A rig-trim-simulation for sail boats, 420 class. You can change the length of the forestay, or the length of the shrouds. The spreaders are also adjustable. The graphics, implemented here as a 3D model, always shows the current situation. By introducing a joint at point D, the spreader attachment position on the mast, the structure is modeled as a mechanical linkage. The simulation is instructive, but nothing is otherwise claimed. (It can however be shown that an attempt to compute with even higher precision is not useful.) The full application runs on the desktop.