Making carbon fiber spar from chop strains

[pbmaise]

Originally Posted by jimbunyard

Seems like a bad idea for all the reasons above plus, if I did the math right, at a weight of .62 oz per cubic inch for the resin alone, your 6"OD x 3"ID spar will displace 254 cubic inches per foot, which works out to 9.84 lbs per foot. So a ten foot spar would weigh close on to 100 lbs.

Why not use the materials available in your part of the world to make a composite spar that is lighter, stronger, prettier and probably longer lived?

There are likely dozens of species of strong, (relatively) light, rot resistant hardwoods available in the tropics that wood (haha) make a good spar, especially when coupled with an appropriate construction method such as this one, called a 'birdsmouth spar'.




Combined with epoxy and a layer of glass on the outside (for protection, though considered properly it could also add considerable strength to an already very strong structure), these are strong and very light for their dimensions. And they're surprisingly easy to build, all you need is a table saw (or router though that is more difficult), some hose clamps, and a method of rounding the spar off after it's built. For the oar shafts I've built using this method I just use a hand plane and a DA for the rounding.

So you can use things that are readily available, labor and lumber, and get something that might better serve your needs. Obviously just a suggestion...

Another look at how it's done.

I am getting a quote for hollow wooden spars made in Philippines. I am thinking of having two sets made and carry the second set flat on supports at deck level. I plan to cross Pacific from West to East and much of my trip will be up wind. Having one of these spars break 1/3rd of way on a long beat would likely force me to turn back. So two sets of spars or one set vastly over capacity sounds good to me.

A 100 lb spar sounds light to me. I don't know the exact weight of my sail but it is around 225 lbs. If a crew member is on top installing the cover that is even more weight. Most of that weight must free span across my pilot house. Another task for the upper spar is lifting the dinghy on board. I have a solid fiberglass sailing dinghy and it weighs a lot.

The lower smaller spar replaces the original boom that was about 14 inches tall, and 5 inches wide. That boom weighed around 200 lbs.

I requested the spar builder quote me a solid wood smaller spar. I have Dyneema filament that I can epoxy onto the wood to make it even stronger.

I also found a factory in China willing to make me spars.

It is looking less likely I will make my own.

[reed1v]

For all that work just build a spruce mast with a fiberglass skin.

[Cadence]

Originally Posted by pbmaise

I am not very adept with working with large sheets of carbon fiber fabric.

If I want to make a quick carbon fiber spar, would the following work.

1. Stand PVC pipe vertical (one will be 4" diameter and other 6" diameter with 3" diameter hollow core)

2. Pack with chop carbon fiber strains.

3 Tamp down to pack tightly, or just let fall loosely?

3. Pour epoxy slow cure resin from the top (10 feet section at a time).

4. Let cure.

5. Use Dremel to cut away PVC pipe.



I am merely looking for a resulting spar stronger than wood and epoxy tough. I realize it won't be the strongest spar compared to working with fabric.

Questions

1. Will it work?

2. Will resin gravity flow down the pipe and air escape up... or do I need a vacuum pump at bottom? The epoxy I plan to work with takes several hours to cure. It is concrete crack filler epoxy.

Thanks

Surely this is in jest. If not very foolish.

[pbmaise]

Okay facts people facts based on scientific research.

I searched today literature and found what I am most interested in is called the flexural strength.

I want a spar that is stiff and resist bending without breaking.

Tension strength is of no concern whatsoever . Compression strenth for a spar that isn't a mast supporting a high load is also of little concern.

So if you are interested in facts based on actual research continue reading. I will share with you my findings.

#1 Normally you hear only of tension and compression strength, however for a gaff and boom it is flexural strength that is key.

#2 It is measured as follows:

Flexural tests are performed in accordance with ASTM D790-98 [6]. Tests are conducted on an Instron-4204 testing machine with the cross head speed set at 1.4 mm/min

#3 Spar flexural strength has little to do with the orientation of the fibers. Which direction should strains be oriented when a spar can flex in both X and Y direction? Yes yes, for a column supporting a weight from above I can see your points. Therefore, yes a mast should use oriented strains for higher strength.

This said remember this is not an airplane. I am a cruising sailor crossing the Pacific and not racing an ultralight. I have a 70 ton vessel and don't give a hoot if my spar weighs twice or three times more then it can be made with ultralight materials in perfect arrangement.
(I have no vacuum machine and no access to a professional shop. Standing a mold vertical seems logical. )

Please, don't get me wrong I do what your input and seriously still considering

Continuing with idea of random chop in epoxy (may use injectable epoxy which has extremely slow cure and seems logical to allow voids to natural bubble out via gravity.

Switch to wood.

Have professional shop in China build spars using orientated carbon fiber as everyone is clearly suggesting despite fact no one mentions flexural strength.

See page 60 of this thesis which discusses flexural strength of carbon fiber compositions.



Notice in particular that flexural strength DECREASES after the volume ratio of fiber to resin exceeds 34%.

Flexural strength for carbon fiber in a PET matrix deceased from 240 mPa at 34% down to 140 mPa at 41%.

Obviously this shows packing as much carbon fiber as possible into a spar is detremental.

https://theses.lib.vt.edu/theses/ava...thesis_ETD.pdf

[Pauls]

Hi Paul,

OK. Here is some engineering info on your project:
1. You are correct, as regards your mast, that resistance to bending is what masts are designed for. This is because they are loaded in compression from rigging tension and they must not buckle.
2. Gaffs are loaded partly in compression, however they usually are also seeing bending loads from the sail, peak halyards, etc.
3. When a bending load is applied to a spar the spar surface on the outside of the bend is in tension. The opposite surface, on the inside of the bend, is in compression. The STRENGTH of the spar is a direct function of the tension and compression strength of the material it is made from.
4. Stiffness, better described as resistance to elongation, is different from strength. In engineering this is quantified as Youngs Modulus.
Masts are designed using Euler's formulas. You can look them up. They take into account the load on the mast, shape and dimensions of the mast section, and Youngs Modulus.
5. Epoxy resin has very low strength. It is structurally of almost no use. The fibers which are used in a layup provide almost all of the strength. So fiber orientation along the direction of stress is very important. Chopped fibers have random orientation, short lengths. They are the worst structural fiber type available.

Practical advice - this is coming from somebody who can/does design this stuff and who has built a lot of it. If you are building a conventional stayed mast, where you are (I am familiar with the Phillipines), make it of wood. If you can get good wood glues the birdsmouth recommendation above is very good. It will be much lighter and stronger than your original idea. That said, gaffers have sailed for centuries with solid wood masts and they still work just as well today.

I would under no conditions trust a company to build a carbon spar unless they have the engineering ability to do the analysis and the quality control to ensure it is made well. China is not known for quality control, and with high stress composite structures this is a really big deal.

Wood is simple, easily inspectable, easy to work with, and has a good strength to weight ratio.

[TurninTurtle]

Structure is structure...

You want a spar to resist bending and the fibers/grain need to be parallel or nearly parallel along the length in essentially a tube formation.


You get that by extruding... as in aluminum tube
You get it by laying up fibers along a round former of some sort, primarily lengthwise
You get it by assembling a hollow "box" spar of some sort such as the "birdsmouth"

You certainly don't get it by stuffing chop strand between 2 tubes and then trying to draw resin in to glue it all together.

For some applications, the same aluminum tubes are used in aircraft main spars and boat spars or masts... They see similar loads.
A sail can be looked at as a vertical, flexible "Rogallo" wing.
https://en.wikipedia.org/wiki/Rogallo_wing

[pbmaise]

Pauls thanks for advice and recommendations.

My rotating mast is already existing and aluminum. The spars attach behind the mast.

See starclassics.org to see the type of rig I am going for.

I am indeed reading all the advice and leaning towards two sets of wooden spar's built by a professional boat builder with loads of experience. My vessel hardly heels to dump the wind so that is why I am so keen on making sure everything is solid.

Shipping costs from China would most likely make cost prohibitive.

I forgot one on my list of things you can find that I can't. Ready-mix! I am making a temporary mooring to sit on while I send my chain to Davao to be regalvanized. Last time I waited three months for it to get back. Arrrg!

Anyone want to help me hand mix 2000 lbs of concrete on my deck. Rebar I have cut off the boat.

[pbmaise]

Shucks I like to sometimes pioneer new roads. I listened to all of your good advice and purchased two wooden spars skinned with West System epoxy and fiberglass.