Here is the answer, obtained by experimentation. This is a fundamental question of applied physics, namely when "viscous drag" becomes a dominant force.
It is amazing how heavily this investigation was regulated:
The most troublesome part of the experiment was getting permission to do it in the first place. Cussler and Gettelfinger had to obtain 22 separate kinds of approval, including persuading the local authorities that it was okay to put their syrup down the drain afterwards.