On the CFD modelling uncertainty of ACC yachts

The hydrodynamics of two fully appended ACC yachts are modelled in free to sink and trim conditions, with zero leeway and heel angles, at Froude numbers ranging from 0.22 to 0.44. The verification and validation procedure to assess the numerical and modelling errors, respectively, is performed for the hull resistance of one of the two hulls at Froude number 0.22. The numerical uncertainty is found to be 2.2% at 95% confidence level. The numerical/experimental error is lower than 0.8%, which is lower than the validation uncertainty (2.4%) and therefore the computed resistance is validated. This numerical setup is then used to compute the resistance of the two hulls across all the speeds. The maximum numerical/experimental error is 1.3% and 4.1% for the two hulls respectively. The low numerical uncertainty allows ranking the two hulls based on the lowest resistance for most of the boat speeds, except in a range between about 9.5 and 11 knots full scale, where there the probability that the ranking is correct is less than 95%. In fact, the experiment showed that the computed ranking is incorrect at Froude number 0.37 (about 10 knots full scale). In order to narrow this uncertain speed range, the grid resolution should be increased.