This article aims at helping you better understand the main differences between the 3 big types of resin and the 2 most frequently used fibers for boatbuilding. A simple and simplified explanation of the main differences between polyester, vinylester and epoxy resins, and between glass fiber, typically E-Glass, and carbon fiber will provide you with a good mapping of what can be achieved in terms of mechanic resistance and stiffness.
I’m trying here to point out key elements for sailing yachts buyers or amateur boat builders to take into account when deciding what composite to choose regarding their own program, budget and care for lightness, speed at sea and durability.
Fibers: Carbon versus E-Glass
Carbon fibers have the highest specific stiffness of any commercially available fibers, very high strength in both compression and tension and a high resistance to corrosion, creep and fatigue. Comparatively, E-Glass fibers have a really bad resistance to fatigue and will quickly loose some of their properties.
One can manufacture lighter pieces in carbon fiber than with a traditional E-glass, because the mechanical properties of carbon fibers are much greater than those of E-glass, meaning you need less fiber, in grams per square meter, to get to the same structural strength and rigidity. Basically, it allows, on any given structural composite piece to reduce the number of layers or their grammage to get a lighter structure. Of course, there are different types and grades of carbon fiber (aeronautical uses very high-tech and thin fibers) as there are different types and qualities for glass fiber, E-Glass being the most frequently used. Carbon fibers range from 1K to 24K, 1K being the best quality (used on fighter aircrafts) offering the best mechanical properties (we talk about Ultra High Modulus). Detailing the different qualities and fibers inside the carbon family would request a full book, but buyers must remember to ask for a detailed definition of what types of carbon fibers were used for the construction of the yacht, it gives an idea of how hard the builder is fighting weight.
Also, by its rigidity, carbon is a better fiber for sailing yachts as less sail power, or keelforce, is absorbed and lost by the softness of the structure (hull and deck). Fiberglass or E-Glass create a bigger waste of energy, and a heavier structure, resulting in less accelerating power.
There are a few downsize to the use of carbon. For instance, carbon has a quite low elasticity (elongation before brake) and will brittle more quickly in case of a shock. This is why I would not recommend the use of carbon on thin objects, such as boat’s wheels.
You should also know that carbon fiber is a much better electricity conductor than glass fiber, meaning the effort and investments to make a proper isolation from hull and deck equipment, especially aluminium parts, will be more important. If you are not planning to get a specialist to take care of the isolation of your system, than I would recommend to both small builders and users to stay away from full carbon hulls…unless you accept to change your winches or deck equipment on a frequent basis…galvanic corrosion between carbon and aluminium works really fast, and aluminium will quickly corrode to death.
Finally, due to the complex fabrication process, carbon fibers still cost 10 times more than the standard E-Glass fiber used by most of the boat builders, but their price tend to go down as production rises (The wind energy industry is a heavy consumer).
Resins: Polyester vs. Vinylester vs. Epoxy
Polyester, Vinylester and Epoxy are the main resins available for boat building.
First of all, I would say Polyester and vinylester are much easier to work and can tolerate mistakes, but epoxy can’t. Epoxy requires very precise mixing, and after mixing Epoxy resin and hardener, temperature rises very quickly, reducing the amount of workable time. Epoxy also makes post curing compulsory. Finally, epoxy resin is by far the most expensive, its price being related to its mechanical performances.
Polyester is the cheapest resin, and the one with the worst mechanical properties and sensibility, or resistance to water. Epoxy completely resists moisture where polyester does not, hence the osmosis. Because the resin is very fragile, it will brake quickly under a lot of stress. The “Polyester and E-Glass” mix is the cheapest but also the heavier, weakest and less durable composite. It’s not recommended for offshore cruising.
It is useless to use carbon mixed with polyester; the very low “elongation at break” of the resin will not allow the carbon mechanical properties to be solicited before the resin breaks, creating a weak point in which humidity will penetrate quickly and delamination will occur. Polyester is easy to work on, with all techniques from traditional wet-laid process to vacuum infusion or RTM (closed moulding technique), it will not heat up too fast and the acceptable range of temperature while using Polyester is wider.
Vinylester has better mechanical properties and percentage of elongation at break than polyester, which means it will not brake before the fibers reinforcement has been solicited. Vinylester also offers a better resistance to moisture. You can gain a little weight with a good architect and builder but more likely you will have a stronger hull or deck using the same grammage. Vinylester is 1,5 to 2 times more expensive than basic polyester, but the safety of it can worth the investment on the long term.
Polyester and vinylester don’t necessary require post curing, although it’s always better to reach the full mechanical properties in the mould. If you don’t cure the pieces you manufacture in polyester or vinylester, you will probably get only 60 to 70% of the mechanical properties (strength, stiffness) but your boat will slowly post cure under on sunny days. That can create a few marking on hulls that you would avoid by curing the hull or deck in it’s mould after manufacturing it.
Epoxy resin is 2 to 3 times more expensive than vinylester and much more complicated to process for builders. It requires a steady temperature, a much greater precision and control of mixing (resin and hardener), and also can lead to a massive exothermic reaction. Left too long in the pot, a mix will quickly heat up. It’s an exothermic reaction (see a video) and the temperature can, within 5 minutes, rise from 40 to 50 C° to more than 150°C creating physical risks for the people around. A post curing is mandatory for pieces manufactured in epoxy, generally between 50 to 80°C for vacuum infusion to 120°C for pre-pregs. But, if you find the budget and gather good specialists, then you can manufacture extra strong and extra light boats.
The epoxy resin is very stiff, has a very high elongation at break that allows fibers to be fully solicited, hence delivering a very strong and stiff hull or deck with light grammage of fibers (don’t use anything else but epoxy if you pay the premium for carbon). Epoxy remains great in time and keeps its high mechanical properties much longer than polyester or even vinylester and the water cannot infiltrate it, making it safer for structure exposed to risks of in water collision. Finally, epoxy resin resists much better to high temperature.
All in all, a carbon/epoxy hull costs 5 or 6 times the price of a standard E-Glass/Polyester hulls, and 2 to 3 times the price of an E-Glass/vinylester hull but delivers without any doubt the best technical and durable hulls and decks, with the ability to gain a lot of weight at constant strength. If you go for carbon/epoxy, find a really good and experimented builder that can rely on modern tools, skilled workers and a tempered environment.
A good alternative is vinylester with E-glass fiber and local carbon reinforcements, if you don’t care too much about weight and can afford a prime for safety (osmosis prevention) and better durability the vinylester offers compared to polyester.
Once you have chosen your fibers and resin, it’s time to care about choosing the foam to put in the sandwich, to add stiffness while not adding too much weight.
Core Material: Forget Balsa and hesitate between PVC and SAL Polymer:
The flexural stiffness of any panel is proportional to the cube of its thickness. By increasing the thickness of the laminate with a low density material, architects and builders can increase the laminate’s stiffness for very little additional weight. The core material also helps to improve thermal isolation.
There are many types of foam but the most common are:
- Honeycomb, such as Nomex® (very low weight, very high mechanical properties but also very high price – 3 times more than PVC or SAN – and difficult to use. But if you can afford it…it’s a perfect addition to a pre-preg hull).
- Closed cell PVC like Areix®,
- SAN Polymer like Corecell®,
- Or the cheaper Balsa (I exclude this one as it has a relatively high density and absorbs large quantities of resin during infusion)
The choice of the foam is equally important as the choice of fiber and resin, to get a good laminate. Regarding the properties, I would define good foam as foam:
- That will absorb a minimum of water if the hull is damaged
- That will absorb, during the vacuum infusion process, a minimum of resin while letting it flow between fiber’s layers properly.
- With a good shear strength, compression resistance and stiffness,
- With a good resistance and stability at high temperature (a dark navy hull can go really hot under the Caribbean sun). Some core material are more sensitive to high temperature than others and will quickly, and permanently, loose some mechanical properties.
- Ideally with a good elongation at break, to absorb shocks between the layers of fiber.
Good quality PVC foam (such as the widely used Aireix®) and SAN Polymer both meet the above requirements. SAN polymer, such as Corecell® are said to offer a higher Elongation at break, which is a good thing to limit damages in case of a shock.
Foam will usually be used in curved areas and need to be shaped to follow the curves.
The most frequently used and easier way to shape the foam is to use grid-scored sheets. It’s very easy but a lot of resin goes between each square of the grid, increasing the structure’s weight without making it any better.
Thermoforming is more expensive and requires both additional tooling and real skills, but the result is a much lighter structure.
What’s in it for Offshore sailors ?
Why being so demanding with the laminate ? Because the lighter your structure is, at constant mechanical resistance and stiffness, the faster your boat will be. Carbon opens new possibilities for sailing yachts, the opportunity to reduce the structural weight of hulls and deck while maintaining the mechanical properties (strength, stiffness) and improving their durability. Keep in mind that the heavier the composite structure is, the heavier the keel must be to maintain the displacement/keel ratio. It means basically that the weight you gain on you structure can be gained a second time on the keel without reducing the displacement/keel ratio.
The virtuous circle of weight
If a boat without its keel (structure + Equipment + rigging and deck fitting) weights, for instance 12 Tons (it’s a big boat), you need an 8 tons keel to reach a 40% displacement ratio.
Now, here is the virtuous circle. The 12 tons of the boat without it’s keel are roughly 6 tons of structure and 6 tons of equipment. Imagine you skip from E-Glass and Polyester to Carbon-Epoxy, use a good core foam (as opposed to the cheap old solutions usually used by the majority of builders), and a well-executed vacuum infusion process: In result, you’ll gain around 40%* in weight for the same mechanical strength. This is a reduction by 2,4 Tons in our example.
*Note that those kind of gains can only be achieved by skilled and well-equipped yard, mastering the infusion and reaching good FVF around 55% or more (FVF stands for Fiber Volume Fraction, i.e the percentage of fibers in the total weight of a laminate pre-preg used in the aerospace industry can reach FVF 60 to 70%). Resin is a bonding agent for fibers, an excess of it won’t make the bonding better since tough job is done by fibers. Unfortunately, it is not rare to see FVF under 40% (in 1kg of laminate, more than 600 grams are resin…most probably mediocre resin such as ortho Polyester) in yards that do not master vacuum infusion properly or still laminate using the wet laid process, in which an operator manually apply the resin (a huge amount of it to be sure) on each layer of fiber.
Back to our example. Since you gained 2,4 Tons on the structure (20%), you can automatically reduce the weight of the keel by 2 Tons (20% of the initial keel) while maintaining your displacement/keel ratio at 40%. It means, in that example, that by reducing the weight of the composite structure (lighter carbon/epoxy hull, deck, bulkheads) by 2,4 Tons, you can actually make the whole boat 4,4 Tons lighter. You’ll also gain a few more kilos by adapting mast, rigging, anchor, etc. to the new weight of the boat. You have gained stiffness in the trade.
For offshore cruising yachts, the weight gained in the structure can also be transferred, to comfort and sailing equipment, without making a too heavy boat.
This virtuous circle, achievable thanks to carbon/epoxy and vacuum infusion, opens new possibilities for producing “state of the art” Light displacement offshore sailing yachts, faster, as safe and more durable than traditional heavy displacement sailing yachts.
More and more builders are now mixing E-Glass and carbon. The carbon is used in monolithic areas, where a lot of fiber layers are piled up. It’s a good option if you can’t afford a full carbon option. It allows quick weight gains, but does not really make the boat much stiffer. The mix carbon/Kevlar is also a good solution for the outside layer, to improve resistance to shocks (one of the rare field, with pricing, where carbon is actually not better than E-Glass).
I hope this can help you in your choices, please post a comment if you want to react, I hope my English, especially when it comes to technical words, didn’t make this post too hard to read.
Thanks for reading,
If you want to increase your knowledge about composite, I strongly recommend the book “Introduction to composite materials design” by Ever J. Barbero.
Tough it’s quite a big book (562 pages) for an “introduction to”, it’s easy to read, well illustrated, properly organized and up to date (second edition from 2010, so they include modern pre-pregs, vacuum infusion, aeronautical grades of carbon fibers, etc. ).
The book covers all the subjects you might be interested in as someone looking to understand more about your boat when it comes to composite.
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