Weighing the Options
As mentioned previously, the choice of tooling material and shape depends on the construction process of the parts to be moulded.
To decide on construction method we look at the desired properties of the finished product.
The hull can be thought of as a box girder that has to resist global bending loads and other localised forces at specific points such as stay attachments, beam junctions, foil housings and where the crew stands.
In a box girder the outer edges take tension and compression and the connecting faces work mainly in shear, preventing the load bearing edges from moving relative to each-other.
This is an efficient arrangement because the corners are furthest away from the neutral axis so can be thought of as having the best leverage.
The curvature of the edges also makes them less prone to local buckling.
The members that make up the long edges of the truss are substantial but the diagonal members are comparatively dainty.
To build on the analogy, an A Cat hull relies on additional unidirectional fibres running along the turns of the bilge and the gunnels to take global bending loads efficiently.
The panels between the four outer edges will have fibres running diagonally between the edges in a pattern similar to the diagonal elements of a truss.
Truss boom on an IACC yacht. To resist global bending loads, the long outer edges take tension and compression.
The connecting panels use diagonally aligned fibres to prevent relative movement of the edges.
Image credit unknown.
Where forces are applied at a mechanical connection point such as a stay attachment or beam junction, the load path can be resolved locally with additional reinforcement and possibly a bulkhead or ring frame.
Where the load is hydrostatic or hydrodynamic, panel stiffness needs to be considered more globally.
In both cases, if the panels are inherently stiff, then less additional support is required for a given deformation.
Panel stiffness is therefore important to global stiffness as well as to maintaining the local design shape.
Thickening a panel increases its stiffness.
For reasons similar to those governing material distribution in a truss, the material furthest away from the neutral plane of the panel works most efficiently.
This is why a comparatively weak material such as foam or a low density material such as honeycomb can be used in the middle of the panel in conjunction with strong/stiff materials such as carbon fibre for the skins.
So stiff is good and thick is stiff.
Thickness is best achieved using sandwich construction.
This brings us to our first major decision: what core material to use in the sandwich.
The two candidates are foam and honeycomb…