June 16, 2018
When awaiting the completion of a new Gunboat, it’s natural to obsess about sailing her – the thrill of helming a performance cat on an evening sail, a regatta, or a voyage around the world. You imagine the balance and responsiveness through your fingers, pressing against light windward helm, in full command and collecting some ocean miles. Continuing to share insight into the evolutions in design and engineering seen in the latest Gunboat model, in this article about the steering system we can truly begin to think about the much-anticipated sailing experience!
For the Gunboat 68, we set out to design a new steering system that, out of the factory, would deliver the well-balanced feel and robust construction desired for the range of use of a performance catamaran in both racing and cruising settings.
“We reached out to the existing Gunboat fleet to discuss how best to evolve the steering system,” says Benoit Lebizay, Gunboat Managing Partner. “Historically most Gunboats have been working to optimize the factory-delivered systems, each one taking a different approach to improving helm sensitivity for a better connection between the helmsman and the boat. So, in order to take the Gunboat 68 to the next level and map out a path to achieving this, we started with a blank sheet of paper.”
“There were multiple challenges,” says Gunboat’s Head of Engineering Stéphane Renard. “Our end-goal was a balanced steering system that is well integrated. It has to deliver a great feel at the helm and offer options for adjustment to give the owner the exact response level they want. It also has to be easy to install and maintain, but also built ready for the options of T-rudders and tillers, and of course the rudders have to be retractable too. We even looked at worst-case scenarios – in a collision the rudder will be sacrificial but the hull itself remains watertight. It’s a unique and demanding set of criteria so we partnered with engineering experts and, in fact, designed other aspects of the boat around the space requirements of the steering system.”
Modeling of the full Gunboat 68 steering system
The drawback of systems with rope or cables in them is that you get play, or ‘lash’, in the system. It isn’t a fully rigid response. The only way to get micro tolerances is to use inflexible, mechanically linked components.
For the section of the steering system extending from the carbon wheel at the helm station to the rudders, we chose a transmission rod system from a company with a spotless reputation for quality and reliability. Two gearboxes drive a carbon torque tube that runs aft between longitudinal beams under the salon sole, through the aft bulkhead to a splitting gearbox between the aft bulkhead and the aft beam. From there, torque tubes run port and starboard to transmission boxes above each rudder, and these drive a draglink attached to the ruddercassettes.
Using torque tubes and gearboxes, rather than chain-and-cable, means there is no play in the system at all, and hardly any friction. Plus, both rudders are rigidly connected with draglinks and torque tubes so both remain perfectly aligned. Each tiny adjustment at the wheel runs through the system to the rudders with 100% efficiency. Likewise, the slightest change in pressure on the rudders is faithfully reproduced at the wheel, delivering the true feeling of connection between the helmsman and the boat.
Designing the rudder bearings presented the most complicated engineering challenge, and we went straight to the top to solve it. We partnered with Mer Forte, the marine engineering office set up by Michel ‘le professeur’ Desjoyeaux, offshore sailing legend and two-time Vendée Globe-winner. Mer Forte’s record of delivery in Grand Prix projects like the America’s Cup, Ultim’ trimarans and IMOCAs was the qualification we looked for in a partner for the Gunboat 68.
“When we started thinking about the Gunboat 68 steering system,” says Desjoyeaux, “we started by looking at the specifications, and the requirements they had in common with our racing projects: reliability, light weight, ergonomics, and performance. The shoal-draft sailing parameter was the most interesting one because it pushes the limits of technology, and I love that! I’m excited to get my hands on the helm of the 68 and experience how good the boat feels!”
Michel Desjoyeaux, offshore sailing legend and founder of Mer Forte
The solution worked out between Gunboat and Mer Forte is both remarkably innovative in the way that we integrated it in the hull and reassuringly simple in its mechanics. To work from the top down, the draglink drives a cassette, and a tiller, either for racing or emergency steering, attaches to the head of the cassette.
The cassette contains top and bottom bearings, both plastic and self-aligning with no moving parts, and the top one provides 3° of adjustable rake so that the owner can fine-tune the feedback required. For instance, on a powerful spinnaker reach with lots of pressure on the rudders, you can rake the top bearing aft (which increases the amount of blade ahead of the axis) to tune out some of that weight for a softer, lighter sensation at the wheel. Rake the bearing forward in a light-wind beat to get more feedback. This also futureproofs the system should the owner later choose T-foils – the thrust loads of which the cassette has been designed to handle with ease.
The Gunboat 68 self-aligning bearing
The lifting-and-locking mechanism uses a notched system on the leading edge of the blade, all above the waterline, with the top-notch being fully deployed and the lowest notch fully retracted. Each blade weighs around 25kg and, with its positive buoyancy and super- low-friction bearings, it is easily lifted using the hand strap at the top.
Cassette and blade turn in a fixed bucket, shaped to optimize the flooded space, reducing weight. The same shape is also visible on the closing plate attached to the bottom of the hull, optimizing hydrodynamic efficiency by closing off the bottom of the bucket and providing a smooth underwater surface.
The rudder bearings are housed in the sealed off transom hatch, as seen above on 6801 in production and during sea trials.
The entire structure sits in a pre-preg carbon top plate laminated to the hull sides. In the event of a high-speed collision, loads will be transferred from the blade (which will break off), through the bucket into the structure, and dissipated through the hull. The bucket remains watertight.
All of our steering system goals are achieved in this one elegant and simple package.
The rudder blades themselves were designed and structurally engineered by VPLP
“When Gunboat approached us with their request, it was quite a unique design. It’s a lifting rudder system, a race boat design that’s fully integrated into the hull rather than transom-mounted. We enjoyed finding the best solution with the Gunboat and Mer Forte teams and the end result is really quite ground-breaking for a cruising boat and also exciting for a performance boat.”
The Gunboat 68 team and Mer Forte designers checking out the rudder stock molds
Half rudder blade mold for the Gunboat 68
“Many rudder bearing systems involve various individual pieces that need to be aligned in the factory,” says William Jelbert, Chief Operating Officer at Gunboat. “The design from Mer Forte involves far fewer components, so there is significantly less room for alignment errors. The full assembly is delivered to us along with an installation fixture that we align to our reference lines in the hull, which guarantees perfect alignment. For the steering system, because it is so modular, the pieces can be dry-fitted, quickly adjusted, and easily maintained. We love this system – it’s simple, strong, and reliable – and delivers exceptional feel at the helm, which is the standard we know Gunboat owners expect on a brand new yacht.”