By Eric Hall
As an engineer by training, it's my nature to approach an idea from an analytical point of view. My first question is always "What is the best way to achieve our goal?" When it came to finding the best way to build advanced carbon structures, aerospace research told us in 1992, and continues to confirm, that the best way is to use pre-preg materials cured at elevated temperatures and pressures in an autoclave. Our own testing confirmed that the autoclave is unbeatable for producing optimum composite structures. This is what led to the initial investment in the autoclave in 1992 and to our acquisition of long (46-meter) autoclaves in each of our factories today. Also in 1992, the America3 design team, working with Hall, did an extensive study comparing a number of different molding methods. We concluded that aluminum mandrel molding is the best way to create a tube in the autoclave.
Strong and Smooth
The tube we create on a mandrel is seamless and, due to aluminum mandrel expansion with heat, the fibers are stretched straighter than with any other method. Fiber straightness is basic to advanced composite laminate performance: the straighter the fibers, the stronger and stiffer the laminate. Mandrel molding, however, is not a simple process. To eliminate the risk of wrinkling on the non-molded surface (in this case, the outer surface of the mast) extreme care is required during the carbon lay-up process. In Hall's proprietary process, each layer of carbon is carefully compacted before and during the autoclave process to ensure wrinkle-free laminates even in thick-walled tubes.
Mandrel-molded, seamless masts have tooled, smooth inside surfaces. From an engineering standpoint, these surfaces provide highly accurate and predictable areas for mounting close-tolerance computer machined fittings and halyards slide smoothly inside the mast.
Lightweight and Aerodynamic
The seamless mast is globally more aerodynamic than masts made using other systems. The key element in rig/sail aerodynamics is the mast's ability to bend optimally for sail design shape - at minimum weight. Without the baggage of lap joints, glue, screws and filler required by masts with full-length seams, a Hall seamless mast is lighter for equal bending stiffness. Yachts with lighter masts heel less, maximizing sailplan aerodynamic efficiency.
And we are not restricted by our tooling when it comes to placing strengthening fibers in the laminate - fibers are applied only where needed. Seamless masts have external reinforcements (internal reinforcements would inhibit extraction of the mandrel and inhibit proper fit of internal fittings). Wherever there is a tang, spreader fitting, or hound box, there are external reinforcements (doublers). Do they add drag? Computer Fluid Dynamic (CFD) studies show that such doublers increase mast section drag (without sail) by only hundredths of a percent. When the same section is attached to a sail, the drag increase is virtually immeasurable. Compared to the very measurable benefit of the lighter masts that have them, these doublers are a non-issue.
Shape predictions in pre-preg laminates are extremely accurate; even with thick laminates, predictions are accurate to tenths of a millimeter. The surfaces are so smooth that we're able to apply a clear-coat finish to the surface with no weight-adding filler.
What You See Is What You Get
What you see through the remarkably accurate and aerodynamically smooth finish of a clear-coat mast tube is the fascinating array of the structural fibers themselves. In addition, seamless masts have an unbelievably good track record. The combination of autoclave curing at 6 bar pressure and profile homogeneity creates extremely tough mast tubes. We invite you to visit either of our factories and see our process. We guarantee that you'll agree - for tubular structures, nothing beats seamless.
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