The Turtle was used in 1775, during the American Revolutionary War, as the first submersible vessel for warfare. 

The model's main body is split in half and the 6 “accessory” pieces, including propellers, breathing pipes, a drill, and a large explosive charge, are made separate for ease of printing and for ease of replacement as many of the parts are delicate.

*Note that the octagonal prism used to connect both halves of the main body is a tight fit, but it can be pushed into the hole with just your bare hands.

For extra historical background and context:

   David Bushell designed the Turtle as a one-man, hand-propelled submersible to attack HMS warships stationed at New York Harbor. For context, the skinny pillar at the front represents the drill that would be used to make a hole on the underside of an HMS warship. After which, the large rectangular block in the back would attach to the ship's hull and blow it up as it is actually an explosive charge. However, the British warship's hull was too tough for the drill to pierce, and the mission was a failure.