Complex sentence: Such was the difficulty of this manoeuvre that it could only have been contemplated with intimate knowledge of the coastline, and the anchorage ensured that the only wind that might drive the ships off would bring them clear to the open sea.
Split into simpler sentences: Such was the difficulty of this manoeuvre that it could only have been contemplated with prior knowledge of the coastline. The anchorage ensured that the only wind that might drive the ships off would bring them clear to the open sea.