Vismara 56 Mills Design summary
Introduction
This report describes the design process used to develop the Vismara 56 Mills hull, appendages and sail plan, including a brief description of the tools (CFD & VPP) used to investigate the hull shape performance. This has involved the following tasks: Set-up of the evaluation loop (CFD & VPP) Design and testing of hull shapes Evaluation of sail plan
Design outline
The Vismara 56 Mills performance cruising design has been prepared for a well-known Italian owner, to conform to a number of requirements based on his experience with previous boats. The brief described a high-performance cruising boat, capable of being sailed shorthanded or with non-sailing family and friends, with comfortable interior and exterior spaces. The boat would be likely to daysail off Sardinia as well as make longer passages around the Mediterranean. Particular emphasis was laid on performance in higher wind strengths, with a desire to avoid reefing below 20kt and to rely on furling headsails of various sizes to cover the full range of wind speeds. The extensive interior accommodation developed by Vismara Marine with Lucio Micheletti inevitably occupied a significant percentage of the weight tally, reducing the VCG available for the stable upwind sailing in higher winds desired by the client. To compensate for this and to provide the interior volume desired it was clear that a beamy hull shape was required, with a wide beam waterline to produce the form stability necessary to match a reasonable sail plan.
Candidate evaluation: tools and methods Hull shapes are assessed using the North Sails VPP, taking in drag data from the Das Boot panel code CFD solver and aero coefficients from the KND library appropriate for a design of this type. These lift and drag inputs are then output as predicted boat speeds across a matrix of wind speeds and angles. The matrix we used weights the four wind angles equally, reflecting the all- around use envisaged for the design: VMG beating, 80° reaching, 120° reaching and VMG running.
CFD
A major component of the R&D programme is the generation of hydro coefficients which the VPP will use to predict the candidate hull shape performance. CFD is now considered the most efficient way of generating these hydro coefficients as it is possible to evaluate a large number of candidates in a limited amount of time and it is not bound by the scaling issues tied to tank testing. Two types of CFD were used in this program: Panel Code evaluations, thanks to a simplification of the governing equations (in essence their validity excludes turbulent flows), are able to deliver a solution in a matter of minutes. This allows a wide range of potential solutions to be evaluated quickly. The panel code used is the Das Boot code developed by Michael Richelson, used in conjunction with his North Sails VPP that integrates the lift and drag solutions from Das Boot with the other speed producing factors such as sail area, coefficients, stability etc to produce the final performance output. The Panel Code CFD uses the double rudders developed for this design.
Coefficient Cp and LCB, narrower beam, more powerful stern and bow shapes, higher and lower chines to identify a hull shape that could perform well across the wind range. As progress was made the improved shapes became the basis of the next iteration to combine the positive attributes. Toward the end of the process a reduction in the target displacement was made to improve performance and reduction in waterline length to reduce fears of transom immersion in light airs, and finally an aftward adjustment to LCB was made to allow the hull to trim correctly based on the preliminary weight study. The final candidate 130 (above) shows the evolution towards fuller sections forwards, lower chine height (approx 120mm) adding stability when heeled, as well as the raised freeboard to match the Vismara Marine model. Mark Mills
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3D Reynolds Averaged Navier Stokes Equations (RANSE) yield the most accurate solutions at the cost of a large volume of computation (thousands of iterations on million elements grids). In return, one can expect a high-quality assessment of the wave and viscous drag. A new use of RANS in waves has been developed by KND, the Vismara 56 being the second project it has been used on (the original single rudder solution was used for RANS wave modelling to simplify the meshing). Both solutions must be combined to ensure efficiency and accuracy of the prediction. All candidate hull shapes were evaluated using CFD Panel Code to maximise resources in investigating a wide range of possibilities, while RANS CFD was used to refine the solutions on the final candidate hull to avoid serious pitfalls – for example, panel code is prone to over-predict the performance of powerful solutions such as chine height and stem fullness.
Development of hull shapes
Within the starting parameters of a target length of 17m, target weight in lightship trim initially of 14,800kg (revised after discussions with Vismara to 14,000kg (the weight being saved with the use of an integrated furniture system and carefully minimised cable runs) and the max draft of 2.80m, an initial hull shape was developed to start the process based on the final SuperNikka hull. 112 was a chined-shape drawing from Open Class design where reaching is prioritised with a wide hull shape, medium- height chines and a reasonably wide waterline beam to produce a stable all-round performer.
Various iterations were tried such as altered Prismatic
Technical Brief
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