Rough plate in the friction measurement stand of SVA.

The properties of the wetted surface of a ship significantly influences the ship resistance and is especially time-dependent due to fouling. Fouling can result in an increase of the resistance of up to 50 %. Therefore ships are treated with anti fouling coatings. In the quest for new anti fouling coatings research work is done to find ways to also reduce friction with these coatings. SVA Potsdam owns a friction measurement stand providing the capability to determine the friction coefficient of arbitrary coatings. Even rough-textured or structured surfaces can be assessed.

To this end the texture or coating is applied on two brass plates. Rectified water flows through the slim channel that is formed by these plates. From the measured pressure drop between two locations inside the channel the friction coefficient can be calculated in dependency of velocity resp. Reynolds number.



At Reynolds numbers below 10^6 the measured values will be located inside the area that is limited by the laminar (Blasius) resp. turbulent flow (ITTC 1957) and the transition line.



Beside the investigation of coatings for ships SVA Potsdam analyses the influence of micro structures on the frictional resistance. An example of such structure is the skin of a shark, which is shown here as a silicon moulding of a dogfish.

Silicon moulding of shark skin.

Theoretical background

Following Froude’s hypothesis, the total resistance is the sum of different resistance fractions, viz. frictional resistance and residual resistance. The residual resistance is the sum of all fractions that scale with the displacement of the ship. According to Froude, the residual resistance of the full scale ship can be inferred simply by scaling the residual resistance of the model with the ratio of displacement. In contrast, the scaling of the frictional resistance depends on the surface structure, the scale and the absolute velocities (Reynolds numbers of model and full scale ship). In the following only the frictional resistance will be discussed. For the scaling of the frictional resistance the wetted surface of a ship is related to a flat plate of equal area at the same Reynolds number. There are models for the calculation of the friction coefficient for the smooth (and rough) plate depending on the flow state (laminar, turbulent, transitional) that are deduced from theoretical considerations and experiments. The friction coefficient can then be calculated from the wall shear stress W, the density and the velocity of the fluid.