INNER STRENGTH – GUNBOAT 68 INTERNAL STRUCTURE

January 30, 2018

Leading up to the launch of the first Gunboat 68, with construction in full swing in La Grande Motte, France, we continued our series of articles on different aspects of design and construction following the progress of 6801. Building from the Exterior Design Brief, let’s look at the strong internal structure engineered to support that head turning design.

From the beginning of the brand, Gunboat’s unique trait was applying Grand Prix race boat building and sailing technology to what is at its core, a highly-capable, offshore cruising boat. As technologies have advanced over the years, so have the methodologies employed by Gunboat. Tri-radial sails gave way to 3DL, then on to 3Di. Nitronic steel rigging has given way to Kevlar then on to Carbon and ECsix. Similarly, the build methodology of the first Gunboat (E-glass with vacuum-bagged/wet-layup over Corecell) was years ahead of its time, but is now more common.

From the start of the Gunboat 68 project, we continued to innovate and push the boundaries of modern production boatbuilding. With that said, we also had to balance the market’s price sensitivity and the realities of the huge number of hours it takes to build a complex, 18T/40,000 lb machine! So, from a blank slate, we meticulously evaluated the full range of options – from build methodology (male plug vs female tooling) to material choices (infused carbon and foam core vs all pre-preg carbon/Nomex), to how much to outsource (all built in-house vs 100% outsourced).

If you take a cursory glance at the material choices, pre-preg offers about a 10-15% weight saving on a composite structure. While it’s easy to get excited about that at face value, the devil is always in the details. On a typical Gunboat, the composite structure only makes up about 30% of the total displacement. A percentage of a percentage gets pretty small pretty quickly, to the point that building in full pre-preg carbon did not justify the additional costs and complexity involved.

The best option for the Gunboat 68 was to build the majority of the boat in-house, leveraging our expertise in infusion for the larger composite elements, then integrate advanced components commissioned from industry-leading partners. We outsourced flat panel parts of the structure that could be built in pre-preg on a vacuum table and easily shipped – reducing our risk and the need for in-house, high-temperature molds for prepreg carbon fiber – and gained the benefits of flat panel pre-preg structural elements.

COLLABORATIVE DESIGN

The interior structure had to achieve many things to support both the high-performance loading and the luxury expected of the Gunboat 68. Fortunately, VPLP Design (Marc Van Peteghem and Vincent Lauriot-Prévost) were highly qualified to deliver both. Between their two divisions in Paris and Vannes, they have been responsible for designing some of the world’s most innovative, record-breaking race boats, as well as an incredible range of cruising catamarans. Their team in Paris was responsible for the weight study and layout, and built in all the systems and luxury features a Gunboat must have. After identifying a realistic weight based on cruising expectations, the performance experts in Vannes designed the optimal lines, structure and sail power. And throughout, the creative loop was further refined by the engineering and build expertise of Gunboat’s in-house team.

TECHNICAL ADVANTAGES

Once the key structural elements were designed, the rest of the internal structure could be optimized accordingly. We weighed the option of outsourcing these advanced components to an industry partner. A breakthrough came when FIBRE Mechanics presented us with the opportunity to employ a new highly automated and low waste methodology to build flat panels using prepreg carbon. The methodology uses robotically controlled fibre cutting and laser-projection for accurate placement of each piece, at a quality that is just exceptional!

Now we had a repeatable and semi-automated process can handle very fine laminate details. This opened up opportunities for our engineers, as decisions on the internal structure (and ultimately performance gains) could be further optimized.

“In every way, the Gunboat 68 project is a step ahead. So when Gunboat approached us to build the interior structure, we knew they were expecting something out of the ordinary.

We have spent years talking about combining the carbon pre-preg materials used on racing yachts with aerospace and automotive production processes – the Gunboat Team asked us to stop talking and to get on with it.

So with the help of our partners, we have made full use of CNC cutters, laser projection and autoclave cure ovens, to build the most advanced primary structure yet seen on a series produced yacht.”

– Geoff Stock, Managing Director, FIBRE Mechanics

THE ORDER OF OPERATIONS

Key structural elements were designed first with the rest of the structure optimized to suit, resulting in some distinct advantages.

Using this highly accurate prepreg placement opened doors (literally and figuratively!) to the naval architecture of the Gunboat 68. Xavier Guilbaud from VPLP remarked, “It’s thanks to the all-encompassing structural approach that the areas under high loads, specifically in open spaces (sunroof, windows, and doorways), could be evaluated and subsequently engineered to a more favorable outcome.”

This also allowed higher structural tolerances of the forward cockpit design than on prior models, giving VPLP the opportunity to move the mast further aft. “The mast is positioned further aft than on the previous Gunboat models to increase the aspect ratio of the mainsail. It also allows easier fine tuning on the mainsail  trim with smaller loads to handle. Adding rake to the rig gives the main a near-vertical leech for a better-looking and more efficient main when reefed. All this makes a more efficient sail plan, better helm balance in any sail configuration, and easier to handle and trim,” Guilbaud explained.

Big doors and windows, great sightlines and water drainage, with increased structural support – these are just some of many examples where the design loop allowed us to make refinements that ultimately maximize performance, quality, and weight savings.

ACHIEVING MORE STRENGTH WITH LESS WEIGHT

Using advanced Finite Element Analysis (FEA) software, VPLP then optimized the engineering of each bulkhead to manage the loads it would bear. Normally this would be a nightmare for production, with everything having to be custom built, but with the fiber cut robotically, we have a superior product at a fraction of the man-hours.

Gunboat’s COO William Jelbert sums up the advantages, “The guys at FIBRE Mechanics brought us a repeatable process with clear-cut quality control. In addition to weight saving, pre-preg carbon gives you an enhanced level of quality and improved tolerances. Outsourcing our interior structure to a supplier capable of handling minute details and very light materials allowed us to implement these components easily into our production line. No two bulkheads are the same in this boat, every bulkhead has been optimized for the load it will see. On top of that, we can control cost and labor hours to shrink the build time.”

FROM THE BUILD TEAM

The Gunboat 68 project requires ‘out-of-the-box’ thinking on all fronts. Moving production to France entailed the construction of a new factory and the recruitment of a team, as well as the build of a new boat. From the outset, it was also an opportunity to explore all avenues and manage this ramp up as smoothly as possible. For instance, with accurate 3D modelling we can create jigs to ease installation. We’re constantly finding methods for more precision and less of the tape measure!

When FIBRE Mechanics’ components arrive at the Gunboat facility in France, our team uses wooden templates and reference lines scribed into the molds to accurately locate and test fit the bulkhead shapes in the boat, before trimming the edges of the final product to shape. The bulkheads are then bonded to the strict tolerance rules of the structural glue joints and vacuum pressure is applied to the carbon taping.

For the internal structure, we worked closely with VPLP and the exterior and interior designers to find the right compromise between aperture sizes and structural weight, befitting for Gunboat’s essential performance DNA, thoroughbred “racing bones”. Then the technologies employed by FIBRE Mechanics further optimized internal structure to control weight, support performance gains, build efficiency and control cost.

For boatbuilders, and hopefully those who took the time to read this article, these bare bones photos showing what lies beneath the finished details hold a glamour of their own.

Article #2 in our series offering insight on the design and build of the Gunboat 68; Originally published Jan 2018 chronicling the production stages of 6801, updated with additional build photos from the series in 2020.

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