Quick advice needed - enlarging a long blind hole

@rob denney has carbon mast build experience

see if he has a comment

but unless the resin tds has a post cure stated and was followed, I’d say the resin was not fully cured unless you got up into 60C temps or like 150F

and the other mast from a week or month ago is also not fully cured until oven

I just read the datasheet and for the lowest hdt; they want an 80C post cure. I’d say the mast will bend less if you cook it. The epoxy is allowing some of it.

 

Perhaps... Unfortunately I don't really have a way of reaching such temperatures in a controlled way. Sunlight only gets it up to 50°C, and that is one side only. I used some plastic tarp and heaters for curing epoxy faster few years ago when I build my last boat, but that only gets to 40°C or so.

I don't think it's worth trying to save this mast. Even if we could achieve some improvement, I don't think it would solve anything in the end. It is bendy like a straw, and we'd need orders of magnitude more stiffness to make it usable...

 

I believe the late Dr Stuart Walker had some kind of system on his International 14 mast in the 1960's.I don't believe it caught on.

 

I understand that the archetypal sail used tree trunks to support the sails. Since I don't come from a sailing background, I don't think a single mast spar is a good solution at all. It's common and has been replicated forever, but does it really make sense ? Especially freestanding structure.

 

You could use 6” metal duct and pipe pressure cooker steam into the tube and get pretty close to 80C. I’ve read somewhere that these masts have designed deflection, but I think the reason your stays bent is the epoxy is still elastic. Once the epoxy is not elastic; if the mast is not built properly, it will crack. Unfortunately, I only know a bit about epoxy and cannot offer any insight into the actual laminate requirements. This thread has wandered and weaved, so not sure if you posted the laminate design.

Also, if you try the metal duct; you’ll need to keep the mast arrow straight during cooking and run a slow cooldown. Probably something to stand it off the metal duct as well.

I have to share with you, I post cured two boat hulls in a 40 foot modified ocean container. We had to build a heat input wall and ramps to get the hulls in and I used small remote tiles for heat sensing and used the further one for the control. It went very well and took about 4 hours to heat up the control and 2 hours cooking, and 2-3 hours cooldown. Cooking that little mast will be really simple in comparison. I was afraid of melting the boat or catching it on fire shooting the heat into the chamber. We had a couple industrial rated fans in the oven as well.

See if you can get anyone to suggest a laminate and/or critique the one you used, but I honestly think the bending over the short distance is caused by uncured epoxy.

 

An example of the intricacies of composite design; pultruded carbon tubing is hell for stout in compression not so good when bending is involved, there is no transfer of shears from the fibers loaded in compression and tension on opposite sides of the tube, I've got a good example of such sitting in the shop from QB's adventure in the 2024 Texas 200. Getting any reliable answers to these statically indeterminate questions is far beyond my abilities.

This discussion has prompted me to consider doing some controlled samples.
EG; Beams with cores of, XPS 2# density foam, Gpet 5# density foam, Corecell M foam, 3D printed PETG cores with various infill patterns, density and orientation. All 1/2" thickness with 22 Oz uni glass; stuff I have on hand.

Impetuous is I'm considering using some 3D printed in some more structural bits around some rudder assemblies rather than the cleats, brackets and other bits I've done so far. I've had some real failures in the past with things buckling when I tried them out before the epoxy has fully set, a matter of a week to ten days with the epoxy I was using.

 

Interesting suggestion with that duct pipe, I suppose this is doable with the means I have. But still, I don't think it should be necessary to post-cure this epoxy at higher temperatures. The datasheet clearly states that it is not required, this epoxy will cure in ambient temperatures just as well in 2 weeks time, there is no benefit to be gained with an oven, except maybe better retention of mechanical properties at elevated temperatures, which is not really important in my case.

Nevertheless, the main question remains: if I were to build a birdsmouth mast, and apply carbon on top, how do I calculate how much I can reduce the wall thickness of the wood to have approximately the same stiffness as without carbon? I'm thinking of adding two wraps of 200gsm twill weave cloth, or one wrap of 200gsm unidirectional and one wrap of twill weave on top. Since the surface area of the mast is around 1 m^2, that would result in 800g increase in weight. So at the very least I assume I can reduce the birdsmouth wall thickness by the amount that would save 800g (exchange 800g of wood for 800g of carbon+epoxy). But since carbon is so much better at this, it stands to reason that I can probably reduce the wall thickness far more, before the mast returns to the previous (pre-carbon) stiffness. Question is, how much I can reduce that wall? Does anyone know how to calculate this?

 

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.

 

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.

 

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...

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.

 

It's a shame you can't easily access some small diameter pultruded rod, the homebuilt aircraft guys use it.
It's easier to calculate properties with standard structural math and the properties are aren't subject to the uncertainties of home built construction.

I'm sure the composite guys are correct but if I build another wing sail the tail booms will be tapered biaxial, it's so much easier to do. The wing itself will have uni carbon, either uni tape or pultruded rod.

 

I apologize in advance for this comment, but carbon works in tension, so you need to deal with compression and torsion as well.

You are in this thread I link. It has lots of good advice.

Estimating strength and stiffness of a carbon-wrapped 2-piece wooden mast https://www.boatdesign.net/threads/estimating-strength-and-stiffness-of-a-carbon-wrapped-2-piece-wooden-mast.66814/page-2

 

I can, actually. There are plenty of cheap pultruded rods/tubes that I can order from Aliexpress for small scale testing. Not sure how useful that would be though, because I won't be able to make a pultruded mast myself. Even making the entire mast with unidirectional+some hoops or sleeves for compression and torsion strength would likely require at least 2mm thick wall, which is a lot of carbon, very expensive. Birdsmouth mast plus a little bit of carbon seems much more affordable and less risky to make, but unlike these pultruded rods from Ali, I can't really get any such samples, I would have to make them myself, and that would be a lot of work with many variables, requiring a lot of samples, with enough specimens and accurate enough testing equipment to get enough useful datapoints to make any conclusions...

Yeah, you're right. I started that thread myself, it was mostly focused on the joinery of a 2 piece mast, but it did have some useful info. Unfortunately even with that info I was never able to work out any formulas to estimate how wood with carbon would behave. But I'll review it again, thank you.

 

What I had in mind was to reinforce your tapered thinwall birdmouth wood or structural foam mast with pultruded carbon rod or strips capped with a biaxial sleeve which could be glass or carbon. The advantages are the pultruded bits have far greater physical properties than any wet laminated structure so less is needed and it is easy to taper the strips up the mast to optimize material usage. The rods I had in mind were ~ 2-3 mm diameter and came as a continuous piece in a roll. The sleeve over the rods fix the strips in place and prevent buckling and I suspect help help transfer shears around the mast.

Edit: It took me a while to chase down this info, my memory not as sharp as it used to be. Graphlite Carbon Rod by Jim Marske https://www.ihpa.ie/carbon-dragon/index.php/home-top/articles/83-graphlite-carbon-rod

 

I do not understand this. It would mean a carbon fibre beam would be impossible as it involves tension and compression.

Obviously on its own the compressive strength of carbon fibre is nil, however once contained by epoxy resin which prevents buckling it can be very strong.

In the table below see column 3 Std CF fabric tension 600 compression 570 which it is almost the same, however it can differ significantly e.g Kevlar UD only 22%.

Even more surprising is Boron UD which is much stronger in compression than it is in tension.

https://acpcomposites.com/wp-conten...rties-of-Carbon-Fiber-Composite-Materials.pdf