Saturday, 19 May 2007

Carbon epoxy laminate for multihulls

Our Seaon 96crb is fully built in pre-preg carbon fibre (for more on information on our building technology please check production” on our web site). Occasionally we get comments that suggest a laminate based on carbon fibre is more fragile than a glass fibre polyester laminate and carbon there fore not being suitable for normal boat handling e.g. when docking.

Let us look at some “theory” behind a laminate.
Most modern boats are built with a sandwich laminate meaning that instead of a solid laminate it is built with a laminate on each side on both sides of a foam core (early days balsa was used – and to some extent still is). Separating the inner and outer laminate skin by a foam core gives a lighter laminate than building a solid laminate.

When dimensioning a laminate, considerations have to be taken to the various forces that will impact on the laminate such as structural loads and practical boat handling e.g. local impact when docking. Furthermore the type of fibre and resin (normally polyester, vinyl ester and epoxy) selected will affect the dimensioning of the laminate. Typically carbon fibre is approx 1,5 times stronger and approx 2,5 times stiffer than glass fibre compared to weight, i.e. a carbon laminate of the same weight is 2, 5 stiffer than the glass fibre laminate meaning it will distribute physical impact over a wider area of the core. Practically this means if the same force is used for impact on a glass fibre and carbon fibre laminate of the same weight, it will create a deeper deformation in the core of the glass fibre based laminate.

The total laminate strength however is also determined by the combination of fibre and resin (the matrix). Polyester has a maximum elongation of approx 1,5-2% before it breaks. The equivalent value for epoxy is approx 4-6%. This means that a polyester based laminate will be limited to a deformation of 1,5- 2% before breaking. If the laminate is based on epoxy it has the potential of making use of the full strength of the fibre.

The rumour of carbon fibre being fragile may be based more on an imagination of a thin eggshell layer of carbon than on real life experience. For practical reasons however, it is not always possible to use the full advantages of the additional strength of carbon fibre and thus saving weight because of the laminate strength needed for practical handling (e.g. docking, walking on deck etc) of the boat exceeds the dimensioning due to structural forces.

In addition to the above it is sometimes claimed it is more difficult to repair an epoxy carbon fibre laminate. Thin laminates in general are more difficult to repair whether based on polyester and glass fibre or epoxy and carbon fibre, however it is not more difficult to repair an epoxy and carbon fibre laminate.

Summing up - An epoxy carbon laminate is ideal for a light weight boat since it is considerably stronger than the equivalent polyester glass fibre laminate. This is also the reason why we have used an epoxy carbon laminate for the Seaon 96crb. Using carbon fibre for a racing monohull may well be worth the efforts since every 1/100 of a knot counts, however the weight gain in percent is considerably smaller for a monohull than for a multihull since the keel weight is a major part of the total weight. On a multihull every saved kilo has direct impact on the performance. As an example, saving 350 kilo on a multihull such as the Seaon reaching a total of 1400 kilo is a weight reduction of close to 25%. Saving 350 kilo on an e.g. 34 foot monohull performance cruiser/racer coming down to a total of 4000 kilo is only a saving short of 10%.

We strongly believe using an epoxy carbon fibre laminate for the Seaon 96crb has been very well worth the effort. Together with polyurethane painted instead of gel coated surfaces, we have decreased the need for power equivalent to approx. 13 square meters of sail area which also allows us to use a shorter mast. A light boat is more fun to sail in light winds and the reduced need for sail area and mast length will make the boat more comfortable to handle at increased wind power.