Seakeeping and manoeuvring
Assessing seakeeping and manoeuvring performance at an early stage is even more important for wind powered vessels than for conventional ships, since there is little design experience to lean on. When the driving force comes from sails instead of a propeller, ship dynamics change considerably. SSPA has developed advanced computational tools and test methodologies to assess course keeping, turning ability, motions, and acceleration in waves.
CONDUCTING SEAKEEPING MODEL TESTS:
ON THE HORNS OF THE TRILEMMA
To obtain meaningful results during any model test, the flow physics need to be modelled correctly. In the case of a conventionally propelled ship this leads to the “dilemma of model testing”, famously solved in the mid-19th century by William Froude, when he proposed to combine model tests for “wavemaking resistance” with empirical corrections of the “frictional resistance”.
In the case of a model testing wind powered vessels Froude’s dilemma turns into a trilemma because aerodynamic similitude requires identical Reynolds numbers of model and full-scale sail systems.
To solve the trilemma, SSPA has developed a hybrid testing methodology where rpm and azimuth-controlled fans are used to mimic the aerodynamic forces from the sails. The approach has successfully been used to study the seakeeping % maneuvering behaviour of Oceanbird.
ADVANCED SEAKEEPING SIMULATIONS
An alternative to model testing in waves is computer simulations. A popular choice here is RANS-based methods. Such simulations come with a relatively large computational cost. On the other end of the spectrum are simple Velocity Prediction Programs (VPPs). These are computationally efficient but do not resolve all the flow physics.
To strike the balance between computational effort on one side and accuracy of results on the other, SSPA and FLOWTECH International have developed SHIPFLOW motions 7 for sailing ships. The method behind this tool is based on coupling an unsteady 3D fully nonlinear potential flow hydrodynamic model to a hybrid 2D RANS/3D lifting-line aerodynamic model.
Applications of SHIPFLOW motions 7 include structural fatigue analysis of wind propulsion systems, sail dynamics, parametric roll, sheeting strategies and appendage configuration studies.
F. C. Gerhardt, M. Kjellberg, I. Wigren, S. Werner, M. Razola, The horns of the trilemma: seakeeping model tests for a wind powered vessel, In the Proceedings of the RINA Conference Wind Propulsion, 2021, London, UK
Kjellberg, M., Gerhardt, F., Werner, S., Sailing in waves: A numerical method for analysis of seakeeping performance and dynamic behavior of a wind powered ship, accepted to the 24th Chesapeake Sailing Yacht Symposium, 2022, Annapolis.