Or an experiment? Maybe eventually finding its niche, even if it doesn't solve climate change, but why would the impact be zero? If it does not use oil, then this is CO2 not released.
> The 136-metre-long vessel had to rely partly on its auxiliary motor and its remaining sail after the aft sail was damaged in a storm shortly after departure.
Last summer we motored ~10% of the miles.That includes full day leg when there was practically no wind and few hours occasionally with combined powering. Working towards less or fractional oil based power can have significant benefits.
I expect that it's much worse than zero impact. Don't forget that they've got to build and maintain the ship, feed the crew, and so forth. It seems likely that what they're not using in the actual propulsion of the ship, they're expending anyway - and then some - because they've lost the economies of scale of giant cargo ships.