[null and void]

Good point, and I didn’t know that little factoid.

It doesn’t even require any particular malice or neglect or extra care on the parts of the respective captains. The slavers started with cargoes of healthy able-bodied young men and women; the coffin ships, the very young, the elderly, the sickly and half-starved refugees yearning to breathe free.

[Sherman Logan]

For one period for which there are reasonably accurate records, the mortality on slave ships varied from 2% to 11%, with the average around 6%.

http://www.collegeboard.com/prod_downloads/yes/4297_MODULE_12.pdf

I would not be at all surprised if mortality on immigrant ships during this period was in the same vicinity or higher. To be fair, this was near the end of the legal slave trade period. Mortality was probably a lot higher in earlier periods.

This is not to say there wasn't an immense death toll caused by the trade. Slaves also died during the capture process, the trek to the coast, while in the barracoons waiting to be sold, and during the "seasoning" process in the New World. The cumulative toll was probably well above 50%.

But the notion that slave ships, as such, were immensely deadly just isn't true, on average, though there were no doubt horrible exceptions.

At the time, however, long ocean voyages were dangerous for anybody.