The loads on both a model of a smooth cable and a helical strake (also known as scrouton spiral) were measured in a wind tunnel. The coefficients of drag, lift and transverse force were ascertained for an angle of attack between 0 and 360 degrees. Results show that the drag coefficient increases by the spiral in each case whereas the lift coefficient changes only insignificantly. A defined transverse force is caused by the spiral. To find an explanation of this force by the circulation a simple mathematical model is given. Finally the behaviour of a twisted rope in water calculated by a known code is predicted in case of steady current.

From the point of view of mechanics, trawls are considered as extremely flexible and partly extensible rope and net structures which are exposed to flow. Form and loads of such gears mainly depend on the corresponding velocity of inflow and also on the so-called rigging elements that are required for the horizontal and vertical spreading of the fishing gear. At the same time the fishing gear is acting on the surrounding fluid. These reactions can on the one hand lead to unsteady states in the fishing gear. On the other hand changes of pressure and velocity can be detected by the fish and can possibly influence the selectivity of the fishing gear. This lecture is focused on the presentation of special numerical and experimental methods both for calculating large net systems and for analysing the reactions of the structure to the fluid.

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In Germany the combination of environmentally friendly changed wind energy with aquaculture is seen as an opportunity for multiple use of natural marine resources. Loads and stresses of “open ocean” offshore installations depend on wind velocity. For measurements of wind velocity within the field of wind energy conversion the only cost effective device is the cup anemometer. It has a linear relation between number of revolutions and velocity. Questionable is the measuring result if the wind vector is not within the ‘measuring plane’. Results of CFD calculations of the flow around an anemometer cup are given. With the force coefficients (lift and drag coefficients) an anemometer in inclined flow is calculated. It will be shown that the measured velocity within the measuring plane is higher than the cosine component of velocity.

Modern nets consist of meshes made of threads or twines with spirals or helical strakes. Fluid-structure interactions have been investigated in Rostock for a long time applying different theoretical models. Because of great net flexibility there is a need of calculation methods which consider the main physical qualities. This is done by the approximation of wake of threads by results from circular cylinders and influence of circulation, which is known from measurements of transverse force. Results of measurements with two models with and without spirals are given for comparison.