@Schulz
Sea, Air movement and Time all are constraining each other and they must be a compromise.
In 1941 GCD, time is set at 3 months/turn, but this is only for technology, and as @iratoric says
steamship of the time could easily make multiple Atlantic crossings in one month
1941GCD part answer was ships move faster.
Most Aircraft should move 2-4 TT/SZ, as this works for land, so SZ must fit this.