that sweet spot where you you don't need to go up or down.
As of today, it has been 12 days since that mast was made, though the first loading was about 7 days after it was made. The temperature was around 26°C (79 F) on average. Before you say that isn't enough (it isn't), I still have that other mast that was too large in diameter - that one has been curing for almost a month now, never loaded until a few days ago, and it too exhibits the same behavior (permanent bend under load)
As for birdsmouth, well, based on Skene's formula 20% would be overkill in my case because I have very large diameter, 15% seems to be just right for a safety factor of 2. However, this is without carbon. If I stay with 15% and add carbon, even like you suggested, that is a bunch of extra weight, and will make the mast stiffer than it needs to be. So like I said, I want to compensate by reducing that wall thickness even more, otherwise there is no point in adding carbon. Like I said, weight is super critical here...
As for biaxial, I am simultaneusly consulting on Discord with composites guys, and they said to ABSOLUTELY NOT use biaxial. They say that the fiber orientation penalty is far greater than one would assume. Here is a chart they provided to prove the point:
As you can see, a biaxial sleeve that matches the diameter of the mast would have a fiber angle of 45°, which means only 25% of the stiffness that uni would provide. One would need to use a ridiculously oversized sleeve, stretched to it's limit to reduce that fiber angle - and even then the penalty will likely be very large. The largest sleeve I found available to me is 125mm, and for 80mm mast that would place fiber angle at around ~28°, which still means losing almost half of the stiffness that uni would provide...
There are some composites guys here who can answer, but they are infrequent visitors.
Epoxy is NOT fully cured in two weeks. In my reading, I learned epoxy will increase in properties as much as 8%.
For the mast; the way to design deflection, in the absence of data or prior knowledge, is testing. You build a short section and wrap and skip the post cure if you must, but I’d probably wrap it in a heat blanket for a day, then apply a load to say a 6’ section to determine the equation, not a simple percentage.
If the deflection equation is insufficient; add more cloth.
Yeah, I guess you're right, I just got two things going against me - carbon is expensive, and I don't really have the best facilities to make accurate enough measurements from smaller scale models. I might have to do this anyway, but I am really hoping someone with some knowledge will turn up. Having some equations to get in the ballpark would help a lot.
