Some claim that the boat feels heavier or sluggish when moving upstream but more seem to feel this effect when moving downstream. When moving downstream the current adds to the boat's velocity relative to the water making the shore pass by more quickly. This, I suppose, could subtract or add to perceptions of sluggishness but it is an effect which is purely subjective.
What is real is air resistance; the relative "wind" produced by the current. A current produces a wind speed exactly equal to the speed of the current so that, if you were simply sitting in your boat drifting down, you would notice a wind "blowing" upstream--actually drifting you upstream. This relative wind is a help when rowing upstream and a hindrance when rowing downstream. In order to maintain the same speed relative to the water you must work harder going down than up.
I have run the ROWING model for a single (heavy man) in a current of one
knot (0.51 m/s) while holding the (water) shell speed and the rower's
oarhandle effort constant. The results are as follows:
Upstream Downstream ---------- ---------- Current (and wind speed), m/s (-) 0.51 (+) 0.51 Shell speed (avg. rel. water), m/s 4.62 4.62 Peak oarhandle effort, avg., N 677 677 Stroke rating, 1/min 26.70 27.69 Total rower power, Watts 539 565 a 4.8% increase System mechanical efficiency 0.642 0.632 Shell speed relative to shore, m/s 4.11 5.13 Air resist. as % of water resist. 10 10I should add that this model stream is everywhere uniform; containing no crossection differentials having the water under the shell moving at a different speed from the water at the oarblades.