The three paintings on the predella of the Perugia Triptych depict the story of St Nicholas.
This is the second predella painting. In the left two-thirds, we see St. Nicholas' negotiations to secure wheat for Bari (see the Golden Legend) during a famine. To the left, we see the miracle of the tempest.
Also available at this site: the first predella painting and a detail from the third.
More of St.