Are the boats traveling with or against the current?  What other factors might make travel slow going?  During most of the Northern Song dynasty, transport on the Grand Canal included ferrying cargo overland because of shallow portions closer to the capital of Kaifeng.  How do you think the type and size of boats (which, in the lower reaches of the canal system, were accommodated in much greater variety) and the volume of traffic impacted navigability through the capital city?