Geo-Sim Models

Simulations can be very useful in verifying and improving the scaling techniques used in towing tank tests. It is very easy to get wetted lengths and areas from simulations, where as these are difficult to get in the tank because the flotation of the vessels varies a lot.

We conducted a large Geo-Sim model study during our last America's Cup campaign. We focused on verification and validation of simulations and tank scaling techniques using 1/3, 2/3, and 3/3 (full scale) models. We found that:

  • "Froude-Scaling" works well. Pressure forces scale consistently with Froude number
  • Frictional forces on hulls and bluff bodies like bulbs scale well using a "Hughes friction line" multiplied by a factor
  • Frictional forces of lifting surfaces such as rudders and foils scale well with a "Prandtl line" multiplied by a factor

The diagrams below show friction and pressure drag for each of the three scale yachts. The first diagram plots drag coefficients against Reynolds number and the second plots drag coefficients against Froude number. We observe that pressure coefficients coincide for the 3 scales and friction coefficients vary parallel to a Hughes friction line.