DESIGNER / A.KANAI & SDK
Photo: Kazushige Nakajima
Seahorse Magazine June 2022
Thing of beauty
Akihiro Kanai from ACTechnoiogy describes the development of a contemporary 30ft racing yacht, the K30W, designed to be competitive under IRC and in the HP30 class while also employing a sustainable building method, with wood used throughout as the primary building material.
The new wooden racer, launched in December 2021. is a development of ideas first explored with a 24ft wooden boat, W24, which we designed in 2012. The goal was to achieve the high performance of a modern racing yacht while taking advantage of the benefits offered by modem wood construction.
Based on the design of the all-carbon racer K36-Samural, but with improved performance, K30W brought onboard Naoyuki Umekawa of Wood Friender as builder and Steve Koopman of SDK who worked with us on the K36-Samurai as structural engineer.
The impetus for this project was to create an even more extreme wooden racer than our previous 24-footers, and to show that the result can be lightweight and rigid, and successfully race against current composite-built yachts under the IRC rating system.
The freedom and flexibility that IRC allows designers within a relatively simple framework Is a great basis to be creative in the design of a wooden racing boat. The owner of W24, Yoshiki Jinno, fully endorsed the new project.
When first starting out as a CFD researcher 30 years ago my first task was undertaking research on wavemaking drag reduction; I have since used CFD as a tool in the development of America's Cup boats and countless other subsequent vessels of all types in the pursuit of performance.
These efforts have led to fuel efficiency improvements in commercial vessels, reducing fuel consumption by designing new hull shapes, sometimes introducing sails as secondary propulsion. As a return to the old days of sailing boats I had also been thinking about the possibility of wooden boats for the past 10 years with Wood Friender. As a result K30W was born as the latest wooden racing boat.
Despite its wooden construction K30W employs the very latest technologies in the huII and deck shapes, structural arrangements and building methods.
As a standard design process a total of 250 different hull and deck shapes were assessed through an intensive CFD and VPP study of the hydrodynamic performance of the hull and appendages, plus the aerodynamic performance of the deck in upwind and downwind conditions. *
The balance between hull drag, stability and sail power at each wind speed is always the central factor in determining hull shape and sailplan. Downwind a boat's fore and aft trim always changes a lot with the wind speed, but a good CFD tool should account for this dynamic variation with appropriate grids, including at or near the planing condition for better accuracy.
This changing prismatic curve downwind and upwind Is an important focal shaping factor to determine the global shape. The front deck shape was carefully designed to give a good flow with low resistance and low vortex onto the jib. However, I did need to consult the builder during the design development about how he could achieve the complex 3D surface curvature with wood materials!
A modem racing yacht with stability provided by a deep bulb keel and crew hiking on the rail and highly loaded rig and appendages is subject to much greater structural loads when compared to more traditional wooden boats. Traditional scantling rules for planking, framing and appendages do not properly apply. The structure needs to be carefully engineered to withstand the calculated loads; for the K30W the ISO 122216 rules (Category B, offshore) were used as guidance to establish appropriate loads, material properties and safety factors.
The hull, deck and internal structure are made mainly of mahogany, red cedar and Baltic birch plywood, while the rest of the keel fin, rudder, bowsprit, mast and other parts are carbon fibre/ epoxy.
The structural layout is similar to that of other modem composite racing yachts but with closer spacing of the transverse frames to accommodate the thinner wood planking vs thicker cored composite panels. Two longitudinal webs run between the transom and main bulkhead, supporting both the cockpit and hull panels.
The substantial heeling and grounding loads induced by the deep, high-aspect keel fin are supported by extending the top of the keel fin to the underside of the cockpit sole. This avoids having to use large wooden laminations in the bilge to support the keel loads. The carbon fin is contained withm a thin fibreglass trunk to maintain watertight integrity and a thicker integral flange around the bottom of the trunk distributes the keel loads into the surrounding wood hull planking. The keel fin is also used to extend distribution of the mast step compress ion load over the height of the main bulkhead.
The hull was cold-moulded with four layers of wood, including the first layer of planking, each epoxy bonded with vacuum bagging. The inner and outer veneers, of red cedar and mahogany respectively, are oriented longitudinally to span the transverse frames and to provide a visually pleasing orientation for the clear final finish. The inner veneers of red cedar are oriented at +/-45° to provide shear and cross-grain strength. The frames, deck, cockpit and chamfer panels are all made of Baltic birch plywood.
The shroud chainplates are of typical carbon uni strap construction lapping onto the hull sides. However, to accommodate the relativity low peel strength of wood grain the carbon uni straps are interleaved within the planking veneers, doubling bonding area, and are extended further down the topsides than would be found on a more contemporary composite structure.
In regard to the global stiffness of a wooden boat compared to a carbon boat, the modulus of carbon is much higher than that of wood, but the wood thickness is multiple times thicker than the combined outer and inner skins of a carbon boat. So the overall global bending stiffness is similar - and much higher than that of a conventional E-glass laminate design.
Umekawa of Wood Friender believes exploring using trees, which naturally create the structure of wood without any difficulty, is something we need more of in the age of carbon neutrality.
Since this period of building coincided with great advances in digital fabrication, new technologies such as 3D CAD. CNC router, laser cutter, 3D printer, 3D scanner and reverse engineering all enabled us to combine traditional wooden shipbuilding techniques with the latest digital technologies. For example, the frames of Baltic birch plywood were all CNC-cut and fabricated, ensuring accuracy and saving a significant amount of time in assembly.
The deck layout was designed to make the large cockpit ergonomically efficient, with the sliding hatch placed on the floor of the cockpit instead of on deck. The minimal small pit is located aft of the mast, and the forward vertical side of the cockpit is also the rear of the mast bulkhead, which contributes to a lighter weight and lower centre of gravity.
The boat manager, Tetsuya Sasaki, introduced the latest deck gear and control systems for the K30W, including the latest B&G Nemesis displays, Spinlock softgrip clutches, UnREEL revolution reels for spinnaker halyards and Brother system for spin sheets. Some custom parts for bushes and dogbones were developed from scratch by the team members.
The first sailing was in 20kt of very cold northerly winds on Lake Biwa in the middle of Japan. The boat showed good balance, stiffness and a nice soft-feeling steering through waves without any creaking sounds at all (in general I think that wooden boats have a very different way of absorbing vibrations in waves).
We expected to join the HP30 race at Cowes Week in the UK this year for the start of our K30W campaign, but had to postpone to next year due to the unstable international situation and other factors. But we look forward to the K30W soon competing alongside other high-performance boats in the IRC and HP30 fleets.