I've been reading up on the subject, and all the 19th century expeditions to find the fabled Source (culminating in John Hanning Speke's 1858 discovery of Lake Victoria) seem to have been conducted overland from the south or east. Did any explorer ever attempt to sail south from Alexandria to find the Source? My reading suggests that it was possible, as the winds blow southerly up the river in winter.
Couldn't be done, alas. There were six cataracts ( or white-water rapids ) on the Nile that were very difficult to get past with any kind of boat, and the sudd marsh ( a giant marsh or set of marshes) in South Sudan that was a maze of small channels, dead-ends.
Although they're very dated as histories of the time and place, the two books The White Nile and The Blue Nile by Alan Moorehead still present a good and very readable account of the difficulties of exploring the river and its two main branches.