Model Testing - What is the point? A review of yacht testing and the implications for yacht designers

With the ever evolving design space, the need for quality data for designers has never been more important. To realize this need, designers typically tum to one of two techniques either: experimental fluid dynamics, typically using either towing tank or wind tunnel testing; or computational fluid dynamics. It is becoming increasingly easy, especially with the ever increasing power of computers and the visually attractive output, to take the decision to undertake computational fluid dynamics analyses. Whilst on the surface this may seem an easy decision to make, certainly easier than to undertake towing tank tests, there are some strong arguments to suggest that simply undertaking a computational fluid dynamics analysis as an a-priori solution is not without problems or dangers. Extrapolation of experimental fluid dynamics drag results to full-scale for marine craft has traditionally involved attributing components of the total force to "viscous" and "inviscid" regimes. These components are then scaled with the Reynolds and Froude numbers respectively using a zero Froude number extrapolation to separate the two. However, this division of forces does not correlate exactly with shear and normal pressures on a body due to fluid flow, which sum to give drag. Nor can each of the "viscous" and "inviscid" components be attributed to strictly Reynolds and Froude number dependent effects, respectively.