Query: What did the French navy lack in engaging coastal defenses?
Context: It was discovered that Prussia had recently built defences around the big North German ports, including coastal artillery batteries with Krupp heavy artillery, which with a range of 4,000 yards (3,700 m), had double the range of French naval guns. The French Navy lacked the heavy guns to engage the coastal defences and the topography of the Prussian coast made a seaborne invasion of northern Germany impossible.