carbon glass balsa core

I'm playing with a sandwich layup over a male mold- was wondering about peoples favorite combinations.
Please let me know what types and weights of glass you like to use.

2o ft catamaran
sailboat

glass, end grain balsa, carbon fiber weave, glass. West epoxy

also, I've vacuum bagged a small part before. Any hints on bagging a large project?
Thnaks,
Rob

 

There are no objections against a glass/carbon combo and West as a matrix. Prepare the balsa; give 'm a lick of poxy b'fore final application.

 

You have to be very carefull with the glass carbon mix. Carbon is much stiffer and breaks at much smaler strains than glass. If you load up the structure the carbon takes most of the load because it is much stiffer whereas the glass does not much. You have to limit the allowable strain to the carbon strain. Otherwise the carbon breaks and if the glass can't hold the load because you thought it was reinforced by the carbon it will break too.
Balsa is cheap and nasty. It can and probably will soak up water and it will also soak up expansive epoxy resin. By the time you allowed for the cost of the extra resin and for water repairs around fittings a good foam core would be cheaper. Just look through this forum and search for water damage in balsa sandwich. Also balsa is very brittle and therefore not good for impact damage.

 

Thanks for the response- I'm learning that there is a lot to learn out there!
I'm happy to use either balsa or foam- the main reason for the choice is to be able to sand to shape and fairness before putting the finish layers on. Can you reccomend a foam that you have worked with? I'm guessing it would have to be 3/8 or 1/4 thick. I do like the ease of laying up foam
If I go with balsa, I assumed that on a thin piece it would soak up enough epoxy to keep water damage local and repairable.
The boat is drysailed, and we have a dozen wooden sharks (cedar) there that we are diligent with on hull maintenence.

I hadn't given any thought to the "compatability" of carbon and glass. You bring up a great point, and we sail these boats hard. My thought on the carbon was as much for looks as well as strength- I didn't want to paint this boat. I'll re-think this as well as make a few test samples.
Rob

 

The main advent of a glass/carbon composite is to stop the Carbon from failing quite so explosively in compression. "Carbon-finished" boats look great, but tend to get incredibly hot in the sun. Heating doesn't do the resin any good once it's cured.

Balsa is absolutely fine as a core material, but you'll probably find it's a bit heavy. Epoxy will work with "white" foam (normal insulation foam) which is easy to cut with a hot wire. There are many different types of foam available. most should be ok for epoxy, but check the chemical makeup anyway.

In the past I have used expanding cavity foam for fillets and the like, but the voids tend to be large where it is not under direct constraint. It would be perfect for moulding rudders and keels though.

In terms of moulding the hull(s), you will get a better finish if you lay up the hull(s) in a female mould. taking a female mould is easy if you have a male plug. and since this is a Catamaran, you can mould a hull/deck (or either side) and use a layup on a piece of thick cardboard to join the hulls, held against the hull with string pulled from the outside. There are several ways of joining hulls. that's just one.

Good Luck,

Tim B.

 

There are basically three different types of STRUCTURAL foam. Cross linked PVC, linear PVC and SAN foam. Cross linked PVC has high shear strength (that's what you want in a foam) but is very brittle (It cracks if you have a big enough impact). Linear PVC is very tough but has low shear strength. SAN foam has good shear strength and good toughness.
Finishing in clear carbon is very difficult. If you male mould what are you going to use to smooth all the little dips due to the weave of the fabric? You would have to use clear epoxy which is very hard to sand and then paint the whole thing with some UV protective coat. The hull will get hot in the sun which is a problem with PVC cores. The gas that is used to foam them expands and can pop the skins off the core. Also the strength of the core drops signifficantly under heat. SAN foams are much better in hot climates.

 

Balsa has excellent impact resistance

Hi,

I agree with Karsten's comments on the use of carbon

But I have always thought that Balsa has excellent impact resistance and is a fine material in that particular respect. Or is there something I am overlooking - always happy to learn more?!

I tried to find some or similar testing for different core materials but below is the best I could find on the net (a case study for selection of materials for a bicycle helmet)


Figure 2 is a materials selection chart generated by CES, which enables selection of foams for energy absorption. It shows the maximum compressive strain (the 'densification strain'), eD, plotted against another measure of the compressive strength: it is sc(0.25), the 'plateau stress' at 25% strain level. The ability of a foam to absorb energy is measured by the product eD. sc(0.25). Materials above the selection line absorb the most energy per unit volume.

The main limitation of balsa for the lightest structures is that it has a relatively high density compared with foams which can be produced in a huge range of different densities.

Also it is important to take care to minimise resin absorbtion - thus added weight and expense. It is not difficult to get around at all, but you have to be aware.

A third aspect is that if you have a weight critical production item balsa may mean that the weight has a tendancy to vary from item to item a little more than foam. ie varies in density itself and the variation in resin absorbtion. So if a really accurate reproduceable weight is essential, then balsa would probably not be a good choice either.

As far as water absorbtion - closed cell foams won't absorb water, whereas balsa will. However be aware that balsa is a standard component of many high quality production boats, so water absorbtion cannot be a serious problem otherwise it would have been discounted from use a long time ago.

Best Regards
Michael
Web Site

 

Michael,
Thank you for the info. I've been making some sample balsa/glass sandwiches, and I am really impressed with the strength. As soon as I get some carbon, I will try that.I have to be as strong as two layers of 1/8 cedar and light glass. I'm closer than I thought I would be. If I stay lighter, I can afford more internal sturcture to stiffen the hull.
I am going to play with some insulation foam as well, although I am leaning towards balsa due to familiarity. (building all those model airplanes paid off!)
the minimum class weight is 450lbs. I'm getting confident that I can meet that goal.
Rob

 

Karsten,
Thanks for your thoughts. We've used west clear finish - usually three coats-sanded fair between coats. Sunday morning long boarding parties between coats.
http://www.sharkcatamaranclass.org/classifiedadd.htm#614
We use spar urathane for uv protection on the decks, but I dont think John Rogers uses it on the hulls.

I'll play with more sandwhich samples. I just received the hull station plans, so once I get the hull thickness determined, It will be time to make my first mold!