Testing Foils with Chainfalls and a Capsize

In the latest issue of Professional BoatBuilder (No. 156), you can turn to page 40 to read naval architect/engineer Doug Schickler’s story about Gunboat’s G4. The foiling catamaran’s original and complex design made it essential to conduct a significant amount of finite element analysis (FEA) prior to construction. Schickler explains how he and the rest of the project’s design and engineering team used many computer simulation models to calculate how the design would take on global loads.

While that story focuses mostly on the work that took place before the build, the testing of finished materials takes place too—though one was planned and one wasn’t.

Of this complex project, writes Schickler, G4’s two foiling daggerboards were the most challenging to design. To find the right design that could handle the foiling loads, builder Holland Composites built and tested five scaled-down models. Then, when the best design had been selected and the actual full-size foils were built, they needed to be tested again. Above you can watch a sped-up video of that test, which Holland performed in a purpose-built apparatus with a chainfall that simulated combined loads and sideforce of the G4’s foils under sail.

The second test was also high-speed. This time, however, it wasn’t the camera that was going fast but the boat:

While the video of the G4’s capsize on April 18 during the final day of racing Les Voiles de St-Barth has gone viral, and is certainly dramatic, it’s also worth watching the remaining 40 or so seconds. A jump to “a few hours later …” shows the boat upright and at the dock.

Though the capsize was a setback, it also provided helpful insights, writes Schickler. During the capsize, for instance, the daggerboards were subjected to much higher stresses in the area where they leave the hull, which helped to validate margins that the builders had built into the laminate. It showed also that the G4 could recover from a capsize without sinking and with its rig mostly intact: the boat was able to race again within one week (though some of its electrical systems were not yet back in full working order).

Schickler concludes: “While I can’t recommend this as a planned testing protocol, the capsize was a practical real-world test of what can happen to any boat. The G4 survived it well.”

Read “Flight Engineering” by Doug Schickler in Professional BoatBuilder No. 156, which is free for qualifying members of the marine industry. Subscribe here.

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