This article was contributed by Meade Gougeon, Gougeon Brothers, Inc.
Choosing a Laminate Construction
When we decided to re-open our wind blade manufacturing plant in 1990, which had been idle for three years due to the wind industry's dramatic 1986 crash, we accepted that the wind industry was still a long way from regaining its former health. We needed to share plant overhead with a second product until the wind business returned to former levels. In addition to wind blades, we began manufacturing a unique, trailerable 32' long catamaran called the "G-32."
My brother, Jan, and I had long thought there would be a large market for a lightweight, trailerable catamaran that was fast, fun to sail, had weekend accommodations for 2 or 3 and was priced below $35,000. We had no intention of achieving large production; if we could arrange to build two per week we could accomplish our financial goals, three per week would be profitable.
The G-32 was designed so that the entire structure could be built in two large molds, with an upper and lower molding that were to be joined at the centerline in one large bonding operation. This allowed a minimal investment in tooling and molds, making low-volume production financially feasible.
The single biggest design challenge was to build the G-32 light enough to be towed by a mid-sized car. To achieve this, the combined weight of the boat and trailer could not exceed 2,200 lbs. Our approach to this problem focused on two areas; first, we designed out of the boat as much unnecessary weight as possible. We kept surface area to a minimum and carefully designed the structure to accommodate anticipated loads. For instance, the cabin served as a large torque box to give necessary torsion resistance between the two hulls. Second, we used the lightest weight construction technology we could afford within our cost goals. We could spend no more than $10 per lb. for molded shells, and these could not weigh more than 800 lbs. total.
We have had many years of experience building high-tech, "one-off", racing sailboats where exotic fibers, expensive core materials and epoxy resins were used to create very light but expensive boats. The molded parts (hulls, decks, bulkheads, etc.) of these custom boats could easily cost $50 to $100 per lb.
Typically these high-tech boats were laminated over male molds requiring multiple vacuum bag applications to achieve a lightweight compacted laminate. Three separate vacuum bag applications were a minimum, with the two skins and the separating core all requiring separate compaction to assure a quality laminate. Boats using pre-preg materials, such as the recent America's Cup yacht, might require 6 to 10 vacuum bag applications to de-bulk each layer, and then a final compaction of all layers before an oven-controlled cure can take place.
The making and application of vacuum bags is labor intensive, and this has restricted their use in production boat building where female molds are always used. In the past, attempts to vacuum bag cores into hand-laid outer skins in female molds has caused excessive print-through, further reducing interest among builders.
The importance of vacuum bagging a complex laminate with core materials cannot be overestimated. A vacuum-bag compacted laminate will easily save 30 to 50 percent of laminate weight over the same materials laid up by hand with only the viscosity of the resins and core bond adhesives to hold the laminate together until it cures.
With the goal of achieving a lightweight laminate at a low cost for the G-32, we chose a manufacturing method that had not been tried before. It featured the following:
1. Epoxy resin technology to achieve maximum physical properties, together with designing in a process time of up to four hours to allow complete laminate assembly in large complex molds before a single vacuum bag was applied to compact the entire laminate at once.
2. Existing polyester-based gelcoat technology was used in female molding, to achieve low-cost finished surfaces with a minimum of print-through. (We solved the significant problem of bonding the polyester gelcoat surface and the epoxy laminate by using a specially-developed tie-coat substance. This was applied between the two dissimilar materials during the manufacturing process.)
3. All fibrous reinforcing materials were to be wet out mechanically with a roll-coater machine. This reduced labor and ensured precise resin control within the fiber matrix. This "wet preg" material could then be efficiently inserted in the mold lay-up, saving a great deal of labor over wetting-out by hand in the mold.
4. The specially-designed long open time epoxy resin could be post-cured at temperatures not exceeding 140°F to achieve high physical properties.
Choosing a Laminate Composition
Early in the G-32's design phase, we began carefully evaluating materials to make the best panel available when considering cost, panel performance, labor to assemble and finished outer surface quality.
The materials listed in Figure 1 were assessed in a matrix to establish 30 combinations to be initially evaluated.
Evaluation of reinforcing fiber materials focused on stitched "E" glass of various styles and weights. Several other common woven "E" glass products were also evaluated, but this only confirmed the superior performance of stitched materials as well worth a small increase in price. These stitched materials were mainly supplied by three companies, Brunswick Technology, Inc., Knytex Division of Hexcel Corp., and Advanced Textiles, Inc.
Core materials are the most expensive component of most marine laminates, thus we made a special effort to evaluate all options. Four different types of core materials in several thicknesses were evaluated, with a concentration on the low-density foams and end grain balsa materials with which we'd already had considerable experience. The core materials suppliers were Baltec Corp., Nida Core Corp., and Termanto, Inc. (Klegecell foam).
The PRO-SET® epoxy resin system to be employed was a product developed in our own labs expressly for use with this new manufacturing process. We set the following goals for this product:
1. Long open-time of 4 hours at 75°F.
2. Post-curable at temperatures not to exceed 140°F to achieve high physical properties.
3. Low sensitization potential and low fume levels for the safety and well being of workers.
4. Resin viscosity and wet out appropriate for use in a high speed roll-coater machine.
5. Mixed resin and hardener cost that did not exceed $3.50 per pound.
Thirty panels were constructed with weights ranging from 0.76 lb. per sq. ft. to 1.2 lbs. per sq. ft. The largest variable in panel weight was resin usage levels necessary to properly wet out the various weights and types of woven and stitched reinforcement fabrics. Core materials also had a varying effect on resin usage, relating to the quality of the surface finish to which they were to be bonded. Generally, even with vacuum pressure, resin and gelcoat usage would about equal the combined dry weight of the fabric and core to be used. We quickly found that a smooth-surfaced core material in combination with a tightly stitched reinforcement fabric could easily save 20 percent in resin weight over lesser quality materials of the same type.
Prices for material ingredients for finished panels ranged from $1.42 to $3.40 per sq. ft. Core materials dominated material cost, thus, choosing an effective core was of paramount importance in the selection process.
After careful evaluation of material costs, panel weights and handling issues, a final matrix of 12 material combinations were chosen to produce panels for testing on our Hydromat test stand, to evaluate panel stiffness and ultimate strength. The Hydromat test evaluation procedure was in its beginning stages in 1990, but even then was sophisticated enough to provide sufficient comparative data to warrant good decision making (see Figure 2).
Initial testing showed that stiff panels were not necessarily stronger than less-stiff panels, but generally cost more because of increased core mass. A key question to the G-32 design effort was how much stiffness was necessary for the boat to perform properly in the most highly strained operating conditions.
Further design changes were made to reduce flat areas requiring additional stiffness, so that a less stiff, lower cost panel might be used. Adequate panel strength requirements were more difficult to determine, as actual loads for large catamarans are hard to measure and are highly cyclic, raising the issue of fatigue life.
Of the 12 panels tested for ultimate strength and stiffness, we chose the four shown in Figure 3 for fatigue life evaluation, again using the new Hydromat test system to perform classical fatigue evaluation.
We determined that long-term panel resistance to sustained cyclic fatigue loads of 5 psi and intermittent slamming loads approaching 15 psi were necessary for long term survival of the G-32 structure. This criterion was not established in any scientific way, such as actual measurement with strain gauges (which is difficult to do). It instead reflected our experience with the performance of several material combinations used to build numerous craft with various success and failure over the years. This is the typical, historical, trial-and-error approach to boat design and engineering, which has not served the industry well since the recent revolutionary development of composite materials. New testing methods, such as the Hydromat, are an attempt to better evaluate composite materials performance in a way that closely simulates actual load conditions, so that more informed decisions can be made.
We began initial fatigue testing by subjecting the chosen panels to short cycle progressive fatigue loading as depicted in Figure 4. This test initially subjects the panels to 1,000 lbs. of loading for 50 fatigue cycles under R=.1, which is then increased 50 lbs. every 50 cycles until failure. This starting load of 1,000 lbs. translates into 15 psi on the panel being tested. In the case of panel #48, as depicted in Figure 4, failure occurred at 3,154 lbs. at 2,225 cycles. Most importantly, our 15 psi intermittent slam load criterion was far exceeded with the 3,154 lbs. maximum load, which translates into 41 psi. With this confidence, we chose
panel #48 for evaluation in high-cycle classical fatigue. We decided to run this test at a constant 1,500 lbs., or 21 psi, until failure occurred at R=.1. We had hoped to achieve 100,000 cycles at this loading, and achieved approximately 130,000 cycles before a progressive failure process began. This confirmed that our 5 psi long-term durability and our 15 psi intermittent load requirements could be met easily by panel #48.
As can be seen by the data, panel #48 was not the strongest panel evaluated in any of the structural areas of stiffness, ultimate strength, or fatigue. It was, however, more than adequate to support the structural needs of the G-32 boat project. Of more importance, panel #48 scored well in three other areas; cost, weight, and ease of manufacturing.
Although we'd hand-built a custom G-32 prototype in late 1990 to prove the design concept, we did not begin production until October 1991. With the use of the ingredients of panel #48, we were able to build the largest boat part, the upper half molding, at about 370 lbs. The lower molding, with slightly less surface area, weighed in at 335 lbs. The surface area of this structure was 645 sq. ft., and we met our goal of building it under 800 lbs. At .92 lb. per sq. ft., panel #48 ingredients could have provided a weight for both moldings of only 593 lbs. with 645 sq. ft. of surface area. Of the actual 705 lbs., the excess 110 lbs. was mainly accounted for by the addition of various interior structural members, such as the main bulkhead, and also the lack of perfect control over the finish gelcoat application.
One of the most difficult parts of the project was achieving the high-gloss finish consumers have come to expect in their cars and boats. We tried many alternatives to polyester-based gelcoat technology. The best, a high-solids activated urethane, was inconsistent. Thus we were forced to adopt the well-proven gelcoat products that have been the industry standard for many years. For us, the dilemma was that gelcoats are heavy and are typically applied at a 20 to 40 mil thickness to achieve best results. With our panel studies, we were able to get good results at 15 mils, but print-through became unacceptable at thinner applications.
We considered molding without gelcoat, then spray painting after demolding, but the added expense was not in line with our original cost goal. The solution was to have a highly trained individual doing the best gelcoat application possible.
We did not quite achieve our cost goal of $10 per lb. of finished laminate. In late 1992, before we'd achieved significant production of the G-32, we decided to sell our wind energy division. This eliminated the main reason for the G-32 boat project, and it shut down shortly after. At that stage, only 14 G-32's had been built, thus the "learning curve" for this type of project had not progressed very far. We are confident that we would have made our goal with further production, as others have since proven it possible.
The epoxy-based boat construction technology developed with the G-32 is now widely used in the construction of high-performance boats. Our PRO-SET® epoxy resin systems are used to build large racing sailboats and power boats in female molds. This development is revolutionary in the marine industry because it has dramatically reduced the price of manufacturing epoxy-based boats, compared to the cost of building custom boats on male molds. For the builder, there are other sound reasons to adopt these epoxy-based manufacturing methods. The styrene emission issue, which has long been a big problem for the industry, is solved. Using these methods and materials, builders can also achieve complete control over open time and cure rates. The use of roller-coaters to mechanically wet out reinforcement fabric has significantly reduced labor costs and resin waste.
Because of these advantages, in the future 5 to 10 percent of boats may be built with epoxy-based technology. Meanwhile, the 14 G-32's that were built have performed as expected, with no structural failures of any kind. A bonus is that their light weight, combined with long slim hulls, has made for exceedingly fast boats that have won many races.
(This file last updated April 1, 1996)
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