OC have been using Maxsurf, the world’s leading yacht design software since drawing the lines of our first racing boat, the 35’ trimaran Fiery Cross, in 1987. Maxsurf has since been developed further for use in the design of commercial craft and warships. It is a powerful 3-dimensional surface modelling system allowing for systematic and rapid optimisation of hull lines, appendages, decks and superstructures. With a customised interface, Maxsurf generates hull forms to high degrees of complexity, including curves and dynamically trimmed surfaces.  The highest number of surfaces we have modelled so far as part of a hull, appendage and deck is seventy-two. Deliverables are hull lines, tanks, watertight compartmentation, tables and transfer files for 2D/3D drafting programs and naval architecture design suites such as Hydromax. Design analysis of either a new yacht or modification of an existing design in respect of hull, appendage or rig/sail plan requires an understanding of the fundamental parameters of its geometry such as wetted surface area, waterline beam, prismatic coefficient, righting moment, etc. In order to undertake this work, we transfer hull, deck, superstructure and appendage files to another program in our yacht design suite called Hydromax.

The prediction of a yacht’s behaviour using VPP software is applicable throughout the design process from preliminary studies, to complex analysis of tank/wind tunnel/CFD results and final hull, appendage and rig/sail selection. We often analyse designs in different weather scenarios using either our database or weather data supplied by the project team. The software can determine accurately the daily passage making qualities of a cruising yacht as well as optimise a yacht for inshore, Atlantic or round the world races. The benefits of changes are easily recognised referencing outputs such as; yacht polar diagrams, boat speed, sail/ballast cross-overs, mainsail reef conditions etc. The program is very flexible and as well as for all our own designs we have used it many times in consultancy work for others. An early and challenging example was for the 2001 America’s Cup Jubilee Regatta at Cowes for which OC designed a new foil package for USA 61 (Tom Blackaller’s Freemantle Americas Cup 12 metre with the twin rudder system), after analysis using the vpp with ten years averaged weather for the East Solent. A more mainstream study for us would be sail damage scenarios and the affect this would have on different races for IMOCA 60s and Class 40s, given their sail wardrobe rule limitations.

When not using test data, VPP software is restricted to its internal mathematical algorithms to calculate values of aero/hydro lift and drag. In broad brush terms they do this very well and for similar hull forms and rig types the results provide good indicators of comparative performance. However, there is a limit to the accuracy of any generic VPP that relies on its own algorithms. The analysis for a moderate displacement IRC/ORC 45 with a relatively standard hull form (not greatly dissimilar from those used in the Delft series of model tests from which most VPP internal algorithms are derived) is quite different from an IMOCA 60 or high speed, high Froude number racing yacht, with foils, canting keel and/or T foil rudders (see photo). A design office that uses model testing has access therefore to more reliable data for input into any VPP analysis. The result is that the likelihood of error is eliminated when comparing different designs, and outputs such as speeds, sail cross-overs etc are more accurate. In the bigger picture, model testing, particularly for larger, higher budget projects is very cost effective. They allow a design office to be more innovative by mitigating risk, confirming hypothesis and eliminating the possibility of a ‘bad idea’ from reducing the performance of an otherwise fast design.

CFD is the study of fluid mechanics that uses numerical methods to solve and analyse flow. Computers are used to perform the calculations required to simulate the interaction of fluids with surfaces, in the case of yacht design; appendages, hulls, sails and spars. With the advent of high-speed and lower cost computing CFD is applied to many more problems and on smaller budget projects than ever before. Owen Clarke Design has been using CFD as a tool since the design of Kingfisher in 1999. Since then all our major R and D programs have included CFD analysis. Our smaller, lower budget projects have benefited from that research, particularly in terms of rudder, keel and sailplan design. The graphic shows the pressure distribution on an IMOCA 60 hull with curved foils, modelled using RANS solver based software, CFX. Although the hull and appendages were both modelled, compared and calibrated against the tank test results, the primary aim of the investigation was the interaction of the three appendages, distribution of side/lift forces and the optimisation of their attack angles. One of the advantages of CFD over model testing is this ability to look at the lift forces and centres of pressures on individual appendages and sails in detail, as well as providing a visualisation of the results.

CFD was first used in the aerospace and aeronautics industries, so for naval architects it is not only a tool for development of hulls, keels, rudders and foils, but also for spars and sail-plans. Through a long association over a number of years with high technology suppliers such as Southern Spars and North Sails France our clients have benefited from the use of CFD based aero software packages such as Flow and Membrane. Most of our 60’ and 40’ sail wardrobes have been developed using this software and it has been used to help define sailing loads in rigging, sheets and halyards since first used for the design of our IMOCA 60, Ecover 2 in 2003. The cost of using CFD has reduced significantly over time and is now used across the full range of our work. We continue to monitor developments in CFD, are invested in it and advise clients where it’s appropriate and cost effective to use. Auto-meshing tools and Cloud based software have cut processing times and cost in recent years, now bringing affordable hull force solutions within the reach of cruising yacht budgets. We are using CFD on our current twin hybrid drive motor sailer to evaluate powering requirements and recently completed a CFD and VPP consultancy that delivered new keel and rudder designs for this modern classic yacht