Friction forces acting on the ship hull are an important if not the most important contribution to the total resistance of a vessel, and they become more important with reduced ship speeds. The following graph shows the ratio between viscous and pressure resistance for a container vessel as a function of speed:
Viscous resistance or friction forces are determined mainly by surface quality and hence roughness and the actual material properties. As the surface quality of a ship hull will degrade over time, roughness will increase and so does the frictional component of the overall resistance. These effects can be predicted with CFD. FreSCo+ provides information on the skin friction distribution on the hull as a function of – increasing – hull roughness over time. The following figure shows the skin friction distribution on a tanker hull for hydrodynamically smooth (upper) conditions compared to a rough surface (lower).
The increased surface roughness not only increases the frictional resistance but also has an influence on the pressure distribution and the development of the ship’s wake at higher Reynolds Numbers. The following figure indicates the evolution the boundary layer in the stern part of the tanker hull for different roughnesses. The smooth surface (green contours) indicates a thinner boundary layer than the rough one (red contours) which is also reflected in the pressure distribution where the smooth hull (top) shows a higher pressure recovery at the stern.
Today, these CFD prediction capabilities are used for life-cycle assessments as well as in research work focusing on detailed boundary layer studies with the aim of stabilising the laminar regime of the boundary layer at the bow of a ship (see the FLIPPER project under RESARCH).
Actual material properties of coatings are another area of high interest. These can only be tested experimentally. Using a special set up in HSVA’s large cavitation tunnel HYKAT shown below the model basin is best suited to conduct high Reynolds Number tests for novel coatings.
In the past this facility has been successfully used to test not only modern silicone paints but also patterned surfaces such as high tech Riblet foils.