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THE TRADITIONAL LAUNCHING
Aircraft Carrier, Cavour
The REX launching at Sestri Levante
It was for this reason that during the launching ceremony the ship was blessed, to underline the solemnity of the ritual that assigned to the ship a prestige that could not be compared with that of any other means of transportation. Today shipping is a common activity and it generally involves no risks, or rather, in the case of cruise ships, it even represents a moment of entertainment and enjoyment. Therefore launching is a moment of celebration of the vessel and of the shipyard, rather than a blessing ceremony of the ship and of the seamen who will entrust their lives to her.
That having been said, the technical difficulties of the launch and its appeal remained unchanged throughout the centuries, as unchanged are the physical laws that govern the phenomenon.
In this article we shall describe the traditional launch, stern first, even if there are other ways to launch a ship such as sideways or in the dry-dock.
In the traditional launch the ship is launched stern first for several reasons. In general, the shape of the stern is rounder than the streamlined bow profile, thus providing greater resistance when entering the water. Moreover, the stern provides more buoyancy to lift the ship from the cradle faster than the bow, thus favoring the gradual detachment from the slipway. Finally, the greater beam astern helps to stabilize the ship crosswise as she enters the water and to avoid dangerous listings. For centuries, before dry-dock construction, the launch stern first determined the design of the keel line. As a matter of fact, the keel slope was determined by the slipway slope: since the decks and the superstructures were built on a horizontal plane for practical reasons, the keel was sloped as the slipway - generally with an approximate 6° inclination.
Before the launch, accurate calculations are made to make sure that the ship moves when the last block is removed, that she accelerates gradually without slipping out of control, that she does not enter the water too fast or lift the bow excessively when sliding down.
The basic concept that regulates the dynamics of a ship launch is the study of the longitudinal balance of the three main forces acting on the ship in that moment: the force of gravity, the reaction of the slipway and the upthrust on the submerged hull.
The force of gravity is easy to calculate for it remains constant in direction and intensity during the launch and it is always applied to the center of gravity of the ship. In order to have a more precise data of the weight, before launching the designer inspects the ship to estimate the missing loads and the loads to be removed compared with the loading component, so as to have updated information of the weight and of the center of gravity of the ship ready to be launched.
The other two forces change in intensity and in point of application during the ship sliding and these variations are precisely what cause the phenomena described below. In order to schematize what happens (or what might happen) during the launch, we can "freeze" the most important moments with our imagination. The detachment is when the ship's cradle starts sliding down the slipway; the rotation is when the ship starts detaching from the slipway; the tipping, when the bow lifts from the slipway together with the cradle; the final phase when the bow sinks into the water before leaving the slipway.
Let us start describing the detachment phase. The preparation must be accurate, since launching is an operation that in case a problem emerges it can be neither interrupted nor slowed down. The weight of the ship must be moved from the keel blocks and shores that supported it during construction to the cradle, a supporting structure made of two enormous wooden side slides that will make the ship slide down the slipway with the help of a lubricating grease. Before letting the ship slide down, the blocks are gradually removed. The aft blocks are the most dangerous ones because they support an enormous load. They will be removed last, just before the launch. For this reason, in the past, the removal operation of the aft blocks was dangerous and it was not easy to find personnel willing to carry it out. Not always the declivity of the slipway is sufficient to overcome the initial friction: the use of hydraulic rams or other auxiliary means may be necessary for the cradle to gain momentum. Once the ship starts sliding down the slipway the heat that is created by the friction between the slipway and the cradle is such that it may even fire up the lubricating grease under the cradle's side slides. From the point of view of the forces acting on the ship, after the detachment and during the sliding down phase, the force of gravity and the reaction of the slipway act on the ship. The two forces are not balanced, since the plane of the slipway is sloped and the resulting force is exactly the one that constantly accelerates the ship towards the water (figure 1). At this point, all depends on the length of the slipway. If the slipway were infinitely long there would be no special problem, as a matter of fact as the stern enters into the water it is subject to an increasing upthrust, until it succeeds in lifting from the cradle while it continues sliding down: this phase is called rotation. In this case (figure 2), while the force of gravity does not change, the hydrostatic upthrust increases with the diving of the stern, until
S x (a+b) > P x a
that is to say, until the momentum due to floating is greater than the momentum due to the weight and the stern starts floating. On the contrary, when the slipway is not long enough for the stern to float in the water and allow rotation, the opposite phenomenon occurs: the stern falls downwards with the cradle (since there is no more slipway to support them) and the bow lifts in the tipping phase.
If the tipping lasts a short time no problem occurs, otherwise transverse stability of the ship may be jeopardized. Calculating the minimum necessary length of the slipway to avoid tipping is one of the most important operations that must be done before launching. In the most difficult cases, an extension of the slipway is built to eliminate or reduce this phenomenon.
If we schematize this phenomenon in terms of forces (figure 3) we may say that tipping occurs when the center of gravity of the ship is on the vertical axis of the end of the slipway and
P x a > S x (a+b)
that is to say, the momentum due to the ship's weight is greater than that due to the hydrostatic thrust and the stern tends to submerge more in the water.
The final launching phase is the complete lifting of the ship and of the cradle. In this case, if the slipway is short the cradle at the bow is suddenly missing support and the bow dives downwards.
The diagram of the forces (figure 4) helps us understand this final phase if we consider what happens when the cradle leaves the slipway. If, when the cradle is on the vertical axis of the slipway, the ship is no more balanced between weight and upthrust, that is if
R > 0
then, as soon as the reaction of the slipway is missing, the bow dives into the water after which the natural floating trim is reached. As in the previous case, if the height of the dive is little, no problem occurs during launching, even in the final phase, but this height has to be accurately calculated.
The considerations made so far describe launching from the technical point of view, showing a part of the preparation work that is done before such a delicate operation. Yet, what calculations and diagrams cannot transmit is the sensation of majesty that arises from observing the famous launchings of the past: from REX to Michelangelo. The photographs that were handed down to us express the roaring of the wood and of the chains that slide down the slipway, the clouds of water sprayed towards the sky and the acre smell of lubricating grease burnt by the friction of the cradle's side slides.
The crowd, packed on the quay, applauding when the ship enters the water, not only cheers the arrival of the new ship but also pays the right homage to all those who, from shipwright to designer, contributed to making the launching a mechanism as precise as a watch.