As evidenced by the many inches of copy devoted to new Class 40 projects in recent issues of this magazine it should be apparent to all that the Class 40 market segment is continuing to grow in spite of the current economic situation. There are many reasons for this meteoric growth but at its core the Class 40 rule promotes fast, high performance offshore capable boats that are well suited to short-handed sailing. This is achieved within a strict and relatively stable set of rule controls that effectively limit build costs and produce equitable performance between different boats. The final ingredient is a diverse calendar of events covering the spectrum between inshore crewed racing, coastal short distance sprints, trans-Atlantic races and even the option of around the world racing for the most adventurous.
At Farr Yacht Design we have been watching developments in the Class 40 arena with interest for some time, looking for the right opportunity to bring our extensive Volvo Ocean Race and Open 60 offshore design experience to bear on this exciting class.
In October 2009 we contracted with Lapo Ancillotti's BTBoats of New Zealand to develop a new limited production Class 40 design to be constructed in New Zealand by Cookson Boats.
As we go to press the hull tooling is well underway and it is anticipated that the first boat will splash down in August of 2010.
The Class 40 presents some unique design challenges that require some analysis and optimization.

While seen by many as a small Open 60, they are quite a bit heavier for their length than a modern Open 60 and this has important implications in the development of the hull characteristics. The boats are very powerful [the righting moment at 20 degrees is more than double a Farr 40 One Design!] largely a result of the large maximum beam, outboard water ballast and reasonably deep draft. The high beam to draft ratio results in a hull with large wetted area and so there is a premium on minimizing this, especially when upright so as to keep light airs performance acceptable. The rule limitations present a rather low sail area for a boat of this power. While this serves the boats well in stronger breezes, especially offshore, it poses some challenges to maintaining sufficient light airs performance. This is one reason for the increased use of Code 0 sails tacked on the sprit to provide sufficient light air horsepower.

In order to gain a proper understanding of some of these trade-offs we felt a focused computational fluid dynamics (CFD) study was required. Working with our long time collaborator Len Imas of Stevens Institute of Technology we completed an intensive CFD study of Class 40 hull forms exploring the performance effects of chines, transom immersion, longitudinal hull shaping, bow fullness and section style. Over the course of multiple shape evolutions the insights from these simulations [both forces and flow visualization information] allowed us to refine the hull shape resulting in drag reductions of 3 - 5% over much of the speed

range at typical sailing heel angles. Additional studies were completed on the sensitivity of the boat to trim and in optimizing the placement of water ballast.

The idea of applying such computational fluid dynamics technologies to a problem like the Class 40> would have been a non-starter only a few short years ago but after many years of applying these tools we feel it is now feasible to complete such studies in an efficient and cost effective manner. Calm water drag reductions of these magnitudes are significant and serve to remind us that even subtle shape changes can have significant impact on yacht performance.

However, small boats such as the Class 40 do not really sail in calm water and in fact spend much of their time sailing in waves that are of significant height relative to the boat size. This has some very significant implications on the hull form shaping, driving towards reduced transom immersion levels upright and at heel, generally more keel spring throughout and a focus on bow shaping and overall bow fineness to maintain effective length without incurring added resistance penalties.
One of our prime objectives in the development of this design was to create a hull that has exceptional handling characteristics when sailing both upright and heeled. This allows the boat to be driven harder even in adverse conditions and to dynamically maintain bow up trim for better handling with less reliance on water ballast.

These qualities translate into higher sustained average speeds, especially when running and high speed reaching in deep ocean conditions. The challenge is to incorporate these features without sacrificing light airs performance or adversely impacting upwind performance especially in waves.
The resulting hull shape is, like many of the latest generation of designs, at the maximum beam corner of the rule space. The boat features a pronounced almost full length chine with very evenly radiused sections below to minimize wetted surface and provide a hull with a very even and consistent heel response in drag and helm load. The transom immersion levels and chine placement have been carefully tuned relative to expected heel angles and water ballast weight additions to avoid excessive drag in light airs but maximize effective length when sailing at heel and speed. In order to achieve a dynamic bow up attitude when sailing at heel and speed the LCB of the hull is reasonably aft, also moving the keel and rig aft. The section shape leading into the chine forward is designed to maximize the ability of the chine to shed water away from the hull and to provide significant hydrodynamic bow lift when entering a wave without requiring the added building complexity of a full VO70 style strake. This is also a more all-round performance solution for sailing upwind in waves, although a boat targeted specifically for upwind biased races may be better suited with alternate chine/section shape treatments.

This design has been carefully refined to meet the Class 40 rule requirements including Category 0 compliance and new aft escape hatch requirements from the outset. A detailed focus on the construction process and engineering of the yachts structure has allowed us to reach the minimum rule displacement and maximum 90 degree pull down test requirement placing the boat in the optimal position relative to the rule limits.
The deck geometry and deck layout incorporate input and feedback from a variety of areas to produce a clean, functional and ergonomically efficient

arrangement well suited to short-handed sailing in distance races. The arrangement incorporates the dual companionway, central line tunnel concepts originally pioneered on our Open 60 Design 498 - Virbac Paprec, and a winch layout specifically suited to the unique Class 40 rule restrictions and demands. Careful attention has been paid to developing an ergonomic deck layout and efficientsteering systems to maximize crew protection while allowing the crew to hand-steer and trim the sails easily.
This logic has been extended through to the mast design and rigging layout where significant emphasis has been placed on maximizing the crew's ability to control the sail trim and mast bend. This is highlighted by the selection of a keel stepped mast with a boom vang arrangement to improve mainsail control. The mast is a 2 or 3 spreader highly swept carbon mast with chainplates located inboard to allow sufficiently narrow sheeting angles for the Code 0 sails.
Detailed studies of sail sizes and parameters have been completed to develop a well balanced sail plan with appropriate head stay control. Care has been taken to ensure an even usage profile over each of the sails to maximize sail life. We have also developed a set of candidate sail inventories that can be selected to mode the boat more appropriately for different races. This is of particular importance when sailing under Class 40 rules where the sail inventory is limited, magnifying the importance of each sail and emphasizing the need to match a sail inventory to the expected conditions of a particular event.

In developing this design we have opted where possible to create a very flexible platform that can be customized for a client's specific needs and sailing programs. The bow sprit may either be fully articulating or fixed with a separate spinnaker pole. Chainplate attachments and rig configuration have been designed to allow the option of an inboard diagonal shrouds. Crew protection is facilitated by a removable dodger that can be used for offshore sailing or left behind for coastal or inshore sailing.

The interior arrangement has been designed for racing functionality with ample berths, head and galley for offshore races.
The navigation station is positioned at the forward end of the main compartment with easy access through the dual companionways. The interior features the 4 rule required fixed bottom berths that will be supplemented with pipe berths when racing. The engine is situated forward in the main compartment with its box forming the navigation seat. Various options for interior layout, galley placement etc. will be available to suit individual owners. The head is located forward of the mast frame. Attention has been paid to developing an internal structural arrangement that will permit easy moving of gear and sails to alter the yachts trim.

The keel is a high aspect fin with "T" bulb incorporating FYD's proprietary foil sections to achieve a good balance of drag and lift

 

efficiency. The fin is built from fabricated steel with a fiber glass fairing attached to a squished lead bulb. The keel attachment has been engineered to simplify boat assembly. The bulb shape is an extension of a series of bulb style studies using transitional flow computational fluid dynamics studies to minimize drag while taking into account the significant accelerations that occur due to boat pitching motions in real life conditions.
The design features twin rudders mounted under the hull with a direct linkage steering system with a single tiller. The rudders are located under the boat to simplify weight/construction and installation.

The use of Open 60 style kick up rudder system can be incorporated for around the world racing but the material construction constraints of the rule make this quite a heavy and costly consideration.
The rudder sections and planform shapes have been optimized to provide exceptional control while minimizing cavitation at high speed and are optimized within the structural limitations of the rule on rudder stock materials.
We are excited to have the opportunity to develop a new design for this dynamic class and look forward to seeing the first boat in the water later this year.