Once africans boarded the slave ships for the journey to the americas, what had become of them?

About 30% of them died during the voyage

Explanation: The journey was between 6 to 8 weeks, with the Africans tightly packed together to maximise the profits for any given trip. This led to poor hygiene and before long there was dysentery on the ships. Dysentery was the single biggest killer on board slave ships during the middle passage and on average 30% of the human cargo didn't make it to America.

The survivors of the inhumane voyage were sold into servitude, mostly plantations for agriculture. In some colonial states like Virginia, slavery even became a heritable condition based on the mother's status.

One reason why this is believable is because di Coppo Di Stefano Buonaiuti was in Florence at the time when the Black Death hit Florence in 1338. One reason why this passage isn't believable is because he was only two when it happened so might not remember it all.