Monday, 20 September 2010

Tech talk: PJ on DNA materials and DNA construction





When we started the project we had a clear vision on the quality of build:
We wanted to produce topboats, just under minimum weight, which would keep their stifness and strength for many years even after years of racing in tough conditions.

The Marstrom Tornado’s were our example of ‘best-quality-for-money' 'best-buy’ racing boats.
Compared to other Olympic classes that boat was (and still is) an unique product.
We all know the stories of top teams sailing more than one Olympics on the same boat.

We asked our self the following questions:
- What would it take to achieve such quality standard in the A class?
- How to build a strong and durable boat, even when used in though conditions?
- How to build a boat that transforms the image of  nice-but-fragile boats into an image of great plug-and-play lively racing machines?

75 kgs is the mininimum all up sailing weight. Aftter substracting the weight for the rig, appendage, fittings and trampoline, we are left with the ideal weight for a bare but painted platform of 40-42 Kgs.

- How to construct a platform on target, which is as stiff, as strong and as durable as possible?

Basically the stiffer the boat, the faster (less loss of energy).
Extreme stiffnes has usually one downnside , that’s durability. When you make the construction a bit more forgiving , then certain flex can reduce peak loads, which increases durability. It's a tricky engineering area, because we did not have an AC budget to do the necessary strain gauge testing .
Fortunately we have plenty of experience in sailing A class boats of all different constructions. So the design decisions were made intuitively but based on a thorough empirical base. We felt that flexing boats were not durable at all, and that stiff boats were more durable as long the materials were in the right spot .
- What are the most flexing areas in an A class platform?
Three main areas: flexing in hulls, flexing in the beams  (both torsion and bending) and panel deflection of the hull shell itself .
There is an optimal balance between the beam stiffness and the hull stifness especially at the conjunction of these parts.
In the DNA design we tried to achieve the most integrated construction possible. We chose not to use‘beampockets‘ as most builders normally do, but designed special shaped beams, which exactly fitted into the hull, which made it possible to bond and laminate the beams directly to the hull skins and bulkheads, which saved weight and increased overall stiffness.
For building the hull shells we decided to use the best practice building method : 100 C curing carbon pre-preg combined with a 10 mm Nomex Core.
Nomex honeycomb has high panel stiffness and good processing capabilities.
A 10 mm nomex core weighs 480 grs / m2 . For comparison a common used 80 kgs/ m3 pvc Core of 6 mm weighs the same , but the higher thickness of the honeycomb gives a quadratic increase of stiffness,which means 2,7 X higher panel stiffness if  the same skins are used. You can't beat that .
Another advantage over foam is the higher sheer stiffness of the core.
Everything stiffer leads to less deflection, less fatigue and increased longevity.
Choosing nomex honey comb  as core is a choice for extra process control as well :
It is of paramount importance that the fine honeycomb cell walls are bonded 100 % to the skins. The resin in the prepreg skins should form small fillets to all cell walls during curing. This sounds simple, but you need the exact right resin type and content, the right resin viscosity, the right curing schedules and  the right vacuum pressure. Besides perfect core bonding, we were looking for an excellent, pinhole free outside surface quality
and a 100 % porosity free inner skin. Finally we tried to create an impact resistant outer skin, to prevent the pinching of the outerskin by small stones and shells.

All this requirements may sound logical and straightforward, but getting the exact right materials is normally quite a challenge. By closely co-operating with Holland Composites' long time prepreg supplier we werre able to develop a custom formulated pre-preg.
Pretty unique, because most pre-preg companies are not interested in developing a custom product for such a small application area as building multihulls.
Our prepreg has exellent bonding and flow characterics, with  results even Holland Composites did not see  in 20 years of advanced composites building.

The outer skin is 300 grs sqm 3K standard modulus twill weave cloth pre-preg , the inner skins is a 200grs sqm twill weave pre-preg which results in a good impact vs overall stiffness performance .
On certain spots the outer skin is reinforced with UD tapes of different weights and some lightweight surface layer of 48 grs sqm E glass weave .
On high stress areas the skin thickness is increased up to 6 times the standard skin eg. areas around  the beams , trapezing area, shroud attachment points, rudders and daggerboard bearing areas.
The hulls are oven cured at 1 bar vacuum pressure and a staged curing cycle leving the parts on ‘flow temperature’ for some hours before kicking it to 100 C for the final cure .

A special designed and CNC cutted assembly frame is used for the assembly of the hull shells, beams, bulkheads, subdecks, deck frames and daggerboard cases into the DNA platform
After the bonding of the parts, the platform is postcured at 50 C for 24 hrs to make sure the secondary bondings are 100% too.

Next article on this Blog  by PJ will dig deeper into the construction method, and design of the curved daggerboards, rudder blades and  rudderstocks.

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