What are three adjectives of U-boats?

Here are three adjectives that could describe U-boats, depending on the context:

1. Stealthy: U-boats were designed for covert operations, using their ability to submerge to avoid detection.

2. Deadly: U-boats were formidable weapons of war, responsible for sinking many ships during both World Wars.

3. Innovative: U-boat technology was cutting-edge for its time, with features like diesel-electric propulsion and sonar.