What kind of material is it made of? The same type of material the hull of the ship is made of, steel.
What kind of ships have a bulbous bow? Passenger, container ships, car ships, military, most any ship that draws more than 15 ft of water.
How long have they been around?
Most ships that were built from the 1980s and later have these bubbles installed on them. The bubbles were used as early as the 1930s, but were known as experimental.
You mentioned the bulbous bow is fuel-efficient, how so? Depending on the metrics of the ship, it can reduce the drag through the water up to 15%. Less drag, less friction, and less fuel usage.
How deep does the bulbous bow go? The bubble goes down to the keel (bottom) of the ship.
Can a harbor tug hit the bulbous bow if it gets too close to it? Yes, and when it does, it will shake the whole ship …. steel touching steel is not a good thing.
Could a tug hitting it compromise the integrity of the hull? Of course – any outside force can. Look at what a sheet of ice did to the titanic.