The term float-out, as pertaining to ship building, refers to filling the dry-dock with water and pulling the ship out with tug boats. This float-out procedure typically follows the laying of the keel and the near completion of the ship's hull. Once removed from the dry-dock, the ship is routinely placed back in the dry-dock to be painted and to have the propulsion system, the propellers and any other final assembly completed. This early float-out procedure has virtually replaced and eliminated the traditional launching of ships by sliding them dramatically into the water off a ramp.
Modern ship-building techniques often include the building of a double hull to protect against puncture from ice, grounding and other floating debris. This design, however, creates the possibility of a leak forming between the inner and outer hull. If left to early ship-building techniques without the float-out, the leaks could be virtually unnoticed when the ship was launched after final construction. This could potentially lead to disaster. This problem is virtually eliminated by using the float-out method of ship building to complete a ship's hull.
It is not uncommon for a ship's outer hull to be incomplete at the initiation of the float-out. Once floating, the hull can be completed along with most of the interior decks and the superstructure. Components such as the drive system can be installed in the lower decks, however, it is not until the final dry-dock session that items such as propeller shafts, propellers and the rudder system are often installed. It is also during this period of construction — following the initial float-out — that the hull is painted and the waterline markers are added to the painted detail of the ship's hull. Shaft water seals and other water-related seals and pumps are typically checked during the secondary float-out period.
While not as romantic and thrilling as watching a large ship slide down a launching ramp and into the water, the size and weight of the modern ship make that process nearly impossible. A great deal of damage, including breaking the ship's spine, could result from such a launch. The process of the float-out allows the ship's hull to be equally supported by the rising water as the dry-dock is slowly filled, preventing any unnecessary flexing or bowing of the vessel as it becomes waterborne. This technique also allows a ship to be constructed and built in one area of the world and officially named and launched at a dock in another.