A whiskey plank is the last plank which needs to be put in place to finish a ship's hull. Although a ship is far from finished once the plank has been installed, mounting it marks a major milestone in the construction of a ship, and many shipbuilders treat it as a cause for celebration. This term is most widely used in the boating community, although it sometimes crops up in other industries as well.
You may also hear the whiskey plank referred to as the shutter plank, since it closes up the hull once it has been put in place. After a whiskey plank has been put in, the hull can be treated, sealed, and painted, and work can begin on the inner fixtures of the ship. Because this marks a turning point in construction, it is traditional to take a brief moment to celebrate the mounting of the whiskey plank; this celebration has traditionally involved shots of whiskey all round.
In the early stages of construction of a ship, it can be hard to see what's going on, and it sometimes seems like nothing is really happening, as planks get nailed or riveted on here and there. Once the whiskey plank goes on, however, the ship really seems to start to take shape, and people can visualize it out on the water. Often, the ship's owners and the owners of the shipyard come to see the whiskey plank installed, and sometimes they will even take a few ceremonial swings of the hammer to celebrate the event.
In shipbuilding communities, the term may also spill over into other forms of construction, thanks to the nautical saturation of regional slang. People may talk about putting the whiskey plank on a home or business, for example, expressing a celebratory air and indicating that a major step has been reached. The term is also sometimes used among woodworkers, especially those who work on traditional wooden ships and boats.
Finishing a ship after completing the hull can still take months, as the interior needs to be fitted, but pausing to mark the completion of the hull is still traditional in many shipyards, with or without whiskey. The hull will also typically require more work, to ensure that the ship is sound and seaworthy.