PUTTING THE POWER DOWN – GUNBOAT 68 DAGGERBOARDS

February 28, 2018

Alongside production of 6801, so far in our series of articles we have looked at the hulls talking about exterior design and the strength of the internal structure. This time we’re looking at the all-important daggerboards – the means by which the power of the rig is transferred through the structure and into forward motion through the water.

With both performance and evolution being part of Gunboat’s DNA, a deep level of research went into design and engineering of daggerboards.

“When we started thinking about daggerboards, in addition to speed and comfortable motion, we set a few additional specifications. Aesthetically they needed to interfere as little as possible with the interior. When raised for shallow water access they needed to deliver a draft of 1.2m (4ft),” says Gunboat’s Benoit Lebizay. “Safety is a priority for us in every aspect of the boat, so the latter limitation is a safeguard to protect the saildrives, which have  a draft of 1.1m (3ft 7in). Further, if grounded or in a collision at speed, the daggerboard casings needed to be able to withstand that impact while the daggerboards themselves shear off without compromising the hull.”

It was a tall order that we set out to deliver without compromise – a foil package that ensures the fastest, most comfortable and above all, safest ride on the Gunboat 68!

FINDING THE RIGHT BOARD DESIGN

When we started off down this road in mid-2016, the world had already gone foil- crazy: curved boards or straight, L- or J-profile, tips or no tips, and the full range of development and designs. A lot was already known about the performance of the various options, and most of that knowledge is held by our design partners VPLP. We mined all of their extensive experience to come up with the solution for the Gunboat 68 that found the perfect balance of performance, stability, and ease of use.

“We ran a whole range of board concepts,” says Vincent Lauriot-Prévost, the ‘LP’ of VPLP. “While we remained focused on modeling side force and vertical lift, and we also factored in the practical parameters of the brief such as reliability, interference with the interior, the simplicity of maneuvers and the versatility of cruising and racing versions. All the candidates we tested came from either ORMA trimarans, foiling and non-foiling IMOCA monohulls and the past three America’s Cup multihull editions. Ultimately it was about finding the right formula for this program.”

All in all we did fourteen iterations of board VPPs, running them with different displacements, rig heights, symmetric and asymmetric boards – every variable there is! We also checked true wind hull fly speeds to stick to our safety targets: the performance-rigged Gunboat 68 shouldn’t fly a hull below 16 knots TWS, or 20kts for the cruising version. To be clear, our world-cruising and regatta-worthy Gunboat 68 is not a fully foiling, flying catamaran – we’re talking about the ability to push for performance and fly a hull, which she is structurally capable to do.

Without flying the platform, boards with foils or tips could not only generate lift, reducing wetted surface area and therefore drag, but could also help to improve comfort and stabilize the platform by preventing ‘heaving’ (sudden vertical movement when sailing through waves). We wanted to explore the options.

First we looked at curved boards. They create lift and have the added bonus of matching the curved profile of the outside hull, making the board cases less intrusive to the living space below. VPLP ran a VPP study on a variety of curved boards with and without tips. They all showed that curved boards bring the center of lift inboard, actually reducing righting moment, which will make a hull fly earlier. Safety is a cornerstone of the Gunboat philosophy though, so much so that we increased the beam of the Gunboat 68, and therefore the righting moment, to make her a safer boat. Further, the build complications of curved molds added more complexity for very little gain. We opted against curved boards.

Image: The results of the VPP study of curved boards sent us in a different direction.

“Then we looked at L-shaped boards with down tips to provide more lift,” says VPLP’s Xavier Guilbaud. “They were quite promising in terms of performance for downwind and reaching races like the Caribbean 600, but it was too radical for a round-the-cans regatta – and you end up with a draft of 2m (6ft 7in), a dealbreaker.”

It wasn’t just a draft issue. The VPP showed L-tips provided more lift, but at normal boat speeds below 20 knots, they didn’t generated enough lift to reduce wetted surface area and increased drag significantly. Above 20 knots they would progressively generate lift up to around 30-40 percent of displacement, but it seemed difficult to justify adding L-tips as a standard option. Regardless of draft, they didn’t suit the Gunboat 68’s normal operating parameters.

Curved and L-shaped boards were obviously not the right option for the Gunboat 68 although everyone liked the way they reduced heaving, which adds a lot of comfort to the ride.

“We were seeing the results we wanted once we suggested using straight boards but canting them inboard,” says Xavier. “Canted, they increased lift and reduced heaving. We increased the length by 850mm and saw significant performance gains across 90 percent of the wind angles. Upwind the asymmetric boards always performed better above 15 knots TWS, but only if your crew switched boards on every tack. This revealed another advantage of symmetric boards: you can leave them both down, ideal for cruising and sailing shorthanded.” – Xavier Guildbaud, Naval Architect, VPLP – Vannes

“It may surprise some people, but the choice which best fit the brief of this high-performance cruising catamaran was simple canted straight boards,” Xavier continues. “We engineered three configurations for the catalog that give owners the full range of options, without ever needing to make extreme structural modifications to their boat: short symmetrical boards for cruising, high-aspect ratio long asymmetrical boards for pure racing, and in the middle the long symmetrical boards which were selected on Gunboat 6801. They are the perfect compromise for the owner wanting to run both cruising and racing programs, and they all fit in the same trunk geometry, structure, and longitudinal location.”

Image: A diagram showing some of the board profiles that were part of the VPLP study.

“Gunboat has always been out front in the performance cruising market, plus the occasional owner going turbo, so we’re ready to push the boundaries,” adds Lauriot-Prévost. “In the future that may include concepts like straight boards with horizontal tips or heave-damping Z-foils for performance and comfort at sea, but for Gunboat 6801 none of those options fit the original brief for reasons like the inability to fit the boards from deck or satisfy the draft limit. VPLP’s toolbox is full of different concepts – it is all about where we put the cursor between practicality and performance.”

BUILDING THE BOARDS

All the VPP work had shown that the best option for the owner of Gunboat 6801 was long symmetrical straight boards. Working with VPLP, our engineering team decided the boards would be raised by a line on an integrated and Gunboat-styled gantry crane using electric winches in the forward cockpit. The boards needed to be built in pre-preg carbon and cured in an autoclave, to optimize strength, weight, and quality and produce consistent parts with perfect consolidation. We also wanted everything – daggerboard, casing, gantry and bearings – delivered as a single assembly meeting the required structural strength. This important component helps transfer serious loads between bulkheads, hull and deck – it’s not just there to keep the water out!

To build the boards, we sought a partner with a consistent track record of excellence in producing carbon components. We wanted a partner for the long run and searched wide only to circle back to Lorima in France. The Lorient-based specialist is perhaps best known for its super high- performance carbon masts but Lorima seriously impressed us with the quality of the appendages it makes for race boats, most notably for the Multi 50 class.

“We’re very pleased to be working with Gunboat because we know their ambition matches our own,” says Lorima president, Vincent Marsaudon. “We also know VPLP well. Before 2009 about 80 percent of Lorima’s work was on racing projects, and that’s how our relationship with them evolved. We’ve done Ultim masts, booms and foils for them. Then when foiling took off, we were chosen as a trusted partner to build foils for the Multi 50 class – with our ultrasound inspection and load testing to ensure they met specifications.”

Photo below: Loadtesting the Gunboat 68 board at Lorima

FROM THE BUILD TEAM

“Because the case and bearings come as one finished piece, the bearings are all perfectly aligned and it makes the job of installation very accurate,” says Gwenael Bardou, Composites and Assembly Manager at Gunboat. “We get a precisely   molded unit, with the components already structurally integrated, reducing how much reinforcement is needed in the boat, which saves weight.”

Above: Port daggerboard trunk, trunk installed in the Gunboat 68, and the corresponding 3D model. Scott Gray pf Rigging Projects on board Gunboat 6801 Condor.
Below: Gunboat 68 boards loadtesting at Lorima. Daggerboard installation on Gunboat 6802.

And how strong is it? The lower bearing in the daggerboard casing (and therefore the daggerboard itself) can handle loads of up to 20 tons. To put that another way, you could hang six Lincoln Navigators or eight Range Rovers from the daggerboard.

In this exciting era for foil development (and for fast sailing in general), we thoroughly enjoyed the no-stone-unturned approach to finding the right boards for the Gunboat 68, and her rock-solid performance on the water!

Article #3 in our series offering insight on the design and build of the Gunboat 68; Originally published Feb 2018 chronicling production stages of 6801, updated with additional build photos from the series in 2020.

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PUTTING THE POWER DOWN – GUNBOAT 68 DAGGERBOARDS
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