Sailing A New Course in Machine Technology

Many months ago, as yachting teams began to prepare for the prestigious America's Cup competitions, an unusual alliance of sailors and engineers began sharing carefully guarded design secrets with trusted manufacturing partners such as Southern Spars Group.

Southern produces booms and masts, often referred to as spars, for some of the world's fastest yachts, including several syndicates that vied to challenge Team New Zealand for possession of the coveted America's Cup this month.

Based on the collective talents of this mast maker, a system integrator, and a machine builder, a new, specially designed autoclave emerged to build masts that would stand up to the performance requirements and stresses of racing at the highest level of competition.

Carbon composite spars are stronger and lighter than their alloy predecessors. In yacht racing, the stronger and lighter the spar, the faster and more competitive the yacht. Wooden or metal masts would work, but they would be much heavier and larger in section. The new composite masts are lighter and stiffer and so, pound for pound, stronger.

In the case of the America's Cup Class, there are rules that limit the weight of the mast, but in general terms the stiffest, smallest, and lightest would be the best.

The length of the spars, which can measure more than 150 ft., is only part of the production challenge. Masts have tapered shapes and are manufactured from two long shells joined to make one piece. To maximize strength, the manufacturer must ensure absolute uniformity from the wider and thicker base to the narrower and thinner tip.

Strength is important. There is a compression force of about 120,000 lbs. at the bottom of an America's Cup mast (Figure 1). "When we go to schools and talk to students about it, we compare it to about 25 Chevy Suburbans sitting on the top of the mast," said Scott Vogel, a designer/engineer for Southern Spars.

Custom-designed autoclaves are critical to the manufacturing of the masts, and the Southern Spars Group was the first to have extra long autoclaves built for its use to improve uniformity in carbon fiber spars.

Gilbert Engineering, a general engineering/machining business that specializes in pressure vessel handling equipment, in Auckland, New Zealand, partnered with Southern Spars to build a custom autoclave.

The autoclave is 160 ft. long and 5 ft. in diameter (Figure 2). It is internally lagged with a mechanical load in/load out system that is electronically heated.

Although Gilbert Engineering usually builds turnkey systems, this autoclave was specifically built in New Zealand for Southern Spars. "The autoclave we designed was a three-stage system that required varied interior temperatures over an extended period of time," says Ron Gilbert, managing director of Gilbert Engineering. "Because the shape of the mast is not uniform, one section might require a higher heat than another."

Racing strategy begins with equipment design, so the ability to control the composite material cure process gives a competitive edge that Southern Spars can pass on to the racing teams.

It takes both extensive knowledge of composites and a high skill level to produce quality carbon fiber spars. Carbon fiber spars are manufactured from multiple layers of carbon fiber and resin placed in a mold. The orientation of these layers combined with the type of fibers and resin system used produces rigs of varying shapes, stiffness, and strengths.

"The curing process can be difficult to control," explains Vogel, "in part because of the variety of resin chemistries used and the substantial variations in thickness required to generate optimum performance in any given spar."

Quality relies heavily on the fabricator's ability to prevent air pockets from forming in the laminate and to remove any pockets that do form before the resin is cured. This prevents weak spots. Two processes are used to remove these air pockets: vacuum bag molding and autoclave molding. Having previously considered competing technologies, Southern Spars this time saw an opportunity to improve quality by applying both methods.

In vacuum bag molding, the laminate is covered with a thin polymer release film, a porous bleeder fabric, and a polymer membrane bag. The bag is sealed to the mold around the edges of the part, and a vacuum is applied between the bag and the lay-up.

The vacuum can greatly reduce trapped gas, but does not necessarily eliminate it all. So, at Southern, operators use autoclaves to further process the vacuumed-bagged spars.

With the heat and pressure capabilities of a tightly controlled autoclave, the company applies its expertise in composites to create an environment capable of removing any remaining gas pockets while curing the resin. As the autoclave heats, the resin alters to a liquid state, allowing any remaining voids to be eliminated by pressure condensation and/or molecular diffusion into the liquid resin. This produces a tightly compacted, void-free laminate.