Epoxy/glass mast for a cruising yacht?

[Inquisitor]

Robjl,

Nice Title

The short answer… unfortunately not. I was talking about 37% Fiber Volume (FV) and the 55% you (and Eric and the Priestess) are talking about is Fiber Weight (FW). They are the same thing.

The long answer… There are two standards by which the ratio of fibers to epoxy can be described… FW and FV. It’s mixing the two parts either by weighing the components or by the volume of the components. If the densities of the two components are equal the ratio is the same. There are several good reasons to use FV… but really, it just comes down to measuring FW directly is a lot easier than FV directly… and boat people are practical and AE’s are masochist. Eric has already done the math for us in his #39 post. (repeated here)

E-glass:
40% f.w. (fiber content by weight) = 22% f.v. (fiber content by volume)
50% f.w. = 30% f.v.
60% f.w. = 39% f.v.

…and the numbers you supplied match his comments. Although the chasm between marine and AE composites is huge, I have no reason to doubt Eric and the Priestess’ assessment. Hand-lay-up is just… that… less… efficient.

Here are two pictures to describe the differences between marine and AE composites. The first one shows a polished edge of a 40% VF (~61% FW). If you cross your eyes you could WAG the fibers being about 40% of the area. This is the best that we can do with marine composites.

The second one shows a ply at 70% FV (also higher magnification). In AE, we use to get closer to 78% FV and the theoretical maximum is 91% FV.

In a nut shell, for our mast, stiffness (not strength) is the most important. The Priestess supplied “modulus” (of stiffness) is 24.8 GPa.
Using basically the same stuff, AE just by compressing and sucking out the excess epoxy gets the modulus up to 53.7 GPa!

This is a stiffness factor of: 2.2
The stiffness for increasing the diameter by 50% was another factor of 5.1

In my world (you may call it Oz) I was expecting ELEVEN times more bending stiffness that we can now get. All you can expect now is a very limp… noodle (yeah, that’s not my first choice of word)

Sorry man! I think the last fiber in the thread has broken.

P.S. Hey! But we both learned a lot. You got enough math to make you puke and I got enough marine experience to daze me.

 

[Robjl]

Another idea busted...

Thanks Inquisitor and the others who contributed....but maybe if we used CF for a home build with that cheap CF.... noooo don't take me there.
I'm back to the alloy stick.
Regards All
Rob.jl

 

[oceangboy2000]

fiberglass spars in dignys

first off great work to everyone so far! i have only been a member of this site for a few days now, stumbling across it while looking for help with foils. its been a great learning experiance reading and plugging numbers into the excel spreadsheet. i am a student in my senior year of study in naval arch. graduating in may. i have been working on a small catamaran design to produce this winter and im now extremly interested in fiberglass for the rig. Any thoughts on a shroudless mast of say 15ft? thanks in advance for what im sure will be more then helpful.

 

[Inquisitor]

I am not familiar with a naval architecture curriculum, but I’d assume there is extensive mechanic of materials type courses somewhere along the way. Its moment distribution along the height would probably diminish roughly with the square of the height and be zero at the top. As such the bending stiffness and strength and thus cross sectional properties would diminish similarly. A 15’ cantilever mast with a variable cross-section would be a great candidate for glass/epoxy.

However, the spreadsheet requires a known aluminum mast and the safety factors associated with it. Not having that, you would have to have all the loads that could hit the mast. Worst case righting moments in both side-to-side and fore-aft and some idea of shock loads. If you can come up with that, I could walk you through the composite portions.

 

[oceangboy2000]

Homemade?

Thanks for the quick response! Its pretty much just what I wanted to hear. As for the buildability of this mast though...i am looking to do much of this project literaly in house, but while still calculating and knowing exactly what is going into my boat. Is this mast buildable, by me, to specifications i can calculate?
And yes my courses have included much strength of materials and structure work. Thanks again for your help ~dan

 

[Robjl]

Try this site for ideas Oceangboy2000,
http://www.sailingsource.com/cherub/masts.htm
it is for carbon fibre... but you may get some ideas.
regards
Rob

 

[bilbobaggins]

Alternative sticks.....

I'm finding this thread fascinating. Why? I have, under a tarp in my garden, two ply/glass/epoxy hulls waiting for me to do something useful to them. A 'project' that's stalled..... and I'm trying to take it all forward again.

Now, these hulls are elliptical in cross-section, and 13.5m long. Max beam is 1m, while max height is 1.95m Each weighs about 740kg. and some weight could come out of the excess bulkhead material. I'd very much like to build and fit a pair of 'sticks' in a biplane config. These would need to be about 17-18m tall, to carry enough area ( ~ 85-95sq.m ) in a ~3.5:1 aspect ratio for swift cruising.

'Rugged' is a word I don't see often in structural engineering tomes, but it encompasses what I want in a boat that I would take out of sight of the lifeboat station. And 'KISS'..... I've got quite a lot of sailing experience, enough to know what I'd find un/acceptable with a 360 degree empty horizon, big wind and seas, and night falling.

A lot of years ago, a good aeronautical engineer taught me that "you can throw lots of money at a design problem, or you can throw lots of thinking...."
So I welcome the continuing discussion on the relative merits of differing mast materials, 'cos I'm going to use some of what I learn here, and take it sailing with me. So, in thinking of a plug material for an essentially glass stick, could someone offer thots on rigid foam, birchply, cedar.....?

 

[BillyDoc]

Some thoughts on actually making the stick

After all the engineering is done you still have to make the damn thing, and that can be a real problem. I found this thread late and don't know if the following will help, but it is an area I have some experience in so I’ll toss it out here for what it's worth.

I have been able to make composite pieces with extremely high fiber densities (better than 80%, by volume) using the following technique:

Pack the fibers (whatever they are) into a mold WITHOUT the resin and compact them as much as possible. Air is a lot easier to squeeze out than resin. Also, as you will see below, the fiber can be wetted (with water) if this facilitates the fiber placement.

Pull a hard vacuum on the mold and contents. By this, I mean a vacuum of at least 10^-6 torr. Depending on the shape and size, hold this vacuum for an extended period of time . . . for something like a mast, perhaps for two or three days. This is because: if you used water to help lay it up you want to boil it off and remove it, and if you are only removing gasses even a slight vacuum will get you to the point where individual molecules are bouncing off of the surfaces containing them (the fibers) and NOT off of each other . . . resulting in no gas "flow" (viscous flow) at all, but a mere random process as individual molecules bounce around and eventually find a way out. This takes a lot of time. Also, the vacuum will cause chemisorbed molecules on all surfaces to boil off and re-fill the chamber . . . for a long time. There is a lot of surface in a bundle of fiber. Cleaning the fiber surfaces in this way does indeed improve performance, though.

Once the vacuum phase is completed, some opening to a pre-degassed, low viscosity resin can be made and atmospheric pressure used to achieve resin transfer.

If all this is done properly, every little space in the fiber bundle will be completely filled with resin, and resin bonding on every surface will be extremely good.

The problem is actually pulling this process off with something as long and clumsy as a mast.

This is how I would approach it. First a "core" could be constructed of light aluminum to get something to place the fiber on/around. For example, a thin-wall tube could be rolled into an elliptical shape, then slit longitudinally and re-welded to taper it. Make the mast a few inches too long, so you can just saw the ends off later. Both ends of this core should be welded shut, with a vent tube placed in one end perpendicular to the core’s long axis. You MUST pressurize this and check for leaks. And fix them if you find them. ANY leak will totally screw this process up! This core will be removed after curing with a little acid and time. After all required fibers are in place the core and fibers could be wrapped in another piece of thin aluminum (roofer’s flashing comes to mind) so that with the mast lying horizontal the outer aluminum forms a trough with the opening upward into which the resin can be poured, and the vent from the inner tube protrudes upward from this trough. It is quite all right to use clamps on this trough where it is close to the fiber, as it is very unlikely that you could clamp it to the point where resin would not get past the interface.

Now comes the daunting part. A long vacuum chamber could be made with an appropriately sized length of pipe, as long as it is a metal pipe with a wall thickness sufficient to withstand the vacuum. Plastic pipe will leak. Your mast, core, and outer wrap are placed into this chamber.

Pull vacuum on this chamber, wait a couple of days, then while the vacuum is still high you open a valve to your resin (which you just mixed and de-gassed) which is routed to the trough you formed along the length of the mast. Atmospheric pressure will force the resin into this trough, and after it stops fizzing and settles down, (oh, I forget to mention that a little window in your vacuum chamber is a good idea) you can release the vacuum in the chamber to atmospheric pressure and watch as the now completely de-gassed resin is forced from the trough into the fiber.

This process is very sensitive to leaks in the mold, but if you can design it so that the entire mast is effectively submerged in resin before the pressure is allowed to come up on it, the resin makes a great seal and you get perfection every time.

The outer aluminum can likely be peeled off after the whole thing is cured, and the inner tube can be removed with acid.

I can’t imagine going to all this trouble for a couple of masts . . . but what do I know. Maybe someone wants to go into the mast business. The patent for this process is now expired (http://poiesisresearch.com/Pdfs/5,248,467.pdf) so have fun!

Bill

 

[stonebreaker]

This is a fascinating thread. I don't know jack about composites, but I did have a redneck engineering idea on the manufacturing process.

I'm addicted to the Discovery Channel. I remember watching some NASA types make a home-built rocket in an attempt to capture an altitude/speed record at an amateur rocket meet. They made their rocket tube out of carbon fiber. As I recall, it was about 12ft/3m long and about a foot in diameter. They used a cardboard tube as a core, and then instead of vacuum bagging it, they wrapped it diagonally with a strip of cloth, some sort of wicking material that would absorb the excess resin without sticking to the carbon fiber, very tightly, to squeeze out the excess resin and compact the fibers. They then cured it in a homemade oven. When the tube was finished, they soaked it in a swimming pool to soften and remove the cardboard tube.

I was thinking about inquisitor's use of an autoclave. I wonder if you could use convoluted drain pipe to make a long thin oven? Around here they use metal tubing that has a convoluted pattern (to help bear dirt loads when it's buried?) to channel rainwater under roads and such. Seems to me you could build an oven out of this stuff. Use a smaller diameter inside a bigger one, then run hot air between them to bake the resin. Maybe wrap the outside with fiberglass insulation and put it inside an even larger tube? Then heat it with one of those propane shop heaters - "heat cannons" they call them around here. Click on the pic for a link to the site I found it on.



Here's a link to the type of tubing I was talking about: http://www.rcspipe.com/Steel Pipe.htm