Quick advice needed - enlarging a long blind hole

[laukejas]

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

Okay, I didn't get it previously, but now I do. This sounds quite crazy, but it could work. However, I couldn't find any pultruded carbon fiber in rolls either on Ali, Ebay, or any EU sites, apart from a few that only deal in bulk. Do you know where this stuff is available?

Off topic, for those of you who enjoy horror, my dad took a photo of me sailing with that current carbon mast today. This was in around 5 m/s (11mph) of wind. I can only imagine how fast this boat would be if that mast didn't turn into a bow... I have to sail her with pretty much zero downhaul, otherwise the mast would bend even more due to compression.

EDIT: Also, gaze upon my improvised mast un-bending jig. Leave it overnight like this, and the mast is straight again, good for another 2-3 hours of sailing before it becomes like in the pic above.

 

[gonzo]

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.

That is not correct. Compressive strenght and buckling are two completely different things. Compressive is a material property, and buckling is a structural property. They are related on the overall design though.

 

[Rumars]

Laukejas, it's your core that's the problem, it takes a set and there isn't enough carbon to bend it back. First line of defense is to paint it white, see if that hepls. If not switch to a different core material like wood. The carbon is obviously strong enough, otherwise it would have broken by now.

 

[fallguy]

3D printed something? What is the core? This thread is hard to follow as it shifted from a hole to a bend.

 

[laukejas]

Laukejas, it's your core that's the problem, it takes a set and there isn't enough carbon to bend it back. First line of defense is to paint it white, see if that hepls. If not switch to a different core material like wood. The carbon is obviously strong enough, otherwise it would have broken by now.

Appreciate your suggestion, but I seriously doubt that the core could have any influence. That core is extremely weak and bendy. It cannot even support it's own weight for a length of over 1 meter. Carbon should be orders of magnitude stiffer than the core, and whatever force the core might impart would dwarf in comparison to carbon...

As for painting, yeah, I suspected temperature too, but I also sailed it on some cool, sunless days, and unfortunately the behavior is exactly the same...

And yeah, I do want to switch to wood. Birdsmouth core with a few wraps of unidirectional and twill weave. I'm still waiting, hoping someone here knows how to estimate the stiffness and strength of such a structure, so that I can optimize it for weight... Otherwise there is no point in adding carbon, as a "traditional" birdsmouth mast of 80mm with 15-20% wall would be stiff enough on it's own (but too heavy). I've asked this very question on 3 more forums, yet no one seems to know.

3D printed something? What is the core? This thread is hard to follow as it shifted from a hole to a bend.

The core is 3D printed PLA. It was basically only useful as an internal mold over which I draped carbon sleeves. Ideally it could be scooped out, but there is no practical way to do it. In any case, it is so incredibly weak that it contributes basically nothing to the mast once the carbon+epoxy are cured.

 

[latestarter]

That is not correct. Compressive strenght and buckling are two completely different things. Compressive is a material property, and buckling is a structural property. They are related on the overall design though.

I was viewing the buckling occurring at a microscopic level.

The point I am making is the epoxy resin prevents lateral movement so the carbonfibre's compressive strength is utilised.

It takes little strength to prevent buckling. IIRC only 2.5% of the force in the top flange of a steel I beam is required to resist buckling.

The main issue I raised previously was that carbon fibre in practice has compressive strength, if you don't believe that why are the manufacturers claiming it has.

 

[Skip Johnson]

Okay, I didn't get it previously, but now I do. This sounds quite crazy, but it could work. However, I couldn't find any pultruded carbon fiber in rolls either on Ali, Ebay, or any EU sites, apart from a few that only deal in bulk. Do you know where this stuff is available?

Alas, the website associated with the graphlite pultruded carbon no longer exist, I'll look further but offer no hope.

On another matter, I assumed your printed core was providing "some" shear transfer, PLA inside a black tube would probably end up as a solid mass at the bottom of the mast if you left out in the sun for a few days ;-)

I really think you would transform your existing mast if you wrapped it with a glass or carbon sleeve, the +/- 45 degree fibers will lock the existing uni fibers in place in relation to each other, Right now the only thing doing that is the epoxy which isn't near stiff enough. An Aluminum tube is isotropic and doesn't have that problem. What you have now is the equivalent of a group of long aluminum fibers rubber banded together in a cylindrical shape.

BTW, nice looking boat.

 

[laukejas]

Alas, the website associated with the graphlite pultruded carbon no longer exist, I'll look further but offer no hope.

On another matter, I assumed your printed core was providing "some" shear transfer, PLA inside a black tube would probably end up as a solid mass at the bottom of the mast if you left out in the sun for a few days ;-)

I really think you would transform your existing mast if you wrapped it with a glass or carbon sleeve, the +/- 45 degree fibers will lock the existing uni fibers in place in relation to each other, Right now the only thing doing that is the epoxy which isn't near stiff enough. An Aluminum tube is isotropic and doesn't have that problem. What you have now is the equivalent of a group of long aluminum fibers rubber banded together in a cylindrical shape.

BTW, nice looking boat.

When I tried to cure the mast under the sun, it would only reach 50°C at best (measured it). With the boat on the water, wind blowing past, etc., I don't think the mast could heat up any more than that. For comparison, the PLA becomes maliable at 70°C, and liquid enough to flow on it's own at around 130˜°C. So I think it is staying intact inside the mast. Hopefully.

You are right, some uni would definitely help this mast, but it is already at 5 kg, adding anything more to it would be a waste of materials as it would make the mast too heavy. This boat is super unstable (by design, intentionally), so I am really trying to keep the weight down. Of course, the current mast, even though light, is a total disaster. Aluminum mast would be 6 kg, a whole kilo heavier, and even worse with a slightly higher COM due to no taper. But maybe that birdsmouth + carbon idea has some merit. I finally got a reply on another forum (reddit) about calculating wood+carbon mast stiffness, and I think I finally figured it out. I will post again soon with some numbers.

And thanks for the compliment about the boat. I am really pleased how she turned out. Unstable girl. Dangerous but fun.

 

[fallguy]

So, you used a bendy plastic for the core and are wondering why it is so bendy?

What is the mass of the core vs rest of mass?

I apologize, but you used a core that is somewhere between rubber and black polyethylene tubing for Young’s Modulus. So, the thing to do it ditch the core and the mass of it and make it all glass or start over.

 

[gonzo]

It takes little strength to prevent buckling. IIRC only 2.5% of the force in the top flange of a steel I beam is required to resist buckling.

That is not correct. Check Euler's column formula for critical loading.

 

[latestarter]

That is not correct. Check Euler's column formula for critical loading.

Once the individual carbon fibres are encased in epoxy their effective length is minute so the full stress is permissible.

Obviously the member itself, made up of many fibres would need to be considered.

My example of flange buckling in a beam was based on BS449-2 Chapter 4 clause 26d I admit it was not the best example in this case.

Anyway laukejas problem is about bending.

 

[laukejas]

So, you used a bendy plastic for the core and are wondering why it is so bendy?

What is the mass of the core vs rest of mass?

I apologize, but you used a core that is somewhere between rubber and black polyethylene tubing for Young’s Modulus. So, the thing to do it ditch the core and the mass of it and make it all glass or start over.

I think I mentioned it in this rather long thread, but the core is so weak that it has absolutely no chance of overcoming the stiffness of carbon fiber that sits on top, the mechanical properties of the whole structure are completely dominated by the carbon. Therefore, there is zero chance that the core is causing this bendyness. The mast was designed so that there is enough carbon to take up the entire load. So yes, the core could be ditched without any changes to how the mast behaves, except that it would be very difficult to remove it in any practical way. I left it in just for simplicity, even though that is a substantial mass penalty.

The mass of the core is 1.1 kg, and carbon+epoxy is 3.8kg. The ratio of fiber to epoxy was approximately 60:40.

 

[seasquirt]

A different core could make a difference. My unusual suggestion is a circle of individual fibre bundles under tension (pre-stressed) to internally brace against bending. My example from nature is tall cacti plants. A 20 foot tall spiky water container prevents bend in the wind with an internal circle of very tough strands, continuous from base to growing tip. The flesh handles the compression, and the internal circle of half a dozen or more strands handles the tension, like pre-stressed cables. They are very tough to cut.

Include it in your cf coated thin birdsmouth mast. A crown cap with 6 - 8 high tension strength strands, almost parallel, glued in the top, going through the mast's length almost parallel, and out the bottom, through a holed base plate, to be tensioned when bottom plate is final glued. Light weight solution that may work for you.

Or, make a cf mast inner, small diameter, and build a birdsmouth around it. The wood takes the compression, and the cf takes the tension, in the same way as the cactus inner tension ring example. If anyone starts making these to sell, I want a cut.

 

[gonzo]

Once the individual carbon fibres are encased in epoxy their effective length is minute so the full stress is permissible.

Obviously the member itself, made up of many fibres would need to be considered.

My example of flange buckling in a beam was based on BS449-2 Chapter 4 clause 26d I admit it was not the best example in this case.

Anyway laukejas problem is about bending.

Have you checked Euler's? What you say makes no sense to me. This mast is, from the materials perspective, a matrix and should be calculated as such.

 

[Skip Johnson]

-Clip-

And thanks for the compliment about the boat. I am really pleased how she turned out. Unstable girl. Dangerous but fun.

Ah youth

Back to the mast, and basics. An unstayed mast is a 'simple' cantilever beam; a stayed mast is more a loaded column. If I was going to build as light as reasonably possible cantilever mast, the first order solution is to have as large a base diameter as possible. Then a tapered mast with a core of structural foam, probably birdmouthed. The trick then is keep the wiggly thing straight while laminating the structural skin over the foam. My previous project was done by laminating a pair of unidirectional 12k carbon tows on opposite sides of mast blank and then repeating at 90 degrees round the circumference. The result was a blank that was adequately stiff but not near strong enough. I then laminated biaxial carbon sleeve two layers on the bottom third. It's been a while ago so I don't remember the exact numbers but the thing was just over 8' long, weighed ounces and eventually was used as a mizzen sprit on a 25' mono hull rescue boat when it's wooden mizzen shattered in gale force winds. The thing was unbelievably stiff, no discernable deflection pushing down on mast supported at ends. For a longer span mast uni with a biaxial wrap would make more sense, the problem is having adequate tension on the fibers when laminating.