Two Piece Female Moulds
Beauty, as the saying goes is more than skin deep. It is the same with masts. The internal structures of masts may be unseen, but they play a vital role in their integrity and performance.
Because Southern Spars pays as much attention to the interior of the mast as the exterior, we build our medium to large custom rigs in female, rather than male, moulds.
The Benefits of Female Moulding
• All structural reinforcing and local patching is added on the inside of the tube or within the basic laminate, leaving the outside surface perfectly smooth for maximum aerodynamic efficiency without significant external fairing.
• Allows the design and installation of individual internal and external components such as sheave boxes, spreader attachments, tangs, wiring, hydraulics and webs to be optimised.
• Components are more accurately placed and fitted, minimising local patch reinforcement sizing, eliminating potential chafe points and ensuring the lightest detailing and finish. A strategically placed web in the mast dramatically improves control over the high torsional and flexural loads demanded of this type of structure.
• Under debulking pressure and heat, carbon-fibre laminates are expanded into the female mould and fibres are aligned correctly, reducing the danger of fibre crimping and voids.
• Female moulding allows designers to optimise stiffness in the extremities of a section, leaving the outside surface of the mast completely smooth and aerodynamic, with less requirement for fairing which just adds weight.
• Increasingly, modern rigs are designed as wing shapes that have cored sidewalls. It is extremely difficult to construct a male-moulded wing mast with the same weight and stiffness properties as a female-moulded mast.
• Before the two halves of the mast are joined together we have visually inspected every aspect of the interior structure. This way, our customers can have absolute confidence it is exactly as it should be, both on the inside and outside. As part of our quality assurance, electronic records of the mast interior are made prior to joining.
• Female moulding allows tools to be planted in the exterior of the mast to create recesses. Fittings such as chafe guards can be planted flush in the recesses, maintaining the aerodynamics of the exterior.
It is more expensive to build the tooling for female moulding but, in every other respect, female moulding is recognized as the optimised method for creating precision, high-tech laminates to the most demanding specifications, ie components for Formula One race cars, fighter jets and aero-space applications. All serious contenders in the America’s Cup and Volvo Ocean Races sail with female-moulded, joined spar sections. Even our competitors use female moulds for racing masts.
Some argue that eliminating the overlapping join makes the mast stronger. This is only correct if the join in a two-piece mast is poorly executed. We have never seen a shear failure in the join of any of our masts. In fact, a flange join has the effect of stiffening the side wall against local buckling, as it provides a web/stringer up the side of the mast.
No additional weight is gained because the material in the flange lap is taken from the inefficient 'quarters’ of the section and strategically placed further from the neutral axis of the tube. The weight savings gained from having access to the inside of the mast to fit components most efficiently easily overcome the weight in fasteners and adhesive in the join.
An advantage of carbon fibre construction is that the structure can be very precisely engineered, with more material in the areas of high load and less material in areas of reduced load, which is best suited for larger spars [see diagram below].