Can anyone clear up if they are loading two ships at once when...

Can anyone clear up if they are loading two ships at once when there is an overlap? I had worked out that to midnight on the 23/4 we had 37 barges travel from river to Anchorage. With 3 x 3.5kt & 1 x 7kt barges the average barge is roughly 4.3kt. You can see the larger barge as it takes 2.5x as long. With 37 x 4.3kt is 161kt which is more than two ships worth. But the second ship didn't leave until early this morning. To me that indicates either the barges are not at capacity or two ships are being loaded at once which does work out with some of the timings. Often two barges arrive and depart around the same time. This would also make sense trying to make the most of each tide.