Building a Tartan or Legacy yacht hull
There are many ways to build a sail or motor yacht. And at the end of the day, they all float and perform to a certain level. However, just as both the Kia Rio and the Audi A8 Quattro have 4 tires and a steering wheel, their DNA is distinctly different and they deliver differing levels of performance and luxury. The same can be said of the differences in how boats are built and ultimately, how they perform. Tartan and Legacy owners demand a rare combination of performance and luxury, achieving this lofty goal starts with the very first building blocks, the materials and process used for laminating the hull.
In the late 50’s and early 60’s, reinforced fiberglass lamination revolutionized the pleasure boat industry. The primary materials used were chopped strand mat and heavy, coarsely woven fabrics. Chopped strand mat is the least expensive reinforcement material, it soaks up a lot of resin and the short fibers provide little strength. Woven fabrics are stronger, but still require a lot resin. The end result for these early hulls is that the bulk of the laminate was chopped strand mat (low strength/high weight) with a few layers of heavy woven reinforcements (higher strength but also heavy). Compounding the problem with these materials is that to achieve stiffness the laminate had to be made too thick. The simplest factor to determine strength to weight in a fiberglass hull is how much glass fiber there is in a laminate versus how much resin. The higher the glass fiber content the better. A hand laminated hull based on chopped strand mat and woven reinforcements typically contains 25% glass fiber and 75% polyester resin. Although there have been some small improvements in these materials, incredibly, many builders still build their hulls this way, why? Low cost with little concern for the impact on performance.
Chopped strand mat and coarsely woven fabrics were the norm in the early years of fiberglass boat building, these materials produce a heavy and flexible hull, incredibly many builders still use these antiquated materials and processes in their hulls, why? They are low cost and performance is not a guiding principle for these builders.
There has been a lot of advancement in fiberglass composite technology since 1960. Largely driven by the aerospace industry, fiberglass structures can be built that are stronger, lighter and more durable than their predecessors. Doesn’t it just make sense to apply these advances to yacht construction? It does if your customers demand great performance AND luxurious accommodations. Tartan and Legacy hulls are built using a matrix of uni-directional reinforcements carefully aligned for maximum benefit based on how a hull is loaded. These very high strength reinforcements comprise the inside and outside laminations over a light weight synthetic coring producing a composite that has the same high strength characteristics of an I-beam. Equally important, the laminate is infused under vacuum in one shot with the best marine resin available, modified epoxy/vinylester. Modified epoxy/vinylester resin has far superior adhesive qualities, elasticity (important for better laminate fatigue resistance caused by the cyclic loading of the hull) and chemical/water resistant properties than the more commonly used polyester resin used by most builders, it is also three time more expensive. But if your goal is to deliver on the promise of great performance coupled with luxury, it is the only resin that makes sense.
Uni-directional fibers make up the inside and outside skins over a synthetic coring to produce a very strong light and stiff I-beam structure
In engineering a great hull, materials are only half the story, the laminating process is of equal importance. Traditional hand laminating produces a laminate that is 75% polyester resin and only 25% glass fiber, polyester resin is heavy and by itself has little strength and is quite brittle. A Tartan or Legacy hull is laminated through vacuum infusion. The entire structural hull layers are carefully fit into the hull dry, with additional plies placed in areas of high stress; this includes the precise CNC cut synthetic coring. After the glass reinforcements are in place a series of resin feed lines and vacuum lines are set into the hull and a plastic vacuum bag film is fit over the entire hull. Once a full vacuum is achieved, the required amount of modified epoxy resin is catalyzed and the resin is pushed into the dry reinforcing materials by one atmosphere of pressure (14 lbs per square inch). Under vacuum the resin fills all of the spaces around fibers and the coring tightly gluing all of the composite materials together in one shot. Other laminating processes involve multiple laminations allowing the resin to cure between each layer; the secondary bond to cured resin is never as strong as the primary bond achieved when a laminate is cured in one shot. Under vacuum the composite is under great pressure, this results in reversing the resin to glass fiber ratio of hand lamination. Instead of 75% resin and 25% glass fiber, the infusion process produces a composite of 30% resin and 70% glass fiber. Tartan and Legacy hulls are simply stronger, lighter and more durable.
The table below is a comparison of a typical bottom laminate for a 45 foot boat. The solid hand laminate is more than twice the weight of the equivalent high tech infused composite found in a Tartan or Legacy and it is not as stiff as the infused composite. Impact testing has also proved that an infused composite with greater fiber content is also more resilient than a single skin hand lamination. Lighter, stiffer and more impact resistant, also means better performance and greater safety.
THE HULL PROCESS
Click to enlarge the process below. Use arrows to navigate/zoom.
We go to great lengths to deliver hulls, both power and sail that are lighter, stronger, stiffer and more durable, and they retain these characteristics longer than their predecessors. And because the modified epoxy resin used in a Tartan or Legacy shrinks less in the curing process, they also retain their good cosmetic exterior finish better. Boats built with polyester resin are prone to post molding distortion and print problems, when a hull is out in the sun, the surface temperature can be as high as 190 degrees. At this temperature, polyester resin goes through a secondary cure and the resin shrinks again. Modified epoxy resins have a much higher resistance to heat and thus avoid this post cure print problem. Walk around a boat yard, and check out older hulls that are painted or gel coated a dark color, chances are you’ll see an assortment of distorted hull surfaces that just aren’t befitting any yacht, shrinking polyester resin is the reason why these hulls have degraded. Your Tartan or Legacy will look good for years.
Power or sail, it just makes sense to use better materials and processes in building a hull. Lighter weight with greater strength is the only way to deliver the performance and comfort that you should demand in your yacht. Sailing yachts that handle better and sail faster or motor yachts that handle better and burn less fuel should be expected, but not all builders make the commitment to deliver on this promise. Expect more and get more from a Tartan or Legacy yacht.