FRP vs CF

[Seafra]

I've recently become interested in boatbuilding, but I've been interested in working with advanced composites for a long time.

Would any of the knowledgeable among here care to add their personal opinions on FRP vs. CF in specific regards to:

-Cost(assuming an epoxy resin)
-Ease of work. I assume it's fairly similar though CF is tougher to cut.
-Performance advantage per $
-In a boat hull...is stiffer always better?

Additionally, my understanding is epoxy resin is 100% UV proof. Has anyone laid out a hull in CF and left it unpainted? There are too many white/light-coloured boats in the world.

Any good online CFRP/epoxy suppliers to recommend?

[SeaSpark]

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[marshmat]

Cost: CF is more than FG.
Ease of work: Depends how many years of experience you have with each.
Performance: Depends on boat. When CF is used it gives slight gains for big bucks.

[frosh]

Hi Seafra, welcome to the forum. I will assume that CF is carbon fibre?
I have built boats in wood, glass fibre and wood, carbon fibre/glass and wood, carbon fibre foam, carbon fibre/glass foam. Always epoxy resin.
The differences are not that huge compared to the cost differences. It also depends a hell of a lot in the particular application. If you are doing a high performance kayak, or an outboard runabout, it is completely a different world between the two. You need to be a bit more specific about what type of boats you are particularly interested in, and then we can progress.
PS. Just lately woven carbon fibre has become scarcer and more expensive!

[Tim B]

I have found that Carbon is actually just as easy to cut as glass fibre as long as you have a sharp pair of scissors. A good reason for boats being white is that a dark surface when left in the sun for any period of time can get very hot. I don't know if it would be hot enough to cause problems with delamination, but you should certainly consider the fact that the material will be working at an elevated temperature (prob 50 deg C or more) and consequently the resin properties will change from standard room temperature properties.

Dark boats look great in the marina but are hard to see at night at sea and tend to be incredibly hot inside.

Tim B.

[Splint]

Hi Seafra,
personally I think unless you're building a hull for racing purposes you should stay away from CF. Mostly due to the cost but you also need to be aware of the performance characteristics of CF, it is strong but has very limited impact resistance so if your hull hits rocks you're in trouble. Sheets of CF are very easy to snap with your fingers, much easier than woven mat or even chop mat fiberglass, CF is good if used in the right methodology and in the right application but appart from that it's a not all that the average punter on the street thinks it is. Also keep in mind that the directions of the rovings are criticle to load exposure. Here's a couple of pictures to demonstrate how easily CF can break. This was an experiment to sample different surface and core materials. A hammer was used to punch the holes in the CF, it took minimal effort to do the damage. CF works well where the load is spread over a large area (think of aeroplane wings).

Cheers
Splint

[marshmat]

CF is incredibly strong and incredibly stiff, but it is also very brittle. As Splint says, it doesn't like point loads. It can't flex to absorb them, without shattering. As several have said now, CF is for specialized applications where weight is of paramount importance. It needs really careful engineering of the laminate schedules in order to work. Unless you know exactly why you need carbon, you probably don't need carbon.

[fiberglass jack]

why dont u go with a kavlar carbon hybrid u can get a 2x2 twill and it has a nice look on it this way u have the best of both worlds

[bntii]

Additionally, my understanding is epoxy resin is 100% UV proof. Has anyone laid out a hull in CF and left it unpainted? There are too many white/light-coloured boats in the world.





you have it backwards....

Epoxy is not at all UV proof. In about 2 months the stuff will go south if left in the sun. All epoxy laminates must be painted. Also the thermal transition temp is low enough that the solar gain on a black CF surface will be too high.

Just paint the hull a nice light green and with some bright blue spots :)

[Tim B]

There are special UV blocking agents to add to epoxy. Personally, I think the heat will be the biggest problem. Even with high-temperature resins, the inside of the boat will still get far too hot.

Tim B.

[bntii]

Never used blocking agents.

Do you have a link to a commercially available UV stabilizer for epoxies?

Thanks

[Seafra]

Yea, my prior experience with carbon fiber comes from motorcycling. It's great for the fairings and non-load bearing parts. I've seen some pics of spectacularly delaminated carbon fiber wheels...high speeds and rotational stress can do that I suppose....


I understand one of the key differences between FRP and CF is in terms of flexibility vs. stiffness. CF is the most brittle when it's made with a poly resin, but never has the flexibility of FRP.

In my initial question, I wondered if stiffer was always better(in terms of sailboat hulls). Would using carbon fiber to reinforce an otherwise FRP hull produce favorable results IYHO? In terms of motorcycling, stiffer is not always better as a certain amount of lateral flex is necessary to act as a tertiary suspension (wheels, dampers, (sub)frame) when leaned over. That is why Carbon Fiber frames made only a brief spark in MotoGP.

Fiberglass Jack, tell me more about Kevlar/Carbon hybrids. Kevlar's used in riding gear for motorcyclists, but it's typically blended with other fabircs to increase abrasion resistance. I suppose abrasion isn't much of a concern in a sailboat. From what I remember Kevlar degrades rapidly in UV light- I had kevlar brake lines on my bike but they were coated with a thick plastic to shield them.

So whatever, this is a boating forum...not a bike forum.

I think for hull color I'll just paint it soft pink...with little flowers running along the gunwales. ;)

[Tim B]

Kevlar is has a lower modulus than Carbon, but much higher tensile strength. That is what makes it useful within a (mostly Carbon) laminate. The failure will not shower little bits of (sharp) Carbon fibre all over the place.

For Yacht hulls, stiffer is always better. It is a necessity for good handling while sailing. Carbon is a fantastic material, and great for highly loaded beams (mast supporting beam) and the like. However, since boat hulls tend to be pretty stiff anyway when they have a full-length deck, it is only really needed in highly loaded areas of the hull. For a small racing skiff (Moth or I14) you may choose to use Carbon because of the weight reduction. Not because it couldn't be made stiff enough from GRP.

Tim B.

[John ilett]

If you build a boat in CF using the same weight of fabrics you will have a much stronger stiffer boat that is very unlikely to explode, snap or be brittle.

When people make the change to stronger materials like carbon they tend to use less of it in comparison and have a much lighter boat.

[Seafra]

John,

Do you think it's common that a boat built out of Carbon Fiber is likely to have less material thus be lighter, stiffer, but also more brittle?

In ocean cruising situations, it seems almost disadvantagous to use carbon fiber for the either the mast/spars and the hull. However, for dinghies and racing scows I think it seems the best option.

You don't get much lighter than pre-preg carbon fiber layup.

[Toot]

Another problem with CF is the fact that fabrics don't give you nearly the strength as straight rovings.

Tensile strength for well-manufactured solid rod will exceed 320ksi. Because of the small size and difficulty with inspection, following appropriate standard practice might be to cut that amount in half for design purposes. So let's assume 160ksi and just be happy that we have a nice safety margin. By comparison, a wet-layup of unidirectional E-Glass will be around 40ksi. Although carbon can be brittle, it is very strong, so it can be used within a laminate structure to give a HUGE boost to strength without exposing it to damage on the exterior.

Graphlite is a tradename for pultruded rods that hit the above stength specification- most cheapo rods do not. Graphlite costs $2 a foot in low volume for a rectangular pultruded rod with the dimensions being: 0.092x.220, or .02024sq.inches, which means each rod has a tensile strength of 3,240 pounds. The cost, in low volume, is $2 per foot. I think in high volume, you can get it for half that amount. That's makes it a really good option for making I-beams and such where you embed the rods into a fiberglass structure. Here's a link to a guy that's doing it with sailplanes... Jim Marske (http://www.continuo.com/marske/ARTICLES/Carbon%20rods/carbon.htm)


By way of comparison, most homemade CF rods will struggle to get above 60ksi and some may fail at half that! And most woven CF fabrics will struggle to outperform S-glass because the crimps/curves in the weave dramatically decreases the strength of the CF, much more so than with fiberglass. I read somewhere that a 1-2 degree variance in the straightness of CF will cut it's strenght in half. Scary!

As a result of the above, I'm pretty well sold on the idea that, at least where cost is a concern, the only justifiable use for CF in a small homebuilding type of operation, be it sailplane, boat, car, plane, skis, or helicopter, is in pre-manufactured rods and then only for unidirectional load absorption. Otherwise, stick to FG or maybe Kevlar.

[John ilett]

John,

Do you think it's common that a boat built out of Carbon Fiber is likely to have less material thus be lighter, stiffer, but also more brittle?

This is usually the case to save weight and cost as the carbon is so expensive. For any boat a builder would hope to improve their boats in any and every way possible so I think there is a compromise of using less material and still having a boat that is lighter and stronger than its fglass equivilant.

The stuff (carbon fibre) is really stong especially when it's pre pregs (2 x better resins) and when it's used properly. With aerospace and race car builds they follow strict guidelines when processing to remove the human error factor so carefull attention is given to fibre orientation with engineered laminates, monitored vacuum and cooking set ups.

When you start to use less material you then have to be sure that the loads are spread (good fibre orientation). Average fglass boats often simply rely on the laminate thickness as a guide to strength.

When you hear of broken carbon parts very often the cause is a processing mistake. Not enough fibre in the right place pointing in the direction. Holes drilled in the wrong place etc.

[Toot]

Right now, for a project I'm working on, I'm 80% settled on the idea of creating an I-beam using the following method-

Take an 8 foot aluminum/steel U-channel, waxing and PVA'ing the inside surface, laying up graphlite rods inside the channel to acquire the necessary strength, laying unidirectional glass rovings on top to make a uniformly-sized piece. Then creating 2 knitted-glass (+45/-45 orientation) flat panels using peel-ply on a flat surface to serve as the spar web. Then bonding the sparcaps in between the sparwebs, with a foam core taking up the rest of the space. I figure I can create a 16 foot long spar, capable of carrying 20G's on a 700 pound aircraft and keep the spar well under 20 pounds.

And best of all, the carbon won't be exposed to the surface, so it's brittleness won't be a factor. It sounds like just about a perfect structure!

Try that with E-glass! ;)

[frosh]

It seems to me that a good compromise cost wise and also durability and weight wise can be achieved as follows.
Make the hull skin a foam sandwich using an appropriate thickness PVC structural foam, Airex, Klegecell etc.
As most of the loadings on the hull skin are pushing inwards or torsional, it makes sense to have the inner reinforcement in the strongest lightest material, such as carbon fibre. This takes advantage of the tensile modulus of carbon which is higher than all other fibres.
The outer skin is probably less loaded than the inner skin, but is exposed to the outside elements, and environmental nasties.
For excellent strength, flexibility, and fairly good abrasion and impact resistance, I like using a biaxial knitted fibreglass. If the orientation of the carbon fibre is 0,90 degrees, then the biaxial glass is best in a different orientation, that is: +45, -45 degrees. Resin will be always epoxy, never polyester.
Any opinions? As this is just my idea of an excellent hull skin composition.