Each curve = boat speed (BSP) as a function of true wind angle (TWA) at one true wind speed (TWS). Move the slider to highlight one wind speed.
The amber dots mark best Velocity-Made-Good upwind and downwind — the optimal TWA to tack or gybe through at that TWS.
Hover the plot to read TWA and interpolated BSP. The no-go zone near 0° is where the boat cannot sail without tacking.
The routing engine uses exactly this lookup, millions of times, to propagate isochrones across a wind field.