When Spirit Yachts and Ben Ainslie’s BAR Technologies set out to create a wooden foiling motor launch, they chose West System and Pro-Set epoxies
Wood is not a material that’s typically associated with the very latest technologies. Yet the Spirit/ BAR 35ft electric foiling motor yacht is proof that timeless natural materials can be the perfect choice for bleeding-edge projects when it is combined with appropriate resin and reinforcing systems.
An ethos that's always driven Spirit Yachts is one of innovation. The Spirit/BAR foiler is a collaboration of like-minded thinking that combines founder Sean McMillan’s design and styling flair with the yard’s outstanding constructional skills, the expertise in high-speed foiling craft at Ben Ainslie’s BAR Technologies and a visionary, receptive owner.
‘The design brief was for a classically styled powerboat with a clear-coated hardwood finish and aesthetics that paid homage to the Gold Cup racers of the early 1900s,’ says Spirit Yachts’ managing director Karen Underwood. This is, however, a subtle deception as the boat lifts onto its foils at a speed of only 14kts.
This is a critical aspect in creating an electric yacht with a long range, as resistance drops so much once foiling that power consumption falls well below that of displacement vessels that can only proceed at very modest speeds. Nevertheless, this commission required a step change in design and engineering to create a boat that looks right on her moorings and at displacement speeds, while also having the right underwater shape for ultra-efficient foiling. An additional complexity is that the foils retract to reduce draught to only 0.75m (2.6in) which gives flexibility to access shallow berths and small harbours.
BAR Technologies brought its America's Cup design and simulation skills to the project, helping to create optimised shapes below the waterline, with low-riding foiling technology employed to minimise resistance and improve ride comfort. The result is a 35ft electric boat with an impressive 100-mile range at 22kts and a top speed of 30kts, despite having a battery bank of a relatively modest size. This compares with the 25-70-mile range at five knots which to date has been typical for electric motor vessels of similar length.
The foiler’s unique styling includes immaculate timber topsides along with a flexible deck layout that allows a six-seat open cockpit to be quickly converted into a two-seater spider configuration.
Epoxy strip-planked construction has long been recognised as an affordable, lightweight and very stiff option and the hull of this 35ft boat weighs just 596kg. It's a method that has roots in the pioneering development and information that Gougeon Brothers gifted to every yacht designer and builder in the form of West System epoxy products and literature. It was refined in the late 1980s by Grimsby (UK)-based boatbuilders Farrow and Chambers, who developed a loose-fitting tongue and groove profile that minimises time needed to plank up a boat and gives optimal space for the glue that holds the planks together. Two thinner double diagonal veneers, also epoxy glued, contribute hugely to the overall stiffness of the structure.
If lightweight, rot resistant timbers such as cedar or Douglas fir are used, the resulting structure has an excellent strength to weight ratio as well as enviable longevity – these are boats with an inherent ability to last for 100 years. The hull is then sheathed in glass and epoxy, creating a layer with excellent resistance to point impacts and thus protecting the timber from water ingress. A host of successful yachts have been built in this way, from a slew of well-known designers including Rob Humphreys and Stephen Jones, with the latter choosing the method for his own spirit of tradition 46 footer.
Today, vacuum bagging of the veneers and sheathing increases clamping pressure, creating an even stiffer and lighter structure. At the same time, CNC machining of timber for strong backs and bulkheads reduces build times, while ensuring a high level of repeatable accuracy.
Above: the two-seat spider top converts to a six-seat cockpit.
These benefits have long been recognised by Spirit Yachts. ‘Our very first Spirit, a 37ft sloop, was built with a hull that could be lifted by four people and had no carbon at all. Thirty years later she is in great shape and sailing well,’ says McMillan. ‘In our experience judicious use of carbon can be beneficial as a stiffener in larger 65ft-plus yachts but it is not needed for strength – the correct timber selection is key and the end result is superior in every sense, weight, feel, cost and crucially its carbon footprint.’
Don’t be fooled by the classic styling of McMillan’s yachts, or the use of natural materials. His passion for innovation is as strong as anyone’s in the industry. Notable previous projects include a gaff sloop with a 1,000sq ft (93m²) sail area that weighed only 200kg, plus zeroG, a foiling twin-winged ground effect flying craft that pushed hard against technological boundaries when it was completed nearly 25 years ago.
Nevertheless, Spirit Yachts and BAR Technologies, a spin-off from Ben Ainslie's America's Cup team, have taken the strip planked concept a stage further for this foiling boat. The principle of the timber core remains the same, using yellow cedar over laminated frames. However, the cedar is sheathed on both sides with carbon to increase structural stiffness while adding very little extra weight. The clear timber finish is achieved with a 3.5mm sipo veneer over the outer layer of carbon. This was then faired and coated with West System 105/207 special coating system before varnishing to give an exquisite clear coat finish that shows the natural materials off to their very best.
Above: the finish is a 3.5mm veneer over the outer layer of carbon.
‘We adopted our tried and tested build methods, just scaled for the size of this craft,’ says Spirit Yachts’ yard supervisor Adrian Gooderham, ‘paying particular attention to the overall weight of the build without compromising strength, using West System epoxy and fillers – all products we have trusted for years.’
Despite its reputation for classic styling, Spirit Yachts has also used carbon fibre for many years for rudders and occasionally keel fins. Composite specialist Graham Eeles explains that for this project, with the carbon fibre fabric applied to both the exterior and internal structure and quite a lot of detailing needed on the interior, the ideal solution was a combination of West System and Pro-Set epoxies and adhesives.
Above: BAR optimised the underwater hull shape as well as the foil design
On reflection McMillan, who is well versed at producing highperformance sailing and motor yacht designs that combine a traditional aesthetic above the waterline with state of the art immersed sections, says that the hull could have been built even lighter. ‘I now consider the amount of carbon sheathing could have been reduced by at least 50 per cent and an equivalent hull built at around 500kg,’ he says.
The involvement of Wessex Resins and Adhesives goes far beyond simply the manufacture and supply of epoxy resin systems. The company also provides technical advice and training to clients, including on-site development of production techniques and staff training at Spirit Yachts. The firm’s technical service deals with a wide range of questions, varying from advice on laminating frames for a 1930s sailboat restoration to construction of carbon fibre violins. ‘It’s both inspirational and challenging,’ says sales director David Johnson.
What of the future? ‘There is significant market demand for sustainable leisure vessels, drawing innovation from the wider maritime sector,’ says BAR Technologies’ CEO John Cooper, adding they were ‘hugely excited to partner with Spirit Yachts to push the boundaries or modern classic yacht performance’ with this foiling project.
Hull weight: 596kg
Draught (foils up): 0.75m
Similarly, McMillan adds: ‘There’s a trend for forward thinking, environmentally aware yacht buyers that has driven exciting changes in the market. BAR’s background in high-tech racing made them the ideal partner for this project, blending heritage with technology to create a sophisticated yet modern vessel.’
We invite you to read on and find out for yourself why Seahorse is the most highly-rated source in the world for anyone who is serious about their racing.
To read on simply SIGN up NOW
Take advantage of our very best subscription offer or order a single copy of this issue of Seahorse.
www.seahorse.co.uk/shop and use the code TECH20
Or for iPad simply download the Seahorse App at the iTunes store