Throughout the height of the Atlantic slave trade (1570–1808), slave ships were normally smaller than traditional cargo ships, with most slave ships weighing between 150 and 250 tons. This equated to about 350 to 450 enslaved Africans on each slave ship, or 1.5 to 2.4 per ton. The English ships of the time normally fell on the larger side of this spectrum and the French on the smaller side. Ships purposely designed to be smaller and more maneuverable were meant to navigate the African coastal rivers into farther inland ports; these ships therefore increased the effects of the slave trade on Africa. Additionally, the ships' sizes increased slightly throughout the 1700s; however the number of enslaved Africans per ship remained the same. This reduction in the ratio of enslaved Africans to ship tonnage was designed to increase the amount of space per person and thus improve the survival chances of everyone on board. These ships also had temporary storage decks which were separated by an open latticework or grate bulkhead, Ship masters would presumably use these chambers to divide enslaved Africans and help prevent mutiny. Some ships developed by the turn of the 19th century even had ventilation ports built into the sides and between gun ports (with hatches to keep inclement weather out). These open deck designs increased airflow and thus helped improve survival rates, diminishing potential investment losses.