Eva, I agree with that. Here's the best way to encourage:
First 100 meters - yell at them to do their first three pulls extremely fast, you have to get the flywheel up to speed, then emphasize the current speed, since this is when the pulling will be fastest
100 to 200 meters - Mark 30 seconds in, mindless comments on form (mostly to distract)
Mark 250 meters and call it half done (actually this may not be exactly true, based on the initial rate of acceleration, as well as the rate of fatigue)
Mark 300 meters, 375 meters, and 1 minute
Closing in on 500 - keep them aware of their projected finish, basically yell as loud as you can to keep going, give them a target like: 'ten more strong strokes!'
I've found that the last 60 meters there's really nothing you can do, the rower has to make their own choice on whether they want to kill themselves 8 more times, or just ride it out. The flywheel will probably get to 500 total on its own anywhere after 450 if you're travelling fast enough.