Some years ago my folks and I flew into Manaus and then on to Sao Gabriel de Cachoiera, (so we had gone up the Rio Negro which joins the Solimoies there at Manaus.) From there we took a skiff with an outboard
up to the border of Venezuela
to see the Casiquiare canal and then continued a short way up into Venezuela. The river surface was like glass the whole way. This was in July and aside from the occasional rain squall, there wasn't a breath of wind
. The trip was absolutely beautiful, but you will be motoring a VERY, VERY LONG WAY. At least on the Rio Negro you probably won't be able to get past the "cachoiera" which means rapids in Portuguese. As far as going up the river, yes you have to avoid shallows in the low water
season, but also you need to know where you are to avoid going up dead end channels, and there are a lot of them. You have probably seen Google Earth
photos of the river. The Rio Negro is nice in that owing to the acidic tea-colored water (hence "black river") there are fewer mosquitoes and piranha, or at least they aren't biting so much there. Before you do it with a sailboat, go visit first to at least be better prepared. Have a good strong diesel
with plenty of tankage!