upcoming foiling version of n°945 that we will soon be presenting! Before finalising our latest design we
undertook what for the Mini class was a relatively comprehensive CFD programme which compared seven well-developed candidate hulls, three of them scows. Inter- estingly, this study confirmed that despite being the most obvious and therefore the most often discussed design characteristic, being a scow or not a scow is not such a distinctive feature performance-wise, even without introducing foils. Our CFD projects take place in coll -
aboration with Advanced Aerodynamic Vessels, which allows us access to the company’s cutting-edge Six Degrees of Freedom Solver, as well as advanced mathematical models,
to accurately
address unsteady aerodynamic and hydro- dynamic cases (these tools were developed and then validated at full scale during the company’s design and later development of three experimental high-speed commercial transportation vessels that exploit passive aerodynamic support). During the past five years this partnership has been very fruit- ful, especially when designing flying mono- hulls and multihulls with the new model- ling challenges they bring in the Imoca/ Volvo, Mini and multihull classes. While developing an all-new Mini 6.50,
careful study of different heel angles and speeds showed that the best non-scow can actually deliver more righting moment than
some of the current competing scow forms. For example: buttocks curvature and rocker angle can have a greater impact on high-speed righting moment and drag than a very full bow shape. Equilibrium in terms of leeway at speed
also has a large impact; finding the opti- mum keel cant and daggerboard toe-in is a major but very often undervalued factor in achieving good boat speed on almost every point of sail… not just when upwind or with sheets slightly cracked. To get those values right on the drawing
board, without requiring lengthy and costly full-scale testing, we rely on CFD but also reverse engineering, using feedback from our previous designs to constantly validate and re-validate the numerical tools. Continual detailed exchanges with the
skippers and sailmakers are also very valuable in defining the right parameters under which to identify the most relevant computations. In terms of bow angle and sheer line,
seen from above our 2018 Mini hull appears quite similar to our previous design from 2012; however, the hull sec- tions are a great deal more powerful as well as much flatter along the bottom to boost high-speed performance with what is a relatively shallow draft hull. Seen in profile, the evolution is much
more radical, including a distinct bow profile with significant forward overhang and reduced static waterline length. Keel
line and buttocks curvature have also been carefully optimised to permit the most effi- cient high-speed sailing trim. Surprisingly, aesthetics were also a factor for us, with the chine running all around the boat leading to a seemingly simple but sleek hull shape. It is here that the parallels with our new Class40 also become rather obvious! The result is a hull whose righting
moment is higher than some of the scows, especially at high speed, yet with less drag than our previous non-scow designs at boat speeds of 6.5kt and above.
Class40 In preparing our latest Class40 design, the Lift 40, the first set of hulls to be CFD analysed included our previous Akilaria RC3 series, plus two of the best current boats from rival designers, plus two more candidate hulls which we extrapolated directly from our new Mini 6.50 shape. It appears that Class40s are quite differ-
ent from the Minis! Despite sharing the same ‘look’, with the chine running around a fulsome bow, our final Class40 design evolves from a second distinct set of hull shapes ending up a long way from our Mini… (and contrary to some of our initial expectations). With the new Class40 the buttocks cur-
vature, waterline length and beam-to-draft ratio have all been significantly adjusted in pursuit of satisfactory all-round perfor- mance; such big changes were driven less w
SEAHORSE 51
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