Why It's Unique: With a 76-foot diameter, Yerba BuenaIsland Tunnel is the world's widest single-bore tunnel, an impressivefeat considering it opened in November of 1936.

This machine would also be capable of making a glass-lined tunnel of 40 ft. in diameter or more.

It is an important way for improving the rapidity of a displacement craft to adopt tunnel stern and increase propeller diameter.