That requires far more experimentation and development, I suspect. Climate control spells as well, in order to deal with the struggles of keeping the ships themselves warm during the winter months.

[Air service was all but dead when Alucard had reawakened. They'll be well stuck in the winter if such things aven't changed.]

Creature attacks would be easy enough to look into, if only by asking a captain of one of the ships along the normal routes. That'd probably reveal something about the development of the thing we're aboard now.