The wooden sailing ships were tiny in comparison to modern liners. The average journey time from the West African coast to the Americas was six to eight weeks and for many slaves this was the worst experience of their lives.
In bad weather the slaves would not be brought up on deck. They were largely untended and kept continuously shackled in the dark holds. They were even served their meals in the hold. It was in these conditions that the slaves suffered the most discomfort, particularly as the air vents often had to be covered.
Owners rarely went on voyages, so it was easier for them to ignore humanitarian concerns.
If the slaves and their rooms can be constantly aired, and they are not detained too long on board, perhaps not many die; but the contrary is often their lot. They are kept down by the weather to breathe a hot and corrupted air, sometimes for a week... the galling of the irons... and the despondency... soon becomes fatal, more instances than one are found of the living and the dead fastened together.John Newton (1788)
The only regulation from British Parliament came very late in the day. The Act is also known as Dolben's Law after the MP and ship inspector who pushed it through Parliament.
The Slave Trade Regulation Act of 1788 reduced the death rate of slaves on ships.