Good to see a big new dual-purpose high-performance yacht aimed at a programme that specifically includes offshore racing. It is surely ironic that, while the entire Maxi 72 fleet pulls out of the Bermuda Race because of the ‘risk of’ boat damage, with a few notable exceptions today it is largely left to boats like the new Chessie (above) and Mike Slade’s Leopard to show what today’s big boats are capable of when released onto the open sea. A healthy offshore component also helps to keep righting moments sensible
l Single-level cockpit and shallow angle companionway l Easily sailed by three people including docking l Shoal draft of 7-8ft l Six-day autonomy l A/C at the dock only l Can run in silent mode (no audible power generation) for day sailing and day racing l Offshore racing: Bermuda, Key West, Block Island, Vineyard, Palm Beach, Jamaica and others under multiple rules l Finally, the boat was to be a monohull that was as fast as possible given the balance needed across the above factors.
The virtuous design spiral
Along with new challenges come new opportunities. In design these can be answered via a symbiosis of experience and tech- nology. Technology is relentless in providing better means and ways. We use high-tech tools and controls to first develop a better design and then manage (fine-tune) the outcome. In a pocket rocket there are a lot of pieces to control; the tying together of the modelling, the engineering and the distribution of mass are critical to realise the project. We choose to break the design process down into discrete sections, allowing us to manage them as tasks within which we achieve concrete results. We use ESWBS (US Navy Ship Works Build System) to organise the sections and tasks. Starting with a detailed spec tied to our weight calculations, we use CFD to model the hull and FEA to model the load structure. It is important that we co-operate and communicate well on the different tasks both within our office as well as with outside technical experts and sup- pliers. Weight control is a ‘team event’ from design to delivery. Unlike a racing boat, which has a singular purity of purpose, Chessie Racinghad a defined length with a corresponding weight (before ballast), freeboard and interior volume. Within the design brief we need to optimise the principal parameters of beam/ stability/sail plan/foils/ballast. We can also optimise shape and some weight options. This optimisation starts what we call a virtuous design spiral. l For Chessie we started with several parametric designs and a VPP. Reviewing that output, we manipulated the parameters, creating a base boat shape that then went into a panel code run with automatic parametric iterations (robot) to more accurately investigate various influences – for instance, beam and sail area vs stability and wetted area/drag.
58 SEAHORSE
l We saw early on that stability was a better driver, and if we could mode the sail plan between day sailing and racing, then the boat would be faster racing and easier to sail when not. Parallel to this, we had three different handicap frameworks to work within – two VPP-based and one a typeforming single number rule. A complex equation.
While not wanting to dive into the single number vs VPP issue here, we have seen what we think is an unintentional effect in IRC around the 62ft+ size. When confronted with big dual-purpose boats, the black box of IRC typeforms and rates their larger sail areas more heavily than it ‘de-rates’ their higher displacement; as a result more complex bigger boats suffer as they add sail area to compensate for the weight of their many systems and all of the luxury add-ons (this perceived imbalance was a factor in the superyacht fleet moving away from IRC in its present form). For the same reasons, J-Class boats rate well in IRC – heavy and with comparatively little in the way of sail area. This is not, however, an issue for IRC grand prix boats –- these boats drop their weight, removing anything not strictly there for performance goals, thereby increasing the responsiveness per kN of drive force from the sails. It is this better responsiveness, as much as the pure sail area/displacement ratio of a raceboat that often allows it to beat very much bigger boats around the racecourse, especially in variable conditions (it also makes for a much more entertaining boat to sail).
The result for us… we didn’t design strictly to IRC regarding sail area because we all wanted our boat to also power up in light air (fun), and plane offshore in a breeze (more fun). l After we had our weights refined we then optimised the shape around this and the better parametric results. At a DLR Displacement/Length ratio (DLR) of 77 and an upwind Sail Area/Displacement Ratio (SADR) of 41, we had a very exciting boat. Even if not Grand Prix, it would be state of the art. l After a verification of our panel code results through some point checking with a RANS program, we settled on a new base boat and then ran another hull series around the most promising parameters, focusing on shape variation this time, again initially by robot and then moving to two perfected ‘drawn’ shapes. We took these two forms and ran them through the North VPP, while finishing the hull study with a two-boat RANS test (this time run at KND Sailing Performance) as a check against our work. Everything aligned and after another spiral we had the design we wanted.
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