Just cause it is a sailing vessel does not mean it does not have an engine.
It probably uses a couple of 100 liters of disel to get in and out of port, driving generators and heating the boat.
On top of that the sails will degrade with time and the ship will need to be repaired and there is allot more crew and manpower needed per unit of cargo.
Everything has environmental cost in CO2 emissions, and the amount of useful work you get from a large container ship per amount CO2 emissions is insane. The only thing that can get you those kinds of numbers is scale.
According to [0], a typical container ship generate 12.5 g of CO2 per ton per km, which means that one trip from France to NY with 350T of cargo would release about 35T of CO2 in the atmosphere.
I'm convinced that this sail boat generates an order of magnitude less of that (I can't find any estimate of it)
Whatever the initial CO2 cost of the construction of this boat is, I'm sure that it's a net benefits if it's used long enough
1.8 is lower than 3g. If those aims are reached I guess they are "more green" than I thought. But the gain is quite small, if you truck that cargo for 160km all the CO2 savings are gone. A modern truck emits 45g of CO2 per km.
It doesn't really make sense to compare a 350T sail boat with a super tanker, a comparable diesel boat produce 36g per ton per km(according to the same report), the "grain de sail II" aims to replace those, and the gains are huge.
On top of that the sails will degrade with time and the ship will need to be repaired and there is allot more crew and manpower needed per unit of cargo.
Everything has environmental cost in CO2 emissions, and the amount of useful work you get from a large container ship per amount CO2 emissions is insane. The only thing that can get you those kinds of numbers is scale.