The City Council or equivalent body may be the decision taker for hosting a TS event, but the port authority will always have a say in it, and that will forever be that they will not suffer major disruptions of their money making activites, i.e. commercial traffic. In Antwerp no serious problem in this respect seems likely. In Lisbon they may have to divert a cruise ship or two to Doca Alcantara, otherwise no big problems either. (The departure parades however may cause slight delays in the Schelde and in the Tejo, something lik 2 - 3 hrs for a handful of vessels.) As for traffic in the busy Southern North Sea and the British Channel - the Tall Ships will not be racing there. The start goes from somewhere near Torquay. From Antwerp to there no racing Tall Ship will get in the way of the commercial traffic in any undue manner.
cheers
Uwe