Comparing the headline contract speed with the average actual speed is the most common mistake in performance monitoring. Without a weather adjustment, every rough passage looks like underperformance.
The right comparison
Build a digital twin that predicts expected speed for the weather actually encountered, then compare the warranty only within good-weather periods. The gap that remains is genuine underperformance — and it is defensible either way.
BF ≤ 4the good-weather band within which warranted speed is typically assessed.