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Sunday, May 6, 2012

Ways to Skin… a Hull

Previous posts on A Class Catamaran material choices hinted at the influences of core type on construction process.
Let’s look at the options in construction method and the unique requirements of each.

Foam Core
Hull panel lamination can take place in one or two steps, depending on whether a perforated core is used. 
A perforated core will involve a weight penalty because the holes used to allow entrapped air to evacuate will end up filled with resin. 
Assuming the resin content of the laminate on the mould side of a non perforated core is carefully controlled, a one step process will give a consistently more efficient panel. 
When considering labour cost, as well as the number of steps involved, the brittleness of foam must be taken into account. If plain foam flat sheet is used, it needs to be formed into the mould prior to lamination. This requires care and is usually done by gingerly heating the core material. 
It is possible to buy structural foam that comes ‘scored’ with cuts that allow it to conform to curved moulds. However the voids left by the cuts (that must splay open to allow the foam to deform) are also likely to trap resin, adding weight to the finished panel.

Honeycomb Core
As discussed in previous posts, our aim in the A Class is to maximize rigidity for the mandated minimum weight. 
We want to create a thick panel with as much fibre in the skins and as little resin content as possible. 
The lower density of honeycomb is more suited to our goal.
Since the bond between honeycomb core and skins relies on the thin edges of each cell being captured in just the right amount of resin, a very controlled process is called for.

Resin
The manageable dimensions, thin skins and simple shape of an A Cat hull are such that similar results can be obtained with prepreg and wet layup techniques. 
The challenge with wet layup is managing resin content with respect to de-bulking, evacuation of entrapped air, core bonding, and drainage from vertical surfaces into areas of the mould prone to pooling. 
More resin is safer in terms of interlaminar and core bonding, but it increases the risk of air entrapment, pooling, and filling of the honeycomb cells.
Resin has to be applied evenly and consistently, balancing the conflicting requirements and taking into account the effect on final laminate resin content of bleed-off into the vacuum stack.

On the other hand, prepreg, though guaranteeing a known and consistent resin content, involves additional steps in de-bulking and the application of glue film layers.

The contractors we are considering for production have facilities available for either prepreg or ‘wetpreg’, where 'room temperature' resin is applied in a controlled fashion before placing the fibres in the mould. 
We have decided to produce moulds capable of handling the temperature and pressure necessary to cure prepreg laminates in order to have the option of prepreg construction. 
Our plan is to experiment with both methods before committing to either. 
The final decision will be influenced by availability and delivery costs (prepregs need to be moved in an uninterrupted 'cold chain'), and how they relate to any performance differences evident in the two methods. 
Stay tuned for our findings!

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