It's never gonna work for a multitude of reasons like you said. Usually I'm not pessimistic but this really seems like: we have an idea, make a video, post it on a site, ask questions later.
They claim a save of 400kW per sail, with a boat cruising at 22kts. Let's do some math:
- 400kW is 400e6 J/s, or 400e6 N.m/s
- 22kts is 11.31m/s
So the forward force must be 400e6/11.31 = 3.5e7N, so around 3500 metric tons of force for a single sail. And with a relative wind coming from the front usually, this means the side force will be much bigger than that.
I am calling this BS