Whereabouts would a man be likely to leave for Germany, a man in a hurry, who wanted a speedy and a secret passage?
Not from any of the big harbours.

And not from the Channel or the West Coast or Scotland, for, remember, he was starting from London.

I measured the distance on the map, and tried to put myself in the enemy's shoes.

I should try for Ostend or Antwerp or Rotterdam, and I should sail from somewhere on the East Coast between Cromer and Dover.
All this was very loose guessing, and I don't pretend it was ingenious or scientific.

I wasn't any kind of Sherlock Holmes.