How reasonable would be it be to cross the Atlantic Ocean in such a aircraft ?
Would it be more efficient ? How much slower ? How would it fare in turbulent seas ?
It's more efficient than a low flying aircraft, due to reduced drag from ground effect. But it's not more efficient than a high flying airplane getting reduced drag from lower air density. So it works on short distances where the airplane would use a lot of fuel to get to high altitude, but on longer distances it wouldn't be more efficient.
It’s far less efficient crossing the Atlantic than modern jets. The advantage of something like this is it doesn’t need to gain altitude to become efficient so it’s decent for very shorter distances.
